Controlling hydrogen pressure in self-regulating nuclear reactors used to treat a subsurface formation

[0001] This patent application claims priority to U.S. Provisional Patent No. 61/104,974 entitled “SYSTEMS, METHODS, AND PROCESSES UTILIZED FOR TREATING SUBSURFACE FORMATIONS” to Vinegar et al. filed on Oct. 13, 2008 and to U.S. Provisional Patent No. 61/168,498 entitled “SYSTEMS, METHODS, AND PROCESSES UTILIZED FOR TREATING SUBSURFACE HYDROCARBON CONTAINING FORMATIONS” to Vinegar et al. filed on Apr. 10, 2009.

[0002] This patent application incorporates by reference in its entirety each of U.S. Pat. No. 6,688,387 to Wellington et al.; U.S. Pat. No. 6,991,036 to Sumnu-Dindoruk et al.; U.S. Pat. No. 6,698,515 to Karanikas et al.; U.S. Pat. No. 6,880,633 to Wellington et al.; U.S. Pat. No. 6,782,947 to de Rouffignac et al; U.S. Pat. No. 6,991,045 to Vinegar et al.; U.S. Pat. No. 7,073,578 to Vinegar et al.; U.S. Pat. No. 7,121,342 to Vinegar et al.; U.S. Pat. No. 7,320,364 to Fairbanks; U.S. Pat. No. 7,527,094 to McKinzie et al.; U.S. Pat. No. 7,584,789 to Mo et al.; U.S. Pat. No. 7,533,719 to Hinson et al.; and U.S. Pat. No. 7,562,707 to Miller. This patent application incorporates by reference in its entirety each of U.S. Patent Application Publication Nos. 2009-0071652 to Vinegar et al. and 2009-0189617 to Burns et al. This patent application incorporates by reference in its entirety U.S. patent application Ser. No. 12/422,088 to Prince-Wright et al.

[0003] 1. Field of the Invention

[0004] The present invention relates generally to methods and systems for production of hydrocarbons, hydrogen, and/or other products from various subsurface formations such as hydrocarbon containing formations.

[0005] 2. Description of Related Art

[0006] Hydrocarbons obtained from subterranean formations are often used as energy resources, as feedstocks, and as consumer products. Concerns over depletion of available hydrocarbon resources and concerns over declining overall quality of produced hydrocarbons have led to development of processes for more efficient recovery, processing and/or use of available hydrocarbon resources. In situ processes may be used to remove hydrocarbon materials from subterranean formations that were previously inaccessible and/or too expensive to extract using available methods. Chemical and/or physical properties of hydrocarbon material in a subterranean formation may need to be changed to allow hydrocarbon material to be more easily removed from the subterranean formation and/or increase the value of the hydrocarbon material. The chemical and physical changes may include in situ reactions that produce removable fluids, composition changes, solubility changes, density changes, phase changes, and/or viscosity changes of the hydrocarbon material in the formation.

[0007] Large deposits of heavy hydrocarbons (heavy oil and/or tar) contained in relatively permeable formations (for example in tar sands) are found in North America, South America, Africa, and Asia. Tar can be surface-mined and upgraded to lighter hydrocarbons such as crude oil, naphtha, kerosene, and/or gas oil. Surface milling processes may further separate the bitumen from sand. The separated bitumen may be converted to light hydrocarbons using conventional refinery methods. Mining and upgrading tar sand is usually substantially more expensive than producing lighter hydrocarbons from conventional oil reservoirs.

[0008] Obtaining permeability in an oil shale formation between injection and production wells tends to be difficult because oil shale is often substantially impermeable. Drilling such wells may be expensive and time consuming Many methods have attempted to link injection and production wells.

[0009] Many different types of wells or wellbores may be used to treat the hydrocarbon containing formation using an in situ heat treatment process. In some embodiments, vertical and/or substantially vertical wells are used to treat the formation. In some embodiments, horizontal or substantially horizontal wells (such as J-shaped wells and/or L-shaped wells), and/or u-shaped wells are used to treat the formation. In some embodiments, combinations of horizontal wells, vertical wells, and/or other combinations are used to treat the formation. In certain embodiments, wells extend through the overburden of the formation to a hydrocarbon containing layer of the formation. In some situations, heat in the wells is lost to the overburden. In some situations, surface and overburden infrastructures used to support heaters and/or production equipment in horizontal wellbores or u-shaped wellbores are large in size and/or numerous.

[0010] Wellbores for heater, injection, and/or production wells may be drilled by rotating a drill bit against the formation. The drill bit may be suspended in a borehole by a drill string that extends to the surface. In some cases, the drill bit may be rotated by rotating the drill string at the surface. Sensors may be attached to drilling systems to assist in determining direction, operating parameters, and/or operating conditions during drilling of a wellbore. Using the sensors may decrease the amount of time taken to determine positioning of the drilling systems. For example, U.S. Pat. No. 7,093,370 to Hansberry and U.S. Patent Application Publication No. 2009-027041 to Zaeper et al., both of which are incorporated herein by reference, describe a borehole navigation systems and/or sensors to drill wellbores in hydrocarbon formations. At present, however, there are still many hydrocarbon containing formations where drilling wellbores is difficult, expensive, and/or time consuming.

[0011] Heaters may be placed in wellbores to heat a formation during an in situ process. There are many different types of heaters which may be used to heat the formation. Examples of in situ processes utilizing downhole heaters are illustrated in U.S. Pat. No. 2,634,961 to Ljungstrom; U.S. Pat. No. 2,732,195 to Ljungstrom; U.S. Pat. No. 2,780,450 to Ljungstrom; U.S. Pat. No. 2,789,805 to Ljungstrom; U.S. Pat. No. 2,923,535 to Ljungstrom; U.S. Pat. No. 4,886,118 to Van Meurs et al.; and U.S. Pat. No. 6,688,387 to Wellington et al.; each of which is incorporated by reference as if fully set forth herein.

[0012] U.S. Pat. No. 7,575,052 to Sandberg et al. and U.S. Patent Application Publication No. 2008-0135254 to Vinegar et al., each of which are incorporated herein by reference, describe an in situ heat treatment process that utilizes a circulation system to heat one or more treatment areas. The circulation system may use a heated liquid heat transfer fluid that passes through piping in the formation to transfer heat to the formation.

[0013] Patent Application Publication No. 2009-0095476 to Nguyen et al., which is incorporated herein by reference, describes a heating system for a subsurface formation that includes a conduit located in an opening in the subsurface formation. An insulated conductor is located in the conduit. A material is in the conduit between a portion of the insulated conductor and a portion of the conduit. The material may be a salt. The material is a fluid at operating temperature of the heating system. Heat transfers from the insulated conductor to the fluid, from the fluid to the conduit, and from the conduit to the subsurface formation.

[0014] In situ production of hydrocarbons from tar sand may be accomplished by heating and/or injecting fluids into the formation. U.S. Pat. No. 4,084,637 to Todd; U.S. Pat. No. 4,926,941 to Glandt et al.; U.S. Pat. No. 5,046,559 to Glandt, and U.S. Pat. No. 5,060,726 to Glandt, each of which are incorporated herein by reference, describe methods of producing viscous materials from subterranean formations that includes passing electrical current through the subterranean formation. Steam may be injected from the injector well into the formation to produce hydrocarbons.

[0015] U.S. Pat. No. 3,170,842 to Kehler, which is incorporated herein by reference, describes a subcritical, nuclear reactor and neutron-producing means as a heat source.

[0016] U.S. Pat. No. 3,237,689 to Justheim and U.S. Pat. No. 3,766,982 to Justheim, both of which are incorporated herein by reference, describe methods of using a nuclear reactor to provide heat to a heat-exchange medium. U.S. Pat. No. 3,598,182 to Justheim, which is incorporated herein by reference, describes a method of obtaining hot hydrogen from a nuclear reactor utilizing hydrogen as a coolant. The hot hydrogen is used to carry the heat and to hydrogenate kerogen.

[0017] U.S. Pat. No. 4,930,574 to Jager, which is incorporated herein by reference, describes a method for tertiary oil recovery and gas utilization by the introduction of nuclear-heated steam into an oil field and the removal, separation and preparation of an escaping oil-gas-water mixture.

[0018] U.S. Patent Application Publication No. 20070181301 to O'Brien, which is incorporated herein by reference, describes a system and method using nuclear energy sources for energy to fracture the oil shale formations and provide sufficient heat and pressure to produce liquid and gaseous hydrocarbon products.

[0019] As discussed above, there has been a significant amount of effort to develop methods and systems to economically produce hydrocarbons, hydrogen, and/or other products from hydrocarbon containing formations. At present, however, there are still many hydrocarbon containing formations from which hydrocarbons, hydrogen, and/or other products cannot be economically produced. Thus, there is a need for improved methods and systems for heating of a hydrocarbon formation and production of fluids from the hydrocarbon formation. There is also a need for improved methods and systems that reduce energy costs for treating the formation, reduce emissions from the treatment process, facilitate heating system installation, and/or reduce heat loss to the overburden as compared to hydrocarbon recovery processes that utilize surface based equipment.

[0020] Embodiments described herein generally relate to systems, methods, and heaters for treating a subsurface formation. Embodiments described herein also generally relate to heaters that have novel components therein. Such heaters can be obtained by using the systems and methods described herein.

[0021] In certain embodiments, the invention provides one or more systems, methods, and/or heaters. In some embodiments, the systems, methods, and/or heaters are used for treating a subsurface formation.

[0022] In certain embodiments, an in situ heat treatment system for producing hydrocarbons from a subsurface formation includes: a plurality of wellbores in the formation; at least one heater positioned in at least two of the wellbores; and a self-regulating nuclear reactor configured to provide energy to at least one of the heaters to heat the temperature of the formation to temperatures that allow for hydrocarbon production from the formation, wherein a temperature of the self-regulating nuclear reactor is controlled by controlling a pressure of hydrogen supplied to the self-regulating nuclear reactor, and wherein the pressure is regulated based upon formation conditions.

[0023] In certain embodiments, a method of producing hydrocarbons from a subsurface formation includes: forming a plurality of wellbores in the formation; positioning at least one heater in at least two of the wellbores; providing energy to at least one of the heaters to heat the temperature of the formation to temperatures that allow for hydrocarbon production from the formation using a self-regulating nuclear reactor; and controlling a temperature of the self-regulating nuclear reactor by controlling a pressure of hydrogen supplied to the self-regulating nuclear reactor; and regulating the pressure of hydrogen supplied to the self-regulating nuclear reactor based upon formation conditions.

[0024] In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments.

[0025] In further embodiments, treating a subsurface formation is performed using any of the methods, systems, or heaters described herein.

[0026] In further embodiments, additional features may be added to the specific embodiments described herein.

[0270] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

[0271] The following description generally relates to systems and methods for treating hydrocarbons in the formations. Such formations may be treated to yield hydrocarbon products, hydrogen, and other products.

[0272] “Alternating current (AC)” refers to a time-varying current that reverses direction substantially sinusoidally. AC produces skin effect electricity flow in a ferromagnetic conductor.

[0273] “Annular region” is the region between an outer conduit and an inner conduit positioned in the outer conduit.

[0274] “API gravity” refers to API gravity at 15.5° C. (60° F.). API gravity is as determined by ASTM Method D6822 or ASTM Method D1298.

[0275] “ASTM” refers to American Standard Testing and Materials.

[0276] In the context of reduced heat output heating systems, apparatus, and methods, the term “automatically” means such systems, apparatus, and methods function in a certain way without the use of external control (for example, external controllers such as a controller with a temperature sensor and a feedback loop, PID controller, or predictive controller).

[0277] “Asphalt/bitumen” refers to a semi-solid, viscous material soluble in carbon disulfide. Asphalt/bitumen may be obtained from refining operations or produced from subsurface formations.

[0278] “Bare metal” and “exposed metal” refer to metals of elongated members that do not include a layer of electrical insulation, such as mineral insulation, that is designed to provide electrical insulation for the metal throughout an operating temperature range of the elongated member. Bare metal and exposed metal may encompass a metal that includes a corrosion inhibiter such as a naturally occurring oxidation layer, an applied oxidation layer, and/or a film. Bare metal and exposed metal include metals with polymeric or other types of electrical insulation that cannot retain electrical insulating properties at typical operating temperature of the elongated member. Such material may be placed on the metal and may be thermally degraded during use of the heater.

[0279] Boiling range distributions for the formation fluid and liquid streams described herein are as determined by ASTM Method D5307 or ASTM Method D2887. Content of hydrocarbon components in weight percent for paraffins, iso-paraffins, olefins, naphthenes and aromatics in the liquid streams is as determined by ASTM Method D6730. Content of aromatics in volume percent is as determined by ASTM Method D1319. Weight percent of hydrogen in hydrocarbons is as determined by ASTM Method D3343.

[0280] “Bromine number” refers to a weight percentage of olefins in grams per 100 gram of portion of the produced fluid that has a boiling range below 246° C. and testing the portion using ASTM Method D1159.

[0281] “Carbon number” refers to the number of carbon atoms in a molecule. A hydrocarbon fluid may include various hydrocarbons with different carbon numbers. The hydrocarbon fluid may be described by a carbon number distribution. Carbon numbers and/or carbon number distributions may be determined by true boiling point distribution and/or gas-liquid chromatography.

[0282] “Chemically stability” refers to the ability of a formation fluid to be transported without components in the formation fluid reacting to form polymers and/or compositions that plug pipelines, valves, and/or vessels.

[0283] “Clogging” refers to impeding and/or inhibiting flow of one or more compositions through a process vessel or a conduit.

[0284] “Column X element” or “Column X elements” refer to one or more elements of Column X of the Periodic Table, and/or one or more compounds of one or more elements of Column X of the Periodic Table, in which X corresponds to a column number (for example, 13-18) of the Periodic Table. For example, “Column 15 elements” refer to elements from Column 15 of the Periodic Table and/or compounds of one or more elements from Column 15 of the Periodic Table.

[0285] “Column X metal” or “Column X metals” refer to one or more metals of Column X of the Periodic Table and/or one or more compounds of one or more metals of Column X of the Periodic Table, in which X corresponds to a column number (for example, 1-12) of the Periodic Table. For example, “Column 6 metals” refer to metals from Column 6 of the Periodic Table and/or compounds of one or more metals from Column 6 of the Periodic Table.

[0286] “Condensable hydrocarbons” are hydrocarbons that condense at 25° C. and one atmosphere absolute pressure. Condensable hydrocarbons may include a mixture of hydrocarbons having carbon numbers greater than 4. “Non-condensable hydrocarbons” are hydrocarbons that do not condense at 25° C. and one atmosphere absolute pressure. Non-condensable hydrocarbons may include hydrocarbons having carbon numbers less than 5.

[0287] “Coring” is a process that generally includes drilling a hole into a formation and removing a substantially solid mass of the formation from the hole.

[0288] “Cracking” refers to a process involving decomposition and molecular recombination of organic compounds to produce a greater number of molecules than were initially present. In cracking, a series of reactions take place accompanied by a transfer of hydrogen atoms between molecules. For example, naphtha may undergo a thermal cracking reaction to form ethene and H₂.

[0289] “Curie temperature” is the temperature above which a ferromagnetic material loses all of its ferromagnetic properties. In addition to losing all of its ferromagnetic properties above the Curie temperature, the ferromagnetic material begins to lose its ferromagnetic properties when an increasing electrical current is passed through the ferromagnetic material.

[0290] “Diad” refers to a group of two items (for example, heaters, wellbores, or other objects) coupled together.

[0291] “Diesel” refers to hydrocarbons with a boiling range distribution between 260° C. and 343° C. (500-650° F.) at 0.101 MPa. Diesel content is determined by ASTM Method D2887.

[0292] “Enriched air” refers to air having a larger mole fraction of oxygen than air in the atmosphere. Air is typically enriched to increase combustion-supporting ability of the air.

[0293] A “fluid” may be, but is not limited to, a gas, a liquid, an emulsion, a slurry, and/or a stream of solid particles that has flow characteristics similar to liquid flow.

[0294] “Fluid injectivity” is the flow rate of fluids injected per unit of pressure differential between a first location and a second location.

[0295] “Fluid pressure” is a pressure generated by a fluid in a formation. “Lithostatic pressure” (sometimes referred to as “lithostatic stress”) is a pressure in a formation equal to a weight per unit area of an overlying rock mass. “Hydrostatic pressure” is a pressure in a formation exerted by a column of water.

[0296] A “formation” includes one or more hydrocarbon containing layers, one or more non-hydrocarbon layers, an overburden, and/or an underburden. “Hydrocarbon layers” refer to layers in the formation that contain hydrocarbons. The hydrocarbon layers may contain non-hydrocarbon material and hydrocarbon material. The “overburden” and/or the “underburden” include one or more different types of impermeable materials. For example, the overburden and/or underburden may include rock, shale, mudstone, or wet/tight carbonate. In some embodiments of in situ heat treatment processes, the overburden and/or the underburden may include a hydrocarbon containing layer or hydrocarbon containing layers that are relatively impermeable and are not subjected to temperatures during in situ heat treatment processing that result in significant characteristic changes of the hydrocarbon containing layers of the overburden and/or the underburden. For example, the underburden may contain shale or mudstone, but the underburden is not allowed to heat to pyrolysis temperatures during the in situ heat treatment process. In some cases, the overburden and/or the underburden may be somewhat permeable.

[0297] “Formation fluids” refer to fluids present in a formation and may include pyrolyzation fluid, synthesis gas, mobilized hydrocarbons, and water (steam). Formation fluids may include hydrocarbon fluids as well as non-hydrocarbon fluids. The term “mobilized fluid” refers to fluids in a hydrocarbon containing formation that are able to flow as a result of thermal treatment of the formation. “Produced fluids” refer to fluids removed from the formation.

[0298] “Freezing point” of a hydrocarbon liquid refers to the temperature below which solid hydrocarbon crystals may form in the liquid. Freezing point is as determined by ASTM Method D5901.

[0299] “Heat flux” is a flow of energy per unit of area per unit of time (for example, Watts/meter²).

[0300] A “heat source” is any system for providing heat to at least a portion of a formation substantially by conductive and/or radiative heat transfer. For example, a heat source may include electrically conducting materials and/or electric heaters such as an insulated conductor, an elongated member, and/or a conductor disposed in a conduit. A heat source may also include systems that generate heat by burning a fuel external to or in a formation. The systems may be surface burners, downhole gas burners, flameless distributed combustors, and natural distributed combustors. In some embodiments, heat provided to or generated in one or more heat sources may be supplied by other sources of energy. The other sources of energy may directly heat a formation, or the energy may be applied to a transfer medium that directly or indirectly heats the formation. It is to be understood that one or more heat sources that are applying heat to a formation may use different sources of energy. Thus, for example, for a given formation some heat sources may supply heat from electrically conducting materials, electric resistance heaters, some heat sources may provide heat from combustion, and some heat sources may provide heat from one or more other energy sources (for example, chemical reactions, solar energy, wind energy, biomass, or other sources of renewable energy). A chemical reaction may include an exothermic reaction (for example, an oxidation reaction). A heat source may also include a electrically conducting material and/or a heater that provides heat to a zone proximate and/or surrounding a heating location such as a heater well.

[0301] A “heater” is any system or heat source for generating heat in a well or a near wellbore region. Heaters may be, but are not limited to, electric heaters, burners, combustors that react with material in or produced from a formation, and/or combinations thereof.

[0302] “Heavy hydrocarbons” are viscous hydrocarbon fluids. Heavy hydrocarbons may include highly viscous hydrocarbon fluids such as heavy oil, tar, and/or asphalt. Heavy hydrocarbons may include carbon and hydrogen, as well as smaller concentrations of sulfur, oxygen, and nitrogen. Additional elements may also be present in heavy hydrocarbons in trace amounts. Heavy hydrocarbons may be classified by API gravity. Heavy hydrocarbons generally have an API gravity below about 20°. Heavy oil, for example, generally has an API gravity of about 10-20°, whereas tar generally has an API gravity below about 10°. The viscosity of heavy hydrocarbons is generally greater than about 100 centipoise at 15° C. Heavy hydrocarbons may include aromatics or other complex ring hydrocarbons.

[0303] Heavy hydrocarbons may be found in a relatively permeable formation. The relatively permeable formation may include heavy hydrocarbons entrained in, for example, sand or carbonate. “Relatively permeable” is defined, with respect to formations or portions thereof, as an average permeability of 10 millidarcy or more (for example, 10 or 100 millidarcy). “Relatively low permeability” is defined, with respect to formations or portions thereof, as an average permeability of less than about 10 millidarcy. One darcy is equal to about 0.99 square micrometers. An impermeable layer generally has a permeability of less than about 0.1 millidarcy.

[0304] Certain types of formations that include heavy hydrocarbons may also include, but are not limited to, natural mineral waxes, or natural asphaltites. “Natural mineral waxes” typically occur in substantially tubular veins that may be several meters wide, several kilometers long, and hundreds of meters deep. “Natural asphaltites” include solid hydrocarbons of an aromatic composition and typically occur in large veins. In situ recovery of hydrocarbons from formations such as natural mineral waxes and natural asphaltites may include melting to form liquid hydrocarbons and/or solution mining of hydrocarbons from the formations.

[0305] “Hydrocarbons” are generally defined as molecules formed primarily by carbon and hydrogen atoms. Hydrocarbons may also include other elements such as, but not limited to, halogens, metallic elements, nitrogen, oxygen, and/or sulfur. Hydrocarbons may be, but are not limited to, kerogen, bitumen, pyrobitumen, oils, natural mineral waxes, and asphaltites. Hydrocarbons may be located in or adjacent to mineral matrices in the earth. Matrices may include, but are not limited to, sedimentary rock, sands, silicilytes, carbonates, diatomites, and other porous media. “Hydrocarbon fluids” are fluids that include hydrocarbons. Hydrocarbon fluids may include, entrain, or be entrained in non-hydrocarbon fluids such as hydrogen, nitrogen, carbon monoxide, carbon dioxide, hydrogen sulfide, water, and ammonia.

[0306] An “in situ conversion process” refers to a process of heating a hydrocarbon containing formation from heat sources to raise the temperature of at least a portion of the formation above a pyrolysis temperature so that pyrolyzation fluid is produced in the formation.

[0307] An “in situ heat treatment process” refers to a process of heating a hydrocarbon containing formation with heat sources to raise the temperature of at least a portion of the formation above a temperature that results in mobilized fluid, visbreaking, and/or pyrolysis of hydrocarbon containing material so that mobilized fluids, visbroken fluids, and/or pyrolyzation fluids are produced in the formation.

[0308] “Insulated conductor” refers to any elongated material that is able to conduct electricity and that is covered, in whole or in part, by an electrically insulating material.

[0309] “Karst” is a subsurface shaped by the dissolution of a soluble layer or layers of bedrock, usually carbonate rock such as limestone or dolomite. The dissolution may be caused by meteoric or acidic water. The Grosmont formation in Alberta, Canada is an example of a karst (or “karsted”) carbonate formation.

[0310] “Kerogen” is a solid, insoluble hydrocarbon that has been converted by natural degradation and that principally contains carbon, hydrogen, nitrogen, oxygen, and sulfur. Coal and oil shale are typical examples of materials that contain kerogen. “Bitumen” is a non-crystalline solid or viscous hydrocarbon material that is substantially soluble in carbon disulfide. “Oil” is a fluid containing a mixture of condensable hydrocarbons.

[0311] “Kerosene” refers to hydrocarbons with a boiling range distribution between 204° C. and 260° C. at 0.101 MPa. Kerosene content is determined by ASTM Method D2887.

[0312] “Modulated direct current (DC)” refers to any substantially non-sinusoidal time-varying current that produces skin effect electricity flow in a ferromagnetic conductor.

[0313] “Naphtha” refers to hydrocarbon components with a boiling range distribution between 38° C. and 200° C. at 0.101 MPa. Naphtha content is determined by ASTM Method D5307.

[0314] “Nitride” refers to a compound of nitrogen and one or more other elements of the Periodic Table. Nitrides include, but are not limited to, silicon nitride, boron nitride, or alumina nitride.

[0315] “Nitrogen compound content” refers to an amount of nitrogen in an organic compound. Nitrogen content is as determined by ASTM Method D5762.

[0316] “Octane Number” refers to a calculated numerical representation of the antiknock properties of a motor fuel compared to a standard reference fuel. A calculated octane number is determined by ASTM Method D6730.

[0317] “Olefins” are molecules that include unsaturated hydrocarbons having one or more non-aromatic carbon-carbon double bonds.

[0318] “Olefin content” refers to an amount of non-aromatic olefins in a fluid. Olefin content for a produced fluid is determined by obtaining a portion of the produce fluid that has a boiling point of 246° C. and testing the portion using ASTM Method D1159 and reporting the result as a bromine factor in grams per 100 gram of portion. Olefin content is also determined by the Canadian Association of Petroleum Producers (CAPP) olefin method and is reported in percent olefin as 1-decene equivalent.

[0319] “Organonitrogen compounds” refer to hydrocarbons that contain at least one nitrogen atom. Non-limiting examples of organonitrogen compounds include, but are not limited to, alkyl amines, aromatic amines, alkyl amides, aromatic amides, pyridines, pyrazoles, and oxazoles.

[0320] “Orifices” refer to openings, such as openings in conduits, having a wide variety of sizes and cross-sectional shapes including, but not limited to, circles, ovals, squares, rectangles, triangles, slits, or other regular or irregular shapes.

[0321] “P (peptization) value” or “P-value” refers to a numerical value, which represents the flocculation tendency of asphaltenes in a formation fluid. P-value is determined by ASTM method D7060.

[0322] “Perforations” include openings, slits, apertures, or holes in a wall of a conduit, tubular, pipe or other flow pathway that allow flow into or out of the conduit, tubular, pipe or other flow pathway.

[0323] “Periodic Table” refers to the Periodic Table as specified by the International Union of Pure and Applied Chemistry (IUPAC), November 2003. In the scope of this application, weight of a metal from the Periodic Table, weight of a compound of a metal from the Periodic Table, weight of an element from the Periodic Table, or weight of a compound of an element from the Periodic Table is calculated as the weight of metal or the weight of element. For example, if 0.1 grams of MoO₃ is used per gram of catalyst, the calculated weight of the molybdenum metal in the catalyst is 0.067 grams per gram of catalyst.

[0324] “Phase transformation temperature” of a ferromagnetic material refers to a temperature or a temperature range during which the material undergoes a phase change (for example, from ferrite to austenite) that decreases the magnetic permeability of the ferromagnetic material. The reduction in magnetic permeability is similar to reduction in magnetic permeability due to the magnetic transition of the ferromagnetic material at the Curie temperature.

[0325] “Physical stability” refers to the ability of a formation fluid to not exhibit phase separation or flocculation during transportation of the fluid. Physical stability is determined by ASTM Method D7060.

[0326] “Pyrolysis” is the breaking of chemical bonds due to the application of heat. For example, pyrolysis may include transforming a compound into one or more other substances by heat alone. Heat may be transferred to a section of the formation to cause pyrolysis.

[0327] “Pyrolyzation fluids” or “pyrolysis products” refers to fluid produced substantially during pyrolysis of hydrocarbons. Fluid produced by pyrolysis reactions may mix with other fluids in a formation. The mixture would be considered pyrolyzation fluid or pyrolyzation product. As used herein, “pyrolysis zone” refers to a volume of a formation (for example, a relatively permeable formation such as a tar sands formation) that is reacted or reacting to form a pyrolyzation fluid.

[0328] “Residue” refers to hydrocarbons that have a boiling point above 537° C. (1000° F.).

[0329] “Rich layers” in a hydrocarbon containing formation are relatively thin layers (typically about 0.2 m to about 0.5 m thick). Rich layers generally have a richness of about 0.150 L/kg or greater. Some rich layers have a richness of about 0.170 L/kg or greater, of about 0.190 L/kg or greater, or of about 0.210 L/kg or greater. Lean layers of the formation have a richness of about 0.100 L/kg or less and are generally thicker than rich layers. The richness and locations of layers are determined, for example, by coring and subsequent Fischer assay of the core, density or neutron logging, or other logging methods. Rich layers may have a lower initial thermal conductivity than other layers of the formation. Typically, rich layers have a thermal conductivity 1.5 times to 3 times lower than the thermal conductivity of lean layers. In addition, rich layers have a higher thermal expansion coefficient than lean layers of the formation.

[0330] “Smart well technology” or “smart wellbore” refers to wells that incorporate downhole measurement and/or control. For injection wells, smart well technology may allow for controlled injection of fluid into the formation in desired zones. For production wells, smart well technology may allow for controlled production of formation fluid from selected zones. Some wells may include smart well technology that allows for formation fluid production from selected zones and simultaneous or staggered solution injection into other zones. Smart well technology may include fiber optic systems and control valves in the wellbore. A smart wellbore used for an in situ heat treatment process may be Westbay Multilevel Well System MP55 available from Westbay Instruments Inc. (Burnaby, British Columbia, Canada).

[0331] “Subsidence” is a downward movement of a portion of a formation relative to an initial elevation of the surface.

[0332] “Sulfur compound content” refers to an amount of sulfur in an organic compound. Sulfur content is as determined by ASTM Method D4294.

[0333] “Superposition of heat” refers to providing heat from two or more heat sources to a selected section of a formation such that the temperature of the formation at least at one location between the heat sources is influenced by the heat sources.

[0334] “Synthesis gas” is a mixture including hydrogen and carbon monoxide. Additional components of synthesis gas may include water, carbon dioxide, nitrogen, methane, and other gases. Synthesis gas may be generated by a variety of processes and feedstocks. Synthesis gas may be used for synthesizing a wide range of compounds.

[0335] “TAN” refers to a total acid number expressed as milligrams (“mg”) of KOH per gram (“g”) of sample. TAN is as determined by ASTM Method D3242.

[0336] “Tar” is a viscous hydrocarbon that generally has a viscosity greater than about 10,000 centipoise at 15° C. The specific gravity of tar generally is greater than 1.000. Tar may have an API gravity less than 10°.

[0337] A “tar sands formation” is a formation in which hydrocarbons are predominantly present in the form of heavy hydrocarbons and/or tar entrained in a mineral grain framework or other host lithology (for example, sand or carbonate). Examples of tar sands formations include formations such as the Athabasca formation, the Grosmont formation, and the Peace River formation, all three in Alberta, Canada; and the Faja formation in the Orinoco belt in Venezuela.

[0338] “Temperature limited heater” generally refers to a heater that regulates heat output (for example, reduces heat output) above a specified temperature without the use of external controls such as temperature controllers, power regulators, rectifiers, or other devices. Temperature limited heaters may be AC (alternating current) or modulated (for example, “chopped”) DC (direct current) powered electrical resistance heaters.

[0339] “Thermally conductive fluid” includes fluid that has a higher thermal conductivity than air at standard temperature and pressure (STP) (0° C. and 101.325 kPa).

[0340] “Thermal conductivity” is a property of a material that describes the rate at which heat flows, in steady state, between two surfaces of the material for a given temperature difference between the two surfaces.

[0341] “Thermal fracture” refers to fractures created in a formation caused by expansion or contraction of a formation and/or fluids in the formation, which is in turn caused by increasing/decreasing the temperature of the formation and/or fluids in the formation, and/or by increasing/decreasing a pressure of fluids in the formation due to heating.

[0342] “Thermal oxidation stability” refers to thermal oxidation stability of a liquid. Thermal oxidation stability is as determined by ASTM Method D3241.

[0343] “Thickness” of a layer refers to the thickness of a cross section of the layer, wherein the cross section is normal to a face of the layer.

[0344] “Time-varying current” refers to electrical current that produces skin effect electricity flow in a ferromagnetic conductor and has a magnitude that varies with time. Time-varying current includes both alternating current (AC) and modulated direct current (DC).

[0345] “Triad” refers to a group of three items (for example, heaters, wellbores, or other objects) coupled together.

[0346] “Turndown ratio” for the temperature limited heater in which current is applied directly to the heater is the ratio of the highest AC or modulated DC resistance below the Curie temperature to the lowest resistance above the Curie temperature for a given current. Turndown ratio for an inductive heater is the ratio of the highest heat output below the Curie temperature to the lowest heat output above the Curie temperature for a given current applied to the heater.

[0347] A “u-shaped wellbore” refers to a wellbore that extends from a first opening in the formation, through at least a portion of the formation, and out through a second opening in the formation. In this context, the wellbore may be only roughly in the shape of a “v” or “u”, with the understanding that the “legs” of the “u” do not need to be parallel to each other, or perpendicular to the “bottom” of the “u” for the wellbore to be considered “u-shaped”.

[0348] “Upgrade” refers to increasing the quality of hydrocarbons. For example, upgrading heavy hydrocarbons may result in an increase in the API gravity of the heavy hydrocarbons.

[0349] “Visbreaking” refers to the untangling of molecules in fluid during heat treatment and/or to the breaking of large molecules into smaller molecules during heat treatment, which results in a reduction of the viscosity of the fluid.

[0350] “Viscosity” refers to kinematic viscosity at 40° C. unless otherwise specified. Viscosity is as determined by ASTM Method D445.

[0351] “VGO” or “vacuum gas oil” refers to hydrocarbons with a boiling range distribution between 343° C. and 538° C. at 0.101 MPa. VGO content is determined by ASTM Method D5307.

[0352] A “vug” is a cavity, void or large pore in a rock that is commonly lined with mineral precipitates.

[0353] “Wax” refers to a low melting organic mixture, or a compound of high molecular weight that is a solid at lower temperatures and a liquid at higher temperatures, and when in solid form can form a barrier to water. Examples of waxes include animal waxes, vegetable waxes, mineral waxes, petroleum waxes, and synthetic waxes.

[0354] The term “wellbore” refers to a hole in a formation made by drilling or insertion of a conduit into the formation. A wellbore may have a substantially circular cross section, or another cross-sectional shape. As used herein, the terms “well” and “opening,” when referring to an opening in the formation may be used interchangeably with the term “wellbore.”

[0355] A formation may be treated in various ways to produce many different products. Different stages or processes may be used to treat the formation during an in situ heat treatment process. In some embodiments, one or more sections of the formation are solution mined to remove soluble minerals from the sections. Solution mining minerals may be performed before, during, and/or after the in situ heat treatment process. In some embodiments, the average temperature of one or more sections being solution mined may be maintained below about 120° C.

[0356] In some embodiments, one or more sections of the formation are heated to remove water from the sections and/or to remove methane and other volatile hydrocarbons from the sections. In some embodiments, the average temperature may be raised from ambient temperature to temperatures below about 220° C. during removal of water and volatile hydrocarbons.

[0357] In some embodiments, one or more sections of the formation are heated to temperatures that allow for movement and/or visbreaking of hydrocarbons in the formation. In some embodiments, the average temperature of one or more sections of the formation are raised to mobilization temperatures of hydrocarbons in the sections (for example, to temperatures ranging from 100° C. to 250° C., from 120° C. to 240° C., or from 150° C. to 230° C.).

[0358] In some embodiments, one or more sections are heated to temperatures that allow for pyrolysis reactions in the formation. In some embodiments, the average temperature of one or more sections of the formation may be raised to pyrolysis temperatures of hydrocarbons in the sections (for example, temperatures ranging from 230° C. to 900° C., from 240° C. to 400° C. or from 250° C. to 350° C.).

[0359] Heating the hydrocarbon containing formation with a plurality of heat sources may establish thermal gradients around the heat sources that raise the temperature of hydrocarbons in the formation to desired temperatures at desired heating rates. The rate of temperature increase through mobilization temperature range and/or pyrolysis temperature range for desired products may affect the quality and quantity of the formation fluids produced from the hydrocarbon containing formation. Slowly raising the temperature of the formation through the mobilization temperature range and/or pyrolysis temperature range may allow for the production of high quality, high API gravity hydrocarbons from the formation. Slowly raising the temperature of the formation through the mobilization temperature range and/or pyrolysis temperature range may allow for the removal of a large amount of the hydrocarbons present in the formation as hydrocarbon product.

[0360] In some in situ heat treatment embodiments, a portion of the formation is heated to a desired temperature instead of slowly heating the temperature through a temperature range. In some embodiments, the desired temperature is 300° C., 325° C., or 350° C. Other temperatures may be selected as the desired temperature.

[0361] Superposition of heat from heat sources allows the desired temperature to be relatively quickly and efficiently established in the formation. Energy input into the formation from the heat sources may be adjusted to maintain the temperature in the formation substantially at a desired temperature.

[0362] Mobilization and/or pyrolysis products may be produced from the formation through production wells. In some embodiments, the average temperature of one or more sections is raised to mobilization temperatures and hydrocarbons are produced from the production wells. The average temperature of one or more of the sections may be raised to pyrolysis temperatures after production due to mobilization decreases below a selected value. In some embodiments, the average temperature of one or more sections may be raised to pyrolysis temperatures without significant production before reaching pyrolysis temperatures. Formation fluids including pyrolysis products may be produced through the production wells.

[0363] In some embodiments, the average temperature of one or more sections may be raised to temperatures sufficient to allow synthesis gas production after mobilization and/or pyrolysis. In some embodiments, hydrocarbons may be raised to temperatures sufficient to allow synthesis gas production without significant production before reaching the temperatures sufficient to allow synthesis gas production. For example, synthesis gas may be produced in a temperature range from about 400° C. to about 1200° C., about 500° C. to about 1100° C., or about 550° C. to about 1000° C. A synthesis gas generating fluid (for example, steam and/or water) may be introduced into the sections to generate synthesis gas. Synthesis gas may be produced from production wells.

[0364] Solution mining, removal of volatile hydrocarbons and water, mobilizing hydrocarbons, pyrolyzing hydrocarbons, generating synthesis gas, and/or other processes may be performed during the in situ heat treatment process. In some embodiments, some processes may be performed after the in situ heat treatment process. Such processes may include, but are not limited to, recovering heat from treated sections, storing fluids (for example, water and/or hydrocarbons) in previously treated sections, and/or sequestering carbon dioxide in previously treated sections.

[0365] FIG. 1 depicts a schematic view of an embodiment of a portion of the in situ heat treatment system for treating the hydrocarbon containing formation. The in situ heat treatment system may include barrier wells 200. Barrier wells are used to form a barrier around a treatment area. The barrier inhibits fluid flow into and/or out of the treatment area. Barrier wells include, but are not limited to, dewatering wells, vacuum wells, capture wells, injection wells, grout wells, freeze wells, or combinations thereof. In some embodiments, barrier wells 200 are dewatering wells. Dewatering wells may remove liquid water and/or inhibit liquid water from entering a portion of the formation to be heated, or to the formation being heated. In the embodiment depicted in FIG. 1, the barrier wells 200 are shown extending only along one side of heat sources 202, but the barrier wells typically encircle all heat sources 202 used, or to be used, to heat a treatment area of the formation.

[0366] Heat sources 202 are placed in at least a portion of the formation. Heat sources 202 may include heaters such as insulated conductors, conductor-in-conduit heaters, surface burners, flameless distributed combustors, and/or natural distributed combustors. Heat sources 202 may also include other types of heaters. Heat sources 202 provide heat to at least a portion of the formation to heat hydrocarbons in the formation. Energy may be supplied to heat sources 202 through supply lines 204. Supply lines 204 may be structurally different depending on the type of heat source or heat sources used to heat the formation. Supply lines 204 for heat sources may transmit electricity for electric heaters, may transport fuel for combustors, or may transport heat exchange fluid that is circulated in the formation. In some embodiments, electricity for an in situ heat treatment process may be provided by a nuclear power plant or nuclear power plants. The use of nuclear power may allow for reduction or elimination of carbon dioxide emissions from the in situ heat treatment process.

[0367] When the formation is heated, the heat input into the formation may cause expansion of the formation and geomechanical motion. The heat sources may be turned on before, at the same time, or during a dewatering process. Computer simulations may model formation response to heating. The computer simulations may be used to develop a pattern and time sequence for activating heat sources in the formation so that geomechanical motion of the formation does not adversely affect the functionality of heat sources, production wells, and other equipment in the formation.

[0368] Heating the formation may cause an increase in permeability and/or porosity of the formation. Increases in permeability and/or porosity may result from a reduction of mass in the formation due to vaporization and removal of water, removal of hydrocarbons, and/or creation of fractures. Fluid may flow more easily in the heated portion of the formation because of the increased permeability and/or porosity of the formation. Fluid in the heated portion of the formation may move a considerable distance through the formation because of the increased permeability and/or porosity. The considerable distance may be over 1000 m depending on various factors, such as permeability of the formation, properties of the fluid, temperature of the formation, and pressure gradient allowing movement of the fluid. The ability of fluid to travel considerable distance in the formation allows production wells 206 to be spaced relatively far apart in the formation.

[0369] Production wells 206 are used to remove formation fluid from the formation. In some embodiments, production well 206 includes a heat source. The heat source in the production well may heat one or more portions of the formation at or near the production well. In some in situ heat treatment process embodiments, the amount of heat supplied to the formation from the production well per meter of the production well is less than the amount of heat applied to the formation from a heat source that heats the formation per meter of the heat source. Heat applied to the formation from the production well may increase formation permeability adjacent to the production well by vaporizing and removing liquid phase fluid adjacent to the production well and/or by increasing the permeability of the formation adjacent to the production well by formation of macro and/or micro fractures.

[0370] More than one heat source may be positioned in the production well. A heat source in a lower portion of the production well may be turned off when superposition of heat from adjacent heat sources heats the formation sufficiently to counteract benefits provided by heating the formation with the production well. In some embodiments, the heat source in an upper portion of the production well may remain on after the heat source in the lower portion of the production well is deactivated. The heat source in the upper portion of the well may inhibit condensation and reflux of formation fluid.

[0371] In some embodiments, the heat source in production well 206 allows for vapor phase removal of formation fluids from the formation. Providing heating at or through the production well may: (1) inhibit condensation and/or refluxing of production fluid when such production fluid is moving in the production well proximate the overburden, (2) increase heat input into the formation, (3) increase production rate from the production well as compared to a production well without a heat source, (4) inhibit condensation of high carbon number compounds (C₆ hydrocarbons and above) in the production well, and/or (5) increase formation permeability at or proximate the production well.

[0372] Subsurface pressure in the formation may correspond to the fluid pressure generated in the formation. As temperatures in the heated portion of the formation increase, the pressure in the heated portion may increase as a result of thermal expansion of in situ fluids, increased fluid generation and vaporization of water. Controlling rate of fluid removal from the formation may allow for control of pressure in the formation. Pressure in the formation may be determined at a number of different locations, such as near or at production wells, near or at heat sources, or at monitor wells.

[0373] In some hydrocarbon containing formations, production of hydrocarbons from the formation is inhibited until at least some hydrocarbons in the formation have been mobilized and/or pyrolyzed. Formation fluid may be produced from the formation when the formation fluid is of a selected quality. In some embodiments, the selected quality includes an API gravity of at least about 20°, 30°, or 40°. Inhibiting production until at least some hydrocarbons are mobilized and/or pyrolyzed may increase conversion of heavy hydrocarbons to light hydrocarbons Inhibiting initial production may minimize the production of heavy hydrocarbons from the formation. Production of substantial amounts of heavy hydrocarbons may require expensive equipment and/or reduce the life of production equipment.

[0374] In some hydrocarbon containing formations, hydrocarbons in the formation may be heated to mobilization and/or pyrolysis temperatures before substantial permeability has been generated in the heated portion of the formation. An initial lack of permeability may inhibit the transport of generated fluids to production wells 206. During initial heating, fluid pressure in the formation may increase proximate heat sources 202. The increased fluid pressure may be released, monitored, altered, and/or controlled through one or more heat sources 202. For example, selected heat sources 202 or separate pressure relief wells may include pressure relief valves that allow for removal of some fluid from the formation.

[0375] In some embodiments, pressure generated by expansion of mobilized fluids, pyrolysis fluids or other fluids generated in the formation may be allowed to increase although an open path to production wells 206 or any other pressure sink may not yet exist in the formation. The fluid pressure may be allowed to increase towards a lithostatic pressure. Fractures in the hydrocarbon containing formation may form when the fluid approaches the lithostatic pressure. For example, fractures may form from heat sources 202 to production wells 206 in the heated portion of the formation. The generation of fractures in the heated portion may relieve some of the pressure in the portion. Pressure in the formation may have to be maintained below a selected pressure to inhibit unwanted production, fracturing of the overburden or underburden, and/or coking of hydrocarbons in the formation.

[0376] After mobilization and/or pyrolysis temperatures are reached and production from the formation is allowed, pressure in the formation may be varied to alter and/or control a composition of formation fluid produced, to control a percentage of condensable fluid as compared to non-condensable fluid in the formation fluid, and/or to control an API gravity of formation fluid being produced. For example, decreasing pressure may result in production of a larger condensable fluid component. The condensable fluid component may contain a larger percentage of olefins.

[0377] In some in situ heat treatment process embodiments, pressure in the formation may be maintained high enough to promote production of formation fluid with an API gravity of greater than 20°. Maintaining increased pressure in the formation may inhibit formation subsidence during in situ heat treatment. Maintaining increased pressure may reduce or eliminate the need to compress formation fluids at the surface to transport the fluids in collection conduits to treatment facilities.

[0378] Maintaining increased pressure in a heated portion of the formation may surprisingly allow for production of large quantities of hydrocarbons of increased quality and of relatively low molecular weight. Pressure may be maintained so that formation fluid produced has a minimal amount of compounds above a selected carbon number. The selected carbon number may be at most 25, at most 20, at most 12, or at most 8. Some high carbon number compounds may be entrained in vapor in the formation and may be removed from the formation with the vapor. Maintaining increased pressure in the formation may inhibit entrainment of high carbon number compounds and/or multi-ring hydrocarbon compounds in the vapor. High carbon number compounds and/or multi-ring hydrocarbon compounds may remain in a liquid phase in the formation for significant time periods. The significant time periods may provide sufficient time for the compounds to pyrolyze to form lower carbon number compounds.

[0379] Generation of relatively low molecular weight hydrocarbons is believed to be due, in part, to autogenous generation and reaction of hydrogen in a portion of the hydrocarbon containing formation. For example, maintaining an increased pressure may force hydrogen generated during pyrolysis into the liquid phase within the formation. Heating the portion to a temperature in a pyrolysis temperature range may pyrolyze hydrocarbons in the formation to generate liquid phase pyrolyzation fluids. The generated liquid phase pyrolyzation fluids components may include double bonds and/or radicals. Hydrogen (H₂) in the liquid phase may reduce double bonds of the generated pyrolyzation fluids, thereby reducing a potential for polymerization or formation of long chain compounds from the generated pyrolyzation fluids. In addition, H₂ may also neutralize radicals in the generated pyrolyzation fluids. H₂ in the liquid phase may inhibit the generated pyrolyzation fluids from reacting with each other and/or with other compounds in the formation.

[0380] Formation fluid produced from production wells 206 may be transported through collection piping 208 to treatment facilities 210. Formation fluids may also be produced from heat sources 202. For example, fluid may be produced from heat sources 202 to control pressure in the formation adjacent to the heat sources. Fluid produced from heat sources 202 may be transported through tubing or piping to collection piping 208 or the produced fluid may be transported through tubing or piping directly to treatment facilities 210. Treatment facilities 210 may include separation units, reaction units, upgrading units, fuel cells, turbines, storage vessels, and/or other systems and units for processing produced formation fluids. The treatment facilities may form transportation fuel from at least a portion of the hydrocarbons produced from the formation. In some embodiments, the transportation fuel may be jet fuel, such as JP-8.

[0381] Formation fluid may be hot when produced from the formation through the production wells. Hot formation fluid may be produced during solution mining processes and/or during in situ heat treatment processes. In some embodiments, electricity may be generated using the heat of the fluid produced from the formation. Also, heat recovered from the formation after the in situ process may be used to generate electricity. The generated electricity may be used to supply power to the in situ heat treatment process. For example, the electricity may be used to power heaters, or to power a refrigeration system for forming or maintaining a low temperature barrier. Electricity may be generated using a Kalina cycle, Rankine cycle or other thermodynamic cycle. In some embodiments, the working fluid for the cycle used to generate electricity is aqua ammonia.

[0382] FIGS. 2 and 3 depict schematic representations of systems for producing crude products and/or commercial products from the in situ heat treatment process liquid stream and/or the in situ heat treatment process gas stream. As shown, formation fluid 212 enters fluid separation unit 214 and is separated into in situ heat treatment process liquid stream 216, in situ heat treatment process gas 218 and aqueous stream 220. In some embodiments, liquid stream 216 may be transported to other processing units and/or facilities.

[0383] In some embodiments, fluid separation unit 214 includes a quench zone. As produced formation fluid enters the quench zone, quenching fluid such as water, nonpotable water, hydrocarbon diluent, and/or other components may be added to the formation fluid to quench and/or cool the formation fluid to a temperature suitable for handling in downstream processing equipment. Quenching the formation fluid may inhibit formation of compounds that contribute to physical and/or chemical instability of the fluid (for example, inhibit formation of compounds that may precipitate from solution, contribute to corrosion, and/or fouling of downstream equipment and/or piping). The quenching fluid may be introduced into the formation fluid as a spray and/or a liquid stream. In some embodiments, the formation fluid is introduced into the quenching fluid. In some embodiments, the formation fluid is cooled by passing the fluid through a heat exchanger to remove some heat from the formation fluid. The quench fluid may be added to the cooled formation fluid when the temperature of the formation fluid is near or at the dew point of the quench fluid. Quenching the formation fluid near or at the dew point of the quench fluid may enhance solubilization of salts that may cause chemical and/or physical instability of the quenched fluid (for example, ammonium salts). In some embodiments, an amount of water used in the quench is minimal so that salts of inorganic compounds and/or other components do not separate from the mixture. In separation unit 214, at least a portion of the quench fluid may be separated from the quench mixture and recycled to the quench zone with a minimal amount of treatment. Heat produced from the quench may be captured and used in other facilities. In some embodiments, vapor may be produced during the quench. The produced vapor may be sent to gas separation unit 222 and/or sent to other facilities for processing.

[0384] In situ heat treatment process gas 218 may enter gas separation unit 222 to separate gas hydrocarbon stream 224 from the in situ heat treatment process gas. Gas separation unit 222 may include a physical treatment system and/or a chemical treatment system. The physical treatment system may include, but is not limited to, a membrane unit, a pressure swing adsorption unit, a liquid absorption unit, and/or a cryogenic unit. The chemical treatment system may include units that use amines (for example, diethanolamine or di-isopropanolamine), zinc oxide, sulfolane, water, or mixtures thereof in the treatment process. In some embodiments, gas separation unit 222 uses a Sulfinol gas treatment process for removal of sulfur compounds. Carbon dioxide may be removed using Catacarb® (Catacarb, Overland Park, Kans., U.S.A.) and/or Benfield (UOP, Des Plaines, Ill., U.S.A.) gas treatment processes. In some embodiments, the gas separation unit is a rectified adsorption and high pressure fractionation unit. In some embodiments, in situ heat treatment process gas is treated to remove at least 50%, at least 60%, at least 70%, at least 80% or at least 90% by volume of ammonia present in the gas stream.

[0385] In gas separation unit 222, treatment of in situ heat conversion treatment gas 218 removes sulfur compounds, carbon dioxide, and/or hydrogen to produce gas hydrocarbon stream 224. In some embodiments, in situ heat treatment process gas 218 includes about 20 vol % hydrogen, about 30% methane, about 12% carbon dioxide, about 14 vol % C₂ hydrocarbons, about 5 vol % hydrogen sulfide, about 10 vol % C₃ hydrocarbons, about 7 vol % C₄ hydrocarbons, about 2 vol % C₅ hydrocarbons, and mixtures thereof, with the balance being heavier hydrocarbons, water, ammonia, COS, thiols and thiophenes. Gas hydrocarbon stream 224 includes hydrocarbons having a carbon number of at least 3. In some embodiments, in situ treatment process gas 218 may be cryogenically treated as described in U.S. Published Patent Application No. 2009-0071652 to Vinegar et al. Cryogenic treatment of an in situ process gas may produce a gas stream acceptable for sale, transportation, and/or use as a fuel. It would be advantageous to separate in situ treatment process gas 218 at the treatment site to produce streams useable as energy sources to lower overall energy costs. For example, streams containing hydrocarbons and/or hydrogen may be used as fuel for burners and/or process equipment. Streams containing sulfur compounds may be used as fuel for burners. Streams containing one or more carbon oxides and/or hydrocarbons may be used to form barriers around a treatment site. Streams containing hydrocarbons having a carbon number of at most 2 may be provided to ammonia processing facilities and/or barrier well systems. In situ heat treatment process gas 218 may include a sufficient amount of hydrogen such that the freezing point of carbon dioxide is depressed. Depression of the freezing point of carbon dioxide may allow cryogenic separation of hydrogen and/or hydrocarbons from the carbon dioxide using distillation methods instead of removing the carbon dioxide by cryogenic precipitation methods. In some embodiments, the freezing point of carbon dioxide may be depressed by adjusting the concentration of molecular hydrogen and/or addition of heavy hydrocarbons to the process gas stream.

[0386] In some embodiments, the process gas stream may include microscopic/molecular species of mercury and/or compounds of mercury. The process gas stream may include dissolved, entrained or solid particulates of metallic mercury, ionic mercury, organometallic compounds of mercury (for example, alkyl mercury), or inorganic compounds of mercury (for example, mercury sulfide). The process gas stream may be processed through a membrane filtration system used for filtering liquid hydrocarbon stream 232 described herein and/or as described in International Application No. WO 2008/116864 to Den Boestert et al., which is incorporated herein by reference, to remove mercury or mercury compounds from the process gas stream described below. After filtration, the filtered process gas stream (permeate) may have a mercury content of 100 ppbw (parts per billion by weight) or less, 25 ppbw or less, 5 ppbw or less, 2 ppbw or less, or 1 ppbw or less.

[0387] In some embodiments, the desalting unit may produce a liquid hydrocarbon stream and a salty process liquid stream. In situ heat treatment process liquid stream 216 enters liquid separation unit 226. Separation unit 226 may include one or more distillation units. In liquid separation unit 226, separation of in situ heat treatment process liquid stream 216 produces gas hydrocarbon stream 228, salty process liquid stream 230, and liquid hydrocarbon stream 232. Gas hydrocarbon stream 228 may include hydrocarbons having a carbon number of at most 5. A portion of gas hydrocarbon stream 228 may be combined with gas hydrocarbon stream 224. Salty process liquid stream 230 may be processed as described in the discussion of FIG. 3. Salty process liquid stream 230 may include hydrocarbons having a boiling point above 260° C. In some embodiments and as depicted in FIG. 2, salty process liquid stream 230 enters desalting unit 234. In desalting unit 234, salty process liquid stream 230 may be treated to form liquid stream 236 using known desalting and water removal methods. Liquid stream 236 may enter separation unit 238. In separation unit 238, liquid stream 236 is separated into bottoms stream 240 and hydrocarbon stream 242. In some embodiments, hydrocarbon stream 242 may have a boiling range distribution between about 200° C. and about 350° C., between about 220° C. and 340° C., between about 230° C. and 330° C. or between about 240° C. and 320° C.

[0388] In some embodiments, at least 50%, at least 70%, or at least 90% by weight of the total hydrocarbons in hydrocarbon stream 242 have a carbon number from 8 to 13. About 50% to about 100%, about 60% to about 95%, about 70% to about 90%, or about 75% to 85% by weight of liquid stream may have a carbon number distribution from 8 to 13. At least 50% by weight of the total hydrocarbons in the separated liquid stream may have a carbon number from about 9 to 12 or from 10 to 11.

[0389] In some embodiments, hydrocarbon stream 242 has at most 15%, at most 10%, at most 5% by weight of naphthenes; at least 70%, at least 80%, or at least 90% by weight total paraffins; at most 5%, at most 3%, or at most 1% by weight olefins; and at most 30%, at most 20%, or at most 10% by weight aromatics.

[0390] In some embodiments, hydrocarbon stream 242 has a nitrogen compound content of at least 0.01%, at least 0.1% or at least 0.4% by weight nitrogen compound. The separated liquid stream may have a sulfur compound content of at least 0.01%, at least 0.5% or at least 1% by weight sulfur compound.

[0391] Hydrocarbon stream 242 enters hydrotreating unit 244. In hydrotreating unit 244, liquid stream 236 may be hydrotreated to form compounds suitable for processing to hydrogen and/or commercial products.

[0392] Liquid hydrocarbon stream 232 from liquid separation unit 226 may include hydrocarbons having a boiling range distribution from about 25° C. to up to about 538° C. or from about 25° C. to about 500° C. at atmospheric pressure. In some embodiments, liquid hydrocarbon stream 232 includes hydrocarbons having a boiling point up to 260° C. Liquid hydrocarbon stream 232 may include entrained asphaltenes and/or other compounds that may contribute to the instability of hydrocarbon streams. For example, liquid hydrocarbon stream 232 is a naphtha/kerosene fraction that includes entrained, partially dissolved, and/or dissolved asphaltenes and/or high molecular weight compounds that may contribute to phase instability of the liquid hydrocarbon stream. In some embodiments, liquid hydrocarbon stream 232 may include at least 0.5% by weight asphaltenes, 1% by weight asphaltenes or at least 5% by weight asphaltenes. In some embodiments, liquid hydrocarbon stream 232 may include at most 5% by volume, at most 3% by volume, or at most 1% by volume of compounds having a boiling point of at least 335° C., at least 500° C. or at least 750° C. at atmospheric pressure.

[0393] In some embodiments, liquid hydrocarbon stream 232 may include small amounts of dissolved, entrained or solid particulates of metals or metal compounds that may not be removed through conventional filtration methods. Metals and/or metal compounds which may be present in the liquid hydrocarbon stream include iron, copper, mercury, calcium, sodium; silicon or compounds thereof. A total amount of metals and/or metal compounds in the liquid hydrocarbon steam may range from 100 ppbw to about 1000 ppbw.

[0394] As properties of the liquid hydrocarbon stream 232 are changed during processing (for example, TAN, asphaltenes, P-value, olefin content, mobilized fluids content, visbroken fluids content, pyrolyzed fluids content, or combinations thereof), the asphaltenes and other components may become less soluble in the liquid hydrocarbon stream. In some instances, components in the produced fluids and/or components in the separated hydrocarbons may form two phases and/or become insoluble. Formation of two phases, through flocculation of asphaltenes, change in concentration of components in the produced fluids, change in concentration of components in separated hydrocarbons, and/or precipitation of components may cause processing problems (for example, plugging) and/or result in hydrocarbons that do not meet pipeline, transportation, and/or refining specifications. In some embodiments, further treatment of the produced fluids and/or separated hydrocarbons is necessary to produce products with desired properties.

[0395] During processing, the P-value of the separated hydrocarbons may be monitored and the stability of the produced fluids and/or separated hydrocarbons may be assessed. Typically, a P-value that is at most 1.0 indicates that flocculation of asphaltenes from the separated hydrocarbons may occur. If the P-value is initially at least 1.0 and such P-value increases or is relatively stable during heating, then this indicates that the separated hydrocarbons are relatively stable.

[0396] Liquid hydrocarbon stream 232 may be treated to at least partially remove asphaltenes and/or other compounds that may contribute to instability. Removal of the asphaltenes and/or other compounds that may contribute to instability may inhibit plugging in downstream processing units. Removal of the asphaltenes and/or other compounds that may contribute to instability may enhance processing unit efficiencies and/or prevent plugging of transportation pipelines.

[0397] Liquid hydrocarbon stream 232 may enter filtration system 246. Filtration system 246 separates at least a portion of the asphaltenes and/or other compounds that contribute to instability from liquid hydrocarbon stream 232. In some embodiments, filtration system 246 is skid mounted. Skid mounting filtration system 246 may allow the filtration system to be moved from one processing unit to another. In some embodiments, filtration system 246 includes one or more membrane separators, for example, one or more nanofiltration membranes or one or more reverse osmosis membranes. Use of a filtration system that operates at below ambient, ambient, or slightly higher than ambient temperatures may reduce energy costs as compared to conventional catalytic and/or thermal methods to remove asphaltenes from a hydrocarbon stream.

[0398] The membranes may be ceramic membranes and/or polymeric membranes. The ceramic membranes may be ceramic membranes having a molecular weight cut off of at most 2000 Daltons (Da), at most 1000 Da, or at most 500 Da. Ceramic membranes may not swell during removal of the desired materials from a substrate (for example, asphaltenes from the liquid stream). In addition, ceramic membranes may be used at elevated temperatures. Examples of ceramic membranes include, but are not limited to, nanoporous and/or mesoporous titania, mesoporous gamma-alumina, mesoporous zirconia, mesoporous silica, and combinations thereof.

[0399] Polymeric membranes may include top layers made of dense membrane and base layers (supports) made of porous membranes. The polymeric membranes may be arranged to allow the liquid stream (permeate) to flow first through the top layers and then through the base layer so that the pressure difference over the membrane pushes the top layer onto the base layer. The polymeric membranes are organophilic or hydrophobic membranes so that water present in the liquid stream is retained or substantially retained in the retentate.

[0400] The dense membrane layer of the polymeric membrane may separate at least a portion or substantially all of the asphaltenes from liquid hydrocarbon stream 232. In some embodiments, the dense polymeric membrane has properties such that liquid hydrocarbon stream 232 passes through the membrane by dissolving in and diffusing through the structure of dense membrane. At least a portion of the asphaltenes may not dissolve and/or diffuse through the dense membrane, thus they are removed. The asphaltenes may not dissolve and/or diffuse through the dense membrane because of the complex structure of the asphaltenes and/or their high molecular weight. The dense membrane layer may include cross-linked structure as described in WO 96/27430 to Schmidt et al., which is incorporated by reference herein. A thickness of the dense membrane layer may range from 1 micrometer to 15 micrometers, from 2 micrometers to 10 micrometers, or from 3 micrometers to 5 micrometers.

[0401] The dense membrane may be made from polysiloxane, poly-di-methyl siloxane, poly-octyl-methyl siloxane, polyimide, polyaramide, poly-tri-methyl silyl propyne, or mixtures thereof. Porous base layers may be made of materials that provide mechanical strength to the membrane. The porous base layers may be any porous membranes used for ultra filtration, nanofiltration, and/or reverse osmosis. Examples of such materials are polyacrylonitrile, polyamideimide in combination with titanium oxide, polyetherimide, polyvinylidenediflouroide, polytetrafluoroethylene, or combinations thereof.

[0402] During separation of asphaltenes from liquid stream 232, the pressure difference across the membrane may range from about 0.5 MPa to about 6 MPa, from about 1 MPa to about 5 MPa, or from about 2 MPa to about 4 MPa. A temperature of the unit during separation may range from the pour point of liquid hydrocarbon stream 232 up to 100° C., from about −20° C. to about 100° C., from about 10° C. to about 90° C., or from about 20° C. to about 85° C. During continuous operation, the permeate flux rate may be at most 50% of the initial flux, at most 70% of the initial flux, or at most 90% of the initial flux. A weight recovery of the permeate on feed may range from about 50% by weight to 97% by weight, from about 60% by weight to 90% by weight, or from about 70% by weight to 80% by weight.

[0403] Filtration system 246 may include one or more membrane separators. The membrane separators may include one or more membrane modules. When two or more membrane separators are used, the separators may be arranged in a parallel-operated (groups of) membrane separators that include a single separation step. In some embodiments, two or more sequential separation steps are performed, where the retentate of the first separation step is used as the feed for a second separation step. Examples of membrane modules include, but are not limited to, spirally wound modules, plate and frame modules, hollow fibers, and tubular modules. Membrane modules are described in Encyclopedia of Chemical Engineering, 4^th Ed., 1995, John Wiley & Sons Inc., Vol. 16, pages 158-164. Examples of spirally wound modules are described in, for example, WO/2006/040307 to Den Boestert et al., U.S. Pat. No. 5,102,551 to Pasternak; U.S. Pat. No. 5,093,002 to Pasternak; U.S. Pat. No. 5,133,851 to Bitter et al.; U.S. Pat. No. 5,275,726 to Feimer et al.; U.S. Pat. No. 5,458,774 to Mannapperuma; and U.S. Pat. No. 7,351,873 to CederlØf et al., all of which are incorporated by reference herein.

[0404] In some embodiments, a spirally wound module is used when a dense membrane is used in filtration system 246. A spirally wound module may include a membrane assembly of two membrane sheets between which a permeate spacer sheet is sandwiched. The membrane assembly may be sealed at three sides. The fourth side is connected to a permeate outlet conduit such that the area between the membranes is in fluid communication with the interior of the conduit. A feed spacer sheet may be arranged on top of one of the membranes. The assembly with feed spacer sheet is rolled up around the permeate outlet conduit to form a substantially cylindrical spirally wound membrane module. The feed spacer may have a thickness of at least 0.6 mm, at least 1 mm, or at least 3 mm to allow sufficient membrane surface to be packed into the spirally wound module. In some embodiments, the feed spacer is a woven feed spacer. During operation, the feed mixture may be passed from one end of the cylindrical module between the membrane assemblies along the feed spacer sheet sandwiched between feed sides of the membranes. Part of the feed mixture passes through either one of the membrane sheets to the permeate side. The resulting permeate flows along the permeate spacer sheet into the permeate outlet conduit.

[0405] In some embodiments, the membrane separation is a continuous process. Liquid stream 232 passes over the membrane due to the pressure difference to obtain filtered liquid stream 248 (permeate) and/or recycle liquid stream 250 (retentate). In some embodiments, filtered liquid stream 248 may have reduced concentrations of asphaltenes and/or high molecular weight compounds that may contribute to phase instability. Continuous recycling of recycle liquid stream 250 through the filter system can increase the production of filtered liquid stream 248 to as much as 95% of the original volume of filtered liquid stream 248. Recycle liquid stream 250 may be continuously recycled through a spirally wound membrane module for at least 10 hours, for at least one day, or for at least one week without cleaning the feed side of the membrane. The flow rate of 250 is used to set a certain required fluid velocity through the membrane modules). The permeate may have a final boiling point of at most 470° C., at most 450° C., or at most 420° C. The permeate may have a final boiling point range from at least 25° C. to about 470° C., from about 50° C. to about 450° C., or at least 75° C. to about 420° C. The permeate may have from about 0.001% to about 5%, from about 0.01% to about 3%, or from about 0.1% to about 1%, by volume of compounds having a boiling point of at least 335° C. The permeate may have undetectable amounts of asphaltenes or substantially undetectable amounts of asphaltenes. The permeate may have a total metal content that is less than about 60% on a weight basis than the metal content of the liquid hydrocarbon stream. For example, the permeate may have a total metal content from about 1 ppbw to about 600 ppbw, from about 10 ppbw to about 300 ppbw, or from about 100 to about 150 ppbw.

[0406] Upon completion of the filtration, asphaltene enriched stream 252 (retentate) may include a high concentration of asphaltenes and/or high molecular weight compounds. In some embodiments, the retentate has at least 50% by volume of compounds having a boiling point of at least 700° C. In an embodiment, the retentate has at least 50%, at least 70%, at least 80%, or at least 90% by volume of compounds having a boiling point of at least 325° C. In an embodiment, the retentate has at least 50% by volume of compounds having a boiling point of at least 350° C., at least 400° C., or at least 700° C. In an embodiment, the permeate has at most 2% by volume of compounds having a boiling point of at least 335° C. and the retentate has at least 25% by volume of compounds having a boiling point of at least 750° C. Asphaltene enriched stream 252 may be provided to separation unit 238 or to other units for further processing.

[0407] At least a portion of filtered liquid stream 248 may be sent to hydrotreating unit 244 for further processing. In some embodiments, at least a portion of filtered liquid stream 248 may be sent to other processing units.

[0408] In some embodiments, at least a portion of or substantially all of filtered liquid stream 248 enters separation unit 254. In separation unit 254, filtered liquid stream 248 may be separated into hydrocarbon stream 256 and liquid hydrocarbon stream 258. Hydrocarbon stream 268 may be rich in aromatic hydrocarbons. Liquid hydrocarbon stream 258 may include a small amount of aromatic hydrocarbons. Liquid hydrocarbon stream 258 may include hydrocarbons having a boiling point up to 260° C. Liquid hydrocarbon stream 258 may enter hydrotreating unit 244 and/or other processing units.

[0409] Hydrocarbon stream 256 may include aromatic hydrocarbons and hydrocarbons having a boiling point up to about 260° C. A content of aromatics in aromatic rich stream 256 may be at most 90%, at most 70%, at most 50%, or most 10% of the aromatic content of filtered liquid stream 248, as measured by UV analysis such as method SMS-2714. Aromatic rich stream 256 may suitable for use as a diluent for undesirable streams that may not otherwise be suitable for additional processing. The undesirable streams may have low P-values, phase instability, and/or asphaltenes. Addition of aromatic rich stream 256 to the undesirable streams may allow the undesirable streams to be processed and/or transported, thus increasing the economic value of the stream undesirable streams. Aromatic rich stream 256 may be sold as a diluent and/or used as a diluent for produced fluids. All or a portion of aromatic rich stream 254 may be recycled to separation unit 226.

[0410] In some embodiments, membrane separation unit 254 includes one or more membrane separators, for example, one or more nanofiltration membranes and/or one or more reverse osmosis membranes. The membrane may be a ceramic membrane and/or a polymeric membrane. The ceramic membrane may be a ceramic membrane having a molecular weight cut off of at most 2000 Daltons (Da), at most 1000 Da, or at most 500 Da.

[0411] The polymeric membrane includes a top layer made of a dense membrane and a base layer (support) made of a porous membrane. The polymeric membrane may be arranged to allow the liquid stream (permeate) to flow first through the dense membrane top layer and then through the base layer so that the pressure difference over the membrane pushes the top layer onto the base layer. The dense polymeric membrane has properties such that as liquid hydrocarbon stream 248 passes through the membrane aromatic hydrocarbons are selectively separated from the liquid hydrocarbon stream to form aromatic rich stream 256. In some embodiments, the dense membrane layer may separate at least a portion of or substantially all of the aromatics from liquid hydrocarbon stream 248. The dense membrane may be a silicon based membrane, a polyamide based membrane and/or a polyol membrane. Aromatic selective membranes may be purchased from W. R. Grace & Co. (New York, U.S.A.), MTR-Inc, California, USA PolyAn (Berlin, Germany), GMT, Rheinfelden, Germany and/or Borsig Membrane Technology (Berlin, Germany).

[0412] Liquid stream 260 (retentate) from membrane separation unit 254 may be recycled back to the membrane separation unit. Continuous recycling of recycle liquid stream 260 idem through nanofiltration system can increase the production of aromatic rich stream 256 to as much as 95% of the original volume of the filtered liquid stream. Recycle liquid stream 260 may be continuously recycled through a spirally wound membrane module for at least 10 hours, for at least one day, for at least one week or until the desired content of aromatics in aromatic rich stream 256 is obtained. Upon completion of the filtration, or when the retentate includes an acceptable amount of aromatics, liquid stream 260 (retentate) from separation unit 254 may be sent to hydrotreating unit 244 and/or other processing units.

[0413] Membranes of separation unit 254 may be ceramic membranes and/or polymeric membranes. During separation of aromatic hydrocarbons from liquid stream 248 in separation unit 254, the pressure difference across the membrane may range from about 0.5 MPa to about 6 MPa, from about 1 MPa to about 5 MPa, or from about 2 MPa to about 4 MPa. Temperature of separation unit 254 during separation may range from the pour point of the liquid hydrocarbon stream 248 up to 100° C., from about −20° C. to about 100° C., from about 10° C. to about 90° C., or from about 20° C. to about 85° C. During a continuous operation, the permeate flux rate may be at most 50% of the initial flux, at most 70% of the initial flux, or at most 90% of the initial flux. A weight recovery of the permeate on feed may range from about 50% by weight to 97% by weight, from about 60% by weight to 90% by weight, or from about 70% by weight to 80% by weight.

[0414] In some embodiments, liquid hydrocarbon streams produced from a formation may include organonitrogen compounds. Organonitrogen compounds are known to poison precious metal catalysts used for treating hydrocarbon streams to make products suitable for commercial sale and/or transportation (for example, transportation fuels and/or lubricating oils). The formation fluids may include nitrogen levels such that process facilities may deem the fluid unsuitable for processing.

[0415] Removal of organonitrogen compounds from the liquid hydrocarbon stream prior to catalytic treatment of the liquid hydrocarbon streams is desirable. Organonitrogen compounds may be removed through catalytic hydrogenation methods and/or solvent extraction methods. Catalytic hydrogenation methods require high temperatures and catalysts that are not subject to poisoning by nitrogen compounds. The catalytic hydrogenation methods may require high temperatures and/or pressures in addition to requiring high amounts of hydrogen. Hydrogen may not be readily available and/or may need to be manufactured. Since hydrogen has to be supplied for denitrogenation, the use of high amounts of hydrogen may increase the overall cost for removal of nitrogen from the fluids such that process facilities deem the fluids unsuitable.

[0416] Liquid hydrocarbon streams may be extracted with aqueous acid streams to produce a hydrocarbon stream having a minimal amount of organonitrogen compounds and an aqueous stream. The aqueous stream may contain organonitrogen salts. Further processing of the aqueous stream (e.g., distillation and/or treatment with base) may result in production of a stream rich in organonitrogen compounds. The stream rich in organonitrogen compounds may be used as diluent for heavy oil and/or sent to other processing units. U.S. Pat. No. 4,287,051 to Curtin describes a method of denitrogenating viscous oils containing a relatively high content of nitrogenous compounds by extracting nitrogenous compounds from a first portion of a viscous oil with an operable acid solvent to produce a raffinate oil having a relatively low concentration of nitrogenous compounds and an extract stream having a high concentration of nitrogenous compounds. The acid solvent is recovered from the extract stream, simultaneously producing a small volume stream of low viscosity oil containing a high concentration of the nitrogenous compounds and referred to as a high nitrogen content oil. The low viscosity high nitrogen content oil is admixed with the remaining first high viscosity bottoms to provide a pumpable mixed stream. Although, aqueous extraction and/or hydrogenation of hydrocarbon streams may produce liquid hydrocarbon streams having a low organonitrogen content, more efficient processes and less costly processes to treat the high nitrogen content oil are desirable. In addition, processes that allow for recycle of waste or low value streams are desirable.

[0417] In some embodiments, as shown in FIGS. 2 and 3, liquid stream 236 includes organonitrogen compounds. In some embodiments, liquid stream 236 includes from about 0.1% by weight to greater than 2% by weight nitrogen compounds. In some embodiments, liquid stream 236 includes from about 0.2% by weight to about 1.5% by weight, or from 0.5% by weight to about 1% by weight, nitrogen compounds. Organonitrogen compounds, for example, alkyl amines, aromatic amines, alkyl amides, aromatic amides, pyridines, pyrazoles, and oxazoles may poison precious metal catalysts used for treating hydrocarbon streams to make products suitable for commercial sale and/or transportation (for example, transportation fuels and/or lubricating oils). Removal of organonitrogen compounds from the liquid hydrocarbon stream prior to catalytic treatment of the liquid hydrocarbon stream may enhance catalyst life of downstream processes. Removal of organonitrogen compounds may allow less severe conditions be used in downstream applications.

[0418] As shown in FIG. 3, liquid stream 236 enters separation unit 262. In some embodiments, liquid stream 236 is passed through one or more filtration units in separation unit 262 to remove solids from the liquid stream. In separation unit 262, liquid stream 236 may be treated with aqueous acid solution 264 to form an aqueous stream 266 and non-aqueous stream 268. In some embodiments, a volume ratio of liquid stream to aqueous acid solution ranges from 0.2 to 0.3, for example, about 0.25. Treatment of liquid stream 236 with aqueous acid solution 264 may be conducted at a temperature ranging from about 90° C. to about 150° C. at pressures ranging from about 0.3 MPa to about 0.4 MPa.

[0419] Non-aqueous stream 268 may include non-organonitrogen hydrocarbons. In some embodiments, non-organonitrogen hydrocarbons include compounds that contain only hydrogen and carbon. In some embodiments, non-aqueous stream 268 contains at most 0.01% by weight organonitrogen compounds. In some embodiments, non-aqueous stream 268 contains from about 200 ppmw to about 1000 ppmw, from about 300 ppmw to about 800 ppmw, or from about 500 ppmw to about 700 ppmw organonitrogen compounds. Non-aqueous stream 268 may enter hydrotreating unit 244 for further processing to make products suitable for transportation and/or sale. In some embodiments, further processing of non-aqueous stream 268 is not necessary.

[0420] Aqueous acid solution 264 includes water and acids suitable to complex with nitrogen compounds (for example, sulfuric acid, phosphoric acid, acetic acid, formic acid, other suitable acidic compounds or mixtures thereof). Aqueous stream 266 includes salts of the organonitrogen compounds, acid, and water. At least a portion of aqueous stream 266 is sent to separation unit 270. In separation unit 270, aqueous stream 266 is separated (for example, distilled) to form aqueous acid stream 264′ and concentrated organonitrogen stream 272. Concentrated organonitrogen stream 272 includes organonitrogen compounds, water, and/or acid. Separated aqueous stream 264′ may be introduced into separation unit 262. In some embodiments, separated aqueous stream 264′ is combined with aqueous acid solution 264 prior to entering the separation unit.

[0421] In some embodiments, at least a portion of aqueous stream 266 and/or concentrated organonitrogen stream 272 are introduced in a hydrocarbon portion or a layer of a subsurface formation that has been at least partially treated by an in situ heat treatment process. Aqueous stream 266 and/or concentrated organonitrogen stream 272 may be heated prior to injection in the formation. In some embodiments, the hydrocarbon portion or layer includes a shale and/or nahcolite (for example, a nahcolite zone in the Piceance Basin). In some embodiments, aqueous stream 266 and/or concentrated organonitrogen stream 272 is used as part of the water source for solution mining nahcolite from the formation. In some embodiments, aqueous stream 266 and/or concentrated organonitrogen stream 272 is introduced in a portion of a formation that contains nahcolite after at least a portion of the nahcolite has been removed. In some embodiments, aqueous stream 266 and/or concentrated organonitrogen stream 272 is introduced in a portion of a formation that contains nahcolite after at least a portion of the nahcolite has been removed and/or the portion has been at least partially treated using an in situ heat treatment process. The hydrocarbon layer may be heated to temperatures above 200° C. prior to introduction of the aqueous stream. Addition of streams that include organonitrogen compounds may increase the permeability of the hydrocarbon layer (for example, increase the permeability of the oil shale layer). Thus, flow of formation fluids from the heated hydrocarbon layer to other sections of the formation may be improved. In the heated formation, the organonitrogen compounds may form non-nitrogen containing hydrocarbons, amines, and/or ammonia and at least some of such non-nitrogen containing hydrocarbons, amines, and/or ammonia may be produced. In some embodiments, at least some of the acid used in the extraction process is produced. Treatment of the liquid stream as described to produce a stream suitable for further processing and introduction of the organonitrogen stream in a portion of the formation provides an improved, economical process to convert streams deemed unsuitable for processing to be converted to commercial products while overall waste is reduced.

[0422] In some embodiments, streams 242, 248, 258, 268, from processes described in FIGS. 2 and 3, enter hydrotreating unit 244 and are contacted with hydrogen in the presence of one or more catalysts to produce hydrotreated liquid streams 274, 276. Hydrotreating may be used to change one or more desired properties of the crude feed to meet transportation and/or refinery specifications using known hydrodemetallation, hydrodesulfurization, hydrodenitrofication techniques. Methods to change one or more desired properties of the crude feed are described in U.S. Published Patent Application No. 2009-0071652 to Vinegar et al.

[0423] In some embodiments, non-aqueous stream 268 is hydrotreated in hydrotreating unit 244 to produce hydrotreated liquid stream 274. Hydrotreated liquid stream 274 has a nitrogen compound content of at most 200 ppm by weight, at most 150 ppm by weight, at most 110 ppm by weight, at most 50 ppm by weight, or at most 10 ppm by weight of nitrogen compounds. The hydrotreated liquid stream may have a sulfur compound content of at most 1000 ppm by weight, at most 500 ppm by weight, at most 300 ppm by weight, at most 100 ppm by weight, or at most 10 ppm by weight of sulfur compounds.

[0424] Asphalt/bitumen compositions are a commonly used material for construction purposes, such as road pavement and/or roofing material. Residues from fractional and/or vacuum distillation may be used to prepare asphalt/bitumen compositions. Alternatively, asphalt/bitumen used in asphalt/bitumen compositions may be obtained from natural resources or by treating a crude oil in a de-asphalting unit to separate the asphalt/bitumen from lighter hydrocarbons in the crude oil. Asphalt/bitumen alone, however, often does not possess all the physical characteristics desirable for many construction purposes. Asphalt/bitumen may be susceptible to moisture loss, permanent deformation (for example, ruts and/or potholes), and/or cracking. Modifiers may be added to asphalt/bitumen to form asphalt/bitumen compositions to improve weatherability of the asphalt/bitumen compositions. Examples, of modifiers include binders, adhesion improvers, antioxidants, extenders, fibers, fillers, oxidants, or combinations thereof. Examples adhesion improvers include fatty acids, inorganic acids, organic amines, amides, phenols, and polyamidoamines. These compositions may have improved characteristics as compared to asphalt/bitumen alone. U.S. Pat. No. 4,325,738 to Plancher et al. describes addition of fractions removed from shale oil that contain high amounts of nitrogen may be used as moisture damage inhibiting agents in asphalt/bitumen compositions. The high nitrogen fractions may be obtained by distillation and/or acid extraction. While the composition of the prior art is often effective in improving the weatherability of asphalt-aggregate compositions, asphalt/bitumen compositions having improved resistance to moisture loss, cracking, and deformation are still needed.

[0425] In some embodiments, a residue stream generated from an in situ heat treatment (ISHT) process and/or through further treatment of the liquid stream generated from an ISHT process is blended with asphalt/bitumen to form an ISHT residue/asphalt/bitumen composition. The ISHT residue/asphalt/bitumen blend may have enhanced water sensitivity and/or tensile strength. The ISHT residue/asphalt/bitumen blend may absorb less water and/or have improved tensile strength modulus as compared to other asphalt/bitumen blends made with adhesion improvers. Absorption of less water by ISHT residue/asphalt/bitumen blends may decrease cracking and/or pothole formation in paved roads as compared to asphalt/bitumen blends made with conventional adhesion improvers. Use of ISHT residue in asphalt/bitumen compositions may allow the compositions to be made without or with reduced amounts of expensive adhesion improvers.

[0426] As shown in FIG. 2, ISHT residue may be generated as bottoms stream 240 from separator 238, and/or bottoms stream 278 from hydrotreating unit 244. ISHT residue may have at least 50% by weight or at least 80% by weight or at least 90% by weight of hydrocarbons having a boiling point above 538° C. In some embodiments, ISHT residue has an initial boiling point of at least 400° C. as determined by SIMDIS750, about 50% by weight asphaltenes, about 3% by weight saturates, about 10% by weight aromatics, and about 36% by weight resins as determined by SARA analysis. In some embodiments, ISHT residue may have a total metal content of about 1 ppm to about 500 ppm, from about 10 ppm to about 400 ppm, or from about 100 ppm to about 300 ppm of metals from Columns 1-14 of the Periodic Table. In some embodiments, ISHT residue may include about 2 ppm aluminum, about 5 ppm calcium, about 100 ppm iron, about 50 ppm nickel, about 10 ppm potassium, about 10 ppm of sodium, and about 5 ppm vanadium as determined by ICP test method such as ASTM Test Method D5185. ISHT residue may be a hard material. For example, ISHT residue may exhibit a penetration of at most 3 at 60° C. (0.1 mm) as measured by ASTM Test Method D243, and a ring-and-ball (R&B) temperature of about 139° C. as determined by ASTM Test Method D36.

[0427] A blend of ISHT residue and asphalt/bitumen may be prepared by reducing the particle size of the ISHT residue (for example, crushing or pulverizing the ISHT residue) and heating the crushed ISHT residue to soften the ISHT particles. The ISHT residue may melt at temperatures above 200° C. Hot ISHT residue may be added to asphalt/bitumen at a temperature ranging from about 150° C. to about 200° C., from about 180° C. to about 195° C., or from about 185° C. to about 195° C. for a period of time to form an ISHT residue/asphalt/bitumen blend.

[0428] The ISHT residue/asphalt/bitumen composition may include from about 0.001% by weight to about 50% by weight, from about 0.05% by weight to about 25% by weight, or from about 0.1% by weight to about 5% by weight of ISHT residue. The ISHT residue/asphalt/bitumen composition may include from about 99.999% by weight to about 50% by weight, from about 99.05% by weight to about 75% by weight, and from about 99.9% by weight to about 95% by weight of asphalt/bitumen. In some embodiments, the blend may include about 20% by weight ISHT residue and about 80% by weight asphalt/bitumen or about 8% by weight ISHT residue and 92% by weight asphalt/bitumen. In some embodiments, additives may be added to the ISHT residue/asphalt/bitumen composition. Additives include, but are not limited to, antioxidants, extenders, fibers, fillers, oxidants, or mixtures thereof.

[0429] The ISHT residue/asphalt/bitumen composition may be used as a binder in paving and/or roofing applications, for example, road paving, shingles, roofing felts, paints, pipecoating, briquettes, thermal and/or phonic insulation, and clay pigeons. In some embodiments, a sufficient amount of ISHT residue may be mixed with asphalt/bitumen to produce an ISHT residue/asphalt/bitumen composition having a 70/100 penetration grade as measured according to EN1426. For example, a mixture of about 8% by weight of ISHT residue and about 91% asphalt/bitumen has a penetration between 70 and 100. The ISHT residue/asphalt/bitumen blend of 70/100 penetration grade is suitable for paving applications.

[0430] Many wells are needed for treating the hydrocarbon formation using the in situ heat treatment process. In some embodiments, vertical or substantially vertical wells are formed in the formation. In some embodiments, horizontal or u-shaped wells are formed in the formation. In some embodiments, combinations of horizontal and vertical wells are formed in the formation.

[0431] A manufacturing approach for forming wellbores in the formation may be used due to the large number of wells that need to be formed for the in situ heat treatment process. The manufacturing approach may be particularly applicable for forming wells for in situ heat treatment processes that utilize u-shaped wells or other types of wells that have long non-vertically oriented sections. Surface openings for the wells may be positioned in lines running along one or two sides of the treatment area. FIG. 4 depicts a schematic representation of an embodiment of a system for forming wellbores of the in situ heat treatment process.

[0432] The manufacturing approach for forming wellbores may include: 1) delivering flat rolled steel to near site tube manufacturing plant that forms coiled tubulars and/or pipe for surface pipelines; 2) manufacturing large diameter coiled tubing that is tailored to the required well length using electrical resistance welding (ERW), wherein the coiled tubing has customized ends for the bottom hole assembly (BHA) and hang off at the wellhead; 3) deliver the coiled tubing to a drilling rig on a large diameter reel; 4) drill to total depth with coil and a retrievable bottom hole assembly; 5) at total depth, disengage the coil and hang the coil on the wellhead; 6) retrieve the BHA; 7) launch an expansion cone to expand the coil against the formation; 8) return empty spool to the tube manufacturing plant to accept a new length of coiled tubing; 9) move the gantry type drilling platform to the next well location; and 10) repeat.

[0433] In situ heat treatment process locations may be distant from established cities and transportation networks. Transporting formed pipe or coiled tubing for wellbores to the in situ process location may be untenable due to the lengths and quantity of tubulars needed for the in situ heat treatment process. One or more tube manufacturing facilities 280 may be formed at or near to the in situ heat treatment process location. The tubular manufacturing facility may form plate steel into coiled tubing. The plate steel may be delivered to tube manufacturing facilities 280 by truck, train, ship or other transportation system. In some embodiments, different sections of the coiled tubing may be formed of different alloys. The tubular manufacturing facility may use ERW to longitudinally weld the coiled tubing.

[0434] Tube manufacturing facilities 280 may be able to produce tubing having various diameters. Tube manufacturing facilities may initially be used to produce coiled tubing for forming wellbores. The tube manufacturing facilities may also be used to produce heater components, piping for transporting formation fluid to surface facilities, and other piping and tubing needs for the in situ heat treatment process.

[0435] Tube manufacturing facilities 280 may produce coiled tubing used to form wellbores in the formation. The coiled tubing may have a large diameter. The diameter of the coiled tubing may be from about 4 inches to about 8 inches in diameter. In some embodiments, the diameter of the coiled tubing is about 6 inches in diameter. The coiled tubing may be placed on large diameter reels. Large diameter reels may be needed due to the large diameter of the tubing. The diameter of the reel may be from about 10 m to about 50 m. One reel may hold all of the tubing needed for completing a single well to total depth.

[0436] In some embodiments, tube manufacturing facilities 280 has the ability to apply expandable zonal inflow profiler (EZIP) material to one or more sections of the tubing that the facility produces. The EZIP material may be placed on portions of the tubing that are to be positioned near and next to aquifers or high permeability layers in the formation. When activated, the EZIP material forms a seal against the formation that may serve to inhibit migration of formation fluid between different layers. The use of EZIP layers may inhibit saline formation fluid from mixing with non-saline formation fluid.

[0437] The size of the reels used to hold the coiled tubing may prohibit transport of the reel using standard moving equipment and roads. Because tube manufacturing facility 280 is at or near the in situ heat treatment location, the equipment used to move the coiled tubing to the well sites does not have to meet existing road transportation regulations and can be designed to move large reels of tubing. In some embodiments the equipment used to move the reels of tubing is similar to cargo gantries used to move shipping containers at ports and other facilities. In some embodiments, the gantries are wheeled units. In some embodiments, the coiled tubing may be moved using a rail system or other transportation system.

[0438] The coiled tubing may be moved from the tubing manufacturing facility to the well site using gantries 282. Drilling gantry 284 may be used at the well site. Several drilling gantries 284 may be used to form wellbores at different locations. Supply systems for drilling fluid or other needs may be coupled to drilling gantries 284 from central facilities 286.

[0439] Drilling gantry 284 or other equipment may be used to set the conductor for the well. Drilling gantry 284 takes coiled tubing, passes the coiled tubing through a straightener, and a BHA attached to the tubing is used to drill the wellbore to depth. In some embodiments, a composite coil is positioned in the coiled tubing at tube manufacturing facility 280. The composite coil allows the wellbore to be formed without having drilling fluid flowing between the formation and the tubing. The composite coil also allows the BHA to be retrieved from the wellbore. The composite coil may be pulled from the tubing after wellbore formation. The composite coil may be returned to the tubing manufacturing facility to be placed in another length of coiled tubing. In some embodiments, the BHAs are not retrieved from the wellbores.

[0440] In some embodiments, drilling gantry 284 takes the reel of coiled tubing from gantry 282. In some embodiments, gantry 282 is coupled to drilling gantry 284 during the formation of the wellbore. For example, the coiled tubing may be fed from gantry 282 to drilling gantry 284, or the drilling gantry lifts the gantry to a feed position and the tubing is fed from the gantry to the drilling gantry.

[0441] The wellbore may be formed using the bottom hole assembly, coiled tubing and the drilling gantry. The BHA may be self-seeking to the destination. The BHA may form the opening at a fast rate. In some embodiments, the BHA forms the opening at a rate of about 100 meters per hour.

[0442] After the wellbore is drilled to total depth, the tubing may be suspended from the wellhead. An expansion cone may be used to expand the tubular against the formation. In some embodiments, the drilling gantry is used to install a heater and/or other equipment in the wellbore.

[0443] When drilling gantry 284 is finished at well site 288, the drilling gantry may release gantry 282 with the empty reel or return the empty reel to the gantry. Gantry 282 may take the empty reel back to tube manufacturing facility 280 to be loaded with another coiled tube. Gantries 282 may move on looped path 290 from tube manufacturing facility 280 to well sites 288 and back to the tube manufacturing facility.

[0444] Drilling gantry 284 may be moved to the next well site. Global positioning satellite information, lasers and/or other information may be used to position the drilling gantry at desired locations. Additional wellbores may be formed until all of the wellbores for the in situ heat treatment process are formed.

[0445] In some embodiments, positioning and/or tracking system may be utilized to track gantries 282, drilling gantries 284, coiled tubing reels and other equipment and materials used to develop the in situ heat treatment location. Tracking systems may include bar code tracking systems to ensure equipment and materials arrive where and when needed.

[0446] Directionally drilled wellbores may be formed using steerable motors. Deviations in wellbore trajectory may be made using slide drilling systems or using rotary steerable systems. During use of slide drilling systems, the mud motor rotates the bit downhole with little or no rotation of the drilling string from the surface during trajectory changes. The bottom hole assembly is fitted with a bent sub and/or a bent housing mud motor for directional drilling. The bent sub and the drill bit are oriented in the desired direction. With little or no rotation of the drilling string, the drill bit is rotated with the mud motor to set the trajectory. When the desired trajectory is obtained, the entire drilling string is rotated and drills straight rather than at an angle. Drill bit direction changes may be made by utilizing torque/rotary adjusting to control the drill bit in the desired direction.

[0447] By controlling the amount of wellbore drilled in the sliding and rotating modes, the wellbore trajectory may be controlled. Torque and drag during sliding and rotating modes may limit the capabilities of slide mode drilling. Steerable motors may produce tortuosity in the slide mode. Tortuosity may make further sliding more difficult. Many methods have been developed, or are being developed, to improve slide drilling systems. Examples of improvements to slide drilling systems include agitators, low weight bits, slippery muds, and torque/toolface control systems.

[0448] Limitations in slide drilling led to the development of rotary steerable systems. Rotary steerable systems allow directional drilling with continuous rotation from the surface, thus making the need to slide the drill string unnecessary. Continuous rotation transfers weight to the drill bit more efficiently, thus increasing the rate of penetration and distance that can be drilled. Current rotary steerable systems may be mechanically and/or electrically complicated with a consequently high cost of delivery.

[0449] Some mechanized drill pipe rotation systems exist such as Slider™ (Slider, LLC, Houston, Tex., U.S.A.), DSCS (directional steering control system) disclosed in U.S. Pat. No. 6,050,348 to Richarson et al., incorporated by reference as if fully set forth herein, and available from Canrig Drilling Technology Ltd. (Magnolia, Tex., U.S.A.), and Wiggle Steer™ (American Augers, Inc., West Salem, Ohio, U.S.A.). These systems replicate the behavior of a driller when the force required to overcome the sliding drag begins to reduce the available weight on bit. The functionality is to “rock” the drilling string forward and backward with rotation to place a portion of the drilling string in rotation and leaving the lower end of the drilling string sliding. This process, however, has drawbacks such as the periodic reversals mean periodic “not rotating” episodes and consequent inefficiency in transfer of force for weight on the drill bit. The rocking also requires “overhead” between drilling string connection torque capacity and operating torque to ensure the drilling string does not become unscrewed. A dual motor rotating steerable system as described herein may reduce or eliminate many of the drawbacks of conventional rotating steerable systems.

[0450] In some embodiments, a dual motor rotary steerable drilling system is used. The dual motor rotary steerable system allows a bent sub and/or bent housing mud motor to change the trajectory of the drilling while the drilling string remains in rotary mode. The dual motor rotary steerable system uses a second motor in the bottom hole assembly to rotate a portion of the bottom hole assembly in a direction opposite to the direction of rotation of the drilling string. The addition of the second motor may allow continuous forward rotation of a drilling string while simultaneously controlling the drill bit and, thus, the directional response of the bottom hole assembly. In some embodiments, the rotation speed of the drilling string is used in achieving drill bit control.

[0451] FIG. 5 depicts a schematic representation of an embodiment of drilling string 292 with dual motors in bottom hole assembly 294. Drilling string 292 is coupled to bottom hole assembly 294. Bottom hole assembly 294 includes motor 296A and motor 296B. Motor 296A may be a bent sub and/or bent housing steerable mud motor. Motor 296A may drive drill bit 298. Motor 296B may operate in a rotation direction that is opposite to the rotation of drilling string 292 and/or motor 296A. Motor 296B may operate at a relatively low rotary speed and have high torque capacity as compared to motor 296A. Bottom hole assembly 294 may include sensing array 300 between motors 296A, motor 296B. Sensing array 300 may include a collar with various directional sensors and telemetry.

[0452] As noted above, motor 296B may rotate in a direction opposite to the rotation of drilling string 292. In this manner, portions of bottom hole assembly 294 beyond motor 296B may have less rotation in the direction of rotation of drilling string 292. In some embodiments, motor 296B is a reverse-rotation low speed motor. The revolutions per minute (rpm) versus differential pressure relationship for bottom hole assembly 294 may be assessed prior to running drilling string 292 and the bottom hole assembly 294 in the formation to determine the differential pressure at neutral drilling speed (when the drilling string speed is equal and opposite to the speed of motor 296B). Measured differential pressure may be used by a control system during drilling to control the speed of the drilling string relative to the neutral drilling speed.

[0453] In some embodiments, motor 296B is operated at a substantially fixed speed. For example, motor 296B may be operated at a speed of 30 rpm. Other speeds may be used as desired.

[0454] In some embodiments, a mud motor is installed in a bottom hole assembly in an inverted orientation (for example, upside-down from the normal orientation). The inverted mud motor may be operated in a reverse direction of rotation relative to other mud motors, a drill bit, and/or a drilling string. For example, motor 296B, shown in FIG. 5, may be installed in an inverted orientation to produce a relative counter-clockwise rotation in portions of bottom hole assembly 294 distal to motor 296B (see counterclockwise arrow).

[0455] FIG. 6 depicts a schematic representation of an embodiment of drilling string 292 including motor 312 in bottom hole assembly 294. Motor 312 may be a low rpm, high torque motor that includes stator 302, rotor 304, and motor shaft 306. Motor shaft 306 couples to driveshaft 310 of drilling string 292 at connection 308. A bit box may be provided at the end of motor shaft 306. Motor shaft 306 and the bit box may face up-hole. The bit box may be fixed relative to drilling string 292. Stator 302 may rotate counter-clockwise relative to drilling string 292.

[0456] Installing a mud motor in an inverted orientation may allow for the use of off-the-shelf motors to produce counter-rotation and/or non-rotation of selected elements of the bottom hole assembly. During drilling, reactive torque from motor 296A is transferred to motor 312. In some embodiments, a threading kit is used (for example, at connection 308) to adapt a threaded mounting for the mud motor to ensure that a secure connection between an inverted mud motor and its mounting is maintained during drilling. For example, the threading kit may reverse the threads (for example, using left hand threads at connection 308). In some embodiments, the connection includes profile-matched sleeve and/or backoff-protected connection.

[0457] In some embodiments, a tool for steerable drilling is at least 4¾ inches with about 25 rpm at 1500 ft-lbs when flowing at 250 gpm. Such a system may be configured to produce at least 2000 ft-lb torque.

[0458] In some embodiments, the rotation speed of drilling string 292 is used to control the trajectory of the wellbore being formed. For example, drilling string 292 may initially be rotating at 40 rpm, and motor 296B rotates at 30 rpm. The counter-rotation of motor 296B and drilling string 292 results in a forward rotation speed (for example, an absolute forward rotation speed) of 10 rpm in the lower portion of bottom hole assembly 294 (the portion of the bottom hole assembly below motor 296B). When a directional course correction is to be made, the speed of drilling string 292 is changed to the neutral drilling speed. Because drilling string 292 is rotating, there is no need to lift drill bit 298 off the bottom of the borehole. Operating at neutral drilling speed may effectively cancel the torque of the drilling string so that drill bit 298 is subjected to torque induced by motor 296A and the formation.

[0459] One of the problems with existing slide drilling processes is that as the drilling string length increases, it may become more difficult to maintain a stable toolface setting due to torsional energy stored in the drilling string. This torsional energy may cause the drilling string to “wind-up” or store rotations. This wind-up may release unpredictably and cause the end of the drilling string to which the motor is attached to rotate independent of the drilling string at the surface. The continuous rotation of drilling string 292 keeps windup of the drilling string consistent and stabilizes drill bit 298. Directional changes of drill bit 298 may be made by changing the speed of drilling string 292. Using a dual motor rotary steerable system allows the changing of the direction of the drilling string to occur while the drilling string rotates at or near the normal operating rotation speed of drilling string 292.

[0460] FIG. 7 depicts cumulative time operating at a particular drilling string rotation speed and direction during drilling in conventional slide mode. Most of the time, the surface rpm is zero (for example, slide drilling) while some of the time the operator rotates the string forward or backward to influence the toolface position of the steerable mud motor downhole. FIG. 8 depicts cumulative time at rotation speed during directional change for the dual motor drilling string during the drill bit direction change. Drill bit control may be substantially the same as for conventional slide mode drilling where torque/rotary adjustment is used to control the drill bit in the desired direction, but to the effect that 0 rpm on the x-axis of FIG. 7 becomes N (the neutral drilling string speed) in FIG. 8.

[0461] The connection of bottom hole assembly 294 to drilling string 292 of the dual motor rotary steerable system depicted in FIG. 5 may be subjected to the net effect of all the torque components required to rotate the entire bottom hole assembly (including torque generated at drill bit 298 during wellbore formation). Threaded connections along drilling string 292 may include profile-matched sleeves such as those known in the art for utilities drilling systems.

[0462] In some embodiments, a control system used to control wellbore formation includes a system that sets a desired rotation speed of drilling string 292 when direction changes in trajectory of the wellbore are to be implemented. The system may include fine tuning of the desired drilling string rotation speed. The control system may be configured to assume full autonomous control over the wellbore trajectory during drilling.

[0463] In certain embodiments, drilling string 292 is integrated with position measurement and downhole tools (for example, sensing array 312) to autonomously control the hole path along a designed geometry. An autonomous control system for controlling the path of drilling string 292 may utilize two or more domains of functionality. In one embodiment, a control system utilizes at least three domains of functionality including, but not limited to, measurement, trajectory, and control. Measurement may be made using sensor systems and/or other equipment hardware that assess angles, distances, magnetic fields, and/or other data. Trajectory may include flight path calculation and algorithms that utilize physical measurements to calculate angular and spatial offsets of the drilling string. The control system may implement actions to keep the drilling string in the proper path. The control system may include tools that utilize software/control interfaces built into an operating system of the drilling equipment, drilling string, and/or bottom hole assembly.

[0464] In certain embodiments, the control system utilizes position and angle measurements to define spatial and angular offsets from the desired drilling geometry. The defined offsets may be used to determine a steering solution to move the trajectory of the drilling string (thus, the trajectory of the borehole) back into convergence with the desired drilling geometry. The steering solution may be based on an optimum alignment solution in which a desired rate of curvature of the borehole path is set, and required angle change segments and angle change directions for the path are assessed (for example, by computation).

[0465] In some embodiments, the control system uses a fixed angle change rate associated with the drilling string, assesses the lengths of the sections of the drilling string, and assesses the desired directions of the drilling to autonomously execute and control movement of the drilling string. Thus, the control system assesses position measurements and controls of the drilling string to control the direction of the drilling string.

[0466] In some embodiments, differential pressure or torque across motor 296A and/or motor 296B is used to control the rate of penetration. A relationship between rate of penetration, weight-on-bit, and torque may be assessed for drilling string 292. Measurements of torque and the rate of penetration/weight-on-bit/torque relationship may be used to control the feed rate of drilling string 292 into the formation.

[0467] Accuracy and efficiency in forming wellbores in subsurface formations may be affected by the density and quality of directional data during drilling. The quality of directional data may be diminished by vibrations and angular accelerations during rotary drilling, especially during rotary drilling segments of wellbore formation using slide mode drilling.

[0468] In certain embodiments, the quality of the data assessed during rotary drilling is increased by installing directional sensors in a non-rotating housing. FIG. 9 depicts an embodiment of drilling string 292 with non-rotating sensor 314. Non-rotating sensor 314 is located behind motor 296. Motor 296 may be a steerable motor. Motor 296 is located behind drill bit 298. In certain embodiments, sensor 314 is located between non-magnetic components in drilling string 292.

[0469] In some embodiments, non-rotating sensor 314 is located in a sleeve over motor 296. In some embodiments, non-rotating sensor 314 is run on a bottom hole assembly for improved data assessment. In an embodiment, a non-rotating sensor is coupled to and/or driven by a motor that produces relative counter-rotation of the sensor relative to other components of the bottom hole assembly. For example, a sensor may be coupled to the motor having a rotation speed equal and opposite to that of the bottom hole assembly housing to which it is attached so that the absolute rotation speed of the sensor is, or is substantially, zero. In certain embodiments, the motor for a sensor is a mud motor installed in an inverted orientation such as described above relative to FIG. 5.

[0470] In certain embodiments, non-rotating sensor 314 includes one or more transceivers for communicating data either into drilling string 292 within the bottom hole assembly or to similar transceivers in nearby boreholes. The transceivers may be used for telemetry of data and/or as a means of position assessment or verification. In certain embodiments, use of non-rotating sensor 314 is used for continuous position measurement. Continuous position measurement may be useful in control systems used for drilling position systems and/or umbilical position control. In certain embodiments, continuous magnetic ranging is possible using the embodiments depicted in FIG. 9. For example, continuous magnetic ranging may include embodiments described herein such as where a reference magnetic field is generated by passing current through one or more heaters, conductors, and/or casing in adjacent holes/wells.

[0471] In some embodiments, an automatic position control system in combination with a rack and pinion drilling system may be used for forming wellbores in a formation. Use of an automatic position control and/or measurement system in combination with a rack and pinion drilling system may allow wellbores to be drilled more accurately than drilling using manual positioning and calibration. For example, the automatic position system may be continuously and/or semi-continuously calibrated during drilling. FIG. 10 depicts a schematic of a portion of a system including a rack and pinion drive system. Rack and pinion drive system 316 includes, but is not limited to, rack 318, carriage 320, chuck drive system 322, and circulating sleeve 324. Chuck drive system 322 may hold tubular 326. Push/pull capacity of a rack and pinion type system may allow enough force (for example, about 5 tons) to push tubulars into wellbores so that rotation of the tubulars is not necessary. A rack and pinion system may apply downward force on the drill bit. The force applied to the drill bit may be independent of the weight of the drilling string (tubulars) and/or collars. In certain embodiments, collar size and weight is reduced because the weight of the collars is not needed to enable drilling operations. Drilling wellbores with long horizontal portions may be performed using rack and pinion drilling systems because of the ability of the drilling systems to apply force to the drilling bit independent of the vertical length of drill string available to provide weight on bit.

[0472] Rack and pinion drive system 316 may be coupled to automatic position control system 328. Automatic position control system 328 may include, but is not limited to, rotary steerable systems, dual motor rotary steerable systems, and/or hole measurement systems. In some embodiments, a measurement system includes one or more sensors, including, but not limited to, magnetic ranging sensors, non-rotating sensors, and/or canted accelerometers. In some embodiments, one or more heaters are included in one or more tubulars of the rack and pinion drive system. In some embodiments, hole measurement systems are positioned in the heaters.

[0473] In some embodiments, a hole measuring system includes one or more canted accelerometers. Use of canted accelerometers may allow for surveying of a shallow portion of the formation. For example, shallow portions of the formation may have steel casing strings from drilling operations and/or other wells. The steel casings may affect the use of magnetic survey tools in determining the direction of deflection incurred during drilling. Canted accelerometers may be positioned in a bottom hole assembly of a drilling system (for example, a rack and pinion drilling system) with the surface as reference of tubular rotational position. Positioning the canted accelerometers in a bottom hole assembly may allow accurate measurement of inclination and direction of a hole regardless of the influence of nearby magnetic interference sources (for example, casing strings). In some embodiments, the relative rotational position of the tubular is monitored by measuring and tracking incremental rotation of the shaft. By monitoring the relative rotation of tubulars added to existing tubulars, more accurate positioning of tubulars may be achieved. Such monitoring may allow tubulars to be added in a continuous manner.

[0474] In some embodiments, a method of drilling using a rack and pinion system includes continuous downhole measurement. A measurement system may be operated using a predetermined and constant current signal. Distance and direction are calculated continuously downhole. The results of the calculations are filtered and averaged. A best estimate final distance and direction is reported to the surface. When received on the surface, the known along-hole depth and tubular location may be combined with the calculated distance and direction to calculate X, Y and Z position data.

[0475] During drilling with jointed pipes, the time taken to shut down circulation, add the next pipe, re-establish circulation, and continuation of hole making may require a substantial amount of time, particularly when using two-phase circulation systems. Handling tubulars (for example, pipes) has historically been a large safety risk where manual handling techniques have been used. Coiled tubing drilling has had some success in eliminating the need for making connections and manual tubular handling; however, the inability to rotate and the limitations on practical coil diameters may limit the extent to which it can be used.

[0476] In some embodiments, a drilling sequence is used in which tubulars are added to a string without interrupting the drilling process. The tubulars may include jointed connections that allow the tubulars to be connected under pressure. Such a sequence may allow continuous rotary drilling with large diameter tubulars. The tubulars may include heaters and/or automatic position control systems described herein.

[0477] A continuous rotary drilling system may include a drilling platform, which includes, but is not limited to, one or more platforms, a top drive system, and a bottom drive system. The platform may include a rack to allow multiple independent traversing of components. The top drive system may include an extended drive sub (for example, an extended drive system manufactured by American Augers, West Salem, Ohio, U.S.A.). The top drive system may be, for example, a rotary drive system or a rack and pinion drive system. The bottom drive system may include a chuck drive system and a hydraulic system. The bottom drive system may operate in a similar manner to a rack and pinion drilling system (for example, the rack and pinion system depicted in FIG. 10). Bottom drive system and top drive system may alternate control of the drilling operation. The chuck drive system may be mounted on a separate carriage. The hydraulic system may include, but is not limited to, one or more motors and a circulating sleeve. The circulating sleeve may allow circulation between tubulars and the annulus. The circulating sleeve may be used to open or shut off production from various intervals in the well. In some embodiments, a system includes a tubular handling system. A tubular handling system may be automated, manually operated, or a combination thereof.

[0478] In some embodiments, a method using a continuous rotary drilling system includes adding a new tubular to an existing tubular coupled to a bottom drive system to form an extended tubular. During drilling, while the bottom drive system controls the drilling operation, a new tubular may be positioned in an opening of the circulating sleeve of the bottom drive system. The new tubular may be coupled to a top drive system. The circulating sleeve of the bottom drive system may allow fluid to flow around the two tubulars. The fluid pressure in the circulating sleeve may be at pressures of up to about 13.8 MPa (2000 psi). The circulation sleeve may include one or more valves (for example, UBD circulation or check valves) that facilitate change and/or flow of circulation. The use of valves may assist in maintaining pressure in the system. The pressure applied to the two tubulars in the circulating sleeve may couple (for example, pressure-fit) the two tubulars to form a coupled tubular without interruption of the drilling process. During and/or after coupling the tubulars together, control of the drilling operation may be transferred from the bottom drive system to the top drive system. Transfer of the drilling operation to the top drive system may allow the bottom drive system to travel up the coupled tubular towards the top drive system without interruption of the drilling process. The bottom drive system may attach to a drive sub of the top drive system and the control of the drilling operation may be transferred from the top drive system to the bottom drive system without interruption of the drilling process. Once drilling control is transferred to the bottom drive system, the top drive system may disconnect from the tubular. The top drive system may then connect to the top of another tubular to continue the process.

[0479] FIGS. 11A-11D depict a schematic of an embodiment of a continuous drilling sequence. FIG. 12 depicts a cut-away view of an embodiment of a circulating sleeve of the bottom drive system depicted in FIGS. 11A-11D. FIG. 13 depicts a schematic of the valve system of the circulating sleeve of the bottom drive system depicted in FIGS. 11A-11D. Referring to FIGS. 11A-11D, the continuous drilling sequence includes bottom (rack and pinion) drive system 316, tubular handling system 330, and top drive system 332. Top drive system 332 includes top circulating sleeve 334 and drive sub 336. Bottom drive system 316 includes bottom circulating sleeve 324 and chuck 322. In some embodiments, the chuck may be on a separate carriage system. As shown in FIG. 11A, top drive system 332 is at reference line Y and bottom drive system 316 is at reference line Z. It will be understood that reference lines Y and Z are shown for illustrative purposes only, and the heights of the drive systems at various stages in the sequence may be different than those depicted in FIGS. 11A-11D.

[0480] As shown in FIG. 11A, existing tubular 326 is coupled to chuck 322 of bottom drive system 316. Bottom drive system controls the drilling operation that inserts existing tubular 326 in a subsurface formation. During the drilling operation, fluid may enter bottom circulating sleeve 324 through port 346 and flow around existing tubular 326. Fluid may remove heat away from chuck 322 and/or existing tubular 326. Bottom circulating sleeve 324 may include side valve 338 (shown in FIG. 13). Side valve 338 may be a check valve incorporated into a side entry flow and check valve port. Use of side valve 338 and/or top valve 348 (shown in FIG. 13) may facilitate change of circulation entry points and creation of a pressurized system (for example, pressures up to about 13.8 MPa).

[0481] As chuck 322 of bottom drive system 316 continues to control drilling using existing tubular 326, new tubular 340 may be aligned with bottom drive system 316 using tubular handling system 330. Once in position, top drive system 332 may be connected to a top end (for example, a box end) of new tubular 340. As shown in FIG. 11B, top drive system 332 lowers and positions or drops a bottom end of new tubular 340 in opening 344 (depicted in FIG. 12) of circulating sleeve 324 of bottom drive system 316. In some embodiments, bottom circulating sleeve 324 includes side valve 338 (shown in FIG. 13) at port 346 and top entry valve 348 at opening 344 (shown in FIG. 13). Regulation of fluid flow through bottom circulating sleeve 324 using valves 338, 348 may control the pressure in the circulating sleeve. In some embodiments, bottom circulating sleeve 324 may include, and/or operate in conjunction with, one or more valves.

[0482] Opening 344 may include one or more tooljoints 350 (see FIG. 12). Tooljoints 350 may guide entry of new tubular 340 in an inner section of circulating sleeve. Since circulating sleeve 324 is pressurized, tooljoints 350 may allow equalization of pressure in the sleeve. Equalization of the pressure facilitates moving new tubular 340 past top entry valve 348 and into bottom circulating sleeve 324.

[0483] Once new tubular 340 is in the chamber of bottom circulating sleeve 324, circulation changes to top drive system 332 and fluid flows through port 352 into top circulating sleeve 334 of top drive system 332. In the chamber of bottom circulating sleeve 324, new tubular 340 and existing tubular 326 are coupled to form coupled tubular 354, shown in FIG. 11C. Coupled tubular 354 includes new tubular 340 and existing tubular 326. After forming coupled tubular 354, chuck 322 of bottom drive system 316 may disconnect from coupled tubular 354, thus relinquishing control of the drilling process to top drive system 332.

[0484] While top drive system 332 controls the drilling process, bottom drive system 316 may be actuated to travel upward (see arrow shown in FIG. 11C) toward top drive system 332 along the length of coupled tubular 354. As bottom circulating system sleeve 324 of bottom drive system 316 comes into proximity with drive sub 336 of top drive system 332, fluid from top drive system 332 may be flowing from top circulating sleeve 334 of top drive system 332 through top valve 348 (shown in FIG. 13). Bottom circulating sleeve 324 may be pressurized and side valve 338 (shown in FIG. 13) may open to provide flow. Top valve 348 (shown in FIG. 13) may shut and/or partially close as side valve 338 opens to provide flow to top circulating sleeve 334. Circulation may be slowed or discontinued through top drive system 332. As circulation is stopped through top drive system 332, top valve 348 may close completely and all fluid may be furnished through side valve 338 from port 346. When bottom drive system 316 reaches the top of coupled tubular 354, bottom drive system 316 may engage drive sub 336. Coupled tubular 354 may disengage from drive sub 336 and engage with chuck 322 while bottom drive system 316 resumes control of the drilling operation. Chuck 322 transfers force to couple tubular 354 to continue the drilling process.

[0485] Once disengaged from coupled tubular 354, top drive system 332 may be raised (see up arrow) relative to bottom drive system 316 (for example, until top drive system 332 reaches reference line Y as shown in FIG. 11D). Bottom drive system 316 may be lowered to push coupled tubular 354 downward into the formation (see down arrows in FIG. 11D). Bottom drive system 316 may continue to be lowered (for example, until bottom drive system 316 has returned to reference line Z). The sequence described above may be repeated any number of times so as to maintain continuous drilling operations.

[0486] Some wellbores formed in the formation may be used to facilitate formation of a perimeter barrier around a treatment area. Heat sources in the treatment area may heat hydrocarbons in the formation within the treatment area. The perimeter barrier may be, but is not limited to, a low temperature or frozen barrier formed by freeze wells, a wax barrier formed in the formation, dewatering wells, a grout wall formed in the formation, a sulfur cement barrier, a barrier formed by a gel produced in the formation, a barrier formed by precipitation of salts in the formation, a barrier formed by a polymerization reaction in the formation, and/or sheets driven into the formation. Heat sources, production wells, injection wells, dewatering wells, and/or monitoring wells may be installed in the treatment area defined by the barrier prior to, simultaneously with, or after installation of the barrier.

[0487] A low temperature zone around at least a portion of a treatment area may be formed by freeze wells. In an embodiment, refrigerant is circulated through freeze wells to form low temperature zones around each freeze well. The freeze wells are placed in the formation so that the low temperature zones overlap and form a low temperature zone around the treatment area. The low temperature zone established by freeze wells is maintained below the freezing temperature of aqueous fluid in the formation. Aqueous fluid entering the low temperature zone freezes and forms the frozen barrier. In other embodiments, the freeze barrier is formed by batch operated freeze wells. A cold fluid, such as liquid nitrogen, is introduced into the freeze wells to form low temperature zones around the freeze wells. The fluid is replenished as needed.

[0488] Grout, wax, polymer or other material may be used in combination with freeze wells to provide a barrier for the in situ heat treatment process. The material may fill cavities (vugs) in the formation and reduces the permeability of the formation. The material may have higher thermal conductivity than gas and/or formation fluid that fills cavities in the formation. Placing material in the cavities may allow for faster low temperature zone formation. The material may form a perpetual barrier in the formation that may strengthen the formation. The use of material to form the barrier in unconsolidated or substantially unconsolidated formation material may allow for larger well spacing than is possible without the use of the material. The combination of the material and the low temperature zone formed by freeze wells may constitute a double barrier for environmental regulation purposes. In some embodiments, the material is introduced into the formation as a liquid, and the liquid sets in the formation to form a solid. The material may be, but is not limited to, fine cement, micro fine cement, sulfur, sulfur cement, viscous thermoplastics, and/or waxes. The material may include surfactants, stabilizers or other chemicals that modify the properties of the material. For example, the presence of surfactant in the material may promote entry of the material into small openings in the formation.

[0489] Material may be introduced into the formation through freeze well wellbores. The material may be allowed to set. The integrity of the wall formed by the material may be checked. The integrity of the material wall may be checked by logging techniques and/or by hydrostatic testing. If the permeability of a section formed by the material is too high, additional material may be introduced into the formation through freeze well wellbores. After the permeability of the section is sufficiently reduced, freeze wells may be installed in the freeze well wellbores.

[0490] Material may be injected into the formation at a pressure that is high, but below the fracture pressure of the formation. In some embodiments, injection of material is performed in 16 m increments in the freeze wellbore. Larger or smaller increments may be used if desired. In some embodiments, material is only applied to certain portions of the formation. For example, material may be applied to the formation through the freeze wellbore only adjacent to aquifer zones and/or to relatively high permeability zones (for example, zones with a permeability greater than about 0.1 darcy). Applying material to aquifers may inhibit migration of water from one aquifer to a different aquifer. For material placed in the formation through freeze well wellbores, the material may inhibit water migration between aquifers during formation of the low temperature zone. The material may also inhibit water migration between aquifers when an established low temperature zone is allowed to thaw.

[0491] In some embodiments, the material used to form a barrier may be fine cement and micro fine cement. Cement may provide structural support in the formation. Fine cement may be ASTM type 3 Portland cement. Fine cement may be less expensive than micro fine cement. In an embodiment, a freeze wellbore is formed in the formation. Selected portions of the freeze wellbore are grouted using fine cement. Then, micro fine cement is injected into the formation through the freeze wellbore. The fine cement may reduce the permeability down to about 10 millidarcy. The micro fine cement may further reduce the permeability to about 0.1 millidarcy. After the grout is introduced into the formation, a freeze wellbore canister may be inserted into the formation. The process may be repeated for each freeze well that will be used to form the barrier.

[0492] In some embodiments, fine cement is introduced into every other freeze wellbore. Micro fine cement is introduced into the remaining wellbores. For example, grout may be used in a formation with freeze wellbores set at about 5 m spacing. A first wellbore is drilled and fine cement is introduced into the formation through the wellbore. A freeze well canister is positioned in the first wellbore. A second wellbore is drilled 10 m away from the first wellbore. Fine cement is introduced into the formation through the second wellbore. A freeze well canister is positioned in the second wellbore. A third wellbore is drilled between the first wellbore and the second wellbore. In some embodiments, grout from the first and/or second wellbores may be detected in the cuttings of the third wellbore. Micro fine cement is introduced into the formation through the third wellbore. A freeze wellbore canister is positioned in the third wellbore. The same procedure is used to form the remaining freeze wells that will form the barrier around the treatment area.

[0493] Fiber optic temperature monitoring systems may also be used to monitor temperatures in heated portions of the formation during in situ heat treatment processes. Temperature monitoring systems positioned in production wells, heater wells, injection wells, and/or monitor wells may be used to measure temperature profiles in treatment areas subjected to in situ heat treatment processes. The fiber of a fiber optic cable used in the heated portion of the formation may be clad with a reflective material to facilitate retention of a signal or signals transmitted down the fiber. In some embodiments, the fiber is clad with gold, copper, nickel, aluminum and/or alloys thereof. The cladding may be formed of a material that is able to withstand chemical and temperature conditions in the heated portion of the formation. For example, gold cladding may allow an optical sensor to be used up to temperatures of 700° C. In some embodiments, the fiber is clad with aluminum. The fiber may be dipped in or run through a bath of liquid aluminum. The clad fiber may then be allowed to cool to secure the aluminum to the fiber. The gold or aluminum cladding may reduce hydrogen darkening of the optical fiber.

[0494] In some embodiments, two or more rows of freeze wells are located about all or a portion of the perimeter of the treatment area to form a thick interconnected low temperature zone. Thick low temperature zones may be formed adjacent to areas in the formation where there is a high flow rate of aqueous fluid in the formation. The thick barrier may ensure that breakthrough of the frozen barrier established by the freeze wells does not occur.

[0495] In some embodiments, a double barrier system is used to isolate a treatment area. The double barrier system may be formed with a first barrier and a second barrier. The first barrier may be formed around at least a portion of the treatment area to inhibit fluid from entering or exiting the treatment area. The second barrier may be formed around at least a portion of the first barrier to isolate an inter-barrier zone between the first barrier and the second barrier. The inter-barrier zone may have a thickness from about 1 m to about 300 m. In some embodiments, the thickness of the inter-barrier zone is from about 10 m to about 100 m, or from about 20 m to about 50 m.

[0496] The double barrier system may allow greater project depths than a single barrier system. Greater depths are possible with the double barrier system because the stepped differential pressures across the first barrier and the second barrier is less than the differential pressure across a single barrier. The smaller differential pressures across the first barrier and the second barrier make a breach of the double barrier system less likely to occur at depth for the double barrier system as compared to the single barrier system. In some embodiments, additional barriers may be positioned to connect the inner barrier to the outer barrier. The additional barriers may further strengthen the double barrier system and define compartments that limit the amount of fluid that can pass from the inter-barrier zone to the treatment area should a breach occur in the first barrier.

[0497] The first barrier and the second barrier may be the same type of barrier or different types of barriers. In some embodiments, the first barrier and the second barrier are formed by freeze wells. In some embodiments, the first barrier is formed by freeze wells, and the second barrier is a grout wall. The grout wall may be formed of cement, sulfur, sulfur cement, or combinations thereof. In some embodiments, a portion of the first barrier and/or a portion of the second barrier is a natural barrier, such as an impermeable rock formation.

[0498] In some embodiments, one or both barriers may be formed from wellbores positioned in the formation. The position of the wellbores used to form the second barrier may be adjusted relative to the wellbores used to form the first barrier to limit a separation distance between a breach or portion of the barrier that is difficult to form and the nearest wellbore. For example, if freeze wells are used to form both barriers of a double barrier system, the position of the freeze wells may be adjusted to facilitate formation of the barriers and limit the distance between a potential breach and the closest wells to the breach. Adjusting the position of the wells of the second barrier relative to the wells of the first barrier may also be used when one or more of the barriers are barriers other than freeze barriers (for example, dewatering wells, cement barriers, grout barriers, and/or wax barriers).

[0499] In some embodiments, wellbores for forming the first barrier are formed in a row in the formation. During formation of the wellbores, logging techniques and/or analysis of cores may be used to determine the principal fracture direction and/or the direction of water flow in one or more layers of the formation. In some embodiments, two or more layers of the formation may have different principal fracture directions and/or the directions of water flow that need to be addressed. In such formations, three or more barriers may need to be formed in the formation to allow for formation of the barriers that inhibit inflow of formation fluid into the treatment area or outflow of formation fluid from the treatment area. Barriers may be formed to isolate particular layers in the formation.

[0500] The principal fracture direction and/or the direction of water flow may be used to determine the placement of wells used to form the second barrier relative to the wells used to form the first barrier. The placement of the wells may facilitate formation of the first barrier and the second barrier.

[0501] FIG. 14 depicts a schematic representation of barrier wells 200 used to form a first barrier and barrier wells 200′ used to form a second barrier when the principal fracture direction and/or the direction of water flow is at angle A relative to the first barrier. The principal fracture direction and/or direction of water flow is indicated by arrow 356. The case where angle A is 0 is the case where the principal fracture direction and/or the direction of water flow is substantially normal to the barriers. Spacing between two adjacent barrier wells 200 of the first barrier or between barrier wells 200′ of the second barrier are indicated by distance s. The spacing s may be 2 m, 3 m, 10 m or greater. Distance d indicates the separation distance between the first barrier and the second barrier. Distance d may be less than s, equal to s, or greater than s. Barrier wells 200′ of the second barrier may have offset distance od relative to barrier wells 200 of the first barrier. Offset distance od may be calculated by the equation:
od=s/2−d*tan(A)  (EQN. 1)

[0502] Using the od according to EQN. 1 maintains a maximum separation distance of s/4 between a barrier well and a regular fracture extending between the barriers. Having a maximum separation distance of s/4 by adjusting the offset distance based on the principal fracture direction and/or the direction of water flow may enhance formation of the first barrier and/or second barrier. Having a maximum separation distance of s/4 by adjusting the offset distance of wells of the second barrier relative to the wells of the first barrier based on the principal fracture direction and/or the direction of water flow may reduce the time needed to reform the first barrier and/or the second barrier should a breach of the first barrier and/or the second barrier occur.

[0503] In some embodiments, od may be set at a value between the value generated by EQN. 1 and the worst case value. The worst case value of od may be if barrier wells 200 of the first freeze barrier and barrier wells 200′ of the second barrier are located along the principal fracture direction and/or direction of water flow (i.e., along arrow 356). In such a case, the maximum separation distance would be s/2. Having a maximum separation distance of s/2 may slow the time needed to form the first barrier and/or the second barrier, or may inhibit formation of the barriers.

[0504] In some embodiments, the barrier wells for the treatment area are freeze wells. Vertically positioned freeze wells and/or horizontally positioned freeze wells may be positioned around sides of the treatment area. If the upper layer (the overburden) or the lower layer (the underburden) of the formation is likely to allow fluid flow into the treatment area or out of the treatment area, horizontally positioned freeze wells may be used to form an upper and/or a lower barrier for the treatment area. In some embodiments, an upper barrier and/or a lower barrier may not be necessary if the upper layer and/or the lower layer are at least substantially impermeable. If the upper freeze barrier is formed, portions of heat sources, production wells, injection wells, and/or dewatering wells that pass through the low temperature zone created by the freeze wells forming the upper freeze barrier wells may be insulated and/or heat traced so that the low temperature zone does not adversely affect the functioning of the heat sources, production wells, injection wells and/or dewatering wells passing through the low temperature zone.

[0505] In situ heat treatment processes and solution mining processes may heat the treatment area, remove mass from the treatment area, and greatly increase the permeability of the treatment area. In certain embodiments, the treatment area after being treated may have a permeability of at least 0.1 darcy. In some embodiments, the treatment area after being treated has a permeability of at least 1 darcy, of at least 10 darcy, or of at least 100 darcy. The increased permeability allows the fluid to spread in the formation into fractures, microfractures, and/or pore spaces in the formation. Outside of the treatment area, the permeability may remain at the initial permeability of the formation. The increased permeability allows fluid introduced to flow easily within the formation.

[0506] In certain embodiments, a barrier may be formed in the formation after a solution mining process and/or an in situ heat treatment process by introducing a fluid into the formation. The barrier may inhibit formation fluid from entering the treatment area after the solution mining and/or in situ heat treatment processes have ended. The barrier formed by introducing fluid into the formation may allow for isolation of the treatment area.

[0507] The fluid introduced into the formation to form a barrier may include wax, bitumen, heavy oil, sulfur, polymer, gel, saturated saline solution, and/or one or more reactants that react to form a precipitate, solid or high viscosity fluid in the formation. In some embodiments, bitumen, heavy oil, reactants and/or sulfur used to form the barrier are obtained from treatment facilities associated with the in situ heat treatment process. For example, sulfur may be obtained from a Claus process used to treat produced gases to remove hydrogen sulfide and other sulfur compounds.

[0508] The fluid may be introduced into the formation as a liquid, vapor, or mixed phase fluid. The fluid may be introduced into a portion of the formation that is at an elevated temperature. In some embodiments, the fluid is introduced into the formation through wells located near a perimeter of the treatment area. The fluid may be directed away from the treatment area. The elevated temperature of the formation maintains or allows the fluid to have a low viscosity so that the fluid moves away from the wells. A portion of the fluid may spread outwards in the formation towards a cooler portion of the formation. The relatively high permeability of the formation allows fluid introduced from one wellbore to spread and mix with fluid introduced from other wellbores. In the cooler portion of the formation, the viscosity of the fluid increases, a portion of the fluid precipitates, and/or the fluid solidifies or thickens so that the fluid forms the barrier to flow of formation fluid into or out of the treatment area.

[0509] In some embodiments, a low temperature barrier formed by freeze wells surrounds all or a portion of the treatment area. As the fluid introduced into the formation approaches the low temperature barrier, the temperature of the formation becomes colder. The colder temperature increases the viscosity of the fluid, enhances precipitation, and/or solidifies the fluid to form the barrier to the flow of formation fluid into or out of the formation. The fluid may remain in the formation as a highly viscous fluid or a solid after the low temperature barrier has dissipated.

[0510] In certain embodiments, saturated saline solution is introduced into the formation. Components in the saturated saline solution may precipitate out of solution when the solution reaches a colder temperature. The solidified particles may form the barrier to the flow of formation fluid into or out of the formation. The solidified components may be substantially insoluble in formation fluid.

[0511] A potential source of heat loss from the heated formation is due to reflux in wells. Refluxing occurs when vapors condense in a well and flow into a portion of the well adjacent to the heated portion of the formation. Vapors may condense in the well adjacent to the overburden of the formation to form condensed fluid. Condensed fluid flowing into the well adjacent to the heated formation absorbs heat from the formation. Heat absorbed by condensed fluids cools the formation and necessitates additional energy input into the formation to maintain the formation at a desired temperature. Some fluids that condense in the overburden and flow into the portion of the well adjacent to the heated formation may react to produce undesired compounds and/or coke. Inhibiting fluids from refluxing may significantly improve the thermal efficiency of the in situ heat treatment system and/or the quality of the product produced from the in situ heat treatment system.

[0512] For some well embodiments, the portion of the well adjacent to the overburden section of the formation is cemented to the formation. In some well embodiments, the well includes packing material placed near the transition from the heated section of the formation to the overburden. The packing material inhibits formation fluid from passing from the heated section of the formation into the section of the wellbore adjacent to the overburden. Cables, conduits, devices, and/or instruments may pass through the packing material, but the packing material inhibits formation fluid from passing up the wellbore adjacent to the overburden section of the formation.

[0513] In some embodiments, one or more baffle systems may be placed in the wellbores to inhibit reflux. The baffle systems may be obstructions to fluid flow into the heated portion of the formation. In some embodiments, refluxing fluid may revaporize on the baffle system before coming into contact with the heated portion of the formation.

[0514] In some embodiments, a gas may be introduced into the formation through wellbores to inhibit reflux in the wellbores. In some embodiments, gas may be introduced into wellbores that include baffle systems to inhibit reflux of fluid in the wellbores. The gas may be carbon dioxide, methane, nitrogen or other desired gas. In some embodiments, the introduction of gas may be used in conjunction with one or more baffle systems in the wellbores. The introduced gas may enhance heat exchange at the baffle systems to help maintain top portions of the baffle systems colder than the lower portions of the baffle systems.

[0515] The flow of production fluid up the well to the surface is desired for some types of wells, especially for production wells. Flow of production fluid up the well is also desirable for some heater wells that are used to control pressure in the formation. The overburden, or a conduit in the well used to transport formation fluid from the heated portion of the formation to the surface, may be heated to inhibit condensation on or in the conduit. Providing heat in the overburden, however, may be costly and/or may lead to increased cracking or coking of formation fluid as the formation fluid is being produced from the formation.

[0516] To avoid the need to heat the overburden or to heat the conduit passing through the overburden, one or more diverters may be placed in the wellbore to inhibit fluid from refluxing into the wellbore adjacent to the heated portion of the formation. In some embodiments, the diverter retains fluid above the heated portion of the formation. Fluids retained in the diverter may be removed from the diverter using a pump, gas lifting, and/or other fluid removal technique. In certain embodiments, two or more diverters that retain fluid above the heated portion of the formation may be located in the production well. Two or more diverters provide a simple way of separating initial fractions of condensed fluid produced from the in situ heat treatment system. A pump may be placed in each of the diverters to remove condensed fluid from the diverters.

[0517] In some embodiments, the diverter directs fluid to a sump below the heated portion of the formation. An inlet for a lift system may be located in the sump. In some embodiments, the intake of the lift system is located in casing in the sump. In some embodiments, the intake of the lift system is located in an open wellbore. The sump is below the heated portion of the formation. The intake of the pump may be located 1 m, 5 m, 10 m, 20 m or more below the deepest heater used to heat the heated portion of the formation. The sump may be at a cooler temperature than the heated portion of the formation. The sump may be more than 10° C., more than 50° C., more than 75° C., or more than 100° C. below the temperature of the heated portion of the formation. A portion of the fluid entering the sump may be liquid. A portion of the fluid entering the sump may condense within the sump. The lift system moves the fluid in the sump to the surface.

[0518] Production well lift systems may be used to efficiently transport formation fluid from the bottom of the production wells to the surface. Production well lift systems may provide and maintain the maximum required well drawdown (minimum reservoir producing pressure) and producing rates. The production well lift systems may operate efficiently over a wide range of high temperature/multiphase fluids (gas/vapor/steam/water/hydrocarbon liquids) and production rates expected during the life of a typical project. Production well lift systems may include dual concentric rod pump lift systems, chamber lift systems and other types of lift systems.

[0519] Temperature limited heaters may be in configurations and/or may include materials that provide automatic temperature limiting properties for the heater at certain temperatures. In certain embodiments, ferromagnetic materials are used in temperature limited heaters. Ferromagnetic material may self-limit temperature at or near the Curie temperature of the material and/or the phase transformation temperature range to provide a reduced amount of heat when a time-varying current is applied to the material. In certain embodiments, the ferromagnetic material self-limits temperature of the temperature limited heater at a selected temperature that is approximately the Curie temperature and/or in the phase transformation temperature range. In certain embodiments, the selected temperature is within about 35° C., within about 25° C., within about 20° C., or within about 10° C. of the Curie temperature and/or the phase transformation temperature range. In certain embodiments, ferromagnetic materials are coupled with other materials (for example, highly conductive materials, high strength materials, corrosion resistant materials, or combinations thereof) to provide various electrical and/or mechanical properties. Some parts of the temperature limited heater may have a lower resistance (caused by different geometries and/or by using different ferromagnetic and/or non-ferromagnetic materials) than other parts of the temperature limited heater. Having parts of the temperature limited heater with various materials and/or dimensions allows for tailoring the desired heat output from each part of the heater.

[0520] Temperature limited heaters may be more reliable than other heaters. Temperature limited heaters may be less apt to break down or fail due to hot spots in the formation. In some embodiments, temperature limited heaters allow for substantially uniform heating of the formation. In some embodiments, temperature limited heaters are able to heat the formation more efficiently by operating at a higher average heat output along the entire length of the heater. The temperature limited heater operates at the higher average heat output along the entire length of the heater because power to the heater does not have to be reduced to the entire heater, as is the case with typical constant wattage heaters, if a temperature along any point of the heater exceeds, or is about to exceed, a maximum operating temperature of the heater. Heat output from portions of a temperature limited heater approaching a Curie temperature and/or the phase transformation temperature range of the heater automatically reduces without controlled adjustment of the time-varying current applied to the heater. The heat output automatically reduces due to changes in electrical properties (for example, electrical resistance) of portions of the temperature limited heater. Thus, more power is supplied by the temperature limited heater during a greater portion of a heating process.

[0521] In certain embodiments, the system including temperature limited heaters initially provides a first heat output and then provides a reduced (second heat output) heat output, near, at, or above the Curie temperature and/or the phase transformation temperature range of an electrically resistive portion of the heater when the temperature limited heater is energized by a time-varying current. The first heat output is the heat output at temperatures below which the temperature limited heater begins to self-limit. In some embodiments, the first heat output is the heat output at a temperature about 50° C., about 75° C., about 100° C., or about 125° C. below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic material in the temperature limited heater.

[0522] The temperature limited heater may be energized by time-varying current (alternating current or modulated direct current) supplied at the wellhead. The wellhead may include a power source and other components (for example, modulation components, transformers, and/or capacitors) used in supplying power to the temperature limited heater. The temperature limited heater may be one of many heaters used to heat a portion of the formation.

[0523] In certain embodiments, the temperature limited heater includes a conductor that operates as a skin effect or proximity effect heater when time-varying current is applied to the conductor. The skin effect limits the depth of current penetration into the interior of the conductor. For ferromagnetic materials, the skin effect is dominated by the magnetic permeability of the conductor. The relative magnetic permeability of ferromagnetic materials is typically between 10 and 1000 (for example, the relative magnetic permeability of ferromagnetic materials is typically at least 10 and may be at least 50, 100, 500, 1000 or greater). As the temperature of the ferromagnetic material is raised above the Curie temperature, or the phase transformation temperature range, and/or as the applied electrical current is increased, the magnetic permeability of the ferromagnetic material decreases substantially and the skin depth expands rapidly (for example, the skin depth expands as the inverse square root of the magnetic permeability). The reduction in magnetic permeability results in a decrease in the AC or modulated DC resistance of the conductor near, at, or above the Curie temperature, the phase transformation temperature range, and/or as the applied electrical current is increased. When the temperature limited heater is powered by a substantially constant current source, portions of the heater that approach, reach, or are above the Curie temperature and/or the phase transformation temperature range may have reduced heat dissipation. Sections of the temperature limited heater that are not at or near the Curie temperature and/or the phase transformation temperature range may be dominated by skin effect heating that allows the heater to have high heat dissipation due to a higher resistive load.

[0524] Curie temperature heaters have been used in soldering equipment, heaters for medical applications, and heating elements for ovens (for example, pizza ovens). Some of these uses are disclosed in U.S. Pat. No. 5,579,575 to Lamome et al.; U.S. Pat. No. 5,065,501 to Henschen et al.; and U.S. Pat. No. 5,512,732 to Yagnik et al., all of which are incorporated by reference as if fully set forth herein. U.S. Pat. No. 4,849,611 to Whitney et al., which is incorporated by reference as if fully set forth herein, describes a plurality of discrete, spaced-apart heating units including a reactive component, a resistive heating component, and a temperature responsive component.

[0525] An advantage of using the temperature limited heater to heat hydrocarbons in the formation is that the conductor is chosen to have a Curie temperature and/or a phase transformation temperature range in a desired range of temperature operation. Operation within the desired operating temperature range allows substantial heat injection into the formation while maintaining the temperature of the temperature limited heater, and other equipment, below design limit temperatures. Design limit temperatures are temperatures at which properties such as corrosion, creep, and/or deformation are adversely affected. The temperature limiting properties of the temperature limited heater inhibit overheating or burnout of the heater adjacent to low thermal conductivity “hot spots” in the formation. In some embodiments, the temperature limited heater is able to lower or control heat output and/or withstand heat at temperatures above 25° C., 37° C., 100° C., 250° C., 500° C., 700° C., 800° C., 900° C., or higher up to 1131° C., depending on the materials used in the heater.

[0526] The temperature limited heater allows for more heat injection into the formation than constant wattage heaters because the energy input into the temperature limited heater does not have to be limited to accommodate low thermal conductivity regions adjacent to the heater. For example, in Green River oil shale there is a difference of at least a factor of 3 in the thermal conductivity of the lowest richness oil shale layers and the highest richness oil shale layers. When heating such a formation, substantially more heat is transferred to the formation with the temperature limited heater than with the conventional heater that is limited by the temperature at low thermal conductivity layers. The heat output along the entire length of the conventional heater needs to accommodate the low thermal conductivity layers so that the heater does not overheat at the low thermal conductivity layers and burn out. The heat output adjacent to the low thermal conductivity layers that are at high temperature will reduce for the temperature limited heater, but the remaining portions of the temperature limited heater that are not at high temperature will still provide high heat output. Because heaters for heating hydrocarbon formations typically have long lengths (for example, at least 10 m, 100 m, 300 m, 500 m, 1 km or more up to about 10 km), the majority of the length of the temperature limited heater may be operating below the Curie temperature and/or the phase transformation temperature range while only a few portions are at or near the Curie temperature and/or the phase transformation temperature range of the temperature limited heater.

[0527] The use of temperature limited heaters allows for efficient transfer of heat to the formation. Efficient transfer of heat allows for reduction in time needed to heat the formation to a desired temperature. For example, in Green River oil shale, pyrolysis typically requires 9.5 years to 10 years of heating when using a 12 m heater well spacing with conventional constant wattage heaters. For the same heater spacing, temperature limited heaters may allow a larger average heat output while maintaining heater equipment temperatures below equipment design limit temperatures. Pyrolysis in the formation may occur at an earlier time with the larger average heat output provided by temperature limited heaters than the lower average heat output provided by constant wattage heaters. For example, in Green River oil shale, pyrolysis may occur in 5 years using temperature limited heaters with a 12 m heater well spacing. Temperature limited heaters counteract hot spots due to inaccurate well spacing or drilling where heater wells come too close together. In certain embodiments, temperature limited heaters allow for increased power output over time for heater wells that have been spaced too far apart, or limit power output for heater wells that are spaced too close together. Temperature limited heaters also supply more power in regions adjacent the overburden and underburden to compensate for temperature losses in these regions.

[0528] Temperature limited heaters may be advantageously used in many types of formations. For example, in tar sands formations or relatively permeable formations containing heavy hydrocarbons, temperature limited heaters may be used to provide a controllable low temperature output for reducing the viscosity of fluids, mobilizing fluids, and/or enhancing the radial flow of fluids at or near the wellbore or in the formation. Temperature limited heaters may be used to inhibit excess coke formation due to overheating of the near wellbore region of the formation.

[0529] In some embodiments, the use of temperature limited heaters eliminates or reduces the need for expensive temperature control circuitry. For example, the use of temperature limited heaters eliminates or reduces the need to perform temperature logging and/or the need to use fixed thermocouples on the heaters to monitor potential overheating at hot spots.

[0530] In certain embodiments, phase transformation (for example, crystalline phase transformation or a change in the crystal structure) of materials used in a temperature limited heater change the selected temperature at which the heater self-limits. Ferromagnetic material used in the temperature limited heater may have a phase transformation (for example, a transformation from ferrite to austenite) that decreases the magnetic permeability of the ferromagnetic material. This reduction in magnetic permeability is similar to reduction in magnetic permeability due to the magnetic transition of the ferromagnetic material at the Curie temperature. The Curie temperature is the magnetic transition temperature of the ferrite phase of the ferromagnetic material. The reduction in magnetic permeability results in a decrease in the AC or modulated DC resistance of the temperature limited heater near, at, or above the temperature of the phase transformation and/or the Curie temperature of the ferromagnetic material.

[0531] The phase transformation of the ferromagnetic material may occur over a temperature range. The temperature range of the phase transformation depends on the ferromagnetic material and may vary, for example, over a range of about 5° C. to a range of about 200° C. Because the phase transformation takes place over a temperature range, the reduction in the magnetic permeability due to the phase transformation takes place over the temperature range. The reduction in magnetic permeability may also occur hysteretically over the temperature range of the phase transformation. In some embodiments, the phase transformation back to the lower temperature phase of the ferromagnetic material is slower than the phase transformation to the higher temperature phase (for example, the transition from austenite back to ferrite is slower than the transition from ferrite to austenite). The slower phase transformation back to the lower temperature phase may cause hysteretic operation of the heater at or near the phase transformation temperature range that allows the heater to slowly increase to higher resistance after the resistance of the heater reduces due to high temperature.

[0532] In some embodiments, the phase transformation temperature range overlaps with the reduction in the magnetic permeability when the temperature approaches the Curie temperature of the ferromagnetic material. The overlap may produce a faster drop in electrical resistance versus temperature than if the reduction in magnetic permeability is solely due to the temperature approaching the Curie temperature. The overlap may also produce hysteretic behavior of the temperature limited heater near the Curie temperature and/or in the phase transformation temperature range.

[0533] In certain embodiments, the hysteretic operation due to the phase transformation is a smoother transition than the reduction in magnetic permeability due to magnetic transition at the Curie temperature. The smoother transition may be easier to control (for example, electrical control using a process control device that interacts with the power supply) than the sharper transition at the Curie temperature. In some embodiments, the Curie temperature is located inside the phase transformation range for selected metallurgies used in temperature limited heaters. This phenomenon provides temperature limited heaters with the smooth transition properties of the phase transformation in addition to a sharp and definite transition due to the reduction in magnetic properties at the Curie temperature. Such temperature limited heaters may be easy to control (due to the phase transformation) while providing finite temperature limits (due to the sharp Curie temperature transition). Using the phase transformation temperature range instead of and/or in addition to the Curie temperature in temperature limited heaters increases the number and range of metallurgies that may be used for temperature limited heaters.

[0534] In certain embodiments, alloy additions are made to the ferromagnetic material to adjust the temperature range of the phase transformation. For example, adding carbon to the ferromagnetic material may increase the phase transformation temperature range and lower the onset temperature of the phase transformation. Adding titanium to the ferromagnetic material may increase the onset temperature of the phase transformation and decrease the phase transformation temperature range. Alloy compositions may be adjusted to provide desired Curie temperature and phase transformation properties for the ferromagnetic material. The alloy composition of the ferromagnetic material may be chosen based on desired properties for the ferromagnetic material (such as, but not limited to, magnetic permeability transition temperature or temperature range, resistance versus temperature profile, or power output). Addition of titanium may allow higher Curie temperatures to be obtained when adding cobalt to 410 stainless steel by raising the ferrite to austenite phase transformation temperature range to a temperature range that is above, or well above, the Curie temperature of the ferromagnetic material.

[0535] In some embodiments, temperature limited heaters are more economical to manufacture or make than standard heaters. Typical ferromagnetic materials include iron, carbon steel, or ferritic stainless steel. Such materials are inexpensive as compared to nickel-based heating alloys (such as nichrome, Kanthal™ (Bulten-Kanthal AB, Sweden), and/or LOHM™ (Driver-Harris Company, Harrison, N.J., U.S.A.)) typically used in insulated conductor (mineral insulated cable) heaters. In one embodiment of the temperature limited heater, the temperature limited heater is manufactured in continuous lengths as an insulated conductor heater to lower costs and improve reliability.

[0536] In some embodiments, the temperature limited heater is placed in the heater well using a coiled tubing rig. A heater that can be coiled on a spool may be manufactured by using metal such as ferritic stainless steel (for example, 409 stainless steel) that is welded using electrical resistance welding (ERW). U.S. Pat. No. 7,032,809 to Hopkins, which is incorporated by reference as if fully set forth herein, describes forming seam-welded pipe. To form a heater section, a metal strip from a roll is passed through a former where it is shaped into a tubular and then longitudinally welded using ERW.

[0537] In some embodiments, a composite tubular may be formed from the seam-welded tubular. The seam-welded tubular is passed through a second former where a conductive strip (for example, a copper strip) is applied, drawn down tightly on the tubular through a die, and longitudinally welded using ERW. A sheath may be formed by longitudinally welding a support material (for example, steel such as 347H or 347HH) over the conductive strip material. The support material may be a strip rolled over the conductive strip material. An overburden section of the heater may be formed in a similar manner.

[0538] In certain embodiments, the overburden section uses a non-ferromagnetic material such as 304 stainless steel or 316 stainless steel instead of a ferromagnetic material. The heater section and overburden section may be coupled using standard techniques such as butt welding using an orbital welder. In some embodiments, the overburden section material (the non-ferromagnetic material) may be pre-welded to the ferromagnetic material before rolling. The pre-welding may eliminate the need for a separate coupling step (for example, butt welding). In an embodiment, a flexible cable (for example, a furnace cable such as a MGT 1000 furnace cable) may be pulled through the center after forming the tubular heater. An end bushing on the flexible cable may be welded to the tubular heater to provide an electrical current return path. The tubular heater, including the flexible cable, may be coiled onto a spool before installation into a heater well. In an embodiment, the temperature limited heater is installed using the coiled tubing rig. The coiled tubing rig may place the temperature limited heater in a deformation resistant container in the formation. The deformation resistant container may be placed in the heater well using conventional methods.

[0539] Temperature limited heaters may be used for heating hydrocarbon formations including, but not limited to, oil shale formations, coal formations, tar sands formations, and formations with heavy viscous oils. Temperature limited heaters may also be used in the field of environmental remediation to vaporize or destroy soil contaminants. Embodiments of temperature limited heaters may be used to heat fluids in a wellbore or sub-sea pipeline to inhibit deposition of paraffin or various hydrates. In some embodiments, a temperature limited heater is used for solution mining a subsurface formation (for example, an oil shale or a coal formation). In certain embodiments, a fluid (for example, molten salt) is placed in a wellbore and heated with a temperature limited heater to inhibit deformation and/or collapse of the wellbore. In some embodiments, the temperature limited heater is attached to a sucker rod in the wellbore or is part of the sucker rod itself. In some embodiments, temperature limited heaters are used to heat a near wellbore region to reduce near wellbore oil viscosity during production of high viscosity crude oils and during transport of high viscosity oils to the surface. In some embodiments, a temperature limited heater enables gas lifting of a viscous oil by lowering the viscosity of the oil without coking the oil. Temperature limited heaters may be used in sulfur transfer lines to maintain temperatures between about 110° C. and about 130° C.

[0540] The ferromagnetic alloy or ferromagnetic alloys used in the temperature limited heater determine the Curie temperature of the heater. Curie temperature data for various metals is listed in “American Institute of Physics Handbook,” Second Edition, McGraw-Hill, pages 5-170 through 5-176. Ferromagnetic conductors may include one or more of the ferromagnetic elements (iron, cobalt, and nickel) and/or alloys of these elements. In some embodiments, ferromagnetic conductors include iron-chromium (Fe—Cr) alloys that contain tungsten (W) (for example, HCM12A and SAVE12 (Sumitomo Metals Co., Japan)) and/or iron alloys that contain chromium (for example, Fe—Cr alloys, Fe—Cr—W alloys, Fe—Cr—V (vanadium) alloys, and Fe—Cr—Nb (Niobium) alloys). Of the three main ferromagnetic elements, iron has a Curie temperature of approximately 770° C.; cobalt (Co) has a Curie temperature of approximately 1131° C.; and nickel has a Curie temperature of approximately 358° C. An iron-cobalt alloy has a Curie temperature higher than the Curie temperature of iron. For example, iron-cobalt alloy with 2% by weight cobalt has a Curie temperature of approximately 800° C.; iron-cobalt alloy with 12% by weight cobalt has a Curie temperature of approximately 900° C.; and iron-cobalt alloy with 20% by weight cobalt has a Curie temperature of approximately 950° C. Iron-nickel alloy has a Curie temperature lower than the Curie temperature of iron. For example, iron-nickel alloy with 20% by weight nickel has a Curie temperature of approximately 720° C., and iron-nickel alloy with 60% by weight nickel has a Curie temperature of approximately 560° C.

[0541] Some non-ferromagnetic elements used as alloys raise the Curie temperature of iron. For example, an iron-vanadium alloy with 5.9% by weight vanadium has a Curie temperature of approximately 815° C. Other non-ferromagnetic elements (for example, carbon, aluminum, copper, silicon, and/or chromium) may be alloyed with iron or other ferromagnetic materials to lower the Curie temperature. Non-ferromagnetic materials that raise the Curie temperature may be combined with non-ferromagnetic materials that lower the Curie temperature and alloyed with iron or other ferromagnetic materials to produce a material with a desired Curie temperature and other desired physical and/or chemical properties. In some embodiments, the Curie temperature material is a ferrite such as NiFe₂O₄. In other embodiments, the Curie temperature material is a binary compound such as FeNi₃ or Fe₃Al.

[0542] In some embodiments, the improved alloy includes carbon, cobalt, iron, manganese, silicon, or mixtures thereof. In certain embodiments, the improved alloy includes, by weight: about 0.1% to about 10% cobalt; about 0.1% carbon, about 0.5% manganese, about 0.5% silicon, with the balance being iron. In certain embodiments, the improved alloy includes, by weight: about 0.1% to about 10% cobalt; about 0.1% carbon, about 0.5% manganese, about 0.5% silicon, with the balance being iron.

[0543] In some embodiments, the improved alloy includes chromium, carbon, cobalt, iron, manganese, silicon, titanium, vanadium, or mixtures thereof. In certain embodiments, the improved alloy includes, by weight: about 5% to about 20% cobalt, about 0.1% carbon, about 0.5% manganese, about 0.5% silicon, about 0.1% to about 2% vanadium with the balance being iron. In some embodiments, the improved alloy includes, by weight: about 12% chromium, about 0.1% carbon, about 0.5% silicon, about 0.1% to about 0.5% manganese, above 0% to about 15% cobalt, above 0% to about 2% vanadium, above 0% to about 1% titanium, with the balance being iron. In some embodiments, the improved alloy includes, by weight: about 12% chromium, about 0.1% carbon, about 0.5% silicon, about 0.1% to about 0.5% manganese, above 0% to about 2% vanadium, above 0% to about 1% titanium, with the balance being iron. In some embodiments, the improved alloy includes, by weight: about 12% chromium, about 0.1% carbon, about 0.5% silicon, about 0.1% to about 0.5% manganese, above 0% to about 2% vanadium, with the balance being iron. In certain embodiments, the improved alloy includes, by weight: about 12% chromium, about 0.1% carbon, about 0.5% silicon, about 0.1% to about 0.5% manganese, above 0% to about 15% cobalt, above 0% to about 1% titanium, with the balance being iron. In certain embodiments, the improved alloy includes, by weight: about 12% chromium, about 0.1% carbon, about 0.5% silicon, about 0.1% to about 0.5% manganese, above 0% to about 15% cobalt, with the balance being iron. The addition of vanadium may allow for use of higher amounts of cobalt in the improved alloy.

[0544] Certain embodiments of temperature limited heaters may include more than one ferromagnetic material. Such embodiments are within the scope of embodiments described herein if any conditions described herein apply to at least one of the ferromagnetic materials in the temperature limited heater.

[0545] Ferromagnetic properties generally decay as the Curie temperature and/or the phase transformation temperature range is approached. The “Handbook of Electrical Heating for Industry” by C. James Erickson (IEEE Press, 1995) shows a typical curve for 1% carbon steel (steel with 1% carbon by weight). The loss of magnetic permeability starts at temperatures above 650° C. and tends to be complete when temperatures exceed 730° C. Thus, the self-limiting temperature may be somewhat below the actual Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. The skin depth for current flow in 1% carbon steel is 0.132 cm at room temperature and increases to 0.445 cm at 720° C. From 720° C. to 730° C., the skin depth sharply increases to over 2.5 cm. Thus, a temperature limited heater embodiment using 1% carbon steel begins to self-limit between 650° C. and 730° C.

[0546] Skin depth generally defines an effective penetration depth of time-varying current into the conductive material. In general, current density decreases exponentially with distance from an outer surface to the center along the radius of the conductor. The depth at which the current density is approximately 1/e of the surface current density is called the skin depth. For a solid cylindrical rod with a diameter much greater than the penetration depth, or for hollow cylinders with a wall thickness exceeding the penetration depth, the skin depth, 6, is:
δ=1981.5*(ρ/(μ*f)^1/2;  (EQN. 2)
in which: δ=skin depth in inches;

[0547] ρ=resistivity at operating temperature (ohm-cm);

[0548] μ=relative magnetic permeability; and

[0549] f=frequency (Hz).

[0550] EQN. 2 is obtained from “Handbook of Electrical Heating for Industry” by C. James Erickson (IEEE Press, 1995). For most metals, resistivity (ρ) increases with temperature. The relative magnetic permeability generally varies with temperature and with current. Additional equations may be used to assess the variance of magnetic permeability and/or skin depth on both temperature and/or current. The dependence of μ on current arises from the dependence of μ on the electromagnetic field.

[0551] Materials used in the temperature limited heater may be selected to provide a desired turndown ratio. Turndown ratios of at least 1.1:1, 2:1, 3:1, 4:1, 5:1, 10:1, 30:1, or 50:1 may be selected for temperature limited heaters. Larger turndown ratios may also be used. A selected turndown ratio may depend on a number of factors including, but not limited to, the type of formation in which the temperature limited heater is located (for example, a higher turndown ratio may be used for an oil shale formation with large variations in thermal conductivity between rich and lean oil shale layers) and/or a temperature limit of materials used in the wellbore (for example, temperature limits of heater materials). In some embodiments, the turndown ratio is increased by coupling additional copper or another good electrical conductor to the ferromagnetic material (for example, adding copper to lower the resistance above the Curie temperature and/or the phase transformation temperature range).

[0552] The temperature limited heater may provide a maximum heat output (power output) below the Curie temperature and/or the phase transformation temperature range of the heater. In certain embodiments, the maximum heat output is at least 400 W/m (Watts per meter), 600 W/m, 700 W/m, 800 W/m, or higher up to 2000 W/m. The temperature limited heater reduces the amount of heat output by a section of the heater when the temperature of the section of the heater approaches or is above the Curie temperature and/or the phase transformation temperature range. The reduced amount of heat may be substantially less than the heat output below the Curie temperature and/or the phase transformation temperature range. In some embodiments, the reduced amount of heat is at most 400 W/m, 200 W/m, 100 W/m or may approach 0 W/m.

[0553] In certain embodiments, the temperature limited heater operates substantially independently of the thermal load on the heater in a certain operating temperature range. “Thermal load” is the rate that heat is transferred from a heating system to its surroundings. It is to be understood that the thermal load may vary with temperature of the surroundings and/or the thermal conductivity of the surroundings. In an embodiment, the temperature limited heater operates at or above the Curie temperature and/or the phase transformation temperature range of the temperature limited heater such that the operating temperature of the heater increases at most by 3° C., 2° C., 1.5° C., 1° C., or 0.5° C. for a decrease in thermal load of 1 W/m proximate to a portion of the heater. In certain embodiments, the temperature limited heater operates in such a manner at a relatively constant current.

[0554] The AC or modulated DC resistance and/or the heat output of the temperature limited heater may decrease as the temperature approaches the Curie temperature and/or the phase transformation temperature range and decrease sharply near or above the Curie temperature due to the Curie effect and/or phase transformation effect. In certain embodiments, the value of the electrical resistance or heat output above or near the Curie temperature and/or the phase transformation temperature range is at most one-half of the value of electrical resistance or heat output at a certain point below the Curie temperature and/or the phase transformation temperature range. In some embodiments, the heat output above or near the Curie temperature and/or the phase transformation temperature range is at most 90%, 70%, 50%, 30%, 20%, 10%, or less (down to 1%) of the heat output at a certain point below the Curie temperature and/or the phase transformation temperature range (for example, 30° C. below the Curie temperature, 40° C. below the Curie temperature, 50° C. below the Curie temperature, or 100° C. below the Curie temperature). In certain embodiments, the electrical resistance above or near the Curie temperature and/or the phase transformation temperature range decreases to 80%, 70%, 60%, 50%, or less (down to 1%) of the electrical resistance at a certain point below the Curie temperature and/or the phase transformation temperature range (for example, 30° C. below the Curie temperature, 40° C. below the Curie temperature, 50° C. below the Curie temperature, or 100° C. below the Curie temperature).

[0555] In some embodiments, AC frequency is adjusted to change the skin depth of the ferromagnetic material. For example, the skin depth of 1% carbon steel at room temperature is 0.132 cm at 60 Hz, 0.0762 cm at 180 Hz, and 0.046 cm at 440 Hz. Since heater diameter is typically larger than twice the skin depth, using a higher frequency (and thus a heater with a smaller diameter) reduces heater costs. For a fixed geometry, the higher frequency results in a higher turndown ratio. The turndown ratio at a higher frequency is calculated by multiplying the turndown ratio at a lower frequency by the square root of the higher frequency divided by the lower frequency. In some embodiments, a frequency between 100 Hz and 1000 Hz, between 140 Hz and 200 Hz, or between 400 Hz and 600 Hz is used (for example, 180 Hz, 540 Hz, or 720 Hz). In some embodiments, high frequencies may be used. The frequencies may be greater than 1000 Hz.

[0556] To maintain a substantially constant skin depth until the Curie temperature and/or the phase transformation temperature range of the temperature limited heater is reached, the heater may be operated at a lower frequency when the heater is cold and operated at a higher frequency when the heater is hot. Line frequency heating is generally favorable, however, because there is less need for expensive components such as power supplies, transformers, or current modulators that alter frequency. Line frequency is the frequency of a general supply of current. Line frequency is typically 60 Hz, but may be 50 Hz or another frequency depending on the source for the supply of the current. Higher frequencies may be produced using commercially available equipment such as solid state variable frequency power supplies. Transformers that convert three-phase power to single-phase power with three times the frequency are commercially available. For example, high voltage three-phase power at 60 Hz may be transformed to single-phase power at 180 Hz and at a lower voltage. Such transformers are less expensive and more energy efficient than solid state variable frequency power supplies. In certain embodiments, transformers that convert three-phase power to single-phase power are used to increase the frequency of power supplied to the temperature limited heater.

[0557] In certain embodiments, modulated DC (for example, chopped DC, waveform modulated DC, or cycled DC) may be used for providing electrical power to the temperature limited heater. A DC modulator or DC chopper may be coupled to a DC power supply to provide an output of modulated direct current. In some embodiments, the DC power supply may include means for modulating DC. One example of a DC modulator is a DC-to-DC converter system. DC-to-DC converter systems are generally known in the art. DC is typically modulated or chopped into a desired waveform. Waveforms for DC modulation include, but are not limited to, square-wave, sinusoidal, deformed sinusoidal, deformed square-wave, triangular, and other regular or irregular waveforms.

[0558] The modulated DC waveform generally defines the frequency of the modulated DC. Thus, the modulated DC waveform may be selected to provide a desired modulated DC frequency. The shape and/or the rate of modulation (such as the rate of chopping) of the modulated DC waveform may be varied to vary the modulated DC frequency. DC may be modulated at frequencies that are higher than generally available AC frequencies. For example, modulated DC may be provided at frequencies of at least 1000 Hz. Increasing the frequency of supplied current to higher values advantageously increases the turndown ratio of the temperature limited heater.

[0559] In certain embodiments, the modulated DC waveform is adjusted or altered to vary the modulated DC frequency. The DC modulator may be able to adjust or alter the modulated DC waveform at any time during use of the temperature limited heater and at high currents or voltages. Thus, modulated DC provided to the temperature limited heater is not limited to a single frequency or even a small set of frequency values. Waveform selection using the DC modulator typically allows for a wide range of modulated DC frequencies and for discrete control of the modulated DC frequency. Thus, the modulated DC frequency is more easily set at a distinct value whereas AC frequency is generally limited to multiples of the line frequency. Discrete control of the modulated DC frequency allows for more selective control over the turndown ratio of the temperature limited heater. Being able to selectively control the turndown ratio of the temperature limited heater allows for a broader range of materials to be used in designing and constructing the temperature limited heater.

[0560] In some embodiments, the modulated DC frequency or the AC frequency is adjusted to compensate for changes in properties (for example, subsurface conditions such as temperature or pressure) of the temperature limited heater during use. The modulated DC frequency or the AC frequency provided to the temperature limited heater is varied based on assessed downhole conditions. For example, as the temperature of the temperature limited heater in the wellbore increases, it may be advantageous to increase the frequency of the current provided to the heater, thus increasing the turndown ratio of the heater. In an embodiment, the downhole temperature of the temperature limited heater in the wellbore is assessed.

[0561] In certain embodiments, the modulated DC frequency, or the AC frequency, is varied to adjust the turndown ratio of the temperature limited heater. The turndown ratio may be adjusted to compensate for hot spots occurring along a length of the temperature limited heater. For example, the turndown ratio is increased because the temperature limited heater is getting too hot in certain locations. In some embodiments, the modulated DC frequency, or the AC frequency, are varied to adjust a turndown ratio without assessing a subsurface condition.

[0562] At or near the Curie temperature and/or the phase transformation temperature range of the ferromagnetic material, a relatively small change in voltage may cause a relatively large change in current to the load. The relatively small change in voltage may produce problems in the power supplied to the temperature limited heater, especially at or near the Curie temperature and/or the phase transformation temperature range. The problems include, but are not limited to, reducing the power factor, tripping a circuit breaker, and/or blowing a fuse. In some cases, voltage changes may be caused by a change in the load of the temperature limited heater. In certain embodiments, an electrical current supply (for example, a supply of modulated DC or AC) provides a relatively constant amount of current that does not substantially vary with changes in load of the temperature limited heater. In an embodiment, the electrical current supply provides an amount of electrical current that remains within 15%, within 10%, within 5%, or within 2% of a selected constant current value when a load of the temperature limited heater changes.

[0563] Temperature limited heaters may generate an inductive load. The inductive load is due to some applied electrical current being used by the ferromagnetic material to generate a magnetic field in addition to generating a resistive heat output. As downhole temperature changes in the temperature limited heater, the inductive load of the heater changes due to changes in the ferromagnetic properties of ferromagnetic materials in the heater with temperature. The inductive load of the temperature limited heater may cause a phase shift between the current and the voltage applied to the heater.

[0564] A reduction in actual power applied to the temperature limited heater may be caused by a time lag in the current waveform (for example, the current has a phase shift relative to the voltage due to an inductive load) and/or by distortions in the current waveform (for example, distortions in the current waveform caused by introduced harmonics due to a non-linear load). Thus, it may take more current to apply a selected amount of power due to phase shifting or waveform distortion. The ratio of actual power applied and the apparent power that would have been transmitted if the same current were in phase and undistorted is the power factor. The power factor is always less than or equal to 1. The power factor is 1 when there is no phase shift or distortion in the waveform.

[0565] Actual power applied to a heater due to a phase shift may be described by EQN. 3:
P=I×V×cos(θ);  (EQN. 3)
in which P is the actual power applied to a heater; I is the applied current; V is the applied voltage; and 0 is the phase angle difference between voltage and current. Other phenomena such as waveform distortion may contribute to further lowering of the power factor. If there is no distortion in the waveform, then cos(θ) is equal to the power factor.

[0566] In certain embodiments, the temperature limited heater includes an inner conductor inside an outer conductor. The inner conductor and the outer conductor are radially disposed about a central axis. The inner and outer conductors may be separated by an insulation layer. In certain embodiments, the inner and outer conductors are coupled at the bottom of the temperature limited heater. Electrical current may flow into the temperature limited heater through the inner conductor and return through the outer conductor. One or both conductors may include ferromagnetic material.

[0567] The insulation layer may include an electrically insulating ceramic with high thermal conductivity, such as magnesium oxide, aluminum oxide, silicon dioxide, beryllium oxide, boron nitride, silicon nitride, or combinations thereof. The insulating layer may be a compacted powder (for example, compacted ceramic powder). Compaction may improve thermal conductivity and provide better insulation resistance. For lower temperature applications, polymer insulation made from, for example, fluoropolymers, polyimides, polyamides, and/or polyethylenes, may be used. In some embodiments, the polymer insulation is made of perfluoroalkoxy (PFA) or polyetheretherketone (PEEK™ (Victrex Ltd., England)). The insulating layer may be chosen to be substantially infrared transparent to aid heat transfer from the inner conductor to the outer conductor. In an embodiment, the insulating layer is transparent quartz sand. The insulation layer may be air or a non-reactive gas such as helium, nitrogen, or sulfur hexafluoride. If the insulation layer is air or a non-reactive gas, there may be insulating spacers designed to inhibit electrical contact between the inner conductor and the outer conductor. The insulating spacers may be made of, for example, high purity aluminum oxide or another thermally conducting, electrically insulating material such as silicon nitride. The insulating spacers may be a fibrous ceramic material such as Nextel™ 312 (3M Corporation, St. Paul, Minn., U.S.A.), mica tape, or glass fiber. Ceramic material may be made of alumina, alumina-silicate, alumina-borosilicate, silicon nitride, boron nitride, or other materials.

[0568] The insulation layer may be flexible and/or substantially deformation tolerant. For example, if the insulation layer is a solid or compacted material that substantially fills the space between the inner and outer conductors, the temperature limited heater may be flexible and/or substantially deformation tolerant. Forces on the outer conductor can be transmitted through the insulation layer to the solid inner conductor, which may resist crushing. Such a temperature limited heater may be bent, dog-legged, and spiraled without causing the outer conductor and the inner conductor to electrically short to each other. Deformation tolerance may be important if the wellbore is likely to undergo substantial deformation during heating of the formation.

[0569] In certain embodiments, an outermost layer of the temperature limited heater (for example, the outer conductor) is chosen for corrosion resistance, yield strength, and/or creep resistance. In one embodiment, austenitic (non-ferromagnetic) stainless steels such as 201, 304H, 347H, 347HH, 316H, 310H, 347HP, NF709 (Nippon Steel Corp., Japan) stainless steels, or combinations thereof may be used in the outer conductor. The outermost layer may also include a clad conductor. For example, a corrosion resistant alloy such as 800H or 347H stainless steel may be clad for corrosion protection over a ferromagnetic carbon steel tubular. If high temperature strength is not required, the outermost layer may be constructed from ferromagnetic metal with good corrosion resistance such as one of the ferritic stainless steels. In one embodiment, a ferritic alloy of 82.3% by weight iron with 17.7% by weight chromium (Curie temperature of 678° C.) provides desired corrosion resistance.

[0570] The Metals Handbook, vol. 8, page 291 (American Society of Materials (ASM)) includes a graph of Curie temperature of iron-chromium alloys versus the amount of chromium in the alloys. In some temperature limited heater embodiments, a separate support rod or tubular (made from 347H stainless steel) is coupled to the temperature limited heater made from an iron-chromium alloy to provide yield strength and/or creep resistance. In certain embodiments, the support material and/or the ferromagnetic material is selected to provide a 100,000 hour creep-rupture strength of at least 20.7 MPa at 650° C. In some embodiments, the 100,000 hour creep-rupture strength is at least 13.8 MPa at 650° C. or at least 6.9 MPa at 650° C. For example, 347H steel has a favorable creep-rupture strength at or above 650° C. In some embodiments, the 100,000 hour creep-rupture strength ranges from 6.9 MPa to 41.3 MPa or more for longer heaters and/or higher earth or fluid stresses.

[0571] In temperature limited heater embodiments with both an inner ferromagnetic conductor and an outer ferromagnetic conductor, the skin effect current path occurs on the outside of the inner conductor and on the inside of the outer conductor. Thus, the outside of the outer conductor may be clad with the corrosion resistant alloy, such as stainless steel, without affecting the skin effect current path on the inside of the outer conductor.

[0572] A ferromagnetic conductor with a thickness of at least the skin depth at the Curie temperature and/or the phase transformation temperature range allows a substantial decrease in resistance of the ferromagnetic material as the skin depth increases sharply near the Curie temperature and/or the phase transformation temperature range. In certain embodiments when the ferromagnetic conductor is not clad with a highly conducting material such as copper, the thickness of the conductor may be 1.5 times the skin depth near the Curie temperature and/or the phase transformation temperature range, 3 times the skin depth near the Curie temperature and/or the phase transformation temperature range, or even 10 or more times the skin depth near the Curie temperature and/or the phase transformation temperature range. If the ferromagnetic conductor is clad with copper, thickness of the ferromagnetic conductor may be substantially the same as the skin depth near the Curie temperature and/or the phase transformation temperature range. In some embodiments, the ferromagnetic conductor clad with copper has a thickness of at least three-fourths of the skin depth near the Curie temperature and/or the phase transformation temperature range.

[0573] In certain embodiments, the temperature limited heater includes a composite conductor with a ferromagnetic tubular and a non-ferromagnetic, high electrical conductivity core. The non-ferromagnetic, high electrical conductivity core reduces a required diameter of the conductor. For example, the conductor may be composite 1.19 cm diameter conductor with a core of 0.575 cm diameter copper clad with a 0.298 cm thickness of ferritic stainless steel or carbon steel surrounding the core. The core or non-ferromagnetic conductor may be copper or copper alloy. The core or non-ferromagnetic conductor may also be made of other metals that exhibit low electrical resistivity and relative magnetic permeabilities near 1 (for example, substantially non-ferromagnetic materials such as aluminum and aluminum alloys, phosphor bronze, beryllium copper, and/or brass). A composite conductor allows the electrical resistance of the temperature limited heater to decrease more steeply near the Curie temperature and/or the phase transformation temperature range. As the skin depth increases near the Curie temperature and/or the phase transformation temperature range to include the copper core, the electrical resistance decreases very sharply.

[0574] The composite conductor may increase the conductivity of the temperature limited heater and/or allow the heater to operate at lower voltages. In an embodiment, the composite conductor exhibits a relatively flat resistance versus temperature profile at temperatures below a region near the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor of the composite conductor. In some embodiments, the temperature limited heater exhibits a relatively flat resistance versus temperature profile between 100° C. and 750° C. or between 300° C. and 600° C. The relatively flat resistance versus temperature profile may also be exhibited in other temperature ranges by adjusting, for example, materials and/or the configuration of materials in the temperature limited heater. In certain embodiments, the relative thickness of each material in the composite conductor is selected to produce a desired resistivity versus temperature profile for the temperature limited heater.

[0575] In certain embodiments, the relative thickness of each material in a composite conductor is selected to produce a desired resistivity versus temperature profile for a temperature limited heater. In an embodiment, the composite conductor is an inner conductor surrounded by 0.127 cm thick magnesium oxide powder as an insulator. The outer conductor may be 304H stainless steel with a wall thickness of 0.127 cm. The outside diameter of the heater may be about 1.65 cm.

[0576] A composite conductor (for example, a composite inner conductor or a composite outer conductor) may be manufactured by methods including, but not limited to, coextrusion, roll forming, tight fit tubing (for example, cooling the inner member and heating the outer member, then inserting the inner member in the outer member, followed by a drawing operation and/or allowing the system to cool), explosive or electromagnetic cladding, arc overlay welding, longitudinal strip welding, plasma powder welding, billet coextrusion, electroplating, drawing, sputtering, plasma deposition, coextrusion casting, magnetic forming, molten cylinder casting (of inner core material inside the outer or vice versa), insertion followed by welding or high temperature braising, shielded active gas welding (SAG), and/or insertion of an inner pipe in an outer pipe followed by mechanical expansion of the inner pipe by hydroforming or use of a pig to expand and swage the inner pipe against the outer pipe. In some embodiments, a ferromagnetic conductor is braided over a non-ferromagnetic conductor. In certain embodiments, composite conductors are formed using methods similar to those used for cladding (for example, cladding copper to steel). A metallurgical bond between copper cladding and base ferromagnetic material may be advantageous. Composite conductors produced by a coextrusion process that forms a good metallurgical bond (for example, a good bond between copper and 446 stainless steel) may be provided by Anomet Products, Inc. (Shrewsbury, Mass., U.S.A.).

[0577] In certain embodiments, it may be desirable to form a composite conductor by various methods including longitudinal strip welding. In some embodiments, however, it may be difficult to use longitudinal strip welding techniques if the desired thickness of a layer of a first material has such a large thickness, in relation to the inner core/layer onto which such layer is to be bended, that it does not effectively and/or efficiently bend around an inner core or layer that is made of a second material. In such circumstances, it may be beneficial to use multiple thinner layers of the first material in the longitudinal strip welding process such that the multiple thinner layers can more readily be employed in a longitudinal strip welding process and coupled together to form a composite of the first material with the desired thickness. So, for example, a first layer of the first material may be bent around an inner core or layer of second material, and then a second layer of the first material may be bent around the first layer of the first material, with the thicknesses of the first and second layers being such that the first and second layers will readily bend around the inner core or layer in a longitudinal strip welding process. Thus, the two layers of the first material may together form the total desired thickness of the first material.

[0578] FIGS. 15-32 depict various embodiments of temperature limited heaters. One or more features of an embodiment of the temperature limited heater depicted in any of these figures may be combined with one or more features of other embodiments of temperature limited heaters depicted in these figures. In certain embodiments described herein, temperature limited heaters are dimensioned to operate at a frequency of 60 Hz AC. It is to be understood that dimensions of the temperature limited heater may be adjusted from those described herein to operate in a similar manner at other AC frequencies or with modulated DC current.

[0579] The temperature limited heaters may be used in conductor-in-conduit heaters. In some embodiments of conductor-in-conduit heaters, the majority of the resistive heat is generated in the conductor, and the heat radiatively, conductively and/or convectively transfers to the conduit. In some embodiments of conductor-in-conduit heaters, the majority of the resistive heat is generated in the conduit.

[0580] FIG. 15 depicts a cross-sectional representation of an embodiment of the temperature limited heater with an outer conductor having a ferromagnetic section and a non-ferromagnetic section. FIGS. 16 and 17 depict transverse cross-sectional views of the embodiment shown in FIG. 15. In one embodiment, ferromagnetic section 358 is used to provide heat to hydrocarbon layers in the formation. Non-ferromagnetic section 360 is used in the overburden of the formation. Non-ferromagnetic section 360 provides little or no heat to the overburden, thus inhibiting heat losses in the overburden and improving heater efficiency. Ferromagnetic section 358 includes a ferromagnetic material such as 409 stainless steel or 410 stainless steel. Ferromagnetic section 358 has a thickness of 0.3 cm. Non-ferromagnetic section 360 is copper with a thickness of 0.3 cm. Inner conductor 362 is copper. Inner conductor 362 has a diameter of 0.9 cm. Electrical insulator 364 is silicon nitride, boron nitride, magnesium oxide powder, or another suitable insulator material. Electrical insulator 364 has a thickness of 0.1 cm to 0.3 cm.

[0581] FIG. 18 depicts a cross-sectional representation of an embodiment of a temperature limited heater with an outer conductor having a ferromagnetic section and a non-ferromagnetic section placed inside a sheath. FIGS. 19, 20, and 21 depict transverse cross-sectional views of the embodiment shown in FIG. 18. Ferromagnetic section 358 is 410 stainless steel with a thickness of 0.6 cm. Non-ferromagnetic section 360 is copper with a thickness of 0.6 cm. Inner conductor 362 is copper with a diameter of 0.9 cm. Outer conductor 366 includes ferromagnetic material. Outer conductor 366 provides some heat in the overburden section of the heater. Providing some heat in the overburden inhibits condensation or refluxing of fluids in the overburden. Outer conductor 366 is 409, 410, or 446 stainless steel with an outer diameter of 3.0 cm and a thickness of 0.6 cm. Electrical insulator 364 includes compacted magnesium oxide powder with a thickness of 0.3 cm. In some embodiments, electrical insulator 364 includes silicon nitride, boron nitride, or hexagonal type boron nitride. Conductive section 368 may couple inner conductor 362 with ferromagnetic section 358 and/or outer conductor 366.

[0582] FIG. 22A and FIG. 22B depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic outer conductor. The outer conductor is clad with a conductive layer and a corrosion resistant alloy. Inner conductor 362 is copper. Electrical insulator 364 is silicon nitride, boron nitride, or magnesium oxide. Outer conductor 366 is a 1″ Schedule 80 446 stainless steel pipe. Outer conductor 366 is coupled to jacket 370. Jacket 370 is made from corrosion resistant material such as 347H stainless steel. In an embodiment, conductive layer 372 is placed between outer conductor 366 and jacket 370. Conductive layer 372 is a copper layer. Heat is produced primarily in outer conductor 366, resulting in a small temperature differential across electrical insulator 364. Conductive layer 372 allows a sharp decrease in the resistance of outer conductor 366 as the outer conductor approaches the Curie temperature and/or the phase transformation temperature range. Jacket 370 provides protection from corrosive fluids in the wellbore.

[0583] In certain embodiments, inner conductor 362 includes a core of copper or another non-ferromagnetic conductor surrounded by ferromagnetic material (for example, a low Curie temperature material such as Invar 36). In certain embodiments, the copper core has an outer diameter between about 0.125″ and about 0.375″ (for example, about 0.5″) and the ferromagnetic material has an outer diameter between about 0.625″ and about 1″ (for example, about 0.75″). The copper core may increase the turndown ratio of the heater and/or reduce the thickness needed in the ferromagnetic material, which may allow a lower cost heater to be made. Electrical insulator 364 may be magnesium oxide with an outer diameter between about 1″ and about 1.2″ (for example, about 1.11″). Outer conductor 366 may include non-ferromagnetic electrically conductive material with high mechanical strength such as 825 stainless steel. Outer conductor 366 may have an outer diameter between about 1.2″ and about 1.5″ (for example, about 1.33″). In certain embodiments, inner conductor 362 is a forward current path and outer conductor 366 is a return current path. Conductive layer 372 may include copper or another non-ferromagnetic material with an outer diameter between about 1.3″ and about 1.4″ (for example, about 1.384″). Conductive layer 372 may decrease the resistance of the return current path (to reduce the heat output of the return path such that little or no heat is generated in the return path) and/or increase the turndown ratio of the heater. Conductive layer 372 may reduce the thickness needed in outer conductor 366 and/or jacket 370, which may allow a lower cost heater to be made. Jacket 370 may include ferromagnetic material such as carbon steel or 410 stainless steel with an outer diameter between about 1.6″ and about 1.8″ (for example, about 1.684″). Jacket 370 may have a thickness of at least 2 times the skin depth of the ferromagnetic material in the jacket. Jacket 370 may provide protection from corrosive fluids in the wellbore. In some embodiments, inner conductor 362, electrical insulator 364, and outer conductor 366 are formed as composite heater (for example, an insulated conductor heater) and conductive layer 372 and jacket 370 are formed around (for example, wrapped) the composite heater and welded together to form the larger heater embodiment described herein.

[0584] In certain embodiments, jacket 370 includes ferromagnetic material that has a higher Curie temperature than ferromagnetic material in inner conductor 362. Such a temperature limited heater may “contain” current such that the current does not easily flow from the heater to the surrounding formation and/or to any surrounding fluids (for example, production fluids, formation fluids, brine, groundwater, or formation water). In this embodiment, a majority of the current flows through inner conductor 362 until the Curie temperature of the ferromagnetic material in the inner conductor is reached. After the Curie temperature of ferromagnetic material in inner conductor 362 is reached, a majority of the current flows through the core of copper in the inner conductor. The ferromagnetic properties of jacket 370 inhibit the current from flowing outside the jacket and “contain” the current. Such a heater may be used in lower temperature applications where fluids are present such as providing heat in a production wellbore to increase oil production.

[0585] In some embodiments, the conductor (for example, an inner conductor, an outer conductor, or a ferromagnetic conductor) is the composite conductor that includes two or more different materials. In certain embodiments, the composite conductor includes two or more ferromagnetic materials. In some embodiments, the composite ferromagnetic conductor includes two or more radially disposed materials. In certain embodiments, the composite conductor includes a ferromagnetic conductor and a non-ferromagnetic conductor. In some embodiments, the composite conductor includes the ferromagnetic conductor placed over a non-ferromagnetic core. Two or more materials may be used to obtain a relatively flat electrical resistivity versus temperature profile in a temperature region below the Curie temperature, and/or the phase transformation temperature range, and/or a sharp decrease (a high turndown ratio) in the electrical resistivity at or near the Curie temperature and/or the phase transformation temperature range. In some cases, two or more materials are used to provide more than one Curie temperature and/or phase transformation temperature range for the temperature limited heater.

[0586] The composite electrical conductor may be used as the conductor in any electrical heater embodiment described herein. For example, the composite conductor may be used as the conductor in a conductor-in-conduit heater or an insulated conductor heater. In certain embodiments, the composite conductor may be coupled to a support member such as a support conductor. The support member may be used to provide support to the composite conductor so that the composite conductor is not relied upon for strength at or near the Curie temperature and/or the phase transformation temperature range. The support member may be useful for heaters of lengths of at least 100 m. The support member may be a non-ferromagnetic member that has good high temperature creep strength. Examples of materials that are used for a support member include, but are not limited to, Haynes® 625 alloy and Haynes® HR120® alloy (Haynes International, Kokomo, Ind., U.S.A.), NF709, Incoloy® 800H alloy and 347HP alloy (Allegheny Ludlum Corp., Pittsburgh, Pa., U.S.A.). In some embodiments, materials in a composite conductor are directly coupled (for example, brazed, metallurgically bonded, or swaged) to each other and/or the support member. Using a support member may reduce the need for the ferromagnetic member to provide support for the temperature limited heater, especially at or near the Curie temperature and/or the phase transformation temperature range. Thus, the temperature limited heater may be designed with more flexibility in the selection of ferromagnetic materials.

[0587] FIG. 23 depicts a cross-sectional representation of an embodiment of the composite conductor with the support member. Core 374 is surrounded by ferromagnetic conductor 376 and support member 378. In some embodiments, core 374, ferromagnetic conductor 376, and support member 378 are directly coupled (for example, brazed together or metallurgically bonded together). In one embodiment, core 374 is copper, ferromagnetic conductor 376 is 446 stainless steel, and support member 378 is 347H alloy. In certain embodiments, support member 378 is a Schedule 80 pipe. Support member 378 surrounds the composite conductor having ferromagnetic conductor 376 and core 374. Ferromagnetic conductor 376 and core 374 may be joined to form the composite conductor by, for example, a coextrusion process. For example, the composite conductor is a 1.9 cm outside diameter 446 stainless steel ferromagnetic conductor surrounding a 0.95 cm diameter copper core.

[0588] In certain embodiments, the diameter of core 374 is adjusted relative to a constant outside diameter of ferromagnetic conductor 376 to adjust the turndown ratio of the temperature limited heater. For example, the diameter of core 374 may be increased to 1.14 cm while maintaining the outside diameter of ferromagnetic conductor 376 at 1.9 cm to increase the turndown ratio of the heater.

[0589] FIG. 24 depicts a cross-sectional representation of an embodiment of the composite conductor with support member 378 separating the conductors. In one embodiment, core 374 is copper with a diameter of 0.95 cm, support member 378 is 347H alloy with an outside diameter of 1.9 cm, and ferromagnetic conductor 376 is 446 stainless steel with an outside diameter of 2.7 cm. The support member depicted in FIG. 24 has a lower creep strength relative to the support members depicted in FIG. 23.

[0590] In certain embodiments, support member 378 is located inside the composite conductor. FIG. 25 depicts a cross-sectional representation of an embodiment of the composite conductor surrounding support member 378. Support member 378 is made of 347H alloy. Inner conductor 362 is copper. Ferromagnetic conductor 376 is 446 stainless steel. In one embodiment, support member 378 is 1.25 cm diameter 347H alloy, inner conductor 362 is 1.9 cm outside diameter copper, and ferromagnetic conductor 376 is 2.7 cm outside diameter 446 stainless steel. The turndown ratio is higher than the turndown ratio for the embodiments depicted in FIGS. 23, 24, and 26 for the same outside diameter, but the creep strength is lower.

[0591] In some embodiments, the thickness of inner conductor 362, which is copper, is reduced and the thickness of support member 378 is increased to increase the creep strength at the expense of reduced turndown ratio. For example, the diameter of support member 378 is increased to 1.6 cm while maintaining the outside diameter of inner conductor 362 at 1.9 cm to reduce the thickness of the conduit. This reduction in thickness of inner conductor 362 results in a decreased turndown ratio relative to the thicker inner conductor embodiment but an increased creep strength.

[0592] FIG. 26 depicts a cross-sectional representation of an embodiment of the composite conductor surrounding support member 378. In one embodiment, support member 378 is 347H alloy with a 0.63 cm diameter center hole. In some embodiments, support member 378 is a preformed conduit. In certain embodiments, support member 378 is formed by having a dissolvable material (for example, copper dissolvable by nitric acid) located inside the support member during formation of the composite conductor. The dissolvable material is dissolved to form the hole after the conductor is assembled. In an embodiment, support member 378 is 347H alloy with an inside diameter of 0.63 cm and an outside diameter of 1.6 cm, inner conductor 362 is copper with an outside diameter of 1.8 cm, and ferromagnetic conductor 376 is 446 stainless steel with an outside diameter of 2.7 cm.

[0593] In certain embodiments, the composite electrical conductor is used as the conductor in the conductor-in-conduit heater. For example, the composite electrical conductor may be used as conductor 380 in FIG. 27.

[0594] FIG. 27 depicts a cross-sectional representation of an embodiment of the conductor-in-conduit heater. Conductor 380 is disposed in conduit 382. Conductor 380 is a rod or conduit of electrically conductive material. Low resistance sections 384 are present at both ends of conductor 380 to generate less heating in these sections. Low resistance section 384 is formed by having a greater cross-sectional area of conductor 380 in that section, or the sections are made of material having less resistance. In certain embodiments, low resistance section 384 includes a low resistance conductor coupled to conductor 380.

[0595] Conduit 382 is made of an electrically conductive material. Conduit 382 is disposed in opening 386 in hydrocarbon layer 388. Opening 386 has a diameter that accommodates conduit 382.

[0596] Conductor 380 may be centered in conduit 382 by centralizers 390. Centralizers 390 electrically isolate conductor 380 from conduit 382. Centralizers 390 inhibit movement and properly locate conductor 380 in conduit 382. Centralizers 390 are made of ceramic material or a combination of ceramic and metallic materials. Centralizers 390 inhibit deformation of conductor 380 in conduit 382. Centralizers 390 are touching or spaced at intervals between approximately 0.1 m (meters) and approximately 3 m or more along conductor 380.

[0597] A second low resistance section 384 of conductor 380 may couple conductor 380 to wellhead 392. Electrical current may be applied to conductor 380 from power cable 394 through low resistance section 384 of conductor 380. Electrical current passes from conductor 380 through sliding connector 396 to conduit 382. Conduit 382 may be electrically insulated from overburden casing 398 and from wellhead 392 to return electrical current to power cable 394. Heat may be generated in conductor 380 and conduit 382. The generated heat may radiate in conduit 382 and opening 386 to heat at least a portion of hydrocarbon layer 388.

[0598] Overburden casing 398 may be disposed in overburden 400. In some embodiments, overburden casing 398 is surrounded by materials (for example, reinforcing material and/or cement) that inhibit heating of overburden 400. Low resistance section 384 of conductor 380 may be placed in overburden casing 398. Low resistance section 384 of conductor 380 is made of, for example, carbon steel. Low resistance section 384 of conductor 380 may be centralized in overburden casing 398 using centralizers 390. Centralizers 390 are spaced at intervals of approximately 6 m to approximately 12 m or, for example, approximately 9 m along low resistance section 384 of conductor 380. In a heater embodiment, low resistance sections 384 are coupled to conductor 380 by one or more welds. In other heater embodiments, low resistance sections are threaded, threaded and welded, or otherwise coupled to the conductor. Low resistance section 384 generates little or no heat in overburden casing 398. Packing 402 may be placed between overburden casing 398 and opening 386. Packing 402 may be used as a cap at the junction of overburden 400 and hydrocarbon layer 388 to allow filling of materials in the annulus between overburden casing 398 and opening 386. In some embodiments, packing 402 inhibits fluid from flowing from opening 386 to surface 404.

[0599] FIG. 28 depicts a cross-sectional representation of an embodiment of a removable conductor-in-conduit heat source. Conduit 382 may be placed in opening 386 through overburden 400 such that a gap remains between the conduit and overburden casing 398. Fluids may be removed from opening 386 through the gap between conduit 382 and overburden casing 398. Fluids may be removed from the gap through conduit 406. Conduit 382 and components of the heat source included in the conduit that are coupled to wellhead 392 may be removed from opening 386 as a single unit. The heat source may be removed as a single unit to be repaired, replaced, and/or used in another portion of the formation.

[0600] For a temperature limited heater in which the ferromagnetic conductor provides a majority of the resistive heat output below the Curie temperature and/or the phase transformation temperature range, a majority of the current flows through material with highly non-linear functions of magnetic field (H) versus magnetic induction (B). These non-linear functions may cause strong inductive effects and distortion that lead to decreased power factor in the temperature limited heater at temperatures below the Curie temperature and/or the phase transformation temperature range. These effects may render the electrical power supply to the temperature limited heater difficult to control and may result in additional current flow through surface and/or overburden power supply conductors. Expensive and/or difficult to implement control systems such as variable capacitors or modulated power supplies may be used to compensate for these effects and to control temperature limited heaters where the majority of the resistive heat output is provided by current flow through the ferromagnetic material.

[0601] In certain temperature limited heater embodiments, the ferromagnetic conductor confines a majority of the flow of electrical current to an electrical conductor coupled to the ferromagnetic conductor when the temperature limited heater is below or near the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. The electrical conductor may be a sheath, jacket, support member, corrosion resistant member, or other electrically resistive member. In some embodiments, the ferromagnetic conductor confines a majority of the flow of electrical current to the electrical conductor positioned between an outermost layer and the ferromagnetic conductor. The ferromagnetic conductor is located in the cross section of the temperature limited heater such that the magnetic properties of the ferromagnetic conductor at or below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor confine the majority of the flow of electrical current to the electrical conductor. The majority of the flow of electrical current is confined to the electrical conductor due to the skin effect of the ferromagnetic conductor. Thus, the majority of the current is flowing through material with substantially linear resistive properties throughout most of the operating range of the heater.

[0602] In certain embodiments, the ferromagnetic conductor and the electrical conductor are located in the cross section of the temperature limited heater so that the skin effect of the ferromagnetic material limits the penetration depth of electrical current in the electrical conductor and the ferromagnetic conductor at temperatures below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. Thus, the electrical conductor provides a majority of the electrically resistive heat output of the temperature limited heater at temperatures up to a temperature at or near the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. In certain embodiments, the dimensions of the electrical conductor may be chosen to provide desired heat output characteristics.

[0603] Because the majority of the current flows through the electrical conductor below the Curie temperature and/or the phase transformation temperature range, the temperature limited heater has a resistance versus temperature profile that at least partially reflects the resistance versus temperature profile of the material in the electrical conductor. Thus, the resistance versus temperature profile of the temperature limited heater is substantially linear below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor if the material in the electrical conductor has a substantially linear resistance versus temperature profile. The resistance of the temperature limited heater has little or no dependence on the current flowing through the heater until the temperature nears the Curie temperature and/or the phase transformation temperature range. The majority of the current flows in the electrical conductor rather than the ferromagnetic conductor below the Curie temperature and/or the phase transformation temperature range.

[0604] Resistance versus temperature profiles for temperature limited heaters in which the majority of the current flows in the electrical conductor also tend to exhibit sharper reductions in resistance near or at the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. The sharper reductions in resistance near or at the Curie temperature and/or the phase transformation temperature range are easier to control than more gradual resistance reductions near the Curie temperature and/or the phase transformation temperature range because little current is flowing through the ferromagnetic material.

[0605] In certain embodiments, the material and/or the dimensions of the material in the electrical conductor are selected so that the temperature limited heater has a desired resistance versus temperature profile below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor.

[0606] Temperature limited heaters in which the majority of the current flows in the electrical conductor rather than the ferromagnetic conductor below the Curie temperature and/or the phase transformation temperature range are easier to predict and/or control. Behavior of temperature limited heaters in which the majority of the current flows in the electrical conductor rather than the ferromagnetic conductor below the Curie temperature and/or the phase transformation temperature range may be predicted by, for example, the resistance versus temperature profile and/or the power factor versus temperature profile. Resistance versus temperature profiles and/or power factor versus temperature profiles may be assessed or predicted by, for example, experimental measurements that assess the behavior of the temperature limited heater, analytical equations that assess or predict the behavior of the temperature limited heater, and/or simulations that assess or predict the behavior of the temperature limited heater.

[0607] In certain embodiments, assessed or predicted behavior of the temperature limited heater is used to control the temperature limited heater. The temperature limited heater may be controlled based on measurements (assessments) of the resistance and/or the power factor during operation of the heater. In some embodiments, the power, or current, supplied to the temperature limited heater is controlled based on assessment of the resistance and/or the power factor of the heater during operation of the heater and the comparison of this assessment versus the predicted behavior of the heater. In certain embodiments, the temperature limited heater is controlled without measurement of the temperature of the heater or a temperature near the heater. Controlling the temperature limited heater without temperature measurement eliminates operating costs associated with downhole temperature measurement. Controlling the temperature limited heater based on assessment of the resistance and/or the power factor of the heater also reduces the time for making adjustments in the power or current supplied to the heater compared to controlling the heater based on measured temperature.

[0608] As the temperature of the temperature limited heater approaches or exceeds the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor, reduction in the ferromagnetic properties of the ferromagnetic conductor allows electrical current to flow through a greater portion of the electrically conducting cross section of the temperature limited heater. Thus, the electrical resistance of the temperature limited heater is reduced and the temperature limited heater automatically provides reduced heat output at or near the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. In certain embodiments, a highly electrically conductive member is coupled to the ferromagnetic conductor and the electrical conductor to reduce the electrical resistance of the temperature limited heater at or above the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. The highly electrically conductive member may be an inner conductor, a core, or another conductive member of copper, aluminum, nickel, or alloys thereof.

[0609] The ferromagnetic conductor that confines the majority of the flow of electrical current to the electrical conductor at temperatures below the Curie temperature and/or the phase transformation temperature range may have a relatively small cross section compared to the ferromagnetic conductor in temperature limited heaters that use the ferromagnetic conductor to provide the majority of resistive heat output up to or near the Curie temperature and/or the phase transformation temperature range. A temperature limited heater that uses the electrical conductor to provide a majority of the resistive heat output below the Curie temperature and/or the phase transformation temperature range has low magnetic inductance at temperatures below the Curie temperature and/or the phase transformation temperature range because less current is flowing through the ferromagnetic conductor as compared to the temperature limited heater where the majority of the resistive heat output below the Curie temperature and/or the phase transformation temperature range is provided by the ferromagnetic material. Magnetic field (H) at radius (r) of the ferromagnetic conductor is proportional to the current (I) flowing through the ferromagnetic conductor and the core divided by the radius, or:
H∞I/r.  (EQN. 4)
Since only a portion of the current flows through the ferromagnetic conductor for a temperature limited heater that uses the outer conductor to provide a majority of the resistive heat output below the Curie temperature and/or the phase transformation temperature range, the magnetic field of the temperature limited heater may be significantly smaller than the magnetic field of the temperature limited heater where the majority of the current flows through the ferromagnetic material. The relative magnetic permeability (μ) may be large for small magnetic fields.

[0610] The skin depth (δ) of the ferromagnetic conductor is inversely proportional to the square root of the relative magnetic permeability (μ):
δ∞(1/μ)^1/2.  (EQN. 5)
Increasing the relative magnetic permeability decreases the skin depth of the ferromagnetic conductor. However, because only a portion of the current flows through the ferromagnetic conductor for temperatures below the Curie temperature and/or the phase transformation temperature range, the radius (or thickness) of the ferromagnetic conductor may be decreased for ferromagnetic materials with large relative magnetic permeabilities to compensate for the decreased skin depth while still allowing the skin effect to limit the penetration depth of the electrical current to the electrical conductor at temperatures below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. The radius (thickness) of the ferromagnetic conductor may be between 0.3 mm and 8 mm, between 0.3 mm and 2 mm, or between 2 mm and 4 mm depending on the relative magnetic permeability of the ferromagnetic conductor. Decreasing the thickness of the ferromagnetic conductor decreases costs of manufacturing the temperature limited heater, as the cost of ferromagnetic material tends to be a significant portion of the cost of the temperature limited heater. Increasing the relative magnetic permeability of the ferromagnetic conductor provides a higher turndown ratio and a sharper decrease in electrical resistance for the temperature limited heater at or near the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor.

[0611] Ferromagnetic materials (such as purified iron or iron-cobalt alloys) with high relative magnetic permeabilities (for example, at least 200, at least 1000, at least 1×10⁴, or at least 1×10⁵) and/or high Curie temperatures (for example, at least 600° C., at least 700° C., or at least 800° C.) tend to have less corrosion resistance and/or less mechanical strength at high temperatures. The electrical conductor may provide corrosion resistance and/or high mechanical strength at high temperatures for the temperature limited heater. Thus, the ferromagnetic conductor may be chosen primarily for its ferromagnetic properties.

[0612] Confining the majority of the flow of electrical current to the electrical conductor below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor reduces variations in the power factor. Because only a portion of the electrical current flows through the ferromagnetic conductor below the Curie temperature and/or the phase transformation temperature range, the non-linear ferromagnetic properties of the ferromagnetic conductor have little or no effect on the power factor of the temperature limited heater, except at or near the Curie temperature and/or the phase transformation temperature range. Even at or near the Curie temperature and/or the phase transformation temperature range, the effect on the power factor is reduced compared to temperature limited heaters in which the ferromagnetic conductor provides a majority of the resistive heat output below the Curie temperature and/or the phase transformation temperature range. Thus, there is less or no need for external compensation (for example, variable capacitors or waveform modification) to adjust for changes in the inductive load of the temperature limited heater to maintain a relatively high power factor.

[0613] In certain embodiments, the temperature limited heater, which confines the majority of the flow of electrical current to the electrical conductor below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor, maintains the power factor above 0.85, above 0.9, or above 0.95 during use of the heater. Any reduction in the power factor occurs only in sections of the temperature limited heater at temperatures near the Curie temperature and/or the phase transformation temperature range. Most sections of the temperature limited heater are typically not at or near the Curie temperature and/or the phase transformation temperature range during use. These sections have a high power factor that approaches 1.0. The power factor for the entire temperature limited heater is maintained above 0.85, above 0.9, or above 0.95 during use of the heater even if some sections of the heater have power factors below 0.85.

[0614] Maintaining high power factors allows for less expensive power supplies and/or control devices such as solid state power supplies or SCRs (silicon controlled rectifiers). These devices may fail to operate properly if the power factor varies by too large an amount because of inductive loads. With the power factors maintained at high values; however, these devices may be used to provide power to the temperature limited heater. Solid state power supplies have the advantage of allowing fine tuning and controlled adjustment of the power supplied to the temperature limited heater.

[0615] In some embodiments, transformers are used to provide power to the temperature limited heater. Multiple voltage taps may be made into the transformer to provide power to the temperature limited heater. Multiple voltage taps allow the current supplied to switch back and forth between the multiple voltages. This maintains the current within a range bound by the multiple voltage taps.

[0616] The highly electrically conductive member, or inner conductor, increases the turndown ratio of the temperature limited heater. In certain embodiments, thickness of the highly electrically conductive member is increased to increase the turndown ratio of the temperature limited heater. In some embodiments, the thickness of the electrical conductor is reduced to increase the turndown ratio of the temperature limited heater. In certain embodiments, the turndown ratio of the temperature limited heater is between 1.1 and 10, between 2 and 8, or between 3 and 6 (for example, the turndown ratio is at least 1.1, at least 2, or at least 3).

[0617] FIG. 29 depicts an embodiment of a temperature limited heater in which the support member provides a majority of the heat output below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. Core 374 is an inner conductor of the temperature limited heater. In certain embodiments, core 374 is a highly electrically conductive material such as copper or aluminum. In some embodiments, core 374 is a copper alloy that provides mechanical strength and good electrically conductivity such as a dispersion strengthened copper. In one embodiment, core 374 is Glidcop® (SCM Metal Products, Inc., Research Triangle Park, N.C., U.S.A.). Ferromagnetic conductor 376 is a thin layer of ferromagnetic material between electrical conductor 408 and core 374. In certain embodiments, electrical conductor 408 is also support member 378. In certain embodiments, ferromagnetic conductor 376 is iron or an iron alloy. In some embodiments, ferromagnetic conductor 376 includes ferromagnetic material with a high relative magnetic permeability. For example, ferromagnetic conductor 376 may be purified iron such as Armco ingot iron (AK Steel Ltd., United Kingdom). Iron with some impurities typically has a relative magnetic permeability on the order of 400. Purifying the iron by annealing the iron in hydrogen gas (H₂) at 1450° C. increases the relative magnetic permeability of the iron. Increasing the relative magnetic permeability of ferromagnetic conductor 376 allows the thickness of the ferromagnetic conductor to be reduced. For example, the thickness of unpurified iron may be approximately 4.5 mm while the thickness of the purified iron is approximately 0.76 mm.

[0618] In certain embodiments, electrical conductor 408 provides support for ferromagnetic conductor 376 and the temperature limited heater. Electrical conductor 408 may be made of a material that provides good mechanical strength at temperatures near or above the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 376. In certain embodiments, electrical conductor 408 is a corrosion resistant member. Electrical conductor 408 (support member 378) may provide support for ferromagnetic conductor 376 and corrosion resistance. Electrical conductor 408 is made from a material that provides desired electrically resistive heat output at temperatures up to and/or above the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 376.

[0619] In an embodiment, electrical conductor 408 is 347H stainless steel. In some embodiments, electrical conductor 408 is another electrically conductive, good mechanical strength, corrosion resistant material. For example, electrical conductor 408 may be 304H, 316H, 347HH, NF709, Incoloy® 800H alloy (Inco Alloys International, Huntington, W.V., U.S.A.), Haynes® HR120® alloy, or Inconel® 617 alloy.

[0620] In some embodiments, electrical conductor 408 (support member 378) includes different alloys in different portions of the temperature limited heater. For example, a lower portion of electrical conductor 408 (support member 378) is 347H stainless steel and an upper portion of the electrical conductor (support member) is NF709. In certain embodiments, different alloys are used in different portions of the electrical conductor (support member) to increase the mechanical strength of the electrical conductor (support member) while maintaining desired heating properties for the temperature limited heater.

[0621] In some embodiments, ferromagnetic conductor 376 includes different ferromagnetic conductors in different portions of the temperature limited heater. Different ferromagnetic conductors may be used in different portions of the temperature limited heater to vary the Curie temperature and/or the phase transformation temperature range and, thus, the maximum operating temperature in the different portions. In some embodiments, the Curie temperature and/or the phase transformation temperature range in an upper portion of the temperature limited heater is lower than the Curie temperature and/or the phase transformation temperature range in a lower portion of the heater. The lower Curie temperature and/or the phase transformation temperature range in the upper portion increases the creep-rupture strength lifetime in the upper portion of the heater.

[0622] In the embodiment depicted in FIG. 29, ferromagnetic conductor 376, electrical conductor 408, and core 374 are dimensioned so that the skin depth of the ferromagnetic conductor limits the penetration depth of the majority of the flow of electrical current to the support member when the temperature is below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. Thus, electrical conductor 408 provides a majority of the electrically resistive heat output of the temperature limited heater at temperatures up to a temperature at or near the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 376. In certain embodiments, the temperature limited heater depicted in FIG. 29 is smaller (for example, an outside diameter of 3 cm, 2.9 cm, 2.5 cm, or less) than other temperature limited heaters that do not use electrical conductor 408 to provide the majority of electrically resistive heat output. The temperature limited heater depicted in FIG. 29 may be smaller because ferromagnetic conductor 376 is thin as compared to the size of the ferromagnetic conductor needed for a temperature limited heater in which the majority of the resistive heat output is provided by the ferromagnetic conductor.

[0623] In some embodiments, the support member and the corrosion resistant member are different members in the temperature limited heater. FIGS. 30 and 31 depict embodiments of temperature limited heaters in which the jacket provides a majority of the heat output below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. In these embodiments, electrical conductor 408 is jacket 370. Electrical conductor 408, ferromagnetic conductor 376, support member 378, and core 374 (in FIG. 30) or inner conductor 362 (in FIG. 31) are dimensioned so that the skin depth of the ferromagnetic conductor limits the penetration depth of the majority of the flow of electrical current to the thickness of the jacket. In certain embodiments, electrical conductor 408 is a material that is corrosion resistant and provides electrically resistive heat output below the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 376. For example, electrical conductor 408 is 825 stainless steel or 347H stainless steel. In some embodiments, electrical conductor 408 has a small thickness (for example, on the order of 0.5 mm).

[0624] In FIG. 30, core 374 is highly electrically conductive material such as copper or aluminum. Support member 378 is 347H stainless steel or another material with good mechanical strength at or near the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 376.

[0625] In FIG. 31, support member 378 is the core of the temperature limited heater and is 347H stainless steel or another material with good mechanical strength at or near the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 376. Inner conductor 362 is highly electrically conductive material such as copper or aluminum.

[0626] In some embodiments, a relatively thin conductive layer is used to provide the majority of the electrically resistive heat output of the temperature limited heater at temperatures up to a temperature at or near the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. Such a temperature limited heater may be used as the heating member in an insulated conductor heater. The heating member of the insulated conductor heater may be located inside a sheath with an insulation layer between the sheath and the heating member.

[0627] FIGS. 32A and 32B depict cross-sectional representations of an embodiment of the insulated conductor heater with the temperature limited heater as the heating member. Insulated conductor 410 includes core 374, ferromagnetic conductor 376, inner conductor 362, electrical insulator 364, and jacket 370. Core 374 is a copper core. Ferromagnetic conductor 376 is, for example, iron or an iron alloy.

[0628] Inner conductor 362 is a relatively thin conductive layer of non-ferromagnetic material with a higher electrical conductivity than ferromagnetic conductor 376. In certain embodiments, inner conductor 362 is copper. Inner conductor 362 may be a copper alloy. Copper alloys typically have a flatter resistance versus temperature profile than pure copper. A flatter resistance versus temperature profile may provide less variation in the heat output as a function of temperature up to the Curie temperature and/or the phase transformation temperature range. In some embodiments, inner conductor 362 is copper with 6% by weight nickel (for example, CuNi6 or LOHM™). In some embodiments, inner conductor 362 is CuNi10Fe1Mn alloy. Below the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 376, the magnetic properties of the ferromagnetic conductor confine the majority of the flow of electrical current to inner conductor 362. Thus, inner conductor 362 provides the majority of the resistive heat output of insulated conductor 410 below the Curie temperature and/or the phase transformation temperature range.

[0629] In certain embodiments, inner conductor 362 is dimensioned, along with core 374 and ferromagnetic conductor 376, so that the inner conductor provides a desired amount of heat output and a desired turndown ratio. For example, inner conductor 362 may have a cross-sectional area that is around 2 or 3 times less than the cross-sectional area of core 374. Typically, inner conductor 362 has to have a relatively small cross-sectional area to provide a desired heat output if the inner conductor is copper or copper alloy. In an embodiment with copper inner conductor 362, core 374 has a diameter of 0.66 cm, ferromagnetic conductor 376 has an outside diameter of 0.91 cm, inner conductor 362 has an outside diameter of 1.03 cm, electrical insulator 364 has an outside diameter of 1.53 cm, and jacket 370 has an outside diameter of 1.79 cm. In an embodiment with a CuNi6 inner conductor 362, core 374 has a diameter of 0.66 cm, ferromagnetic conductor 376 has an outside diameter of 0.91 cm, inner conductor 362 has an outside diameter of 1.12 cm, electrical insulator 364 has an outside diameter of 1.63 cm, and jacket 370 has an outside diameter of 1.88 cm. Such insulated conductors are typically smaller and cheaper to manufacture than insulated conductors that do not use the thin inner conductor to provide the majority of heat output below the Curie temperature and/or the phase transformation temperature range.

[0630] Electrical insulator 364 may be magnesium oxide, aluminum oxide, silicon dioxide, beryllium oxide, boron nitride, silicon nitride, or combinations thereof. In certain embodiments, electrical insulator 364 is a compacted powder of magnesium oxide. In some embodiments, electrical insulator 364 includes beads of silicon nitride.

[0631] In certain embodiments, a small layer of material is placed between electrical insulator 364 and inner conductor 362 to inhibit copper from migrating into the electrical insulator at higher temperatures. For example, a small layer of nickel (for example, about 0.5 mm of nickel) may be placed between electrical insulator 364 and inner conductor 362.

[0632] Jacket 370 is made of a corrosion resistant material such as, but not limited to, 347 stainless steel, 347H stainless steel, 446 stainless steel, or 825 stainless steel. In some embodiments, jacket 370 provides some mechanical strength for insulated conductor 410 at or above the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 376. In certain embodiments, jacket 370 is not used to conduct electrical current.

[0633] For long vertical temperature limited heaters (for example, heaters at least 300 m, at least 500 m, or at least 1 km in length), the hanging stress becomes important in the selection of materials for the temperature limited heater. Without the proper selection of material, the support member may not have sufficient mechanical strength (for example, creep-rupture strength) to support the weight of the temperature limited heater at the operating temperatures of the heater.

[0634] In certain embodiments, materials for the support member are varied to increase the maximum allowable hanging stress at operating temperatures of the temperature limited heater and, thus, increase the maximum operating temperature of the temperature limited heater. Altering the materials of the support member affects the heat output of the temperature limited heater below the Curie temperature and/or the phase transformation temperature range because changing the materials changes the resistance versus temperature profile of the support member. In certain embodiments, the support member is made of more than one material along the length of the heater so that the temperature limited heater maintains desired operating properties (for example, resistance versus temperature profile below the Curie temperature and/or the phase transformation temperature range) as much as possible while providing sufficient mechanical properties to support the heater. In some embodiments, transition sections are used between sections of the heater to provide strength that compensates for the difference in temperature between sections of the heater. In certain embodiments, one or more portions of the temperature limited heater have varying outside diameters and/or materials to provide desired properties for the heater.

[0635] In certain embodiments of temperature limited heaters, three temperature limited heaters are coupled together in a three-phase wye configuration. Coupling three temperature limited heaters together in the three-phase wye configuration lowers the current in each of the individual temperature limited heaters because the current is split between the three individual heaters. Lowering the current in each individual temperature limited heater allows each heater to have a small diameter. The lower currents allow for higher relative magnetic permeabilities in each of the individual temperature limited heaters and, thus, higher turndown ratios. In addition, there may be no return current path needed for each of the individual temperature limited heaters. Thus, the turndown ratio remains higher for each of the individual temperature limited heaters than if each temperature limited heater had its own return current path.

[0636] In the three-phase wye configuration, individual temperature limited heaters may be coupled together by shorting the sheaths, jackets, or canisters of each of the individual temperature limited heaters to the electrically conductive sections (the conductors providing heat) at their terminating ends (for example, the ends of the heaters at the bottom of a heater wellbore). In some embodiments, the sheaths, jackets, canisters, and/or electrically conductive sections are coupled to a support member that supports the temperature limited heaters in the wellbore.

[0637] In certain embodiments, coupling multiple heaters (for example, mineral insulated conductor heaters) to a single power source, such as a transformer, is advantageous. Coupling multiple heaters to a single transformer may result in using fewer transformers to power heaters used for a treatment area as compared to using individual transformers for each heater. Using fewer transformers reduces surface congestion and allows easier access to the heaters and surface components. Using fewer transformers reduces capital costs associated with providing power to the treatment area. In some embodiments, at least 4, at least 5, at least 10, at least 25 heaters, at least 35 heaters, or at least 45 heaters are powered by a single transformer. Additionally, powering multiple heaters (in different heater wells) from the single transformer may reduce overburden losses because of reduced voltage and/or phase differences between each of the heater wells powered by the single transformer. Powering multiple heaters from the single transformer may inhibit current imbalances between the heaters because the heaters are coupled to the single transformer.

[0638] To provide power to multiple heaters using the single transformer, the transformer may have to provide power at higher voltages to carry the current to each of the heaters effectively. In certain embodiments, the heaters are floating (ungrounded) heaters in the formation. Floating the heaters allows the heaters to operate at higher voltages. In some embodiments, the transformer provides power output of at least about 3 kV, at least about 4 kV, at least about 5 kV, or at least about 6 kV.

[0639] FIG. 33 depicts a top view representation of heater 412 with three insulated conductors 410 in conduit 406. Heater 412 may be located in a heater well in the subsurface formation. Conduit 406 may be a sheath, jacket, or other enclosure around insulated conductors 410. Each insulated conductor 410 includes core 374, electrical insulator 364, and jacket 370. Insulated conductors 410 may be mineral insulated conductors with core 374 being a copper alloy (for example, a copper-nickel alloy such as Alloy 180), electrical insulator 364 being magnesium oxide, and jacket 370 being Incoloy® 825, copper, or stainless steel (for example 347H stainless steel). In some embodiments, jacket 370 includes non-work hardenable metals so that the jacket is annealable.

[0640] In some embodiments, core 374 and/or jacket 370 include ferromagnetic materials. In some embodiments, one or more insulated conductors 410 are temperature limited heaters. In certain embodiments, the overburden portion of insulated conductors 410 include high electrical conductivity materials in core 374 (for example, pure copper or copper alloys such as copper with 3% silicon at a weld joint) so that the overburden portions of the insulated conductors provide little or no heat output. In certain embodiments, conduit 406 includes non-corrosive materials and/or high strength materials such as stainless steel. In one embodiment, conduit 406 is 347H stainless steel.

[0641] Insulated conductors 410 may be coupled to the single transformer in a three-phase configuration (for example, a three-phase wye configuration). Each insulated conductor 410 may be coupled to one phase of the single transformer. In certain embodiments, the single transformer is also coupled to a plurality of identical heaters 412 in other heater wells in the formation (for example, the single transformer may couple to 40 or more heaters in the formation). In some embodiments, the single transformer couples to at least 4, at least 5, at least 10, at least 15, or at least 25 additional heaters in the formation.

[0642] Electrical insulator 364′ may be located inside conduit 406 to electrically insulate insulated conductors 410 from the conduit. In certain embodiments, electrical insulator 364′ is magnesium oxide (for example, compacted magnesium oxide). In some embodiments, electrical insulator 364′ is silicon nitride (for example, silicon nitride blocks). Electrical insulator 364′ electrically insulates insulated conductors 410 from conduit 406 so that at high operating voltages (for example, 3 kV or higher), there is no arcing between the conductors and the conduit. In some embodiments, electrical insulator 364′ inside conduit 406 has at least the thickness of electrical insulators 364 in insulated conductors 410. The increased thickness of insulation in heater 412 (from electrical insulators 364 and/or electrical insulator 364′) inhibits and may prevent current leakage into the formation from the heater. In some embodiments, electrical insulator 364′ spatially locates insulated conductors 410 inside conduit 406.

[0643] FIG. 34 depicts an embodiment of three-phase wye transformer 414 coupled to a plurality of heaters 412. For simplicity in the drawing, only four heaters 412 are shown in FIG. 34. It is to be understood that several more heaters may be coupled to the transformer 414. As shown in FIG. 34, each leg (each insulated conductor) of each heater is coupled to one phase of transformer 414 and current is returned to the neutral or ground of the transformer (for example, returned through conductor 416 depicted in FIGS. 33 and 35).

[0644] Return conductor 416 may be electrically coupled to the ends of insulated conductors 410 (as shown in FIG. 35) current returns from the ends of the insulated conductors to the transformer on the surface of the formation. Return conductor 416 may include high electrical conductivity materials such as pure copper, nickel, copper alloys, or combinations thereof so that the return conductor provides little or no heat output. In some embodiments, return conductor 416 is a tubular (for example, a stainless steel tubular) that allows an optical fiber to be placed inside the tubular to be used for temperature and/or other measurement. In some embodiments, return conductor 416 is a small insulated conductor (for example, small mineral insulated conductor). Return conductor 416 may be coupled to the neutral or ground leg of the transformer in a three-phase wye configuration. Thus, insulated conductors 410 are electrically isolated from conduit 406 and the formation. Using return conductor 416 to return current to the surface may make coupling the heater to a wellhead easier. In some embodiments, current is returned using one or more of jackets 370, depicted in FIG. 33. One or more jackets 370 may be coupled to cores 374 at the end of the heaters and return current to the neutral of the three-phase wye transformer.

[0645] FIG. 35 depicts a side view representation of the end section of three insulated conductors 410 in conduit 406. The end section is the section of the heaters the furthest away from (distal from) the surface of the formation. The end section includes contactor section 418 coupled to conduit 406. In some embodiments, contactor section 418 is welded or brazed to conduit 406. Termination 420 is located in contactor section 418. Termination 420 is electrically coupled to insulated conductors 410 and return conductor 416. Termination 420 electrically couples the cores of insulated conductors 410 to the return conductor 416 at the ends of the heaters.

[0646] In certain embodiments, heater 412, depicted in FIGS. 33 and 35, includes an overburden section using copper as the core of the insulated conductors. The copper in the overburden section may be the same diameter as the cores used in the heating section of the heater. The copper in the overburden section may have a larger diameter than the cores in the heating section of the heater. Increasing the size of the copper in the overburden section may decrease losses in the overburden section of the heater.

[0647] Heaters that include three insulated conductors 410 in conduit 406, as depicted in FIGS. 33 and 35, may be made in a multiple step process. In some embodiments, the multiple step process is performed at the site of the formation or treatment area. In some embodiments, the multiple step process is performed at a remote manufacturing site away from the formation. The finished heater is then transported to the treatment area.

[0648] Insulated conductors 410 may be pre-assembled prior to the bundling either on site or at a remote location. Insulated conductors 410 and return conductor 416 may be positioned on spools. A machine may draw insulated conductors 410 and return conductor 416 from the spools at a selected rate. Preformed blocks of insulation material may be positioned around return conductor 416 and insulated conductors 410. In an embodiment, two blocks are positioned around return conductor 416 and three blocks are positioned around insulated conductors 410 to form electrical insulator 364′. The insulated conductors and return conductor may be drawn or pushed into a plate of conduit material that has been rolled into a tubular shape. The edges of the plate may be pressed together and welded (for example, by laser welding). After forming conduit 406 around electrical insulator 364′, the bundle of insulated conductors 410, and return conductor 416, the conduit may be compacted against the electrical insulator 416 so that all of the components of the heater are pressed together into a compact and tightly fitting form. During the compaction, the electrical insulator may flow and fill any gaps inside the heater.

[0649] In some embodiments, heater 412 (which includes conduit 406 around electrical insulator 364′ and the bundle of insulated conductors 410 and return conductor 416) is inserted into a coiled tubing tubular that is placed in a wellbore in the formation. The coiled tubing tubular may be left in place in the formation (left in during heating of the formation) or removed from the formation after installation of the heater. The coiled tubing tubular may allow for easier installation of heater 412 into the wellbore.

[0650] In some embodiments, one or more components of heater 412 are varied (for example, removed, moved, or replaced) while the operation of the heater remains substantially identical. FIG. 36 depicts an embodiment of heater 412 with three insulated cores 374 in conduit 406. In this embodiment, electrical insulator 364′ surrounds cores 374 and return conductor 416 in conduit 406. Cores 374 are located in conduit 406 without an electrical insulator and jacket surrounding the cores. Cores 374 are coupled to the single transformer in a three-phase wye configuration with each core 374 coupled to one phase of the transformer. Return conductor 416 is electrically coupled to the ends of cores 374 and returns current from the ends of the cores to the transformer on the surface of the formation.

[0651] FIG. 37 depicts an embodiment of heater 412 with three insulated conductors 410 and insulated return conductor in conduit 406. In this embodiment, return conductor 416 is an insulated conductor with core 374, electrical insulator 364, and jacket 370. Return conductor 416 and insulated conductors 410 are located in conduit 406 surrounded by electrical insulator 364′. Return conductor 416 and insulated conductors 410 may be the same size or different sizes. Return conductor 416 and insulated conductors 410 operate substantially the same as in the embodiment depicted in FIGS. 33 and 35.

[0652] Mineral insulated (MI) cables (insulated conductors) for use in subsurface applications, such as heating hydrocarbon containing formations in some applications, are longer, may have larger outside diameters, and may operate at higher voltages and temperatures than what is typical in the MI cable industry. For these subsurface applications, the joining of multiple MI cables is needed to make MI cables with sufficient length to reach the depths and distances needed to heat the subsurface efficiently and to join segments with different functions, such as lead-in cables joined to heater sections. Such long heaters also require higher voltages to provide enough power to the farthest ends of the heaters.

[0653] Conventional MI cable splice designs are typically not suitable for voltages above 1000 volts, above 1500 volts, or above 2000 volts and may not operate for extended periods without failure at elevated temperatures, such as over 650° C. (about 1200° F.), over 700° C. (about 1290° F.), or over 800° C. (about 1470° F.). Such high voltage, high temperature applications typically require the compaction of the mineral insulant in the splice to be as close as possible to or above the level of compaction in the insulated conductor (MI cable) itself.

[0654] The relatively large outside diameter and long length of MI cables for some applications requires that the cables be spliced while oriented horizontally. There are splices for other applications of MI cables that have been fabricated horizontally. These techniques typically use a small hole through which the mineral insulation (such as magnesium oxide powder) is filled into the splice and compacted slightly through vibration and tamping. Such methods do not provide sufficient compaction of the mineral insulation or even allow any compaction of the mineral insulation, and are not suitable for making splices for use at the high voltages needed for these subsurface applications.

[0655] Thus, there is a need for splices of insulated conductors that are simple yet can operate at the high voltages and temperatures in the subsurface environment over long durations without failure. In addition, the splices may need higher bending and tensile strengths to inhibit failure of the splice under the weight loads and temperatures that the cables can be subjected to in the subsurface. Techniques and methods also may be utilized to reduce electric field intensities in the splices so that leakage currents in the splices are reduced and to increase the margin between the operating voltage and electrical breakdown. Reducing electric field intensities may help increase voltage and temperature operating ranges of the splices.

[0656] FIG. 38 depicts a side view cross-sectional representation of one embodiment of a fitting for joining insulated conductors. Fitting 422 is a splice or coupling joint for joining insulated conductors 410A, 410B. In certain embodiments, fitting 422 includes sleeve 424 and housings 426A, 426B. Housings 426A, 426B may be splice housings, coupling joint housings, coupler housings. Sleeve 424 and housings 426A, 426B may be made of mechanically strong, electrically conductive materials such as, but not limited to, stainless steel. Sleeve 424 and housings 426A, 426B may be cylindrically shaped or polygon shaped. Sleeve 424 and housings 426A, 426B may have rounded edges, tapered diameter changes, other features, or combinations thereof, which may reduce electric field intensities in fitting 422.

[0657] Fitting 422 may be used to couple (splice) insulated conductor 410A to insulated conductor 410B while maintaining the mechanical and electrical integrity of the jackets (sheaths), insulation, and cores (conductors) of the insulated conductors. Fitting 422 may be used to couple heat producing insulated conductors with non-heat producing insulated conductors, to couple heat producing insulated conductors with other heat producing insulated conductors, or to couple non-heat producing insulated conductors with other non-heat producing insulated conductors. In some embodiments, more than one fitting 422 is used in to couple multiple heat producing and non-heat producing insulated conductors to produce a long insulated conductor.

[0658] Fitting 422 may be used to couple insulated conductors with different diameters, as shown in FIG. 38. For example, the insulated conductors may have different core (conductor) diameters, different jacket (sheath) diameters, or combinations of different diameters. Fitting 422 may also be used to couple insulated conductors with different metallurgies, different types of insulation, or a combination thereof.

[0659] As shown in FIG. 38, housing 426A is coupled to jacket (sheath) 370A of insulated conductor 410A and housing 426B is coupled to jacket 370B of insulated conductor 410B. In certain embodiments, housings 426A, 426B are welded, brazed, or otherwise permanently affixed to insulated conductors 410A, 410B. In some embodiments, housings 426A, 426B are temporarily or semi-permanently affixed to jackets 370A, 370B of insulated conductors 410A, 410B (for example, coupled using threads or adhesives). Fitting 422 may be centered between the end portions of the insulated conductors 410A, 410B.

[0660] In certain embodiments, the interior volumes of sleeve 424 and housings 426A, 426B are substantially filled with electrically insulating material 430. In certain embodiments, substantially filled refers to entirely or almost entirely filling the volume or volumes with electrically insulating material with substantially no macroscopic voids in the volume or volumes. For example, substantially filled may refer to filling almost the entire volume with electrically insulating material that has some porosity because of microscopic voids (for example, up to about 40% porosity). Electrically insulating material 430 may be magnesium oxide, other electrical insulators such as ceramic powders (for example, boron nitride), a mixture of magnesium oxide and another electrical insulator (for example, up to about 50% by volume boron nitride), ceramic cement, mixtures of ceramic powders with certain non-ceramic materials, or mixtures thereof. For example, magnesium oxide may be mixed with boron nitride or another electrical insulator to improve the ability of the electrically insulating material to flow or to improve the dielectric characteristics of the electrically insulating material. In some embodiments, electrically insulating material 430 is material similar to electrical insulation used inside of at least one of insulated conductors 410A, 410B. Electrically insulating material 430 may have substantially similar dielectric characteristics to electrical insulation used inside of at least one of insulated conductors 410A, 410B.

[0661] In certain embodiments, first sleeve 424 and housings 426A, 426B are made up (for example, put together or manufactured) buried or submerged in electrically insulating material 430. Making up sleeve 424 and housings 426A, 426B buried in electrically insulating material 430 inhibits open space from forming in the interior volumes of the portions. Sleeve 424 and housings 426A, 426B have open ends to allow insulated conductors 410A, 410B to pass through. These open ends may be sized to have diameters slightly larger than the outside diameter of the jackets of the insulated conductors.

[0662] In certain embodiments, cores 374A, 374B of insulated conductors 410A, 410B are joined together at coupling 428. The jackets and insulation of insulated conductors 410A, 410B may be cut back or stripped to expose desired lengths of cores 374A, 374B before joining the cores. Coupling 428 may be located in electrically insulating material 430 inside sleeve 424. Coupling 428 may join cores 374A, 374B together, for example, by compression, crimping, brazing, welding, or other techniques known in the art.

[0663] In an embodiment, insulated conductors 410A, 410B are coupled using fitting 422 by first sliding housing 426A over jacket 370A of insulated conductor 410A and, second, sliding housing 426B over jacket 370B of insulated conductor 410B. The housings are slid over the jackets with the large diameter ends of the housings facing the ends of the insulated conductors. Sleeve 424 may be slid over insulated conductor 410B such that it is adjacent to housing 426B. Cores 374A, 374B are joined at coupling 428 to create a robust electrical and mechanical connection between the cores. The small diameter end of housing 426A is joined (for example, welded) to jacket 370A of insulated conductor 410A. Sleeve 424 and housing 426B are brought (moved or pushed) together with housing 426A to form fitting 422. The interior volume of fitting 422 may be substantially filled with electrically insulating material while the sleeve and the housings are brought together. The interior volume of the combined sleeve and housings is reduced such that the electrically insulating material substantially filling the entire interior volume is compacted. Sleeve 424 is joined to housing 426B and housing 426B is joined to jacket 370B of insulated conductor 410B. The volume of sleeve 424 may be further reduced, if additional compaction is desired.

[0664] In certain embodiments, the interior volumes of housings 426A, 426B filled with electrically insulating material 430 have tapered shapes. The diameter of the interior volumes of housings 426A, 426B may taper from a smaller diameter at or near the ends of the housings coupled to insulated conductors 410A, 410B to a larger diameter at or near the ends of the housings located inside sleeve 424 (the ends of the housings facing each other or the ends of the housings facing the ends of the insulated conductors). The tapered shapes of the interior volumes may reduce electric field intensities in fitting 422. Reducing electric field intensities in fitting 422 may reduce leakage currents in the fitting at increased operating voltages and temperatures, and may increase the margin to electrical breakdown. Thus, reducing electric field intensities in fitting 422 may increase the range of operating voltages and temperatures for the fitting.

[0665] In some embodiments, the insulation from insulated conductors 410A, 410B tapers from jackets 370A, 370B down to cores 374A, 374B in the direction toward the center of fitting 422 in the event that the electrically insulating material 430 is a weaker dielectric than the insulation in the insulated conductors. In some embodiments, the insulation from insulated conductors 410A, 410B tapers from jackets 370A, 370B down to cores 374A, 374B in the direction toward the insulated conductors in the event that electrically insulating material 430 is a stronger dielectric than the insulation in the insulated conductors. Tapering the insulation from the insulated conductors reduces the intensity of electric fields at the interfaces between the insulation in the insulated conductors and the electrically insulating material within the fitting.

[0666] FIG. 39 depicts a tool that may be used to cut away part of the inside of insulated conductors 410A, 410B (for example, electrical insulation inside the jacket of the insulated conductor). Cutting tool 436 may include cutting teeth 438 and drive tube 440. Drive tube 440 may be coupled to the body of cutting tool 436 using, for example, a weld or braze. In some embodiments, no cutting tool is needed to cut away electrical insulation from inside the jacket.

[0667] Sleeve 424 and housings 426A, 426B may be coupled together using any means known in the art such as brazing, welding, or crimping. In some embodiments, in the embodiment shown in FIG. 40, sleeve 424 and housings 426A, 426B have threads that engage to couple the pieces together.

[0668] As shown in FIGS. 38 and 40, in certain embodiments, electrically insulating material 430 is compacted during the assembly process. The force to press the housings 426A, 426B toward each other may put a pressure on electrically insulating material 430 of at least 25,000 pounds per square inch, or between 25,000 and 55,000 pounds per square inch, in order to provide acceptable compaction of the insulating material. The tapered shapes of the interior volumes of housings 426A, 426B and the make-up of electrically insulating material 430 may enhance compaction of the electrically insulating material during the assembly process to the point where the dielectric characteristics of the electrically insulating material are, to the extent practical, comparable to that within insulated conductors 410A, 410B. Methods and devices to facilitate compaction include, but are not limited to, mechanical methods (such as shown in FIG. 43), pneumatic, hydraulic (such as shown in FIGS. 44 and 45), swaged, or combinations thereof.

[0669] The combination of moving the pieces together with force and the housings having the tapered interior volumes compacts electrically insulating material 430 using both axial and radial compression. Using both axial and radial compression of electrically insulating material 430 provides more uniform compaction of the electrically insulating material. In some embodiments, vibration and/or tamping of electrically insulating material 430 may also be used to consolidate the electrically insulating material. Vibration (and/or tamping) may be applied either at the same time as application of force to push the housings 426A, 426B together, or vibration (and/or tamping) may be alternated with application of such force. Vibration and/or tamping may reduce bridging of particles in electrically insulating material 430.

[0670] In the embodiment depicted in FIG. 40, electrically insulating material 430 inside housings 426A, 426B is compressed mechanically by tightening nuts 434 against ferrules 432 coupled to jackets 370A, 370B. The mechanical method compacts the interior volumes of housings 426A, 426B because of the tapered shape of the interior volumes. Ferrules 432 may be copper or other soft metal ferrules. Nuts 434 may be stainless steel or other hard metal nut that is movable on jackets 370A, 370B. Nuts 434 may engage threads on housings 426A, 426B to couple to the housings. As nuts 434 are threaded onto housings 426A, 426B, nuts 434 and ferrules 432 work to compress the interior volumes of the housings. In some embodiments, nuts 434 and ferrules 432 may work to move housings 426A, 426B further onto sleeve 424 (using the threaded coupling between the pieces) and compact the interior volume of the sleeve. In some embodiments, housings 426A, 426B and sleeve 424 are coupled together using the threaded coupling before the nut and ferrule are swaged down on the second portion. As the interior volumes inside housings 426A, 426B are compressed, the interior volume inside sleeve 424 may also be compressed. In some embodiments, nuts 434 and ferrules 432 may act to couple housings 426A, 426B to insulated conductors 410A, 410B.

[0671] In certain embodiments, multiple insulated conductors are spliced together in an end fitting. For example, three insulated conductors may be spliced together in an end fitting to couple electrically the insulated conductors in a 3-phase wye configuration. FIG. 41A depicts a side view of a cross-sectional representation of an embodiment of threaded fitting 442 for coupling three insulated conductors 410A, 410B, 410C. FIG. 41B depicts a side view of a cross-sectional representation of an embodiment of welded fitting 442 for coupling three insulated conductors 410A, 410B, 410C. As shown in FIGS. 41A and 41B, insulated conductors 410A, 410B, 410C may be coupled to fitting 442 through end cap 444. End cap 444 may include three strain relief fittings 446 through which insulated conductors 410A, 410B, 410C pass.

[0672] Cores 374A, 374B, 374C of the insulated conductors may be coupled together at coupling 428. Coupling 428 may be, for example, a braze (such as a silver braze or copper braze), a welded joint, or a crimped joint. Coupling cores 374A, 374B, 374C at coupling 428 electrically join the three insulated conductors for use in a 3-phase wye configuration.

[0673] As shown in FIG. 41A, end cap 444 may be coupled to main body 448 of fitting 442 using threads. Threading of end cap 444 and main body 448 may allow the end cap to compact electrically insulating material 430 inside the main body. At the end of main body 448 opposite of end cap 444 is cover 450. Cover 450 may also be attached to main body 448 by threads. In certain embodiments, compaction of electrically insulating material 430 in fitting 442 is enhanced through tightening of cover 450 into main body 448, by crimping of the main body after attachment of the cover, or a combination of these methods.

[0674] As shown in FIG. 41B, end cap 444 may be coupled to main body 448 of fitting 442 using welding, brazing, or crimping. End cap 444 may be pushed or pressed into main body 448 to compact electrically insulating material 430 inside the main body. Cover 450 may also be attached to main body 448 by welding, brazing, or crimping. Cover 450 may be pushed or pressed into main body 448 to compact electrically insulating material 430 inside the main body. Crimping of the main body after attachment of the cover may further enhance compaction of electrically insulating material 430 in fitting 442.

[0675] In some embodiments, as shown in FIGS. 41A and 41B, plugs 452 close openings or holes in cover 450. For example, the plugs may be threaded, welded, or brazed into openings in cover 450. The openings in cover 450 may allow electrically insulating material 430 to be provided inside fitting 442 when cover 450 and end cap 444 are coupled to main body 448. The openings in cover 450 may be plugged or covered after electrically insulating material 430 is provided inside fitting 442. In some embodiments, openings are located on main body 448 of fitting 442. Openings on main body 448 may be plugged with plugs 452 or other plugs.

[0676] In some embodiments, cover 450 includes one or more pins. In some embodiments, the pins are or are part of plugs 452. The pins may engage a torque tool that turns cover 450 and tightens the cover on main body 448. An example of torque tool 454 that may engage the pins is depicted in FIG. 42. Torque tool 454 may have an inside diameter that substantially matches the outside diameter of cover 450 (depicted in FIG. 41A). As shown in FIG. 42, torque tool 454 may have slots or other depressions that are shaped to engage the pins on cover 450. Torque tool 454 may include recess 456. Recess 456 may be a square drive recess or other shaped recess that allows operation (turning) of the torque tool.

[0677] FIG. 43 depicts an embodiment of clamp assemblies 458A,B that may be used to mechanically compact fitting 422. Clamp assemblies 458A,B may be shaped to secure fitting 422 in place at the shoulders of housings 426A, 426B. Threaded rods 462 may pass through holes 460 of clamp assemblies 458A,B. Nuts 468, along with washers, on each of threaded rods 462 may be used to apply force on the outside faces of each clamp assembly and bring the clamp assemblies together such that compressive forces are applied to housings 426A, 426B of fitting 422. These compressive forces compact electrically insulating material inside fitting 422.

[0678] In some embodiments, clamp assemblies 458 are used in hydraulic, pneumatic, or other compaction methods. FIG. 44 depicts an exploded view of an embodiment of hydraulic compaction machine 464. FIG. 45 depicts a representation of an embodiment of assembled hydraulic compaction machine 464. As shown in FIGS. 44 and 45, clamp assemblies 458 may be used to secure fitting 422 (depicted, for example, in FIG. 38) in place with insulated conductors coupled to the fitting. At least one clamp assembly (for example, clamp assembly 458A) may be moveable together to compact the fitting in the axial direction. Power unit 466, shown in FIG. 44, may be used to power compaction machine 464.

[0679] FIG. 46 depicts an embodiment of fitting 422 and insulated conductors 410A, 410B secured in clamp assembly 458A and clamp assembly 458B before compaction of the fitting and insulated conductors. As shown in FIG. 46, the cores of insulated conductors 410A, 410B are coupled using coupling 428 at or near the center of sleeve 424. Sleeve 424 is slid over housing 426A, which is coupled to insulated conductor 410A. Sleeve 424 and housing 426A are secured in fixed (non-moving) clamp assembly 458B. Insulated conductor 410B passes through housing 426B and movable clamp assembly 458A. Insulated conductor 410B may be secured by another clamp assembly fixed relative to clamp assembly 458B (not shown). Clamp assembly 458A may be moved towards clamp assembly 458B to couple housing 426B to sleeve 424 and compact electrically insulating material inside the housings and the sleeve. Interfaces between insulated conductor 410A and housing 426A, between housing 426A and sleeve 424, between sleeve 424 and housing 426B, and between housing 426B and insulated conductor 410B may then be coupled by welding, brazing, or other techniques known in the art.

[0680] FIG. 47 depicts a side view representation of an embodiment of fitting 470 for joining insulated conductors. Fitting 470 may be a cylinder or sleeve that has sufficient clearance between the inside diameter of the sleeve and the outside diameters of insulated conductors 410A, 410B such that the sleeve fits over the ends of the insulated conductors. The cores of insulated conductors 410A, 410B may be joined inside fitting 470. The jackets and insulation of insulated conductors 410A, 410B may be cut back or stripped to expose desired lengths of the cores before joining the cores. Fitting 470 may be centered between the end portions of insulated conductors 410A, 410B.

[0681] Fitting 470 may be used to couple insulated conductor 410A to insulated conductor 410B while maintaining the mechanical and electrical integrity of the jackets, insulation, and cores of the insulated conductors. Fitting 470 may be used to couple heat producing insulated conductors with non-heat producing insulated conductors, to couple heat producing insulated conductors with other heat producing insulated conductors, or to couple non-heat producing insulated conductors with other non-heat producing insulated conductors. In some embodiments, more than one fitting 470 is used in to couple multiple heat producing and non-heat producing insulated conductors to produce a long insulated conductor.

[0682] Fitting 470 may be used to couple insulated conductors with different diameters. For example, the insulated conductors may have different core diameters, different jacket diameters, or combinations of different diameters. Fitting 470 may also be used to couple insulated conductors with different metallurgies, different types of insulation, or a combination thereof.

[0683] In certain embodiments, fitting 470 has at least one angled end. For example, the ends of fitting 470 may be angled relative to the longitudinal axis of the fitting. The angle may be, for example, about 45° or between 30° and 60°. Thus, the ends of fitting 470 may have substantially elliptical cross-sections. The substantially elliptical cross-sections of the ends of fitting 470 provide a larger area for welding or brazing of the fitting to insulated conductors 410A, 410B. The larger coupling area increases the strength of spliced insulated conductors. In the embodiment shown in FIG. 47, the angled ends of fitting 470 give the fitting a substantially parallelogram shape.

[0684] The angled ends of fitting 470 provide higher tensile strength and higher bending strength for the fitting than if the fitting had straight ends by distributing loads along the fitting. Fitting 470 may be oriented so that when insulated conductors 410A, 410B and the fitting are spooled (for example, on a coiled tubing installation), the angled ends act as a transition in stiffness from the fitting body to the insulated conductors. This transition reduces the likelihood of the insulated conductors to kink or crimp at the end of the fitting body.

[0685] As shown in FIG. 47, fitting 470 includes opening 472. Opening 472 allows electrically insulating material (such as electrically insulating material 430, depicted in FIG. 38) to be provided (filled) inside fitting 470. Opening 472 may be a slot or other longitudinal opening extending along part of the length of fitting 470. In certain embodiments, opening 472 extends substantially the entire gap between the ends of insulated conductors 410A, 410B inside fitting 470. Opening 472 allows substantially the entire volume (area) between insulated conductors 410A, 410B, and around any welded or spliced joints between the insulated conductors, to be filled with electrically insulating material without the insulating material having to be moved axially toward the ends of the volume between the insulated conductors. The width of opening 472 allows electrically insulating material to be forced into the opening and packed more tightly inside fitting 470, thus, reducing the amount of void space inside the fitting. Electrically insulating material may be forced through the slot into the volume between insulated conductors 410A, 410B, for example, with a tool with the dimensions of the slot. The tool may be forced into the slot to compact the insulating material. Then, additional insulating material may be added and the compaction is repeated. In some embodiments, the electrically insulating material may be further compacted inside fitting 470 using vibration, tamping, or other techniques. Further compacting the electrically insulating material may more uniformly distribute the electrically insulating material inside fitting 470.

[0686] After filling electrically insulating material inside fitting 470 and, in some embodiment, compaction of the electrically insulating material, opening 472 may be closed. For example, an insert or other covering may be placed over the opening and secured in place. FIG. 48 depicts a side view representation of an embodiment of fitting 470 with opening 472 covered with insert 474. Insert 474 may be welded or brazed to fitting 470 to close opening 472. In some embodiments, insert 474 is ground or polished so that the insert if flush on the surface of fitting 470. Also depicted in FIG. 48, welds or brazes 476 may be used to secure fitting 470 to insulated conductors 410A, 410B.

[0687] After opening 472 is closed, fitting 470 may be compacted mechanically, hydraulically, pneumatically, or using swaging methods to compact further the electrically insulating material inside the fitting. Further compaction of the electrically insulating material reduces void volume inside fitting 470 and reduces the leakage currents through the fitting and increases the operating range of the fitting (for example, the maximum operating voltages or temperatures of the fitting).

[0688] In certain embodiments, fitting 470 includes certain features that may further reduce electric field intensities inside the fitting. For example, fitting 470 or coupling 428 of the cores of the insulated conductors inside the fitting may include tapered edges, rounded edges, or other smoothed out features to reduce electric field intensities. FIG. 49 depicts an embodiment of fitting 470 with electric field reducing features at coupling 428 between insulated conductors 410A, 410B. As shown in FIG. 49, coupling 428 is a welded joint with a smoothed out or rounded profile to reduce electric field intensity inside fitting 470. In addition, fitting 470 has a tapered interior volume to increase the volume of electrically insulating material inside the fitting. Having the tapered and larger volume may reduce electric field intensities inside fitting 470.

[0689] In some embodiments, electric field stress reducers may be located inside fitting 470 to decrease the electric field intensity. FIG. 50 depicts an embodiment of electric field stress reducer 478. Reducer 478 may be located in the interior volume of fitting 470 (shown in FIG. 49). Reducer 478 may be a split ring or other separable piece so that the reducer can be fitted around cores 374A, 374B of insulated conductors 410A, 410B after they are joined (shown in FIG. 49).

[0690] The fittings depicted herein (such as fitting 422, depicted in FIGS. 38 and 40, fitting 442, depicted in FIG. 41, and fitting 470, depicted in FIGS. 47, 48, and 49) may form robust electrical and mechanical connections between insulated conductors. For example, fittings depicted herein may be suitable for extended operation at voltages above 1000 volts, above 1500 volts, or above 2000 volts and temperatures of at least about 650° C., at least about 700° C., at least about 800° C.

[0691] In some embodiments, three insulated conductor heaters (for example, mineral insulated conductor heaters) are coupled together into a single assembly. The single assembly may be built in long lengths and may operate at high voltages (for example, voltages of 4000 V nominal). In certain embodiments, the individual insulated conductor heaters are enclosed in corrosive resistant jackets to resist damage from the external environment. The jackets may be, for example, seam welded stainless steel armor similar to that used on type MC/CWCMC cable.

[0692] In some embodiments, three insulated conductor heaters are cabled and the insulating filler added in conventional methods known in the art. The insulated conductor heaters may include one or more heater sections that resistively heat and provide heat to portions of the formation adjacent to the heater sections. The insulated conductors may include one or more other sections that provide electricity to the heater sections with relatively small heat loss. The individual insulated conductor heaters may be wrapped with high temperature fiber tapes before being placed on a take-up reel (for example, a coiled tubing rig). The reel assembly may be moved to another machine for application of an outer metallic sheath or outer protective conduit.

[0693] In some embodiments, the fillers include glass, ceramic or other temperature resistant fibers that withstand operating temperature of 760° C. or higher. In addition, the insulated conductor cables may be wrapped in multiple layers of a ceramic fiber woven tape material. By wrapping the tape around the cabled insulated conductor heaters prior to application of the outer metallic sheath, electrical isolation is provided between the insulated conductor heaters and the outer sheath. This electrical isolation inhibits leakage current from the insulated conductor heaters passing into the subsurface formation and forces any leakage currents to return directly to the power source on the individual insulated conductor sheaths and/or on a lead-in conductor or lead-out conductor coupled to the insulated conductors. The lead-in or lead-out conductors may be coupled to the insulated conductors when the insulated conductors are placed into an assembly with the outer metallic sheath.

[0694] In certain embodiments, the insulated conductor heaters are wrapped with a metallic tape or other type of tape instead of the high temperature ceramic fiber woven tape material. The metallic tape holds the insulated conductor heaters together. A widely-spaced wide pitch spiral wrapping of a high temperature fiber rope may be wrapped around the insulated conductor heaters. The fiber rope may provide electrical isolation between the insulated conductors and the outer sheath. The fiber rope may be added at any stage during assembly. For example, the fiber rope may be added as a part of the final assembly when the outer sheath is added. Application of the fiber rope may be simpler than other electrical isolation methods because application of the fiber rope is done with only a single layer of rope instead of multiple layers of ceramic tape. The fiber rope may be less expensive than multiple layers of ceramic tape. The fiber rope may increase heat transfer between the insulated conductors and the outer sheath and/or reduce interference with any welding process used to weld the outer sheath around the insulated conductors (for example, seam welding).

[0695] In certain embodiments, an insulated conductor or another type of heater is installed in a wellbore or opening in the formation using outer tubing coupled to a coiled tubing rig. FIG. 51 depicts outer tubing 480 partially unspooled from coiled tubing rig 482. Outer tubing 480 may be made of metal or polymeric material. Outer tubing 480 may be a flexible conduit such as, for example, a tubing guide string or other coiled tubing string. Heater 412 may be pushed into outer tubing 480, as shown in FIG. 52. In certain embodiments, heater 412 is pushed into outer tubing 480 by pumping the heater into the outer tubing.

[0696] In certain embodiments, one or more flexible cups 484 are coupled to the outside of heater 412. Flexible cups 484 may have a variety of shapes and/or sizes but typically are shaped and sized to maintain at least some pressure inside at least a portion of outer tubing 480 as heater 412 is pushed or pumped into the outer tubing. Flexible cups 484 are made of flexible materials such as, but not limited to, elastomeric materials. For example, flexible cups 484 may have flexible edges that provide limited mechanical resistance as heater 412 is pushed into outer tubing 480 but remain in contact with the inner walls of outer tubing 480 as the heater is pushed so that pressure is maintained between the heater and the outer tubing. Maintaining at least some pressure in outer tubing 480 between flexible cups 484 allows heater 412 to be continuously pushed into the outer tubing with lower pump pressures. Without flexible cups 484, higher pressures may be needed to push heater 412 into outer tubing 480. In some embodiments, cups 484 allow some pressure to be released while maintaining pressure in outer tubing 480. In certain embodiments, flexible cups 484 are spaced to distribute pumping forces optimally along heater 412 inside outer tubing 480. For example, flexible cups 484 may be evenly spaced along heater 412.

[0697] Heater 412 is pushed into outer tubing 480 until the heater is fully inserted into the outer tubing, as shown in FIG. 53. Drilling guide 486 may be coupled to the end of heater 412. Heater 412, outer tubing 480, and drilling guide 486 may be spooled onto coiled tubing rig 482, as shown in FIG. 54. After heater 412, outer tubing 480, and drilling guide 486 are spooled onto coiled tubing rig 482, the assembly may be transported to a location for installation of the heater. For example, the assembly may be transported to the location of a subsurface heater wellbore (opening).

[0698] FIG. 55 depicts coiled tubing rig 482 being used to install heater 412 and outer tubing 480 into opening 386 using drilling guide 486. In certain embodiments, opening 386 is an L-shaped opening or wellbore with a substantially horizontal or inclined portion in a hydrocarbon containing layer of the formation. In such embodiments, heater 412 has a heating section that is placed in the substantially horizontally or inclined portion of opening 386 to be used to heat the hydrocarbon containing layer. In some embodiments, opening 386 has a horizontal or inclined section that is at least about 1000 m in length, at least about 1500 m in length, or at least about 2000 m in length. Overburden casing 398 may be located around the outer walls of opening 386 in an overburden section of the formation. In some embodiments, drilling fluid is left in opening 386 after the opening has been completed (the opening has been drilled).

[0699] FIG. 56 depicts heater 412 and outer tubing 480 installed in opening 386. Gap 488 may be left at or near the far end of heater 412 and outer tubing 480. Gap 488 may allow for heater expansion in opening 386 after the heater is energized.

[0700] After heater 412 and outer tubing 480 are installed in opening 386, the outer tubing may be removed from the opening to leave the heater in place in the opening. FIG. 57 depicts outer tubing 480 being removed from opening 386 while leaving heater 412 installed in the opening. Outer tubing 480 is spooled back onto coiled tubing rig 482 as the outer tubing is pulled off heater 412. In some embodiments, outer tubing 480 is pumped down to balance pressure between opening 386 and the outer tubing. Balancing the pressure allows outer tubing 480 to be pulled off heater 412.

[0701] FIG. 58 depicts outer tubing 480 used to provide packing material 402 into opening 386. As outer tubing 480 reaches the “shoe” or bend in opening 386, the outer tubing may be used to provide packing material into the opening. The shoe of opening 386 may be located at or near the bottom of overburden casing 398. Packing material 402 may be provided (for example, pumped) through outer tubing 480 and out the end of the outer tubing at the shoe of opening 386. Packing material 402 is provided into opening 386 to seal off the opening around heater 412. Packing material 402 provides a barrier between the overburden section and the heating section of opening 386. In certain embodiments, packing material 402 is cement or another suitable plugging material. In some embodiments, outer tubing 480 is continuously spooled while packing material 402 is provided into opening 386. Outer tubing 480 may be spooled slowly while packing material 402 is provided into opening 386 to allow the packing material to settle into the opening properly.

[0702] After packing material 402 is provided into opening 386, outer tubing 480 is spooled further onto coiled tubing rig 482, as shown in FIG. 59. FIG. 60 depicts outer tubing 480 spooled onto coiled tubing rig 482 with heater 412 installed in opening 386. In certain embodiments, flexible cups 484 are spaced in the portion of opening 386 with overburden casing 398 to facilitate adequate stand-off of heater 412 in the overburden portion of the opening. Flexible cups 484 may electrically insulate heater 412 from overburden casing 398. For example, flexible cups 484 may space apart heater 412 and overburden casing 398 such that they are not in physical contact with each other.

[0703] After outer tubing 480 is removed from opening 386, wellhead 392 and/or other completions may be installed at the surface of the opening, as shown in FIG. 61. When heater 412 is energized to begin heating, flexible cups 484 may begin to burn or melt off. In some embodiments, flexible cups 484 begin to burn or melt off at low temperatures during early stages of the heating process.

[0704] FIG. 62 depicts an embodiment of a heater in wellbore 490 in formation 492. The heater includes insulated conductor 410 in conduit 382 with material 494 between the insulated conductor and the conduit. In some embodiments, insulated conductor 410 is a mineral insulated conductor. Electricity supplied to insulated conductor 410 resistively heats the insulated conductor. Insulated conductor conductively transfers heat to material 494. Heat may transfer within material 494 by heat conduction and/or by heat convection. Radiant heat from insulated conductor 410 and/or heat from material 494 transfers to conduit 382. Heat may transfer to the formation from the heater by conductive or radiative heat transfer from conduit 382. Material 494 may be molten metal, molten salt, or other liquid. In some embodiments, a gas (for example, nitrogen, carbon dioxide, and/or helium) is in conduit 382 above material 494. The gas may inhibit oxidation or other chemical changes of material 494. The gas may inhibit vaporization of material 494. U.S. Published Patent Application 2008-0078551 to DeVault et al., which is incorporated by reference as if fully set forth herein, describes a system for placement in a wellbore, the system including a heater in a conduit with a liquid metal between the heater and the conduit for heating subterranean earth.

[0705] Insulated conductor 410 and conduit 382 may be placed in an opening in a subsurface formation. Insulated conductor 410 and conduit 382 may have any orientation in a subsurface formation (for example, the insulated conductor and conduit may be substantially vertical or substantially horizontally oriented in the formation). Insulated conductor 410 includes core 374, electrical insulator 364, and jacket 370. In some embodiments, core 374 is a copper core. In some embodiments, core 374 includes other electrical conductors or alloys (for example, copper alloys). In some embodiments, core 374 includes a ferromagnetic conductor so that insulated conductor 410 operates as a temperature limited heater. In some embodiments, core 374 does not include a ferromagnetic conductor.

[0706] In some embodiments, core 374 of insulated conductor 410 is made of two or more portions. The first portion may be placed adjacent to the overburden. The first portion may be sized and/or made of a highly conductive material so that the first portion does not resistively heat to a high temperature. One or more other portions of core 410 may be sized and/or made of material that resistively heats to a high temperature. These portions of core 410 may be positioned adjacent to sections of the formation that are to be heated by the heater. In some embodiments, the insulated conductor does not include a highly conductive first portion. A lead in cable may be coupled to the insulated conductor to supply electricity to the insulated conductor.

[0707] In some embodiments, core 374 of insulated conductor 410 is a highly conductive material such as copper. Core 374 may be electrically coupled to jacket 370 at or near the end of the insulated conductor. In some embodiments, insulated conductor 410 is electrically coupled to conduit 382. Electrical current supplied to insulated conductor 410 may resistively heat core 374, jacket 370, material 494, and/or conduit 382. Resistive heating of core 374, jacket 370, material 494, and/or conduit 382 generates heat that may transfer to the formation.

[0708] Electrical insulator 364 may be magnesium oxide, aluminum oxide, silicon dioxide, beryllium oxide, boron nitride, silicon nitride, or combinations thereof. In certain embodiments, electrical insulator 364 is a compacted powder of magnesium oxide. In some embodiments, electrical insulator 364 includes beads of silicon nitride. In certain embodiments, a thin layer of material clad over core 374 to inhibit the core from migrating into the electrical insulator at higher temperatures (i.e., to inhibit copper of the core from migrating into magnesium oxide of the insulation). For example, a small layer of nickel (for example, about 0.5 mm of nickel) may be clad on core 374.

[0709] In some embodiments, material 494 may be relatively corrosive. Jacket 370 and/or at least the inside surface of conduit 382 may be made of a corrosion resistant material such as, but not limited to, nickel, Alloy N (Carpenter Metals), 347 stainless steel, 347H stainless steel, 446 stainless steel, or 825 stainless steel. For example, conduit 382 may be plated or lined with nickel. In some embodiments, material 494 may be relatively non-corrosive. Jacket 370 and/or at least the inside surface of conduit 382 may be made of a material such as carbon steel.

[0710] In some embodiments, jacket 370 of insulated conductor 410 is not used as the main return of electrical current for the insulated conductor. In embodiments where material 494 is a good electrical conductor such as a molten metal, current returns through the molten metal in the conduit and/or through the conduit 382. In some embodiments, conduit 382 is made of a ferromagnetic material, (for example 410 stainless steel). Conduit 382 may function as a temperature limited heater until the temperature of the conduit approaches, reaches or exceeds the Curie temperature or phase transition temperature of the conduit material.

[0711] In some embodiments, material 494 returns electrical current to the surface from insulated conductor 410 (i.e., the material acts as the return or ground conductor for the insulated conductor). Material 494 may provide a current path with low resistance so that a long insulated conductor 410 is useable in conduit 382. The long heater may operate at low voltages for the length of the heater due to the presence of material 494 that is conductive.

[0712] FIG. 63 depicts an embodiment of a portion of insulated conductor 410 in conduit 382 wherein material 494 is a good conductor (for example, a liquid metal) and current flow is indicated by the arrows. Current flows down core 374 and returns through jacket 370, material 494, and conduit 382. Jacket 370 and conduit 382 may be at approximately constant potential. Current flows radially from jacket 370 to conduit 382 through material 494. Material 494 may resistively heat. Heat from material 494 may transfer through conduit 382 into the formation.

[0713] In embodiments where material 494 is partially electrically conductive (for example, the material is a molten salt), current returns mainly through jacket 370. All or a portion of the current that passes through partially conductive material 494 may pass to ground through conduit 382.

[0714] In the embodiment depicted in FIG. 62, core 374 of insulated conductor 410 has a diameter of about 1 cm, electrical insulator 364 has an outside diameter of about 1.6 cm, and jacket 370 has an outside diameter of about 1.8 cm. In other embodiments, the insulated conductor is smaller. For example, core 374 has a diameter of about 0.5 cm, electrical insulator 364 has an outside diameter of about 0.8 cm, and jacket 370 has an outside diameter of about 0.9 cm. Other insulated conductor geometries may be used. For the same size conduit 382, the smaller geometry of insulated conductor 410 may result in a higher operating temperature of the insulated conductor to achieve the same temperature at the conduit. The smaller geometry insulated conductors may be significantly more economically favorable due to manufacturing cost, weight, and other factors.

[0715] Material 494 may be placed between the outside surface of insulated conductor 410 and the inside surface of conduit 382. In certain embodiments, material 494 is placed in the conduit in a solid form as balls or pellets. Material 494 may melt below the operating temperatures of insulated conductor 410. Material may melt above ambient subsurface formation temperatures. Material 494 may be placed in conduit 382 after insulated conductor 410 is placed in the conduit. In certain embodiments, material 494 is placed in conduit 410 as a liquid. The liquid may be placed in conduit 382 before or after insulated conductor 410 is placed in the conduit (for example, the molten liquid may be poured into the conduit before or after the insulated conductor is placed in the conduit). Additionally, material 494 may be placed in conduit 382 before or after insulated conductor 410 is energized (i.e., supplied with electricity). Material 494 may be added to conduit 382 or removed from the conduit after operation of the heater is initialized. Material 494 may be added to or removed from conduit 382 to maintain a desired head of fluid in the conduit. In some embodiments, the amount of material 494 in conduit 382 may be adjusted (i.e., added to or depleted) to adjust or balance the stresses on the conduit. Material 494 may inhibit deformation of conduit 382. The head of material 494 in conduit 382 may inhibit the formation from crushing or otherwise deforming the conduit should the formation expand against the conduit. The head of fluid in conduit 382 allows the wall of the conduit to be relatively thin. Having thin conduits 382 may increase the economic viability of using multiple heaters of this type to heat portions of the formation.

[0716] Material 494 may support insulated conductor 410 in conduit 382. The support provided by material 494 of insulated conductor 410 may allow for the deployment of long insulated conductors as compared to insulated conductors positioned only in a gas in a conduit without the use of special metallurgy to accommodate the weight of the insulated conductor. In certain embodiments, insulated conductor 410 is buoyant in material 494 in conduit 382. For example, insulated conductor may be buoyant in molten metal. The buoyancy of insulated conductor 410 reduces creep associated problems in long, substantially vertical heaters. A bottom weight or tie down may be coupled to the bottom of insulated conductor 410 to inhibit the insulated conductor from floating in material 494.

[0717] Material 494 may remain a liquid at operating temperatures of insulated conductor 410. In some embodiments, material 494 melts at temperatures above about 100° C., above about 200° C., or above about 300° C. The insulated conductor may operate at temperatures greater than 200° C., greater than 400° C., greater than 600° C., or greater than 800° C. In certain embodiments, material 494 provides enhanced heat transfer from insulated conductor 410 to conduit 382 at or near the operating temperatures of the insulated conductor.

[0718] Material 494 may include metals such as tin, zinc, an alloy such as a 60% by weight tin, 40% by weight zinc alloy; bismuth; indium; cadmium, aluminum; lead; and/or combinations thereof (for example, eutectic alloys of these metals such as binary or ternary alloys). In one embodiment, material 494 is tin. Some liquid metals may be corrosive. The jacket of the insulated conductor and/or at least the inside surface of the canister may need to be made of a material that is resistant to the corrosion of the liquid metal. The jacket of the insulated conductor and/or at least the inside surface of the conduit may be made of materials that inhibit the molten metal from leaching materials from the insulating conductor and/or the conduit to form eutectic compositions or metal alloys. Molten metals may be highly thermal conductive, but may block radiant heat transfer from the insulated conductor and/or have relatively small heat transfer by natural convection.

[0719] Material 494 may be or include molten salts such as solar salt, salts presented in Table 1, or other salts. The molten salts may be infrared transparent to aid in heat transfer from the insulated conductor to the canister. In some embodiments, solar salt includes sodium nitrate and potassium nitrate (for example, about 60% by weight sodium nitrate and about 40% by weight potassium nitrate). Solar salt melts at about 220° C. and is chemically stable up to temperatures of about 593° C. Other salts that may be used include, but are not limited to LiNO₃ (melt temperature (T_m) of 264° C. and a decomposition temperature of about 600° C.) and eutectic mixtures such as 53% by weight KNO₃, 40% by weight NaNO₃ and 7% by weight NaNO₂ (T_m of about 142° C. and an upper working temperature of over 500° C.); 45.5% by weigh KNO₃ and 54.5% by weight NaNO₂ (T_m of about 142-145° C. and an upper working temperature of over 500° C.); or 50% by weight NaCl and 50% by weight SrCl₂ (T_m of about 19° C. and an upper working temperature of over 1200° C.).

[0720]

	TABLE 1
	Material	Tm (° C.)	Tb (° C.)
	Zn	420	907
	CdBr2	568	863
	CdI2	388	744
	CuBr2	498	900
	PbBr2	371	892
	TlBr	460	819
	TlF	326	826
	ThI4	566	837
	SnF2	215	850
	SnI2	320	714
	ZnCl2	290	732

[0721] Some molten salts, such as solar salt, may be relatively non-corrosive so that the conduit and/or the jacket may be made of relatively inexpensive material (for example, carbon steel). Some molten salts may have good thermal conductivity, may have high heat density, and may result in large heat transfer by natural convection.

[0722] In fluid mechanics, the Rayleigh number is a dimensionless number associated with heat transfer in a fluid. When the Rayleigh number is below the critical value for the fluid, heat transfer is primarily in the form of conduction; and when the Rayleigh number is above the critical value, heat transfer is primarily in the form of convection. The Rayleigh number is the product of the Grashof number (which describes the relationship between buoyancy and viscosity in a fluid) and the Prandtl number (which describes the relationship between momentum diffusivity and thermal diffusivity). For the same size insulated conductors in conduits, and where the temperature of the conduit is 500° C., the Rayleigh number for solar salt in the conduit is about 10 times the Rayleigh number for tin in the conduit. The higher Rayleigh number implies that the strength of natural convection in the molten solar salt is much stronger than the strength of the natural convection in molten tin. The stronger natural convection of molten salt may distribute heat and inhibit the formation of hot spots at locations along the length of the conduit. Hot spots may be caused by coke build up at isolated locations adjacent to or on the conduit, contact of the conduit by the formation at isolated locations, and/or other high thermal load situations.

[0723] Conduit 382 may be a carbon steel or stainless steel canister. In some embodiments, conduit 382 may include cladding on the outer surface to inhibit corrosion of the conduit by formation fluid. Conduit 382 may include cladding on an inner surface of the conduit that is corrosion resistant to material 494 in the conduit. Cladding applied to conduit 382 may be a coating and/or a liner. If the conduit contains a metal salt, the inner surface of the conduit may include coating of nickel, or the conduit may be or include a liner of a corrosion resistant metal such as Alloy N. If the conduit contains a molten metal, the conduit may include a corrosion resistant metal liner or coating, and/or a ceramic coating (for example, a porcelain coating or fired enamel coating). In an embodiment, conduit 382 is a canister of 410 stainless steel with an outside diameter of about 6 cm. Conduit 382 may not need a thick wall because material 494 may provide internal pressure that inhibits deformation or crushing of the conduit due to external stresses.

[0724] FIG. 64 depicts an embodiment of the heater positioned in wellbore 490 of formation 492 with a portion of insulated conductor 410 and conduit 382 oriented substantially horizontally in the formation. Material 494 may provide a head in conduit 382 due to the pressure of the material. The pressure head may keep material 494 in conduit 382. The pressure head may also provide internal pressure that inhibits deformation or collapse of conduit 382 due to external stresses.

[0725] In some embodiments, two or more insulated conductors are placed in the conduit. In some embodiments, only one of the insulated conductors is energized. Should the energized conductor fail, one of the other conductors may be energized to maintain the material in a molten phase. The failed insulated conductor may be removed and/or replaced.

[0726] The conduit of the heater may be a ribbed conduit. The ribbed conduit may improve the heat transfer characteristics of the conduit as compared to a cylindrical conduit. FIG. 65 depicts a cross-sectional representation of ribbed conduit 496. FIG. 66 depicts a perspective view of a portion of ribbed conduit 496. Ribbed conduit 496 may include rings 498 and ribs 500. Rings 498 and ribs 500 may improve the heat transfer characteristics of ribbed conduit 496. In an embodiment, the cylinder of conduit has an inner diameter of about 5.1 cm and a wall thickness of about 0.57 cm. Rings 498 may be spaced about every 3.8 cm. Rings 498 may have a height of about 1.9 cm and a thickness of about 0.5 cm. Six ribs 500 may be spaced evenly about conduit 382. Ribs 500 may have a thickness of about 0.5 cm and a height of about 1.6 cm. Other dimensions for the cylinder, rings and ribs may be used. Ribbed conduit 496 may be formed from two or more rolled pieces that are welded together to form the ribbed conduit. Other types of conduit with extra surface area to enhance heat transfer from the conduit to the formation may be used.

[0727] In some embodiments, the ribbed conduit may be used as the conduit of a conductor-in-conduit heater. For example, the conductor may be a 3.05 cm 410 stainless steel rod and the conduit has dimensions as described above. In other embodiments, the conductor is an insulated conductor and a fluid is positioned between the conductor and the ribbed conduit. The fluid may be a gas or liquid at operating temperatures of the insulated conductor.

[0728] In some embodiments, the heat source for the heater is not an insulated conductor. For example, the heat source may be hot fluid circulated through an inner conduit positioned in an outer conduit. The material may be positioned between the inner conduit and the outer conduit. Convection currents in the material may help to more evenly distribute heat to the formation and may inhibit or limit formation of a hot spot where insulation that limits heat transfer to the overburden ends. In some embodiments, the heat sources are downhole oxidizers. The material is placed between an outer conduit and an oxidizer conduit. The oxidizer conduit may be an exhaust conduit for the oxidizers or the oxidant conduit if the oxidizers are positioned in a u-shaped wellbore with exhaust gases exiting the formation through one of the legs of the u-shaped conduit. The material may help inhibit the formation of hot spots adjacent to the oxidizers of the oxidizer assembly.

[0729] The material to be heated by the insulated conductor may be placed in an open wellbore. FIG. 67 depicts material 494 in open wellbore 490 in formation 492 with insulated conductor 410 in the wellbore. In some embodiments, a gas (for example, nitrogen, carbon dioxide, and/or helium) is placed in wellbore 490 above material 494. The gas may inhibit oxidation or other chemical changes of material 494. The gas may inhibit vaporization of material 494.

[0730] Material 494 may have a melting point that is above the pyrolysis temperature of hydrocarbons in the formation. The melting point of material 494 may be above 375° C., above 400° C., or above 425° C. The insulated conductor may be energized to heat the formation. Heat from the insulated conductor may pyrolyze hydrocarbons in the formation. Adjacent the wellbore, the heat from insulated conductor 410 may result in coking that reduces the permeability and plugs the formation near wellbore 490. The plugged formation inhibits material 494 from leaking from wellbore 490 into formation 492 when the material is a liquid. In some embodiments, material 494 is a salt.

[0731] In some embodiments, material 494 leaking from wellbore 490 into formation 492 may be self-healing and/or self-sealing. Material 494 flowing away from wellbore 490 may travel until the temperature becomes less than the solidification temperature of the material. Temperature may drop rapidly a relatively small distance away from the heater used to maintain material 494 in a liquid state. The rapid drop off in temperature may result in migrating material 494 solidifying close to wellbore 490. Solidified material 494 may inhibit migration of additional material from wellbore 490, and thus self-heal and/or self-seal the wellbore.

[0732] Return electrical current for insulated conductor 410 may return through jacket 370 of the insulated conductor. Any current that passes through material 494 may pass to ground. Above the level of material 494, any remaining return electrical current may be confined to jacket 370 of insulated conductor 410.

[0733] Using liquid material in open wellbores heated by heaters may allow for delivery of high power rates (for example, up to about 2000 W/m) to the formation with relatively low heater surface temperatures. Hot spot generation in the formation may be reduced or eliminated due to convection smoothing out the temperature profile along the length of the heater. Natural convection occurring in the wellbore may greatly enhance heat transfer from the heater to the formation. Also, the large gap between the formation and the heater may prevent thermal expansion of the formation from harming the heater.

[0734] In some embodiments, an 8″ (20.3 cm) wellbore may be formed in the formation. In some embodiments, casing may be placed through all or a portion of the overburden. A 0.6 inch (1.5 cm) diameter insulated conductor heater may be placed in the wellbore. The wellbore may be filled with solid material (for example, solid particles of salt). A packer may be placed near an interface between the treatment area and the overburden. In some embodiments, a pass through conduit in the packer may be included to allow for the addition of more material to the treatment area. A non-reactive or substantially non-reactive gas (for example, carbon dioxide and/or nitrogen) may be introduced into the wellbore. The insulated conductor may be energized to begin the heating that melts the solid material and heats the treatment area.

[0735] In some embodiments, other types of heat sources besides for insulated conductors are used to heat the material placed in the open wellbore. The other types of heat sources may include gas burners, pipes through which hot heat transfer fluid flows, or other types of heaters.

[0736] In some embodiments, heat pipes are placed in the formation. The heat pipes may reduce the number of active heat sources needed to heat a treatment area of a given size. The heat pipes may reduce the time needed to heat the treatment area of a given size to a desired average temperature. A heat pipe is a closed system that utilizes phase change of fluid in the heat pipe to transport heat applied to a first region to a second region remote from the first region. The phase change of the fluid allows for large heat transfer rates. Heat may be applied to the first region of the heat pipes from any type of heat source, including but not limited to, electric heaters, oxidizers, heat provided from geothermal sources, and/or heat provided from nuclear reactors.

[0737] Heat pipes are passive heat transport systems that include no moving parts. Heat pipes may be positioned in near horizontal to vertical configurations. The fluid used in heat pipes for heating the formation may have a low cost, a low melting temperature, a boiling temperature that is not too high (for example, generally below about 900° C.), a low viscosity at temperatures below about 540° C., a high heat of vaporization, and a low corrosion rate for the heat pipe material. In some embodiments, the heat pipe includes a liner of material that is resistant to corrosion by the fluid. TABLE 1 shows melting and boiling temperatures for several materials that may be used as the fluid in heat pipes. Other salts that may be used include, but are not limited to LiNO₃, and eutectic mixtures such as 53% by weight KNO₃; 40% by weight NaNO₃ and 7% by weight NaNO₂; 45.5% by weight KNO₃ and 54.5% by weight NaNO₂; or 50% by weight NaCl and 50% by weight SrCl₂.

[0738] FIG. 68 depicts schematic cross-sectional representation of a portion of a formation with heat pipes 502 positioned adjacent to a substantially horizontal portion of heat source 202. Heat source 202 is placed in a wellbore in the formation. Heat source 202 may be a gas burner assembly, an electrical heater, a leg of a circulation system that circulates hot fluid through the formation, or other type of heat source. Heat pipes 502 may be placed in the formation so that distal ends of the heat pipes are near or contact heat source 202. In some embodiments, heat pipes 502 mechanically attach to heat source 202. Heat pipes 502 may be spaced a desired distance apart. In an embodiment, heat pipes 502 are spaced apart by about 40 feet. In other embodiments, large or smaller spacings are used. Heat pipes 502 may be placed in a regular pattern with each heat pipe spaced a given distance from the next heat pipe. In some embodiments, heat pipes 502 are placed in an irregular pattern. An irregular pattern may be used to provide a greater amount of heat to a selected portion or portions of the formation. Heat pipes 502 may be vertically positioned in the formation. In some embodiments, heat pipes 502 are placed at an angle in the formation.

[0739] Heat pipes 502 may include sealed conduit 504, seal 506, liquid heat transfer fluid 508 and vaporized heat transfer fluid 510. In some embodiments, heat pipes 502 include metal mesh or wicking material that increases the surface area for condensation and/or promotes flow of the heat transfer fluid in the heat pipe. Conduit 504 may have first portion 512 and second portion 514. Liquid heat transfer fluid 508 may be in first portion 512. Heat source 202 external to heat pipe 502 supplies heat that vaporizes liquid heat transfer fluid 508. Vaporized heat transfer fluid 510 diffuses into second portion 514. Vaporized heat transfer fluid 510 condenses in second portion and transfers heat to conduit 504, which in turn transfers heat to the formation. The condensed liquid heat transfer fluid 508 flows by gravity to first portion 512.

[0740] Position of seal 506 is a factor in determining the effective length of heat pipe 502. The effective length of heat pipe 502 may also depend on the physical properties of the heat transfer fluid and the cross-sectional area of conduit 504. Enough heat transfer fluid may be placed in conduit 504 so that some liquid heat transfer fluid 508 is present in first portion 512 at all times.

[0741] Seal 506 may provide a top seal for conduit 504. In some embodiments, conduit 504 is purged with nitrogen, helium or other fluid prior to being loaded with heat transfer fluid and sealed. In some embodiments, a vacuum may be drawn on conduit 504 to evacuate the conduit before the conduit is sealed. Drawing a vacuum on conduit 504 before sealing the conduit may enhance vapor diffusion throughout the conduit. In some embodiments, an oxygen getter may be introduced in conduit 504 to react with any oxygen present in the conduit.

[0742] FIG. 69 depicts a perspective cut-out representation of a portion of a heat pipe embodiment with heat pipe 502 located radially around oxidizer assembly 516. Oxidizers 518 of oxidizer assembly 516 are positioned adjacent to first portion 512 of heat pipe 502. Fuel may be supplied to oxidizers 518 through fuel conduit 520. Oxidant may be supplied to oxidizers 518 through oxidant conduit 522. Exhaust gas may flow through the space between outer conduit 524 and oxidant conduit 522. Oxidizers 518 combust fuel to provide heat that vaporizes liquid heat transfer fluid 508. Vaporized heat transfer fluid 510 rises in heat pipe 502 and condenses on walls of the heat pipe to transfer heat to sealed conduit 504. Exhaust gas from oxidizers 518 provides heat along the length of sealed conduit 504. The heat provided by the exhaust gas along the effective length of heat pipe 502 may increase convective heat transfer and/or reduce the lag time before significant heat is provided to the formation from the heat pipe along the effective length of the heat pipe.

[0743] FIG. 70 depicts a cross-sectional representation of an angled heat pipe embodiment with oxidizer assembly 516 located near a lowermost portion of heat pipe 502. Fuel may be supplied to oxidizers 518 through fuel conduit 520. Oxidant may be supplied to oxidizers 518 through oxidant conduit 522. Exhaust gas may flow through the space between outer conduit 524 and oxidant conduit 522.

[0744] FIG. 71 depicts a perspective cut-out representation of a portion of a heat pipe embodiment with oxidizer 518 located at the bottom of heat pipe 502. Fuel may be supplied to oxidizer 518 through fuel conduit 520. Oxidant may be supplied to oxidizer 518 through oxidant conduit 522. Exhaust gas may flow through the space between the outer wall of heat pipe 502 and outer conduit 524. Oxidizer 518 combusts fuel to provide heat that vaporizers liquid heat transfer fluid 508. Vaporized heat transfer fluid 510 rises in heat pipe 502 and condenses on walls of the heat pipe to transfer heat to sealed conduit 504. Exhaust gas from oxidizers 518 provides heat along the length of sealed conduit 504 and to outer conduit 524. The heat provided by the exhaust gas along the effective length of heat pipe 502 may increase convective heat transfer and/or reduce the lag time before significant heat is provided to the formation from the heat pipe and oxidizer combination along the effective length of the heat pipe. FIG. 72 depicts a similar embodiment with heat pipe 502 positioned at an angle in the formation.

[0745] FIG. 73 depicts a perspective cut-out representation of a portion of a heat pipe embodiment with oxidizer 518 that produces flame zone adjacent to liquid heat transfer fluid 508 in the bottom of heat pipe 502. Fuel may be supplied to oxidizer 518 through fuel conduit 520. Oxidant may be supplied to oxidizer 518 through oxidant conduit 522. Oxidant and fuel are mixed and combusted to produce flame zone 526. Flame zone 526 provides heat that vaporizes liquid heat transfer fluid 508. Exhaust gases from oxidizer 518 may flow through the space between oxidant conduit 522 and the inner surface of heat pipe 502, and through the space between the outer surface of the heat pipe and outer conduit 524. The heat provided by the exhaust gas along the effective length of heat pipe 502 may increase convective heat transfer and/or reduce the lag time before significant heat is provided to the formation from the heat pipe and oxidizer combination along the effective length of the heat pipe.

[0746] FIG. 74 depicts a perspective cut-out representation of a portion of a heat pipe embodiment with a tapered bottom that accommodates multiple oxidizers of an oxidizer assembly. In some embodiments, efficient heat pipe operation requires a high heat input. Multiple oxidizers of oxidizer assembly 516 may provide high heat input to liquid heat transfer fluid 508 of heat pipe 502. A portion of oxidizer assembly with the oxidizers may be helically wound around a tapered portion of heat pipe 502. The tapered portion may have a large surface area to accommodate the oxidizers. Fuel may be supplied to the oxidizers of oxidizer assembly 516 through fuel conduit 520. Oxidant may be supplied to oxidizer 518 through oxidant conduit 522. Exhaust gas may flow through the space between the outer wall of heat pipe 502 and outer conduit 524. Exhaust gas from oxidizers 518 provides heat along the length of sealed conduit 504 and to outer conduit 524. The heat provided by the exhaust gas along the effective length of heat pipe 502 may increase convective heat transfer and/or reduce the lag time before significant heat is provided to the formation from the heat pipe and oxidizer combination along the effective length of the heat pipe.

[0747] FIG. 75 depicts a cross-sectional representation of a heat pipe embodiment that is angled within the formation. First wellbore 528 and second wellbore 530 are drilled in the formation using magnetic ranging or techniques so that the first wellbore intersects the second wellbore. Heat pipe 502 may be positioned in first wellbore 528. First wellbore 528 may be sloped so that liquid heat transfer fluid 508 within heat pipe 502 is positioned near the intersection of the first wellbore and second wellbore 530. Oxidizer assembly 516 may be positioned in second wellbore 530. Oxidizer assembly 516 provides heat to heat pipe 502 that vaporizes liquid heat transfer fluid in the heat pipe. Packer or seal 532 may direct exhaust gas from oxidizer assembly 516 through first wellbore 528 to provide additional heat to the formation from the exhaust gas.

[0748] In some embodiments, the temperature limited heater is used to achieve lower temperature heating (for example, for heating fluids in a production well, heating a surface pipeline, or reducing the viscosity of fluids in a wellbore or near wellbore region). Varying the ferromagnetic materials of the temperature limited heater allows for lower temperature heating. In some embodiments, the ferromagnetic conductor is made of material with a lower Curie temperature than that of 446 stainless steel. For example, the ferromagnetic conductor may be an alloy of iron and nickel. The alloy may have between 30% by weight and 42% by weight nickel with the rest being iron. In one embodiment, the alloy is Invar 36. Invar 36 is 36% by weight nickel in iron and has a Curie temperature of 277° C. In some embodiments, an alloy is a three component alloy with, for example, chromium, nickel, and iron. For example, an alloy may have 6% by weight chromium, 42% by weight nickel, and 52% by weight iron. A 2.5 cm diameter rod of Invar 36 has a turndown ratio of approximately 2 to 1 at the Curie temperature. Placing the Invar 36 alloy over a copper core may allow for a smaller rod diameter. A copper core may result in a high turndown ratio. The insulator in lower temperature heater embodiments may be made of a high performance polymer insulator (such as PFA or PEEK™) when used with alloys with a Curie temperature that is below the melting point or softening point of the polymer insulator.

[0749] In certain embodiments, a conductor-in-conduit temperature limited heater is used in lower temperature applications by using lower Curie temperature and/or the phase transformation temperature range ferromagnetic materials. For example, a lower Curie temperature and/or the phase transformation temperature range ferromagnetic material may be used for heating inside sucker pump rods. Heating sucker pump rods may be useful to lower the viscosity of fluids in the sucker pump or rod and/or to maintain a lower viscosity of fluids in the sucker pump rod. Lowering the viscosity of the oil may inhibit sticking of a pump used to pump the fluids. Fluids in the sucker pump rod may be heated up to temperatures less than about 250° C. or less than about 300° C. Temperatures need to be maintained below these values to inhibit coking of hydrocarbon fluids in the sucker pump system.

[0750] In certain embodiments, a temperature limited heater includes a flexible cable (for example, a furnace cable) as the inner conductor. For example, the inner conductor may be a 27% nickel-clad or stainless steel-clad stranded copper wire with four layers of mica tape surrounded by a layer of ceramic and/or mineral fiber (for example, alumina fiber, aluminosilicate fiber, borosilicate fiber, or aluminoborosilicate fiber). A stainless steel-clad stranded copper wire furnace cable may be available from Anomet Products, Inc. The inner conductor may be rated for applications at temperatures of 1000° C. or higher. The inner conductor may be pulled inside a conduit. The conduit may be a ferromagnetic conduit (for example, a ¾″ Schedule 80 446 stainless steel pipe). The conduit may be covered with a layer of copper, or other electrical conductor, with a thickness of about 0.3 cm or any other suitable thickness. The assembly may be placed inside a support conduit (for example, a 1¼″ Schedule 80 347H or 347HH stainless steel tubular). The support conduit may provide additional creep-rupture strength and protection for the copper and the inner conductor. For uses at temperatures greater than about 1000° C., the inner copper conductor may be plated with a more corrosion resistant alloy (for example, Incoloy® 825) to inhibit oxidation. In some embodiments, the top of the temperature limited heater is sealed to inhibit air from contacting the inner conductor.

[0751] FIG. 76 depicts an embodiment of three heaters coupled in a three-phase configuration. Conductor “legs” 534, 536, 538 are coupled to three-phase transformer 414. Transformer 414 may be an isolated three-phase transformer. In certain embodiments, transformer 414 provides three-phase output in a wye configuration. Input to transformer 414 may be made in any input configuration, such as the shown delta configuration. Legs 534, 536, 538 each include lead-in conductors 540 in the overburden of the formation coupled to heating elements 542 in hydrocarbon layer 388. Lead-in conductors 540 include copper with an insulation layer. For example, lead-in conductors 540 may be a 4-0 copper cables with TEFLON® insulation, a copper rod with polyurethane insulation, or other metal conductors such as bare copper or aluminum. In certain embodiments, lead-in conductors 540 are located in an overburden portion of the formation. The overburden portion may include overburden casings 398. Heating elements 542 may be temperature limited heater heating elements. In an embodiment, heating elements 542 are 410 stainless steel rods (for example, 3.1 cm diameter 410 stainless steel rods). In some embodiments, heating elements 542 are composite temperature limited heater heating elements (for example, 347 stainless steel, 410 stainless steel, copper composite heating elements; 347 stainless steel, iron, copper composite heating elements; or 410 stainless steel and copper composite heating elements). In certain embodiments, heating elements 542 have a length of about 10 m to about 2000 m, about 20 m to about 400 m, or about 30 m to about 300 m.

[0752] In certain embodiments, heating elements 542 are exposed to hydrocarbon layer 388 and fluids from the hydrocarbon layer. Thus, heating elements 542 are “bare metal” or “exposed metal” heating elements. Heating elements 542 may be made from a material that has an acceptable sulfidation rate at high temperatures used for pyrolyzing hydrocarbons. In certain embodiments, heating elements 542 are made from material that has a sulfidation rate that decreases with increasing temperature over at least a certain temperature range (for example, 500° C. to 650° C., 530° C. to 650° C., or 550° C. to 650°). For example, 410 stainless steel may have a sulfidation rate that decreases with increasing temperature between 530° C. and 650° C. Using such materials reduces corrosion problems due to sulfur-containing gases (such as H₂S) from the formation. In certain embodiments, heating elements 542 are made from material that has a sulfidation rate below a selected value in a temperature range. In some embodiments, heating elements 542 are made from material that has a sulfidation rate at most about 25 mils per year at a temperature between about 800° C. and about 880° C. In some embodiments, the sulfidation rate is at most about 35 mils per year at a temperature between about 800° C. and about 880° C., at most about 45 mils per year at a temperature between about 800° C. and about 880° C., or at most about 55 mils per year at a temperature between about 800° C. and about 880° C. Heating elements 542 may also be substantially inert to galvanic corrosion.

[0753] In some embodiments, heating elements 542 have a thin electrically insulating layer such as aluminum oxide or thermal spray coated aluminum oxide. In some embodiments, the thin electrically insulating layer is a ceramic composition such as an enamel coating. Enamel coatings include, but are not limited to, high temperature porcelain enamels. High temperature porcelain enamels may include silicon dioxide, boron oxide, alumina, and alkaline earth oxides (CaO or MgO), and minor amounts of alkali oxides (Na₂O, K₂O, LiO). The enamel coating may be applied as a finely ground slurry by dipping the heating element into the slurry or spray coating the heating element with the slurry. The coated heating element is then heated in a furnace until the glass transition temperature is reached so that the slurry spreads over the surface of the heating element and makes the porcelain enamel coating. The porcelain enamel coating contracts when cooled below the glass transition temperature so that the coating is in compression. Thus, when the coating is heated during operation of the heater, the coating is able to expand with the heater without cracking.

[0754] The thin electrically insulating layer has low thermal impedance allowing heat transfer from the heating element to the formation while inhibiting current leakage between heating elements in adjacent openings and/or current leakage into the formation. In certain embodiments, the thin electrically insulating layer is stable at temperatures above at least 350° C., above 500° C., or above 800° C. In certain embodiments, the thin electrically insulating layer has an emissivity of at least 0.7, at least 0.8, or at least 0.9. Using the thin electrically insulating layer may allow for long heater lengths in the formation with low current leakage.

[0755] Heating elements 542 may be coupled to contacting elements 544 at or near the underburden of the formation. Contacting elements 544 are copper or aluminum rods or other highly conductive materials. In certain embodiments, transition sections 546 are located between lead-in conductors 540 and heating elements 542, and/or between heating elements 542 and contacting elements 544. Transition sections 546 may be made of a conductive material that is corrosion resistant such as 347 stainless steel over a copper core. In certain embodiments, transition sections 546 are made of materials that electrically couple lead-in conductors 540 and heating elements 542 while providing little or no heat output. Thus, transition sections 546 help to inhibit overheating of conductors and insulation used in lead-in conductors 540 by spacing the lead-in conductors from heating elements 542. Transition section 546 may have a length of between about 3 m and about 9 m (for example, about 6 m).

[0756] Contacting elements 544 are coupled to contactor 548 in contacting section 550 to electrically couple legs 534, 536, 538 to each other. In some embodiments, contact solution 552 (for example, conductive cement) is placed in contacting section 550 to electrically couple contacting elements 544 in the contacting section. In certain embodiments, legs 534, 536, 538 are substantially parallel in hydrocarbon layer 388 and leg 534 continues substantially vertically into contacting section 550. The other two legs 536, 538 are directed (for example, by directionally drilling the wellbores for the legs) to intercept leg 534 in contacting section 550.

[0757] Each leg 534, 536, 538 may be one leg of a three-phase heater embodiment so that the legs are substantially electrically isolated from other heaters in the formation and are substantially electrically isolated from the formation. Legs 534, 536, 538 may be arranged in a triangular pattern so that the three legs form a triangular shaped three-phase heater. In an embodiment, legs 534, 536, 538 are arranged in a triangular pattern with 12 m spacing between the legs (each side of the triangle has a length of 12 m).

[0758] FIG. 77 depicts a side view representation of an embodiment of a substantially u-shaped three-phase heater. First ends of legs 534, 536, 538 are coupled to transformer 414 at first location 554. In an embodiment, transformer 414 is a three-phase AC transformer. Ends of legs 534, 536, 538 are electrically coupled together with connector 556 at second location 558. Connector 556 electrically couples the ends of legs 534, 536, 538 so that the legs can be operated in a three-phase configuration. In certain embodiments, legs 534, 536, 538 are coupled to operate in a three-phase wye configuration. In certain embodiments, legs 534, 536, 538 are substantially parallel in hydrocarbon layer 388. In certain embodiments, legs 534, 536, 538 are arranged in a triangular pattern in hydrocarbon layer 388. In certain embodiments, heating elements 542 include thin electrically insulating material (such as a porcelain enamel coating) to inhibit current leakage from the heating elements. In certain embodiments, the thin electrically insulating layer allows for relatively long, substantially horizontal heater leg lengths in the hydrocarbon layer with a substantially u-shaped heater. In certain embodiments, legs 534, 536, 538 are electrically coupled so that the legs are substantially electrically isolated from other heaters in the formation and are substantially electrically isolated from the formation.

[0759] In certain embodiments, overburden casings (for example, overburden casings 398, depicted in FIGS. 76 and 77) in overburden 400 include materials that inhibit ferromagnetic effects in the casings. Inhibiting ferromagnetic effects in casings 398 reduces heat losses to the overburden. In some embodiments, casings 398 may include non-metallic materials such as fiberglass, polyvinylchloride (PVC), chlorinated polyvinylchloride (CPVC), or high-density polyethylene (HDPE). HDPEs with working temperatures in a range for use in overburden 400 include HDPEs available from Dow Chemical Co., Inc. (Midland, Mich., U.S.A.). A non-metallic casing may also eliminate the need for an insulated overburden conductor. In some embodiments, casings 398 include carbon steel coupled on the inside diameter of a non-ferromagnetic metal (for example, carbon steel clad with copper or aluminum) to inhibit ferromagnetic effects or inductive effects in the carbon steel. Other non-ferromagnetic metals include, but are not limited to, manganese steels with at least 10% by weight manganese, iron aluminum alloys with at least 18% by weight aluminum, and austentitic stainless steels such as 304 stainless steel or 316 stainless steel.

[0760] In certain embodiments, one or more non-ferromagnetic materials used in casings 398 are used in a wellhead coupled to the casings and legs 534, 536, 538. Using non-ferromagnetic materials in the wellhead inhibits undesirable heating of components in the wellhead. In some embodiments, a purge gas (for example, carbon dioxide, nitrogen or argon) is introduced into the wellhead and/or inside of casings 398 to inhibit reflux of heated gases into the wellhead and/or the casings.

[0761] In certain embodiments, one or more of legs 534, 536, 538 are installed in the formation using coiled tubing. In certain embodiments, coiled tubing is installed in the formation, the leg is installed inside the coiled tubing, and the coiled tubing is pulled out of the formation to leave the leg installed in the formation. The leg may be placed concentrically inside the coiled tubing. In some embodiments, coiled tubing with the leg inside the coiled tubing is installed in the formation and the coiled tubing is removed from the formation to leave the leg installed in the formation. The coiled tubing may extend only to a junction of the hydrocarbon layer and the contacting section, or to a point at which the leg begins to bend in the contacting section.

[0762] FIG. 78 depicts a top view representation of an embodiment of a plurality of triads of three-phase heaters in the formation. Each triad 560 includes legs A, B, C (which may correspond to legs 534, 536, 538 depicted in FIGS. 76 and 77) that are electrically coupled by linkages 562. Each triad 560 is coupled to its own electrically isolated three-phase transformer so that the triads are substantially electrically isolated from each other. Electrically isolating the triads inhibits net current flow between triads.

[0763] The phases of each triad 560 may be arranged so that legs A, B, C correspond between triads as shown in FIG. 78. Legs A, B, C are arranged such that a phase leg (for example, leg A) in a given triad is about two triad heights from a same phase leg (leg A) in an adjacent triad. The triad height is the distance from a vertex of the triad to a midpoint of the line intersecting the other two vertices of the triad. In certain embodiments, the phases of triads 560 are arranged to inhibit net current flow between individual triads. There may be some leakage of current within an individual triad but little net current flows between two triads due to the substantial electrical isolation of the triads and, in certain embodiments, the arrangement of the triad phases.

[0764] In the early stages of heating, an exposed heating element (for example, heating element 542 depicted in FIGS. 76 and 77) may leak some current to water or other fluids that are electrically conductive in the formation so that the formation itself is heated. After water or other electrically conductive fluids are removed from the wellbore (for example, vaporized or produced), the heating elements become electrically isolated from the formation. Later, when water is removed from the formation, the formation becomes even more electrically resistant and heating of the formation occurs even more predominantly via thermally conductive and/or radiative heating. Typically, the formation (the hydrocarbon layer) has an initial electrical resistance that averages at least 10 ohm·m. In some embodiments, the formation has an initial electrical resistance of at least 100 ohm·m or of at least 300 ohm·m.

[0765] Using the temperature limited heaters as the heating elements limits the effect of water saturation on heater efficiency. With water in the formation and in heater wellbores, there is a tendency for electrical current to flow between heater elements at the top of the hydrocarbon layer where the voltage is highest and cause uneven heating in the hydrocarbon layer. This effect is inhibited with temperature limited heaters because the temperature limited heaters reduce localized overheating in the heating elements and in the hydrocarbon layer.

[0766] In certain embodiments, production wells are placed at a location at which there is relatively little or zero voltage potential. This location minimizes stray potentials at the production well. Placing production wells at such locations improves the safety of the system and reduces or inhibits undesired heating of the production wells caused by electrical current flow in the production wells. FIG. 79 depicts a top view representation of the embodiment depicted in FIG. 78 with production wells 206. In certain embodiments, production wells 206 are located at or near center of triad 560. In certain embodiments, production wells 206 are placed at a location between triads at which there is relatively little or zero voltage potential (at a location at which voltage potentials from vertices of three triads average out to relatively little or zero voltage potential). For example, production well 206 may be at a location equidistant from leg A of one triad, leg B of a second triad, and leg C of a third triad, as shown in FIG. 79.

[0767] Certain embodiments of heaters include single-phase conductors in a single wellbore. For example, FIGS. 76 and 77 depict heater embodiments with three-phase heaters that include single-phase conductors in each wellbore. A problem with having a single-phase conductor in the wellbore is current or voltage induction in components of the wellbore (for example, the heater casing) and/or in the formation caused by magnetic fields produced by the single-phase conductor. In a wellbore with the supply and return conductors both located in the wellbore, the magnetic fields produced by the current running through the supply conductor are cancelled by magnetic fields produced by the current running through the return conductor. In addition, the single-phase conductor may induce currents in production wellbores and/or other nearby wellbores.

[0768] FIG. 80 depicts a schematic of an embodiment of a heat treatment system including heater 412 and production wells 206. In certain embodiments, heater 412 is a three-phase heater that includes legs 534, 536, 538 coupled to transformer 414 and terminal connector 556. Legs 534, 536, 538 may each include single-phase conductors. Legs 534, 536, 538 may be coupled together to form a triad heater. In certain embodiments, legs 534, 536, 538 are relatively long heater sections. For example, legs 534, 536, 538 may be about 3000 m or longer in length.

[0769] In some embodiments, as shown in FIG. 80, production wells 206 are located substantially horizontally in the formation and below legs 534, 536, 538 of heater 412. In some embodiments, production wells 206 are located at an incline or vertically in the formation. As shown in FIG. 80, production wells 206 may include two production wells that extend from each side of heater 412 towards the center of the heater substantially lengthwise along the heated sections of legs 534, 536, 538. In some embodiments, one production well 206 extends substantially lengthwise along the heated sections of the legs.

[0770] FIG. 81 depicts a side-view representation of one leg of heater 412 in the subsurface formation. Leg 534 is shown as representative of any leg in of heater 412 in the formation. Leg 534 may include heating element 542 in hydrocarbon layer 388 below overburden 400. In certain embodiments, heating element 542 is located substantially horizontal in hydrocarbon layer 388. Transition section 546 may couple heating element 542 to lead-in cable 540. Lead-in cable 540 may be an overburden section or overburden element of heater 412. Lead-in cable 540 couples heating element 542 and transition section 546 to electrical components at the surface (for example, transformer 414 and/or terminal connector 556 depicted in FIG. 80).

[0771] As shown in FIG. 81, heater casing 564 extends from the surface to at or near end of transition section 546. Overburden casing 398 substantially surrounds heater casing 564 in overburden 400. Surface conductor 566 substantially surrounds overburden casing 398 at or near the surface of the formation.

[0772] In certain embodiments, heating element 542 is an exposed metal or bare metal heating element. For example, heating element 542 may be an exposed ferromagnetic metal heating element such as 410 stainless steel. Lead-in cable 540 includes low resistance electrical conductors such as copper or copper-cladded steel. Lead-in cable 540 may include electrical insulation or otherwise be electrically insulated from overburden 400 (for example, overburden casing 398 may include electrical insulation on an inside surface of the casing). Transition section 546 may include a combination of stainless steel and copper suitable for transition between heating element 542 and lead-in cable 540.

[0773] In some embodiments, heater casing 564 includes non-ferromagnetic stainless steel or another suitable material that has high hanging strength and is non-ferromagnetic. Overburden casing 398 and/or surface conductor 566 may include carbon steel or other suitable materials.

[0774] FIG. 82 depicts a schematic representation of a surface cabling configuration with a ground loop used for heater 412 and production well 206. In certain embodiments, ground loop 568 substantially surrounds legs 534, 536, 538 of heater 412, production well 206, and transformer 414. Power cable 394 may couple transformer 414 to legs 534, 536, 538 of heater 412. The center portion of power cable 394 coupled to center leg 536 may be put into loop 570. Loop 570 extends the center portion of power cable 394 to have approximately the same length as the portions of power cable 394 coupled to side legs 534, 538. Having each portion of power cable 394 approximately the same length inhibits creation of phase differences between the legs.

[0775] In certain embodiments, transformer 414 is coupled to ground loop 568 to ground the transformer and heater 412. In some embodiments, production well 206 is coupled to ground loop 568 to ground the production well.

[0776] FIG. 83 depicts a side view of an overburden portion of leg 534. Lead-in cable 540 is substantially surrounded by heater casing 564 and overburden casing 398 (“casing 564/398”) in the overburden of the formation. Current flow in lead-in cable 540 (represented by +/− symbols at ends the lead-in cable) induces current flow with opposite polarity on casing 564/398 (represented by +/− symbols on line 572). This induced voltage on casing 564/398 is caused by mutual inductance of the casing with all the heater elements in the triad (each of the three-phase elements in the formation). The mutual inductance may be described by the following equation:
M=2×10⁻⁰⁷ ln [S/r];  (EQN. 6)
where M is the mutual inductance, S is the center to center separation between heater elements, and r is the outer radius of the casing. The induced voltage in the casing (V) is proportional to the current (I) and is given by the equation:
ΔV=ωMI.  (EQN. 7)

[0777] Because typically high power is provided through lead-in cable 540 in order to provide power to long heater elements, the induced voltages and currents on casing 564/398 can be relatively high. Large induced currents on the casing may lead to AC corrosion problems and/or leakage of current into the formation. Large currents on the casing, when grounded, may also necessitate large currents in the ground loop to compensate for the currents on the casing. Large currents on the ground loop may be costly and, in some cases, be difficult or unsafe to operate. Large currents on the casing may also lead to high surface potentials around the heaters on the surface. High surface potentials may create unsafe areas for personnel and/or equipment on the surface.

[0778] Simulations may be used to assess and/or determine the location and magnitude of induced casing and ground currents in the formation. For example, simulation systems available from Safe Engineering Services & Technologies, Ltd. (Laval, Quebec, Canada) may be used to assess induced casing and ground currents for subsurface heating systems. Data such as, but not limited to, physical dimensions of the heaters, electrical and magnetic properties of materials used, formation resistivity profile, and applied voltage/current including phase profile may be used in the simulation to assess induced casing and ground currents.

[0779] FIG. 84 depicts a side view of overburden portions of legs 534, 536 grounded to ground loop 568. Legs 534, 536 have opposite polarity such that the currents induced in the casings of the legs also have opposite polarity. The opposite polarity of the casings causes circular current flow between the legs through the overburden. This circular current flow is represented by curve 574. Because legs 534, 536 are grounded to ground loop 568, the magnitude of circular current flow (curve 574) (current density on the casings) is relatively large. For example, current densities in the heater casing may be 1 A/m² or greater. Such current densities may increase the risk of AC corrosion in the heater casing.

[0780] FIG. 85 depicts a side view of overburden portions of legs 534, 536 with the legs grounded to a ground loop. Ungrounding legs 534, 536 reduces the magnitude of the circular current flow between the legs (current density on the casings), as shown by curve 574. For example, the current density on the heater casing may be lowered by a factor of about 2. This reduction in magnitude may, however, not be large enough to satisfy regulatory and/or safety issues with the induced current as the induced current remains near the surface of the formation. In addition, there may be additional regulatory and/or safety issues associated with ungrounding legs 534, 536 such as, but not limited to, increasing wellhead electrical fields above safe levels.

[0781] FIG. 86 depicts a side view of overburden portions of legs 534, 536 with the electrically conductive portions of casings 564/398 lowered selected depth 576 below the surface. As shown by curve 574, lowering the conductive portion of casings 564/398 selected depth 576 reduces the magnitude of the induced current (current density on the casings) and moves the induced current to the selected depth below the surface. Moving the induced current to selected depth 576 below the surface reduces surface potentials and ground currents from the induced currents in the casings. For example, the current density on the heater casing may be lowered by a factor of about 3 by lowering the conductive portion of the casing.

[0782] In certain embodiments, the conductive portions of casings 564/398 are lowered in the formation by using electrically non-conductive materials in the portions of the casings above the conductive portions of the casings. For example, casings 564/398 may include non-conductive portions between the surface and the selected depth and conductive portions below the selected depth. In some embodiments, the electrically non-conductive portions include materials such as, but not limited to, fiberglass or other electrically insulating materials.

[0783] The non-conductive portion of casing 564/398 may only be used to the selected depth because the use of the non-conductive material may not be feasible. The non-conductive material may have low temperature limits that inhibits use of the non-conductive material near the heated section of the heater. Thus, conductive material may need to be used in the lower part of the overburden portion of the heater (the part near the heated section). As the non-conductive material may not be high strength material, to support the weight of the conductive material (for example, stainless steel), the conductive portion may be located as close to the surface as possible. Locating the conductive portion closer to the surface reduces the size of hanging devices or other structures that may be used to support the conductive portion of the casing.

[0784] In certain embodiments, the non-conductive portion of casing 564/398 extends to a depth that is below the surface moisture zone in the formation. Keeping the conductive portion of casing 564/398 below the surface moisture zone inhibits induced currents from reaching the surface.

[0785] In some embodiments, the non-conductive portion of casing 564/398 extends to a depth that is at least the distance between legs 534, 536. For example, for a 40′ (about 12 m) spacing between legs, the non-conductive portion of casing 564/398 may extend at least about 100′ (about 30 m) below the surface. In some embodiments, the non-conductive portion of casing 564/398 extends at least about 15 m, at least about 20 m, or at least about 30 m below the surface. The non-conductive portion of casing 564/398 may extend to a depth of at most about 150 m, about 300 m, or about 500 m from the surface.

[0786] The non-conductive portion of casing 564/398 may extend at most to a selected distance from the heated zone of the formation (the heated portion of the heater). In some embodiments, the selected distance is about 100 m, about 150 m, or about 200 m. In some embodiments, the non-conductive portion of casing 564/398 may extend to a depth that is slightly above or near the beginning of the bend in a u-shaped heater.

[0787] The desired depth of non-conductive portion of casing 564/398 may be assessed based on electrical effects for the formation to be treated and/or electrical properties of the heaters to be used. Simulations, such as those available from Safe Engineering Services & Technologies, Ltd. (Laval, Quebec, Canada), may be used to assess the desired depth of the non-conductive portion of the casing. The desired depth may also be affected by factors such as, but not limited to, safety issues, regulatory issues, and mechanical issues.

[0788] In some embodiments, the overburden portions of legs 534, 536 are moved closer together so that the non-conductive portion of casing 564/398 can be moved to a shallower depth. For example, the overburden portions of legs 534, 536 may be relatively close together while the heated portions of the legs diverge below the overburden to greater separation distances needed for desired heating the formation.

[0789] In certain embodiments, as depicted in FIG. 86, legs 534, 536 are ungrounded with the casings lowered the selected distance. In some embodiments, however, legs 534, 536 are grounded with the casings lowered the selected distance. The grounding or ungrounding of the legs may affect the selected depth to which the casings are lowered.

[0790] When the electrically conductive portions of casings 564/398 are lowered to selected depth 576, ground loop 568 may become the highest field gradient at the surface. In some embodiments, a ground wellbore may be located below the surface and coupled to ground loop 568 (for example, with an insulated conductor (cable)). Coupling ground loop 568 to the ground wellbore below the surface may reduce or eliminate the high field gradient at the surface. The ground wellbore may be at a depth specified, for example, by standard electrical grounding practices known in the art.

[0791] In some embodiments, a subsurface hydrocarbon containing formation may be treated by the in situ heat treatment process to produce mobilized and/or pyrolyzed products from the formation. In some embodiments, a subsurface heater may include two or more flexible cable conductors. The flexible cable conductors may be positioned in a tubular. In some embodiments, the flexible cable conductors are positioned between two tubulars. In certain embodiments, the flexible cable conductors are positioned around an exterior surface of a first tubular. The flexible cable conductors and the first tubular may be positioned in a second tubular. The first and second tubular may form a dual-walled wellbore liner. The flexible cable conductors inside the first and second tubular allows the wellbore liner to be operated as a liner heater.

[0792] In some embodiments, the heater includes a plurality of flexible cable conductors positioned between the first and second tubulars. In certain embodiments, the heater includes between 2 and 16, between 4 and 12, or between 6 and 9 flexible cables. In some embodiments, the flexible cable conductors are wound around the inner first tubular in a roughly spiral pattern (for example, a helical pattern). Flexible cables may be formed from single conductors (for example, single-phase conductors) or multiple conductors (for example, three-phase conductors). Installing the flexible cable conductors in the spiral pattern may produce a more uniform temperature profile and/or relieve mechanical stresses on the conductors. The more uniform temperature profile may increase heater life. Spiraled flexible cable conductors, positioned between two tubulars, may not have the same tendency to expand and contract apart, which may potentially cause eddy currents. Spiraled flexible cable conductors, positioned between two tubulars, may be more easily coiled on a large reel for shipment without the ends of the heaters becoming uneven in length.

[0793] In certain embodiments, the tubulars are coiled tubing tubulars. Integrating the flexible heating cable(s) in the first and second tubulars may allow for installation using a coiled tubing spooler, straightener, and/or injector system (for example, a coiled tubing rig). For example, coiled tubing tubulars may be wound onto the tubing rig during or after construction of the heater and unwound from the tubing rig as the heater is installed into the subsurface formation. This type of installation method may not require additional time typically required to attach the heating cable to a pipe wall during a well intervention, reducing the overall workover cost. The tubing rig may be readily transported from the construction site to the heater installation site using methods known in the art or described herein. Use of the dual walled coiled tubing heating system may allow for retrieval of the system during initial operations.

[0794] In some embodiments, at least a portion of the flexible cables are in contact with the outer second tubular. FIG. 87 depicts a cross-sectional representation of heater 412 including nine single-phase flexible cable conductors 380 positioned between first tubular 578a and second tubular 578b. Forming the heater such that the flexible cable conductors are in contact with the second tubular 578b results in the flexible cables providing conductive heat transfer between the first tubular 578a and the second tubular. In such embodiments, conductive heat transfer functions as the primary method of heat transfer to second tubular 578b.

[0795] In some embodiments, the flexible cables are inhibited from contacting the outer second tubular. FIG. 88 depicts a cross-sectional representation of heater 412 including nine single-phase flexible cable conductors 380 positioned between first tubular 578a and second tubular 578b with spacers 580. Spacers 580 may be positioned between first tubular 578a and second tubular 578b. The spacers may function to maintain separation between the tubulars and inhibit the flexible cables from contacting second tubular 578b. In such embodiments, radiative heat transfer functions as the primary method of heat transfer to second tubular 578b.

[0796] In some embodiments, spacers 580 are formed from an insulating material. For example, spacers may be formed from a fibrous ceramic material such as Nextel™ 312 (3M Corporation, St. Paul, Minn., U.S.A.), mica tape, or glass fiber. Ceramic material may be made of alumina, alumina-silicate, alumina-borosilicate, silicon nitride, boron nitride, or other suitable high-temperature materials.

[0797] In some embodiments, heat transfer material (for example, heat transfer fluid) is located in the annulus between first tubular 578a and second tubular 578b. Heat transfer material may increase the efficiency of the heaters. Heat transfer material includes, but is not limited to, molten metal, molten salt, other heat conducting liquids, or heat conducting gases.

[0798] In some embodiments, the first and/or second tubulars include two or more openings. The openings may allow fluids to be moved upwards and/or downwards through the tubulars. For example, formation fluids may be produced through one of the openings inside the tubulars. Having the openings inside the tubulars may promote heat transfer and/or hydrocarbon accumulation for production assistance (out-flow assurance) or formation heating (in-flow assurance). In some embodiments, the use of spacers enhances flow assurance inside the openings by reducing heat losses to the formation and increasing heat transfer to fluids flowing through the openings.

[0799] In some embodiments, the heater includes two or more portions that function to heat at different power levels and, thus, heat at different temperatures. For example, higher power levels and higher temperatures may be generated in portions adjacent the hydrocarbon containing layer. Lower power levels (for example, <5% of the higher power level) and lower temperatures may be generated in portions adjacent the overburden. In some embodiments, lower power level flexible cables are designed and made utilizing larger diameter and/or different alloys with lower volume resistivities and low-power-producing conductors as compared with the high power level conductors. In some embodiments, the power reduction in the overburden is accomplished by using a conductor with a Curie-temperature power-limiting inherent characteristic (for example, low temperature, temperature limiting characteristics).

[0800] Flexible cables may be formed from single conductors or multiple conductors. In some embodiments, the flexible cables used in the heater include single conductor flexible cables installed between the first and second tubulars (for example, as depicted in FIGS. 87 and 88). The flexible cables may be electrically connected in as single phase conductors or coupled together in groups of 3 in 3-phase configurations (for example, 3-phase wye configurations). The electrical connections may be completed by bonding two conductors and up to nine or more conductors together.

[0801] The single conductor flexible cables may be connected together (for example, bonded) at the un-powered end, creating a single phase heating system (two cables connected) and up to, for example, three, 3-phase heating systems (nine cables connected to three power sources). These connections may be located at the subterranean end of the heating system (for example, near the toe of a horizontal heater wellbore). At the powered connection of the heater, the single-phase cables may be connected to line-to-line voltage (for example, up to 4160 V) for heat generation. 3-phase heaters may be connected electrically on the surface using a 3-phase power transformer. Line-to-neutral voltage for these heaters may be up to about 2402 V (V/√{square root over (3)}) since they are electrically connected at the un-powered subterranean end.

[0802] In some embodiments, the flexible cable used in the heater includes multiple conductor flexible cables installed between the first and second tubulars. For example, the flexible cable may include three multiple conductors configured to be provided power by a 3-phase transformer. FIG. 89 depicts a cross-sectional representation of heater 412 including nine multiple (in FIG. 89, each flexible cable includes three conductors) flexible cable conductors 380 positioned between first tubular 578a and second tubular 578b. FIG. 90 depicts a cross-sectional representation of heater 412 including nine multiple (in FIG. 90, each flexible cable includes three conductors) flexible cable conductors 380 positioned between first tubular 578a and second tubular 578b with spacers 580. Heater 412 depicted in FIG. 90 includes spacers 580. The multiple conductor flexible cables depicted in FIGS. 89 and 90 may be coupled together at the un-powered end (for example, bonded at the un-powered end). These connections may be located at the subterranean end of the heating system (for example, near the toe of a horizontal heater wellbore). Connecting the flexible cable conductors at the un-powered end may create electrically independent, individual heating systems that are powered, up to nine or more at a time, to reduce the heat-up time constant for the desired formation temperature or three at a time to maintain the desired formation temperature. The line to neutral voltage for these heaters may be up to about 2402 V (4160/v3) since they are connected at the un-powered subterranean end.

[0803] The liner heaters, depicted in FIGS. 87, 88, 89, and 90, may include built-in redundancy in either the single conductor or multiple conductor designs. By connecting the flexible cable heaters to a common node at the end of the heating system, the single conductor heating cables may be powered to by-pass a non-working flexible cable, creating a 3-phase or single phase heating system.

[0804] In some embodiments, the liner heater is installed in a wellbore. The heater may allow the heat generated to be primarily transferred by conduction, directly into the near well-bore interface. The heat generation system may be in intimate contact with the near wellbore interface such that the operating temperatures of the heating system may be reduced. Reducing operating temperatures of the heater may extend the expected lifetime of the heater. Lower operating temperatures resulting from integrating the electro-thermal heating system within the dual wall coiled tubular liner may increase the reliability of all components such as: a) outer sheath material; b) ceramic insulation; c) conductor(s) material; d) splices; and e) components. Reducing operating temperatures of the heater may inhibit hydrocarbon coking.

[0805] Because the liner heater is located in the liner portion of the wellbore, the use of a heating system in the interior of the wellbore may be eliminated. Eliminating the need for a heating system in the interior of the wellbore may allow for unobstructed heated oil production through the wellbore. Eliminating the need for a heating system in the interior of the wellbore may allow for the ability to introduce heated diluents or process-inducing additives to the formation through the interior of the wellbore.

[0806] In certain embodiments, portions of the wellbore that extend through the overburden include casings. The casings may include materials that inhibit inductive effects in the casings Inhibiting inductive effects in the casings may inhibit induced currents in the casing and/or reduce heat losses to the overburden. In some embodiments, the overburden casings may include non-metallic materials such as fiberglass, polyvinylchloride (PVC), chlorinated PVC (CPVC), high-density polyethylene (HDPE), high temperature polymers (such as nitrogen based polymers), or other high temperature plastics. HDPEs with working temperatures in a usable range include HDPEs available from Dow Chemical Co., Inc. (Midland, Mich., U.S.A.). The overburden casings may be made of materials that are spoolable so that the overburden casings can be spooled into the wellbore. In some embodiments, overburden casings may include non-magnetic metals such as aluminum or non-magnetic alloys such as manganese steels having at least 10% manganese, iron aluminum alloys with at least 18% aluminum, or austentitic stainless steels such as 304 stainless steel or 316 stainless steel. In some embodiments, overburden casings may include carbon steel or other ferromagnetic material coupled on the inside diameter to a highly conductive non-ferromagnetic metal (for example, copper or aluminum) to inhibit inductive effects or skin effects. In some embodiments, overburden casings are made of inexpensive materials that may be left in the formation (sacrificial casings).

[0807] In certain embodiments, wellheads for the wellbores may be made of one or more non-ferromagnetic materials. FIG. 91 depicts an embodiment of wellhead 392. The components in the wellheads may include fiberglass, PVC, CPVC, HDPE, high temperature polymers (such as nitrogen based polymers), and/or non-magnetic alloys or metals. Some materials (such as polymers) may be extruded into a mold or reaction injection molded (RIM) into the shape of the wellhead. Forming the wellhead from a mold may be a less expensive method of making the wellhead and save in capital costs for providing wellheads to a treatment site. Using non-ferromagnetic materials in the wellhead may inhibit undesired heating of components in the wellhead. Ferromagnetic materials used in the wellhead may be electrically and/or thermally insulated from other components of the wellhead. In some embodiments, an inert gas (for example, nitrogen or argon) is purged inside the wellhead and/or inside of casings to inhibit reflux of heated gases into the wellhead and/or the casings.

[0808] In some embodiments, ferromagnetic materials in the wellhead are electrically coupled to a non-ferromagnetic material (for example, copper) to inhibit skin effect heat generation in the ferromagnetic materials in the wellhead. The non-ferromagnetic material is in electrical contact with the ferromagnetic material so that current flows through the non-ferromagnetic material. In certain embodiments, as shown in FIG. 91, non-ferromagnetic material 582 is coupled (and electrically coupled) to the inside walls of conduit 382 and wellhead walls 584. In some embodiments, copper may be plasma sprayed, coated, clad, or lined on the inside and/or outside walls of the wellhead. In some embodiments, a non-ferromagnetic material such as copper is welded, brazed, clad, or otherwise electrically coupled to the inside and/or outside walls of the wellhead. For example, copper may be swaged out to line the inside walls in the wellhead. Copper may be liquid nitrogen cooled and then allowed to expand to contact and swage against the inside walls of the wellhead. In some embodiments, the copper is hydraulically expanded or explosively bonded to contact against the inside walls of the wellhead.

[0809] In some embodiments, two or more substantially horizontal wellbores are branched off of a first substantially vertical wellbore drilled downwards from a first location on a surface of the formation. The substantially horizontal wellbores may be substantially parallel through a hydrocarbon layer. The substantially horizontal wellbores may reconnect at a second substantially vertical wellbore drilled downwards at a second location on the surface of the formation. Having multiple wellbores branching off of a single substantially vertical wellbore drilled downwards from the surface reduces the number of openings made at the surface of the formation.

[0810] Typical temperature measurement methods may be difficult and/or expensive to implement for use in assessing a temperature profile of a heater located in a subsurface formation for heating in an in situ heat treatment process. The desire is for a temperature profile that includes multiple temperatures along the length or a portion of the heater in the subsurface formation. Thermocouples are one possible solution; however, thermocouples provide only one temperature at one location and one wire is generally needed for each thermocouple. Thus, to obtain a temperature profile along a length of the heater, multiple wires are needed. The risk of failure of one or more of the thermocouples (or their associated wires) is increased with the use of multiple wires in the subsurface wellbore.

[0811] Another possible solution is the use of a fiber optic cable temperature sensor system. The fiber optic cable system provides a temperature profile along a length of the heater. Commercially available fiber optic cable systems, however, typically only have operating temperature ranges up to about 300° C. Thus, these systems are not suitable for measurement of higher temperatures encountered while heating the subsurface formation during the in situ heat treatment process. Some experimental fiber optic cable systems are suitable for use at these higher temperatures but these systems may be too expensive for implementation in a commercial process (for example, a large field of heaters). Thus, there is a need for a simple, inexpensive system that allows temperature assessment at one or more locations along a length of the subsurface heater used in the in situ heat treatment process.

[0812] Current techniques allow for the measurement of dielectric properties of insulation along a length of the insulation (measurement of dielectric properties distributed along the length of the insulation). These techniques provide a profile of the dielectric properties with a spatial resolution (space between measurements) based on the type of insulation and the abilities of the measurement system. These techniques are currently used to assess dielectric properties and detect insulation flaws and/or insulation damage. Examples of current techniques are axial tomography and line resonance analysis. A version of axial tomography (Mashikian Axial Tomography) is provided by Instrument Manufacturing Company (IMCORP) (Storrs, Conn., U.S.A.). Mashikian Axial Tomography is disclosed in U.S. Pat. Application Pub. No. 2008/0048668 to Mashikian, which is incorporated by reference as if fully set forth herein. A version of line resonance analysis (LIRA) is provided by Wirescan AS (Halden, Norway). Wirescan AS LIRA is disclosed in International Pat. Pub. No. WO 2007/040406 to Fantoni, which is incorporated by reference as if fully set forth herein.

[0813] The assessment of dielectric properties (using either the current techniques or modified versions of these techniques) may be used in combination with information about the temperature dependence of dielectric properties to assess a temperature profile of one or more energized heaters (heaters that are powered and providing heat). The temperature dependence data of the dielectric properties may be found from simulation and/or experimentation. Examples of dielectric properties of the insulation that may be assessed over time include, but are not limited to, dielectric constant and loss tangent. FIG. 92 depicts an example of a plot of dielectric constant versus temperature for magnesium oxide insulation in one embodiment of an insulated conductor heater. FIG. 93 depicts an example of a plot of loss tangent (tan 6) versus temperature for magnesium oxide insulation in one embodiment of an insulated conductor heater.

[0814] It should be noted that the temperature dependent behavior of a dielectric property may vary based on certain factors. Factors that may affect the temperature dependent behavior of the dielectric property include, but are not limited to, the type of insulation, the dimensions of the insulation, the time the insulation is exposed to environment (for example, heat from the heater), the composition (chemistry) of the insulation, and the compaction of the insulation. Thus, it is typically necessary to measure (either by simulation and/or experimentation) the temperature dependent behavior of the dielectric property for the embodiment of insulation that is to be used in a selected heater.

[0815] In certain embodiments, one or more dielectric properties of the insulation in a heater having electrical insulation are assessed (measured) and compared to temperature dependence data of the dielectric properties to assess (determine) a temperature profile along a length of the heater (for example, the entire length of the heater or a portion of the heater). For example, the temperature of an insulated conductor heater (such as a mineral insulated (MI) cable heater) may be assessed based on dielectric properties of the insulation used in the heater. Examples of insulated conductor heaters are depicted in FIGS. 32A, 32B, and 33. Since the temperature dependence of the dielectric property measured is known or estimated from simulation and/or experimentation, the measured dielectric property at a location along the heater may be used to assess the temperature of the heater at that location. Using techniques that measure the dielectric properties at multiple locations along a length of the heater (as is possible with current techniques), a temperature profile along that heater length may be provided.

[0816] In some embodiments, as shown by the plots in FIGS. 92 and 93, the dielectric properties are more sensitive to temperature at higher temperatures (for example, above about 900° F., as shown in FIGS. 92 and 93). Thus, in some embodiments, the temperature of a portion of the insulated conductor heater is assessed by measurement of the dielectric properties at temperatures above about 400° C. (about 760° F.). For example, the temperature of the portion may be assessed by measurement of the dielectric properties at temperatures ranging from about 400° C., about 450° C., or about 500° C. to about 800° C., a or about 900° C. These ranges of temperatures are above temperatures that can be measured using commercially available fiber optic cable systems. A fiber optic cable system suitable for use in the higher temperature ranges may, however, provide measurements with higher spatial resolution than temperature assessment by measurement of the dielectric properties. Thus, in some embodiments, the fiber optic cable system operable in the higher temperature ranges may be used to calibrate temperature assessment by measurement of dielectric properties.

[0817] At temperatures below these temperature ranges (for example, below about 400° C.), temperature assessment by measurement of the dielectric properties may be less accurate. Temperature assessment by measurement of the dielectric properties may, however, provide a reasonable estimate or “average” temperature of portions of the heater. The average temperature assessment may be used to assess whether the heater is operating in a safe range. Typically, a heater operating at temperatures below about 400° C., below about 450° C., or below about 500° C. is operating in the safe range.

[0818] Temperature assessment by measurement of dielectric properties may provide a temperature profile along a length or portion of the insulated conductor heater (temperature measurements distributed along the length or portion of the heater). Measuring the temperature profile is more useful for monitoring and controlling the heater as compared to taking temperature measurements at only selected locations (such as temperature measurement with thermocouples). Multiple thermocouples may be used to provide a temperature profile. Multiple wires (one for each thermocouple), however, would be needed. Temperature assessment by measurement of dielectric properties uses only one wire for measurement of the temperature profile, which is simpler and less expensive than using multiple thermocouples. In some embodiments, one or more thermocouples placed at selected locations are used to calibrate temperature assessment by measurement of dielectric properties.

[0819] In certain embodiments, the dielectric properties of the insulation in an insulated conductor heater are assessed (measured) over a period of time to assess the temperature and operating characteristics of the heater over the period of time. For example, the dielectric properties may be assessed continuously (or substantially continuously) to provide real-time monitoring of the dielectric properties and the temperature. Monitoring of the dielectric properties and the temperature may be used to assess the condition of the heater during operation of the heater. For example, comparison of the assessed properties at specific locations versus the average properties over the length of the heater may provide information on the location of hot spots or defects in the heater.

[0820] In some embodiments, the dielectric properties of the insulation change over time. For example, the dielectric properties may change over time because of changes in the oxygen concentration in the insulation over time and/or changes in the water content in the insulation over time. Oxygen in the insulation may be consumed by chromium or other metals used in the insulated conductor heater. Thus, the oxygen concentration decreases with time in the insulation and affects the dielectric properties of the insulation.

[0821] The changes in dielectric properties over time may be measured and compensated for through experimental and/or simulated data. For example, the insulated conductor heater to be used for temperature assessment may be heated in an oven or other apparatus and the changes in dielectric properties can be measured over time at various temperatures and/or at constant temperatures. In addition, thermocouples may be used to calibrate the assessment of dielectric properties changes over time by comparison of thermocouple data to temperature assessed by the dielectric properties.

[0822] In certain embodiments, temperature assessment by measurement of dielectric properties is performed using a computational system such as a workstation or computer. The computational system may receive measurements (assessments) of the dielectric properties along the heater and correlate these measured dielectric properties to assess temperatures at one or more locations on the heater. For example, the computational system may store data about the relationship of the dielectric properties to temperature (such as the data depicted in FIGS. 92 and 93) and/or time, and use this stored data to calculate the temperatures on the heater based on the measured dielectric properties.

[0823] In certain embodiments, temperature assessment by dielectric properties measurement is performed on an energized heater providing heat to the subsurface formation (for example, an insulated conductor heater provided with electric power to resistively heat and provide heat to the subsurface formation). Assessing temperature on the energized heater allows for detection of defects in the insulation on the device actually providing heat to the formation. Assessing temperature on the energized heater, however, may be more difficult due to attenuation of signal along the heater because the heater is resistively heating. This attenuation may inhibit seeing further along the length of the heater (deeper into the formation along the heater). In some embodiments, temperatures in the upper sections of heaters (sections of the heater closer to the overburden, for example, the upper half or upper third of the heater) may be more important for assessment because these sections have higher voltages applied to the heater, are at higher temperatures, and are at higher risk for failure or generation of hot spots. The signal attenuation in the temperature assessment by dielectric properties measurement may not be as significant a factor in these upper sections because of the proximity of these sections to the surface.

[0824] In some embodiments, power to the insulated conductor heater is turned off before performing the temperature assessment. Power is then returned to the insulated conductor heater after the temperature assessment. Thus, the insulated conductor heater is subjected to a heating on/off cycle to assess temperature. This on/off cycle may, however, reduce the lifetime of the heater due to the thermal cycling. In addition, the heater may cool off during the non-energized time period and provide less accurate temperature information (less accurate information on the actual working temperature of the heater).

[0825] In certain embodiments, temperature assessment by dielectric properties measurement is performed on an insulated conductor that is not to be used for heating or not configured for heating. Such an insulated conductor may be a separate insulated conductor temperature probe. In some embodiments, the insulated conductor temperature probe is a non-energized heater (for example, an insulated conductor heater not powered). The insulated conductor temperature probe may be a stand-alone device that can be located in an opening in the subsurface formation to measure temperature in the opening. In some embodiments, the insulated conductor temperature probe is a looped probe that goes out and back into the opening with signals transmitted in one direction on the probe. In some embodiments, the insulated conductor temperature probe is a single hanging probe with the signal transmitted along the core and returned along the sheath of the insulated conductor.

[0826] In certain embodiments, the insulated conductor temperature probe includes a copper core (to provide better conductance to the end of the cable and better spatial resolution) surrounded by magnesium oxide insulation and an outer metal sheath. The outer metal sheath may be made of any material suitable for use in the subsurface opening. For example, the outer metal sheath may be a stainless steel sheath or an inner sheath of copper wrapped with an outer sheath of stainless steel. Typically, the insulated conductor temperature probe operates up to temperatures and pressures that can be withstood by the outer metal sheath.

[0827] In some embodiments, the insulated conductor temperature probe is located adjacent to or near an energized heater in the opening to measure temperatures along the energized heater. There may be a temperature difference between the insulated conductor temperature probe and the energized heater (for example, between about 50° C. and 100° C. temperature differences). This temperature difference may be assessed through experimentation and/or simulation and accounted for in the temperature measurements. The temperature difference may also be calibrated using one or more thermocouples attached to the energized heater.

[0828] In some embodiments, one or more thermocouples are attached to the insulated conductor used for temperature assessment (either an energized insulated conductor heater or a non-energized insulated conductor temperature probe). The attached thermocouples may be used for calibration and/or backup measurement of the temperature assessed on the insulated conductor by dielectric property measurement. In some embodiments, calibration and/or backup temperature indication is achieved by assessment of the resistance variation of the core of the insulated conductor at a given applied voltage. Temperature may be assessed by knowing the resistance versus temperature profile of the core material at the given voltage. In some embodiments, the insulated conductor is a loop and current induced in the loop from energized heaters in the subsurface opening provides input for the resistance measurement.

[0829] In certain embodiments, insulation material properties in the insulated conductor are varied to provide different sensitivities to temperature for the insulated conductor. Examples of insulation material properties that may be varied include, but are not limited to, the chemical and phase composition, the microstructure, and/or the mixture of insulating materials. Varying the insulation material properties in the insulated conductor allows the insulated conductor to be tuned to a selected temperature range. The selected temperature range may be selected, for example, for a desired application of the insulated conductor.

[0830] In some embodiments, insulation material properties are varied along the length of the insulated conductor (the insulation material properties are different at selected points within the insulated conductor). Varying properties of the insulation material at known locations along the length of the insulated conductor allows the measurement of the dielectric properties to give location information and/or provide for self-calibration of the insulated conductor in addition to providing temperature assessment. In some embodiments, the insulated conductor includes a portion with insulation material properties that allow the portion to act as a reflector. The reflector portion may be used to limit temperature assessment to specific portions of the insulated conductor (for example, a specific length of insulated conductor). One or more reflector portions may be used to provide spatial markers along the length of the insulated conductor.

[0831] Varying the insulation material properties adjusts the activation energy of the insulation material. Typically, increasing the activation energy of the insulation material reduces attenuation in the insulation material and provides better spatial resolution. Lowering the activation energy typically provides better temperature sensitivity. The activation energy may be raised or lowered, for example, by adding different components to the insulation material. For example, adding certain components to magnesium oxide insulation will lower the activation energy. Examples of components that may be added to magnesium oxide to lower the activation energy include, but are not limited to, titanium oxide, nickel oxide, and iron oxide.

[0832] In some embodiments, temperature is assessed using two or more insulated conductors. The insulated conductors may have different activation energies to provide a variation in spatial resolution and temperature sensitivity to more accurately assess temperature in the subsurface opening. The higher activation energy insulated conductor may be used to provide better spatial resolution and identify the location of hot spots or other temperature variations more accurately while the lower activation energy insulated conductor may be used to provide more accurate temperature measurement at those locations.

[0833] In some embodiments, temperature is assessed by assessing leakage current from the insulated conductor. Temperature dependence data of the leakage current may be used to assess the temperature based on assessed (measured) leakage current from the insulated conductor. The measured leakage current may be used in combination with information about the temperature dependence of the leakage current to assess a temperature profile of one or more heaters or insulated conductors located in a subsurface opening. The temperature dependence data of the leakage current may be found from simulation and/or experimentation. In certain embodiments, the temperature dependence data of the leakage current is also dependent on the voltage applied to the heater.

[0834] FIG. 94 depicts an example of a plot of leakage current (mA) versus temperature (° F.) for magnesium oxide insulation in one embodiment of an insulated conductor heater at different applied voltages. Plot 586 is for an applied voltage of 4300 V. Plot 588 is for an applied voltage of 3600 V. Plot 590 is for an applied voltage of 2800 V. Plot 592 is for an applied voltage of 2100 V.

[0835] As shown by the plots in FIG. 94, the leakage current is more sensitive to temperature at higher temperatures (for example, above about 950° F., as shown in FIG. 94). Thus, in some embodiments, the temperature of a portion of the insulated conductor heater is assessed by measurement of the leakage current at temperatures above about 500° C. (about 932° F.).

[0836] A temperature profile along a length of the heater may be obtained by measuring the leakage current along the length of the heater using techniques known in the art. In some embodiments, assessment of temperature by measuring the leakage current is used in combination with temperature assessment by dielectric properties measurement. For example, temperature assessment by measurement of the leakage current may be used to calibrate and/or backup temperature assessments made by measurement of dielectric properties.

[0837] In certain embodiments, an insulated conductor using salt as the electrical insulator is used for temperature measurement. The salt becomes an electrical conductor above the melting temperature (T_m) of the salt and allows current to flow through the electrical insulator. FIG. 95 depicts an embodiment of insulated conductor 410 with salt used as electrical insulator 364. Core 374 is copper or another suitable electrical conductor. Jacket 370 is stainless steel or another suitable corrosion-resistant electrical conductor. In one embodiment, core 374 is 0.125″ (about 0.3175 cm) diameter copper surrounded by electrical insulator 364. Electrical insulator 364 is 0.1″ (about 0.25 cm) thick salt insulation surrounded by jacket 370. Jacket 370 is 0.1″ (about 0.25 cm) thick stainless steel. The outer diameter of insulated conductor 410 is then 0.525″ (about 1.33 cm).

[0838] In certain embodiments, electrical insulator 364 includes a salt with a melting temperature (T_m) at a desired temperature. The desired temperature may be a temperature in the range of operation of a subsurface heater or a maximum temperature desired in the opening. For example, the desired temperature may be above about 300° C. or in a range between 300° C., 400° C., about 450° C., or about 500° C. and about 800° C., about 850° C., or about 900° C. Examples of salts include, but are not limited to, Na₂CO₃(T_m=851° C.), Li₂CO₃(T_m=732° C.), LiCl (T_m=605° C.), KOH (T_m=420° C.), KNO₃(T_m=334° C.), NaNO₃(T_m=308° C.), and mixtures thereof. In some embodiments, magnesium oxide (such as porous magnesium oxide) is added to the salt to provide mechanical centering support. The magnesium oxide maintains the integrity and structure of insulated conductor 410 when the salt melts. Porous magnesium oxide allows for electrical connectivity between core 374 and jacket 370 by having the salt distributed in the pores of the magnesium oxide.

[0839] In certain embodiments, a mixture of two or more salts is used in electrical insulator 364 of insulated conductor 410. Varying the composition of the salts in the mixture allows for adjusting and tuning the melting temperature of the mixture to a desired temperature. In some embodiments, the composition of eutectic mixtures of salts is adjusted and tuned to the desired temperature. Eutectic mixtures may allow for finer adjustment and tuning to the desired temperature. Examples of eutectic mixtures that may be used include, but are not limited to, K₂CO₃:Na₂CO₃:Li₂CO₃ and KNO₃:NaNO₃.

[0840] Insulated conductor 410 may be coupled to or located near one or more heaters in a subsurface wellbore to assess the temperature at one or more locations along the length of the insulated conductor at or near the heaters. In some embodiments, insulated conductor 410 is similar in length to the heaters in the subsurface wellbore. In some embodiments, insulated conductor 410 has a shorter length than the heaters. In some embodiments, more than one insulated conductor 410 may be used in the wellbore to assess the temperature at different locations in the wellbore and/or at different temperatures.

[0841] FIG. 96 depicts an embodiment of insulated conductor 410 located proximate heaters 412 in wellbore 490. In some embodiments, insulated conductor 410 is coupled to one or more of heaters 412. For example, insulated conductor 410 may be strapped to the assembly of heaters 412. Heaters 412 may be insulated conductor heaters, conductor-in-conduit heaters, other types of heaters described herein, or combinations thereof.

[0842] To assess a location that is hotter than other portions of insulated conductor 410, voltage is applied to core 374 and jacket 370 of the insulated conductor, as shown in FIG. 97. Below the melting temperature (T_m) of the salt, there is little or no current drawn by core 374 and jacket 370 because the salt is in a solid phase. In the solid phase, the salt acts as an electrical insulator with resistivities above about 10⁶ Ω-cm.

[0843] In some embodiments, hot spot 594 may develop at some location along the insulated conductor 410. Hot spot 594 is hotter than other portions along the length of insulated conductor 410. Hot spot 594 may be caused by a hot spot developing on or near one or more heaters located in the wellbore (for example, heaters 412 depicted in FIG. 96). At hot spot 594, the salt melts and becomes a liquid or molten salt. In the liquid phase, the salt becomes an electrical conductor with resistivities below 1 Ω-cm. Thus, current begins to flow between the surface and hot spot 594, as shown by the arrows in FIG. 97. Once current begins to flow through core 374 and jacket 370 of insulated conductor 410, if the resistance of the core and the jacket are known, the distance from the surface to hot spot 594 (x in FIG. 97) may be assessed by the measured current at the surface.

[0844] In certain embodiments, multiple hotspots may be located using insulated conductor 410. Time domain reflectometry may be used to locate multiple hotspots along insulated conductor 410 because the insulated conductor has a coaxial geometry. FIG. 98 shows insulated conductor 410 with multiple hot spots 594A, 594B. Incident pulse 596 is provided to insulated conductor 410. Reflected pulses 598A, 594B are generated at corresponding hot spots 594A, 594B.

[0845] The conductive molten salt at hot spots 594A, 594B provides a strong impedance mismatch for the reflections. The reflection coefficient for each hotspot can be assessed using EQN. 8:
ρ=(Z_HS−Z₀)/(Z_HS+Z₀);  (EQN. 8)

[0846] where Z_HS is the impedance of the hotspot, and Z₀ is the impedance of the insulated conductor (cable).

[0847] The location of the hotspots (X_HSa, X_HSb) can be assessed by assessing (measuring) the transit time, τ, between the incident and reflected pulses and using EQN. 9:
X_HS=v*τ/2;  (EQN. 9)

[0848] where v=v_c/√(∈) is the propagation velocity, v_c, is the speed of light, and ∈ is the dielectric constant of the salt insulation, which depends upon the salt used and compaction of the insulated conductor. In some embodiments, a hairpin insulated conductor configuration is used. The hairpin configuration allows for testing from both ends of the insulated conductor and increases the accuracy of hotspot location.

[0849] In some embodiments, assessment of the locations of hotspots by assessing the current or pulses applied to salt based insulated conductor 410 is used in combination with temperature assessment using thermocouples and/or fiber optic cable temperature sensor. The thermocouples and/or fiber optic cable temperature sensor may be used for calibration and/or backup measurement of the temperature assessment using the salt based insulated conductor.

[0850] In certain embodiments, a temperature limited heater is utilized for heavy oil applications (for example, treatment of relatively permeable formations or tar sands formations). A temperature limited heater may provide a relatively low Curie temperature and/or phase transformation temperature range so that a maximum average operating temperature of the heater is less than 350° C., 300° C., 250° C., 225° C., 200° C., or 150° C. In an embodiment (for example, for a tar sands formation), a maximum temperature of the temperature limited heater is less than about 250° C. to inhibit olefin generation and production of other cracked products. In some embodiments, a maximum temperature of the temperature limited heater is above about 250° C. to produce lighter hydrocarbon products. In some embodiments, the maximum temperature of the heater may be at or less than about 500° C.

[0851] A heater may heat a volume of formation adjacent to a production wellbore (a near production wellbore region) so that the temperature of fluid in the production wellbore and in the volume adjacent to the production wellbore is less than the temperature that causes degradation of the fluid. The heat source may be located in the production wellbore or near the production wellbore. In some embodiments, the heat source is a temperature limited heater. In some embodiments, two or more heat sources may supply heat to the volume. Heat from the heat source may reduce the viscosity of crude oil in or near the production wellbore. In some embodiments, heat from the heat source mobilizes fluids in or near the production wellbore and/or enhances the flow of fluids to the production wellbore. In some embodiments, reducing the viscosity of crude oil allows or enhances gas lifting of heavy oil (at most about 10° API gravity oil) or intermediate gravity oil (approximately 12° to 20° API gravity oil) from the production wellbore. In certain embodiments, the initial API gravity of oil in the formation is at most 10°, at most 20°, at most 25°, or at most 30°. In certain embodiments, the viscosity of oil in the formation is at least 0.05 Pa·s (50 cp). In some embodiments, the viscosity of oil in the formation is at least 0.10 Pa·s (100 cp), at least 0.15 Pa·s (150 cp), or at least at least 0.20 Pa·s (200 cp). Large amounts of natural gas may have to be utilized to provide gas lift of oil with viscosities above 0.05 Pa·s. Reducing the viscosity of oil at or near the production wellbore in the formation to a viscosity of 0.05 Pa·s (50 cp), 0.03 Pa·s (30 cp), 0.02 Pa·s (20 cp), 0.01 Pa·s (10 cp), or less (down to 0.001 Pa·s (1 cp) or lower) lowers the amount of natural gas or other fluid needed to lift oil from the formation. In some embodiments, reduced viscosity oil is produced by other methods such as pumping.

[0852] The rate of production of oil from the formation may be increased by raising the temperature at or near a production wellbore to reduce the viscosity of the oil in the formation in and adjacent to the production wellbore. In certain embodiments, the rate of production of oil from the formation is increased by 2 times, 3 times, 4 times, or greater over standard cold production with no external heating of formation during production. Certain formations may be more economically viable for enhanced oil production using the heating of the near production wellbore region. Formations that have a cold production rate approximately between 0.05 m³/(day per meter of wellbore length) and 0.20 m³/(day per meter of wellbore length) may have significant improvements in production rate using heating to reduce the viscosity in the near production wellbore region. In some formations, production wells up to 775 m, up to 1000 m, or up to 1500 m in length are used. Thus, a significant increase in production is achievable in some formations. Heating the near production wellbore region may be used in formations where the cold production rate is not between 0.05 m³/(day per meter of wellbore length) and 0.20 m³/(day per meter of wellbore length), but heating such formations may not be as economically favorable. Higher cold production rates may not be significantly increased by heating the near wellbore region, while lower production rates may not be increased to an economically useful value.

[0853] Using the temperature limited heater to reduce the viscosity of oil at or near the production well inhibits problems associated with non-temperature limited heaters and heating the oil in the formation due to hot spots. One possible problem is that non-temperature limited heaters can cause coking of oil at or near the production well if the heater overheats the oil because the heaters are at too high a temperature. Higher temperatures in the production well may also cause brine to boil in the well, which may lead to scale formation in the well. Non-temperature limited heaters that reach higher temperatures may also cause damage to other wellbore components (for example, screens used for sand control, pumps, or valves). Hot spots may be caused by portions of the formation expanding against or collapsing on the heater. In some embodiments, the heater (either the temperature limited heater or another type of non-temperature limited heater) has sections that are lower because of sagging over long heater distances. These lower sections may sit in heavy oil or bitumen that collects in lower portions of the wellbore. At these lower sections, the heater may develop hot spots due to coking of the heavy oil or bitumen. A standard non-temperature limited heater may overheat at these hot spots, thus producing a non-uniform amount of heat along the length of the heater. Using the temperature limited heater may inhibit overheating of the heater at hot spots or lower sections and provide more uniform heating along the length of the wellbore.

[0854] In certain embodiments, fluids in the relatively permeable formation containing heavy hydrocarbons are produced with little or no pyrolyzation of hydrocarbons in the formation. In certain embodiments, the relatively permeable formation containing heavy hydrocarbons is a tar sands formation. For example, the formation may be a tar sands formation such as the Athabasca tar sands formation in Alberta, Canada or a carbonate formation such as the Grosmont carbonate formation in Alberta, Canada. The fluids produced from the formation are mobilized fluids. Producing mobilized fluids may be more economical than producing pyrolyzed fluids from the tar sands formation. Producing mobilized fluids may also increase the total amount of hydrocarbons produced from the tar sands formation.

[0855] FIGS. 99-102 depict side view representations of embodiments for producing mobilized fluids from tar sands formations. In FIGS. 99-102, heaters 412 have substantially horizontal heating sections in hydrocarbon layer 388 (as shown, the heaters have heating sections that go into and out of the page). Hydrocarbon layer 388 may be below overburden 400. FIG. 99 depicts a side view representation of an embodiment for producing mobilized fluids from a tar sands formation with a relatively thin hydrocarbon layer. FIG. 100 depicts a side view representation of an embodiment for producing mobilized fluids from a hydrocarbon layer that is thicker than the hydrocarbon layer depicted in FIG. 99. FIG. 101 depicts a side view representation of an embodiment for producing mobilized fluids from a hydrocarbon layer that is thicker than the hydrocarbon layer depicted in FIG. 100. FIG. 102 depicts a side view representation of an embodiment for producing mobilized fluids from a tar sands formation with a hydrocarbon layer that has a shale break.

[0856] In FIG. 99, heaters 412 are placed in an alternating triangular pattern in hydrocarbon layer 388. In FIGS. 100, 101, and 102, heaters 412 are placed in an alternating triangular pattern in hydrocarbon layer 388 that repeats vertically to encompass a majority or all of the hydrocarbon layer. In FIG. 102, the alternating triangular pattern of heaters 412 in hydrocarbon layer 388 repeats uninterrupted across shale break 600. In FIGS. 99-102, heaters 412 may be equidistantly spaced from each other. In the embodiments depicted in FIGS. 99-102, the number of vertical rows of heaters 412 depends on factors such as, but not limited to, the desired spacing between the heaters, the thickness of hydrocarbon layer 388, and/or the number and location of shale breaks 600. In some embodiments, heaters 412 are arranged in other patterns. For example, heaters 412 may be arranged in patterns such as, but not limited to, hexagonal patterns, square patterns, or rectangular patterns.

[0857] In the embodiments depicted in FIGS. 99-102, heaters 412 provide heat that mobilizes hydrocarbons (reduces the viscosity of the hydrocarbons) in hydrocarbon layer 388. In certain embodiments, heaters 412 provide heat that reduces the viscosity of the hydrocarbons in hydrocarbon layer 388 below about 0.50 Pa·s (500 cp), below about 0.10 Pa·s (100 cp), or below about 0.05 Pa·s (50 cp). The spacing between heaters 412 and/or the heat output of the heaters may be designed and/or controlled to reduce the viscosity of the hydrocarbons in hydrocarbon layer 388 to desirable values. Heat provided by heaters 412 may be controlled so that little or no pyrolyzation occurs in hydrocarbon layer 388. Superposition of heat between the heaters may create one or more drainage paths (for example, paths for flow of fluids) between the heaters. In certain embodiments, production wells 206A and/or production wells 206B are located proximate heaters 412 so that heat from the heaters superimposes over the production wells. The superimposition of heat from heaters 412 over production wells 206A and/or production wells 206B creates one or more drainage paths from the heaters to the production wells. In certain embodiments, one or more of the drainage paths converge. For example, the drainage paths may converge at or near a bottommost heater and/or the drainage paths may converge at or near production wells 206A and/or production wells 206B. Fluids mobilized in hydrocarbon layer 388 tend to flow towards the bottommost heaters 412, production wells 206A and/or production wells 206B in the hydrocarbon layer because of gravity and the heat and pressure gradients established by the heaters and/or the production wells. The drainage paths and/or the converged drainage paths allow production wells 206A and/or production wells 206B to collect mobilized fluids in hydrocarbon layer 388.

[0858] In certain embodiments, hydrocarbon layer 388 has sufficient permeability to allow mobilized fluids to drain to production wells 206A and/or production wells 206B. For example, hydrocarbon layer 388 may have a permeability of at least about 0.1 darcy, at least about 1 darcy, at least about 10 darcy, or at least about 100 darcy. In some embodiments, hydrocarbon layer 388 has a relatively large vertical permeability to horizontal permeability ratio (K_v/K_h). For example, hydrocarbon layer 388 may have a K_v/K_h ratio between about 0.01 and about 2, between about 0.1 and about 1, or between about 0.3 and about 0.7.

[0859] In certain embodiments, fluids are produced through production wells 206A located near heaters 412 in the lower portion of hydrocarbon layer 388. In some embodiments, fluids are produced through production wells 206B located below and approximately midway between heaters 412 in the lower portion of hydrocarbon layer 388. At least a portion of production wells 206A and/or production wells 206B may be oriented substantially horizontal in hydrocarbon layer 388 (as shown in FIGS. 99-102, the production wells have horizontal portions that go into and out of the page). Production wells 206A and/or 206B may be located proximate lower portion heaters 412 or the bottommost heaters.

[0860] In some embodiments, production wells 206A are positioned substantially vertically below the bottommost heaters in hydrocarbon layer 388. Production wells 206A may be located below heaters 412 at the bottom vertex of a pattern of the heaters (for example, at the bottom vertex of the triangular pattern of heaters depicted in FIGS. 99-102). Locating production wells 206A substantially vertically below the bottommost heaters may allow for efficient collection of mobilized fluids from hydrocarbon layer 388.

[0861] In certain embodiments, the bottommost heaters are located between about 2 m and about 10 m from the bottom of hydrocarbon layer 388, between about 4 m and about 8 m from the bottom of the hydrocarbon layer, or between about 5 m and about 7 m from the bottom of the hydrocarbon layer. In certain embodiments, production wells 206A and/or production wells 206B are located at a distance from the bottommost heaters 412 that allows heat from the heaters to superimpose over the production wells but at a distance from the heaters that inhibits coking at the production wells. Production wells 206A and/or production wells 206B may be located a distance from the nearest heater (for example, the bottommost heater) of at most ¾ of the spacing between heaters in the pattern of heaters (for example, the triangular pattern of heaters depicted in FIGS. 99-102). In some embodiments, production wells 206A and/or production wells 206B are located a distance from the nearest heater of at most ⅔, at most ½, or at most ½ of the spacing between heaters in the pattern of heaters. In certain embodiments, production wells 206A and/or production wells 206B are located between about 2 m and about 10 m from the bottommost heaters, between about 4 m and about 8 m from the bottommost heaters, or between about 5 m and about 7 m from the bottommost heaters. Production wells 206A and/or production wells 206B may be located between about 0.5 m and about 8 m from the bottom of hydrocarbon layer 388, between about 1 m and about 5 m from the bottom of the hydrocarbon layer, or between about 2 m and about 4 m from the bottom of the hydrocarbon layer.

[0862] In some embodiments, at least some production wells 206A are located substantially vertically below heaters 412 near shale break 600, as depicted in FIG. 102. Production wells 206A may be located between heaters 412 and shale break 600 to produce fluids that flow and collect above the shale break. Shale break 600 may be an impermeable barrier in hydrocarbon layer 388. In some embodiments, shale break 600 has a thickness between about 1 m and about 6 m, between about 2 m and about 5 m, or between about 3 m and about 4 m. Production wells 206A between heaters 412 and shale break 600 may produce fluids from the upper portion of hydrocarbon layer 388 (above the shale break) and production wells 206A below the bottommost heaters in the hydrocarbon layer may produce fluids from the lower portion of the hydrocarbon layer (below the shale break), as depicted in FIG. 102. In some embodiments, two or more shale breaks may exist in a hydrocarbon layer. In such an embodiment, production wells are placed at or near each of the shale breaks to produce fluids flowing and collecting above the shale breaks.

[0863] In some embodiments, shale break 600 breaks down (is desiccated or decomposes) as the shale break is heated by heaters 412 on either side of the shale break. As shale break 600 breaks down, the permeability of the shale break increases and fluids flow through the shale break. Once fluids are able to flow through shale break 600, production wells above the shale break may not be needed for production as fluids can flow to production wells at or near the bottom of hydrocarbon layer 388 and be produced there.

[0864] In certain embodiments, the bottommost heaters above shale break 600 are located between about 2 m and about 10 m from the shale break, between about 4 m and about 8 m from the bottom of the shale break, or between about 5 m and about 7 m from the shale break. Production wells 206A may be located between about 2 m and about 10 m from the bottommost heaters above shale break 600, between about 4 m and about 8 m from the bottommost heaters above the shale break, or between about 5 m and about 7 m from the bottommost heaters above the shale break. Production wells 206A may be located between about 0.5 m and about 8 m from shale break 600, between about 1 m and about 5 m from the shale break, or between about 2 m and about 4 m from the shale break.

[0865] In some embodiments, heat is provided in production wells 206A and/or production wells 206B, depicted in FIGS. 99-102. Providing heat in production wells 206A and/or production wells 206B may maintain and/or enhance the mobility of the fluids in the production wells. Heat provided in production wells 206A and/or production wells 206B may superimpose with heat from heaters 412 to create the flow path from the heaters to the production wells. In some embodiments, production wells 206A and/or production wells 206B include a pump to move fluids to the surface of the formation. In some embodiments, the viscosity of fluids (oil) in production wells 206A and/or production wells 206B is lowered using heaters and/or diluent injection (for example, using a conduit in the production wells for injecting the diluent).

[0866] In certain embodiments, in situ heat treatment of the relatively permeable formation containing hydrocarbons (for example, the tar sands formation) includes heating the formation to visbreaking temperatures. For example, the formation may be heated to temperatures between about 100° C. and 260° C., between about 150° C. and about 250° C., between about 200° C. and about 240° C., between about 205° C. and 230° C., between about 210° C. and 225° C. In one embodiment, the formation is heated to a temperature of about 220° C. In one embodiment, the formation is heated to a temperature of about 230° C. At visbreaking temperatures, fluids in the formation have a reduced viscosity (versus their initial viscosity at initial formation temperature) that allows fluids to flow in the formation. The reduced viscosity at visbreaking temperatures may be a permanent reduction in viscosity as the hydrocarbons go through a step change in viscosity at visbreaking temperatures (versus heating to mobilization temperatures, which may only temporarily reduce the viscosity). The visbroken fluids may have API gravities that are relatively low (for example, at most about 10°, about 12°, about 15°, or about 19° API gravity), but the API gravities are higher than the API gravity of non-visbroken fluid from the formation. The non-visbroken fluid from the formation may have an API gravity of 7° or less.

[0867] In some embodiments, heaters in the formation are operated at full power output to heat the formation to visbreaking temperatures or higher temperatures. Operating at full power may rapidly increase the pressure in the formation. In certain embodiments, fluids are produced from the formation to maintain a pressure in the formation below a selected pressure as the temperature of the formation increases. In some embodiments, the selected pressure is a fracture pressure of the formation. In certain embodiments, the selected pressure is between about 1000 kPa and about 15000 kPa, between about 2000 kPa and about 10000 kPa, or between about 2500 kPa and about 5000 kPa. In one embodiment, the selected pressure is about 10000 kPa. Maintaining the pressure as close to the fracture pressure as possible may minimize the number of production wells needed for producing fluids from the formation.

[0868] In certain embodiments, treating the formation includes maintaining the temperature at or near visbreaking temperatures (as described above) during the entire production phase while maintaining the pressure below the fracture pressure. The heat provided to the formation may be reduced or eliminated to maintain the temperature at or near visbreaking temperatures. Heating to visbreaking temperatures but maintaining the temperature below pyrolysis temperatures or near pyrolysis temperatures (for example, below about 230° C.) inhibits coke formation and/or higher level reactions. Heating to visbreaking temperatures at higher pressures (for example, pressures near but below the fracture pressure) keeps produced gases in the liquid oil (hydrocarbons) in the formation and increases hydrogen reduction in the formation with higher hydrogen partial pressures. Heating the formation to only visbreaking temperatures also uses less energy input than heating the formation to pyrolysis temperatures.

[0869] Fluids produced from the formation may include visbroken fluids, mobilized fluids, and/or pyrolyzed fluids. In some embodiments, a produced mixture that includes these fluids is produced from the formation. The produced mixture may have assessable properties (for example, measurable properties). The produced mixture properties are determined by operating conditions in the formation being treated (for example, temperature and/or pressure in the formation). In certain embodiments, the operating conditions may be selected, varied, and/or maintained to produce desirable properties in hydrocarbons in the produced mixture. For example, the produced mixture may include hydrocarbons that have properties that allow the mixture to be easily transported (for example, sent through a pipeline without adding diluent or blending the mixture and/or resulting hydrocarbons with another fluid).

[0870] In some embodiments, after the formation reaches visbreaking temperatures, the pressure in the formation is reduced. In certain embodiments, the pressure in the formation is reduced at temperatures above visbreaking temperatures. Reducing the pressure at higher temperatures allows more of the hydrocarbons in the formation to be converted to higher quality hydrocarbons by visbreaking and/or pyrolysis. Allowing the formation to reach higher temperatures before pressure reduction, however, may increase the amount of carbon dioxide produced and/or the amount of coking in the formation. For example, in some formations, coking of bitumen (at pressures above 700 kPa) begins at about 280° C. and reaches a maximum rate at about 340° C. At pressures below about 700 kPa, the coking rate in the formation is minimal Allowing the formation to reach higher temperatures before pressure reduction may decrease the amount of hydrocarbons produced from the formation.

[0871] In certain embodiments, the temperature in the formation (for example, an average temperature of the formation) when the pressure in the formation is reduced is selected to balance one or more factors. The factors considered may include: the quality of hydrocarbons produced, the amount of hydrocarbons produced, the amount of carbon dioxide produced, the amount hydrogen sulfide produced, the degree of coking in the formation, and/or the amount of water produced. Experimental assessments using formation samples and/or simulated assessments based on the formation properties may be used to assess results of treating the formation using the in situ heat treatment process. These results may be used to determine a selected temperature, or temperature range, for when the pressure in the formation is to be reduced. The selected temperature, or temperature range, may also be affected by factors such as, but not limited to, hydrocarbon or oil market conditions and other economic factors. In certain embodiments, the selected temperature is in a range between about 275° C. and about 305° C., between about 280° C. and about 300° C., or between about 285° C. and about 295° C.

[0872] In certain embodiments, an average temperature in the formation is assessed from an analysis of fluids produced from the formation. For example, the average temperature of the formation may be assessed from an analysis of the fluids that have been produced to maintain the pressure in the formation below the fracture pressure of the formation.

[0873] In some embodiments, values of the hydrocarbon isomer shift in fluids (for example, gases) produced from the formation is used to indicate the average temperature in the formation. Experimental analysis and/or simulation may be used to assess one or more hydrocarbon isomer shifts and relate the values of the hydrocarbon isomer shifts to the average temperature in the formation. The assessed relation between the hydrocarbon isomer shifts and the average temperature may then be used in the field to assess the average temperature in the formation by monitoring one or more of the hydrocarbon isomer shifts in fluids produced from the formation. In some embodiments, the pressure in the formation is reduced when the monitored hydrocarbon isomer shift reaches a selected value. The selected value of the hydrocarbon isomer shift may be chosen based on the selected temperature, or temperature range, in the formation for reducing the pressure in the formation and the assessed relation between the hydrocarbon isomer shift and the average temperature. Examples of hydrocarbon isomer shifts that may be assessed include, but are not limited to, n-butane-δ¹³C₄ percentage versus propane-δ¹³C₃ percentage, n-pentane-δ¹³C₅ percentage versus propane-δ¹³C₃ percentage, n-pentane-δ¹³C₅ percentage versus n-butane-δ¹³C₄ percentage, and i-pentane-δ¹³C₅ percentage versus i-butane-δ¹³C₄ percentage. In some embodiments, the hydrocarbon isomer shift in produced fluids is used to indicate the amount of conversion (for example, amount of pyrolysis) that has taken place in the formation.

[0874] In some embodiments, weight percentages of saturates in fluids produced from the formation is used to indicate the average temperature in the formation. Experimental analysis and/or simulation may be used to assess the weight percentage of saturates as a function of the average temperature in the formation. For example, SARA (Saturates, Aromatics, Resins, and Asphaltenes) analysis (sometimes referred to as Asphaltene/Wax/Hydrate Deposition analysis) may be used to assess the weight percentage of saturates in a sample of fluids from the formation. In some formations, the weight percentage of saturates has a linear relationship to the average temperature in the formation. The relation between the weight percentage of saturates and the average temperature may then be used in the field to assess the average temperature in the formation by monitoring the weight percentage of saturates in fluids produced from the formation. In some embodiments, the pressure in the formation is reduced when the monitored weight percentage of saturates reaches a selected value. The selected value of the weight percentage of saturates may be chosen based on the selected temperature, or temperature range, in the formation for reducing the pressure in the formation and the relation between the weight percentage of saturates and the average temperature. In some embodiments, the selected value of weight percentage of saturates is between about 20% and about 40%, between about 25% and about 35%, or between about 28% and about 32%. For example, the selected value may be about 30% by weight saturates.

[0875] In some embodiments, weight percentages of n-C₇ in fluids produced from the formation is used to indicate the average temperature in the formation. Experimental analysis and/or simulation may be used to assess the weight percentages of n-C₇ as a function of the average temperature in the formation. In some formations, the weight percentages of n-C₇ has a linear relationship to the average temperature in the formation. The relation between the weight percentages of n-C₇ and the average temperature may then be used in the field to assess the average temperature in the formation by monitoring the weight percentages of n-C₇ in fluids produced from the formation. In some embodiments, the pressure in the formation is reduced when the monitored weight percentage of n-C₇ reaches a selected value. The selected value of the weight percentage of n-C₇ may be chosen based on the selected temperature, or temperature range, in the formation for reducing the pressure in the formation and the relation between the weight percentage of n-C₇ and the average temperature. In some embodiments, the selected value of weight percentage of n-C₇ is between about 50% and about 70%, between about 55% and about 65%, or between about 58% and about 62%. For example, the selected value may be about 60% by weight n-C₇.

[0876] The pressure in the formation may be reduced by producing fluids (for example, visbroken fluids and/or mobilized fluids) from the formation. In some embodiments, the pressure is reduced below a pressure at which fluids coke in the formation to inhibit coking at pyrolysis temperatures. For example, the pressure is reduced to a pressure below about 1000 kPa, below about 800 kPa, or below about 700 kPa (for example, about 690 kPa). In certain embodiments, the selected pressure is at least about 100 kPa, at least about 200 kPa, or at least about 300 kPa. The pressure may be reduced to inhibit coking of asphaltenes or other high molecular weight hydrocarbons in the formation. In some embodiments, the pressure may be maintained below a pressure at which water passes through a liquid phase at downhole (formation) temperatures to inhibit liquid water and dolomite reactions. After reducing the pressure in the formation, the temperature may be increased to pyrolysis temperatures to begin pyrolyzation and/or upgrading of fluids in the formation. The pyrolyzed and/or upgraded fluids may be produced from the formation.

[0877] In certain embodiments, the amount of fluids produced at temperatures below visbreaking temperatures, the amount of fluids produced at visbreaking temperatures, the amount of fluids produced before reducing the pressure in the formation, and/or the amount of upgraded or pyrolyzed fluids produced may be varied to control the quality and amount of fluids produced from the formation and the total recovery of hydrocarbons from the formation. For example, producing more fluid during the early stages of treatment (for example, producing fluids before reducing the pressure in the formation) may increase the total recovery of hydrocarbons from the formation while reducing the overall quality (lowering the overall API gravity) of fluid produced from the formation. The overall quality is reduced because more heavy hydrocarbons are produced by producing more fluids at the lower temperatures. Producing less fluids at the lower temperatures may increase the overall quality of the fluids produced from the formation but may lower the total recovery of hydrocarbons from the formation. The total recovery may be lower because more coking occurs in the formation when less fluids are produced at lower temperatures.

[0878] In certain embodiments, the formation is heated using isolated cells of heaters (cells or sections of the formation that are not interconnected for fluid flow). The isolated cells may be created by using larger heater spacings in the formation. For example, large heater spacings may be used in the embodiments depicted in FIGS. 99-102. These isolated cells may be produced during early stages of heating (for example, at temperatures below visbreaking temperatures). Because the cells are isolated from other cells in the formation, the pressures in the isolated cells are high and more liquids are producible from the isolated cells. Thus, more liquids may be produced from the formation and a higher total recovery of hydrocarbons may be reached. During later stages of heating, the heat gradient may interconnect the isolated cells and pressures in the formation will drop.

[0879] In certain embodiments, the heat gradient in the formation is modified so that a gas cap is created at or near an upper portion of the hydrocarbon layer. For example, the heat gradient made by heaters 412 depicted in the embodiments depicted in FIGS. 99-102 may be modified to create the gas cap at or near overburden 400 of hydrocarbon layer 388. The gas cap may push or drive liquids to the bottom of the hydrocarbon layer so that more liquids may be produced from the formation. In situ generation of the gas cap may be more efficient than introducing pressurized fluid into the formation. The in situ generated gas cap applies force evenly through the formation with little or no channeling or fingering that may reduce the effectiveness of introduced pressurized fluid.

[0880] In certain embodiments, the number and/or location of production wells in the formation is varied based on the viscosity of fluid in the formation. The viscosities in the zones may be assessed before placing the production wells in the formation, before heating the formation, and/or after heating the formation. In some embodiments, more production wells are located in zones in the formation that have lower viscosities. For example, in certain formations, upper portions, or zones, of the formation may have lower viscosities. In some embodiments, more production wells are located in the upper zones. Producing through production wells in the less viscous zones of the formation may result in production of higher quality (more upgraded) oil from the formation.

[0881] In some embodiments, more production wells are located in zones in the formation that have higher viscosities. Pressure propagation may be slower in the zones with higher viscosities. The slower pressure propagation may make it more difficult to control pressure in the zones with higher viscosities. Thus, more production wells may be located in the zones with higher viscosities to provide better pressure control in these zones.

[0882] In some embodiments, zones in the formation with different assessed viscosities are heated at different rates. In certain embodiments, zones in the formation with higher viscosities are heated at higher heating rates than zones with lower viscosities. Heating the zones with higher viscosities at the higher heating rates mobilizes and/or upgrades these zones at a faster rate so that these zones may “catch up” in viscosity and/or quality to the slower heated zones.

[0883] In some embodiments, the heater spacing is varied to provide different heating rates to zones in the formation with different assessed viscosities. For example, denser heater spacings (less spaces between heaters) may be used in zones with higher viscosities to heat these zones at higher heating rates. In some embodiments, a production well (for example, a substantially vertical production well) is located in the zones with denser heater spacings and higher viscosities. The production well may be used to remove fluids from the formation and relieve pressure from the higher viscosity zones. In some embodiments, one or more substantially vertical openings, or production wells, are located in the higher viscosity zones to allow fluids to drain in the higher viscosity zones. The draining fluids may be produced from the formation through production wells located near the bottom of the higher viscosity zones.

[0884] In certain embodiments, production wells are located in more than one zone in the formation. The zones may have different initial permeabilities. In certain embodiments, a first zone has an initial permeability of at least about 1 darcy and a second zone has an initial permeability of at most about 0.1 darcy. In some embodiments, the first zone has an initial permeability of between about 1 darcy and about 10 darcy. In some embodiments, the second zone has an initial permeability between about 0.01 darcy and 0.1 darcy. The zones may be separated by a substantially impermeable barrier (with an initial permeability of about 10 μdarcy or less). Having the production well located in both zones allows for fluid communication (permeability) between the zones and/or pressure equalization between the zones.

[0885] In some embodiments, openings (for example, substantially vertical openings) are formed between zones with different initial permeabilities that are separated by a substantially impermeable barrier. Bridging the zones with the openings allows for fluid communication (permeability) between the zones and/or pressure equalization between the zones. In some embodiments, openings in the formation (such as pressure relief openings and/or production wells) allow gases or low viscosity fluids to rise in the openings. As the gases or low viscosity fluids rise, the fluids may condense or increase viscosity in the openings so that the fluids drain back down the openings to be further upgraded in the formation. Thus, the openings may act as heat pipes by transferring heat from the lower portions to the upper portions where the fluids condense. The wellbores may be packed and sealed near or at the overburden to inhibit transport of formation fluid to the surface.

[0886] In some embodiments, production of fluids is continued after reducing and/or turning off heating of the formation. The formation may be heated for a selected time. The formation may be heated until it reaches a selected average temperature. Production from the formation may continue after the selected time. Continuing production may produce more fluid from the formation as fluids drain towards the bottom of the formation and/or as fluids are upgraded by passing by hot spots in the formation. In some embodiments, a horizontal production well is located at or near the bottom of the formation (or a zone of the formation) to produce fluids after heating is turned down and/or off.

[0887] In certain embodiments, initially produced fluids (for example, fluids produced below visbreaking temperatures), fluids produced at visbreaking temperatures, and/or other viscous fluids produced from the formation are blended with diluent to produce fluids with lower viscosities. In some embodiments, the diluent includes upgraded or pyrolyzed fluids produced from the formation. In some embodiments, the diluent includes upgraded or pyrolyzed fluids produced from another portion of the formation or another formation. In certain embodiments, the amount of fluids produced at temperatures below visbreaking temperatures and/or fluids produced at visbreaking temperatures that are blended with upgraded fluids from the formation is adjusted to create a fluid suitable for transportation and/or use in a refinery. The amount of blending may be adjusted so that the fluid has chemical and physical stability. Maintaining the chemical and physical stability of the fluid may allow the fluid to be transported, reduce pre-treatment processes at a refinery and/or reduce or eliminate the need for adjusting the refinery process to compensate for the fluid.

[0888] In certain embodiments, formation conditions (for example, pressure and temperature) and/or fluid production are controlled to produce fluids with selected properties. For example, formation conditions and/or fluid production may be controlled to produce fluids with a selected API gravity and/or a selected viscosity. The selected API gravity and/or selected viscosity may be produced by combining fluids produced at different formation conditions (for example, combining fluids produced at different temperatures during the treatment as described above). As an example, formation conditions and/or fluid production may be controlled to produce fluids with an API gravity of about 19° and a viscosity of about 0.35 Pa·s (350 cp) at 5° C.

[0889] In certain embodiments, a drive process (for example, a steam injection process such as cyclic steam injection, a steam assisted gravity drainage process (SAGD), a solvent injection process, a vapor solvent and SAGD process, or a carbon dioxide injection process) is used to treat the tar sands formation in addition to the in situ heat treatment process. In some embodiments, heaters are used to create high permeability zones (or injection zones) in the formation for the drive process. Heaters may be used to create a mobilization geometry or production network in the formation to allow fluids to flow through the formation during the drive process. For example, heaters may be used to create drainage paths between the heaters and production wells for the drive process. In some embodiments, the heaters are used to provide heat during the drive process. The amount of heat provided by the heaters may be small compared to the heat input from the drive process (for example, the heat input from steam injection).

[0890] The concentration of components in the formation and/or produced fluids may change during an in situ heat treatment process. As the concentration of the components in the formation and/or produced fluids and/or hydrocarbons separated from the produced fluid changes due to formation of the components, solubility of the components in the produced fluids and/or separated hydrocarbons tends to change. Hydrocarbons separated from the produced fluid may be hydrocarbons that have been treated to remove salty water and/or gases from the produced fluid. For example, the produced fluids and/or separated hydrocarbons may contain components that are soluble in the condensable hydrocarbon portion of the produced fluids at the beginning of processing. As properties of the hydrocarbons in the produced fluids change (for example, TAN, asphaltenes, P-value, olefin content, mobilized fluids content, visbroken fluids content, pyrolyzed fluids content, or combinations thereof), the components may tend to become less soluble in the produced fluids and/or in the hydrocarbon stream separated from the produced fluids. In some instances, components in the produced fluids and/or components in the separated hydrocarbons may form two phases and/or become insoluble. Formation of two phases, through flocculation of asphaltenes, change in concentration of components in the produced fluids, change in concentration of components in separated hydrocarbons, and/or precipitation of components may result in hydrocarbons that do not meet pipeline, transportation, and/or refining specifications. Additionally, the efficiency of the process may be reduced. For example, further treatment of the produced fluids and/or separated hydrocarbons may be necessary to produce products with desired properties.

[0891] During processing, the P-value of the separated hydrocarbons may be monitored and the stability of the produced fluids and/or separated hydrocarbons may be assessed. Typically, a P-value that is at most 1.0 indicates that flocculation of asphaltenes from the separated hydrocarbons generally occurs. If the P-value is initially at least 1.0, and such P-value increases or is relatively stable during heating, then this indicates that the separated hydrocarbons are relatively stable. Stability of separated hydrocarbons, as assessed by P-value, may be controlled by controlling operating conditions in the formation such as temperature, pressure, hydrogen uptake, hydrocarbon feed flow, or combinations thereof.

[0892] In some embodiments, change in API gravity may not occur unless the formation temperature is at least 100° C. For some formations, temperatures of at least 220° C. may be required to produce hydrocarbons that meet desired specifications. At increased temperatures coke formation may occur, even at elevated pressures. As the properties of the formation are changed, the P-value of the separated hydrocarbons may decrease below 1.0 and/or sediment may form, causing the separated hydrocarbons to become unstable.

[0893] In some embodiments, olefins may form during heating of formation fluids to produce fluids having a reduced viscosity. Separated hydrocarbons that include olefins may be unacceptable for processing facilities. Olefins in the separated hydrocarbons may cause fouling and/or clogging of processing equipment. For example, separated hydrocarbons that contains olefins may cause coking of distillation units in a refinery, which results in frequent down time to remove the coked material from the distillation units.

[0894] During processing, the olefin content of separated hydrocarbons may be monitored and quality of the separated hydrocarbons assessed. Typically, separated hydrocarbons having a bromine number of 3% and/or a CAPP olefin number of 3% as 1-decene equivalent indicates that olefin production is occurring. If the olefin value decreases or is relatively stable during producing, then this indicates that a minimal or substantially low amount of olefins are being produced. Olefin content, as assessed by bromine value and/or CAPP olefin number, may be controlled by controlling operating conditions in the formation such as temperature, pressure, hydrogen uptake, hydrocarbon feed flow, or combinations thereof.

[0895] In some embodiments, the P-value and/or olefin content may be controlled by controlling operating conditions. For example, if the temperature increases above 225° C. and the P-value drops below 1.0, the separated hydrocarbons may become unstable. Alternatively, the bromine number and/or CAPP olefin number may increase to above 3%. If the temperature is maintained below 225° C., minimal changes to the hydrocarbon properties may occur. In certain embodiments, operating conditions are selected, varied, and/or maintained to produce separated hydrocarbons having a P-value of at least about 1, at least about 1.1, at least about 1.2, or at least about 1.3. In certain embodiments, operating conditions are selected, varied, and/or maintained to produce separated hydrocarbons having a bromine number of at most about 3%, at most about 2.5%, at most about 2%, or at most about 1.5%. Heating of the formation at gcontrolled operating conditions includes operating at temperatures between about 100° C. and about 260° C., between about 150° C. and about 250° C., between about 200° C. and about 240° C., between about 210° C. and about 230° C., or between about 215° C. and about 225° C. Pressures may be between about 1000 kPa and about 15000 kPa, between about 2000 kPa and about 10000 kPa, or between about 2500 kPa and about 5000 kPa or at or near a fracture pressure of the formation. In certain embodiments, the selected pressure of about 10000 kPa produces separated hydrocarbons having properties acceptable for transportation and/or refineries (for example, viscosity, P-value, API gravity, and/or olefin content within acceptable ranges).

[0896] Examples of produced mixture properties that may be measured and used to assess the separated hydrocarbon portion of the produced mixture include, but are not limited to, liquid hydrocarbon properties such as API gravity, viscosity, asphaltene stability (P-value), and olefin content (bromine number and/or CAPP number). In certain embodiments, operating conditions in the formation are selected, varied, and/or maintained to produce an API gravity of at least about 15°, at least about 17°, at least about 19°, at least about 20° in the produced mixture. In certain embodiments, operating conditions in the formation are selected, varied, and/or maintained to produce a viscosity (measured at 1 atm and 5° C.) of at most about 400 cp, at most about 350 cp, at most about 250 cp, or at most about 100 cp in the produced mixture. As an example, the initial viscosity of fluid in the formation is above about 1000 cp or, in some cases, above about 1 million cp. In certain embodiments, operating conditions are selected, varied, and/or maintained to produce an asphaltene stability (P-value) of at least about 1, at least about 1.1, at least about 1.2, or at least about 1.3 in the produced mixture. In certain embodiments, operating conditions are selected, varied, and/or maintained to produce a bromine number of at most about 3%, at most about 2.5%, at most about 2%, or at most about 1.5% in the produced mixture.

[0897] In certain embodiments, the mixture is produced from one or more production wells located at or near the bottom of the hydrocarbon layer being treated. In other embodiments, the mixture is produced from other locations in the hydrocarbon layer being treated (for example, from an upper portion of the layer or a middle portion of the layer).

[0898] In one embodiment, the formation is heated to 220° C. or 230° C. while maintaining the pressure in the formation below 10000 kPa. The separated hydrocarbon portion of the mixture produced from the formation may have several desirable properties such as, but not limited to, an API gravity of at least 19°, a viscosity of at most 350 cp, a P-value of at least 1.1, and a bromine number of at most 2%. Such separated hydrocarbons may be transportable through a pipeline without adding diluent or blending the mixture with another fluid. The mixture may be produced from one or more production wells located at or near the bottom of the hydrocarbon layer being treated.

[0899] The in situ heat treatment process may provide less heat to the formation (for example, use a wider heater spacing) if the in situ heat treatment process is followed by a drive process. The drive process may involve introducing a hot fluid into the formation to increase the amount of heat provided to the formation. In some embodiments, the heaters of the in situ heat treatment process may be used to pretreat the formation to establish injectivity for the subsequent drive process. In some embodiments, the in situ heat treatment process creates or produces the drive fluid in situ. The in situ produced drive fluid may move through the formation and move mobilized hydrocarbons from one portion of the formation to another portion of the formation.

[0900] FIG. 103 depicts a top view representation of an embodiment for preheating using heaters before using the drive process (for example, a steam drive process). Injection wells 602 and production wells 206 are substantially vertical wells. Heaters 412 are long substantially horizontal heaters positioned so that the heaters pass in the vicinity of injection wells 602. Heaters 412 intersect the vertical well patterns slightly displaced from the vertical wells.

[0901] The vertical location of heaters 412 with respect to injection wells 602 and production wells 206 depends on, for example, the vertical permeability of the formation. In formations with at least some vertical permeability, injected steam will rise to the top of the permeable layer in the formation. In such formations, heaters 412 may be located near the bottom of the hydrocarbon layer 388, as shown in FIG. 104. In formations with very low vertical permeabilities, more than one horizontal heater may be used with the heaters stacked substantially vertically or with heaters at varying depths in the hydrocarbon layer (for example, heater patterns as shown in FIGS. 99-102). The vertical spacing between the horizontal heaters in such formations may correspond to the distance between the heaters and the injection wells. Heaters 412 are located in the vicinity of injection wells 602 and/or production wells 206 so that sufficient energy is delivered by the heaters to provide flow rates for the drive process that are economically viable. The spacing between heaters 412 and injection wells 602 or production wells 206 may be varied to provide an economically viable drive process. The amount of preheating may also be varied to provide an economically viable process.

[0902] In some embodiments, the steam injection (or drive) process (for example, SAGD, cyclic steam soak, or another steam recovery process) is used to treat the formation and produce hydrocarbons from the formation. The steam injection process may recover a low amount of oil in place from the formation (for example, less than 20% recovery of oil in place from the formation). The in situ heat treatment process may be used following the steam injection process to increase the recovery of oil in place from the formation. In certain embodiments, the steam injection process is used until the steam injection process is no longer efficient at removing hydrocarbons from the formation (for example, until the steam injection process is no longer economically feasible). The in situ heat treatment process is used to produce hydrocarbons remaining in the formation after the steam injection process. Using the in situ heat treatment process after the steam injection process may allow recovery of at least about 25%, at least about 50%, at least about 55%, or at least about 60% of oil in place in the formation.

[0903] In some embodiments, the formation has been at least somewhat heated by the steam injection process before treating the formation using the in situ heat treatment process. For example, the steam injection process may heat the formation to an average temperature between about 200° C. and about 250° C., between about 175° C. and about 265° C., or between about 150° C. and about 270° C. In certain embodiments, the heaters are placed in the formation after the steam injection process is at least 50% completed, at least 75% completed, or near 100% completed. The heaters provide heat for treating the formation using the in situ heat treatment process. In some embodiments, the heaters are already in place in the formation during the steam injection process. In such embodiments, the heaters may be energized after the steam injection process is completed or when production of hydrocarbons using the steam injection process is reduced below a desired level. In some embodiments, steam injection wells from the steam injection process are converted to heater wells for the in situ heat treatment process.

[0904] Treating the formation with the in situ heat treatment process after the steam injection process may be more efficient than only treating the formation with the in situ heat treatment process. The steam injection process may provide some energy (heat) to the formation with the steam. Any energy added to the formation during the steam injection process reduces the amount of energy needed to be supplied by heaters for the in situ heat treatment process. Reducing the amount of energy supplied by heaters reduces costs for treating the formation using the in situ heat treatment process.

[0905] In certain embodiments, treating the formation using the steam injection process does not treat the formation uniformly. For example, steam injection may not be uniform throughout the formation. Variations in the properties of the formation (for example, fluid injectivities, permeabilities, and/or porosities) may result in non-uniform injection of the steam through the formation. Because of the non-uniform injection of the steam, the steam may remove hydrocarbons from different portions of the formation at different rates or with different results. For example, some portions of the formation may have little or no steam injectivity, which inhibits the hydrocarbon production from these portions. After the steam injection process is completed, the formation may have portions that have lower amounts of hydrocarbons produced (more hydrocarbons remaining) than other parts of the formation.

[0906] FIG. 105 depicts a side view representation of an embodiment of a tar sands formation subsequent to a steam injection process. Injection well 602 is used to inject steam into hydrocarbon layer 388 below overburden 400. Portion 604 may have little or no steam injectivity and have small amounts of hydrocarbons or no hydrocarbons at all removed by the steam injection process. Portions 606 may include portions that have steam injectivity and measurable amounts of hydrocarbons are removed by the steam injection process. Thus, portion 604 may have a greater amount of hydrocarbons remaining than portions 606 following treatment with the steam injection process. In some embodiments, hydrocarbon layer 388 includes two or more portions 604 with more hydrocarbons remaining than portions 606.

[0907] In some embodiments, the portions with more hydrocarbons remaining (such as portion 604, depicted in FIG. 105) are large portions of the formation. In some embodiments, the amount of hydrocarbons remaining in these portions is significantly higher than other portions of the formation (such as portions 606). For example, portions 604 may have a recovery of at most about 10% of the oil in place and portions 606 may have a recovery of at least about 30% of the oil in place. In some embodiments, portions 604 have a recovery of between about 0% and about 10% of the oil in place, between about 0% and about 15% of the oil in place, or between about 0% and about 20% of the oil in place. The portions 606 may have a recovery of between about 20% and about 25% of the oil in place, between about 20% and about 40% of the oil in place, or between about 20% and about 50% of the oil in place. Coring, logging techniques, and/or seismic imaging may be used to assess hydrocarbons remaining in the formation and assess the location of one or more of the first and/or second portions.

[0908] In certain embodiments, during the in situ heat treatment process, more heat is provided to the first portions of the formation that have more hydrocarbons remaining than the second portions with less hydrocarbons remaining. In some embodiments, heaters are located in the first portions but not in the second portions. In some embodiments, heaters are located in both the first portions and the second portions but the heaters in the first portions are designed or operated to provide more heat than the heaters in the second portions. In some embodiments, heaters pass through both first portions and second portions and the heaters are designed or operated to provide more heat in the first portions than the second portions.

[0909] In some embodiments, steam injection is continued during the in situ heat treatment process. For example, steam injection may be continued while liquids are being produced from the formation. The steam injection may increase the production of liquids from the formation. In certain embodiments, steam injection may be reduced or stopped when gas production from the formation begins.

[0910] In some embodiments, the formation is treated using the in situ heat treatment process a significant time after the formation has been treated using the steam injection process. For example, the in situ heat treatment process is used 1 year, 2 years, 3 years, or longer (for example, 10 years to 20 years) after a formation has been treated using the steam injection process. During this dormant period, heat from the steam injection process may diffuse to cooler parts of the formation and result in a more uniform preheating of the formation prior to in situ heat treatment. The in situ heat treatment process may be used on formations that have been left dormant after the steam injection process treatment because further hydrocarbon production using the steam injection process is not possible and/or not economically feasible. In some embodiments, the formation remains at least somewhat heated from the steam injection process even after the significant time.

[0911] In certain embodiments, a fluid is injected into the formation (for example, a drive fluid or an oxidizing fluid) to move hydrocarbons through the formation from a first section to a second section. In some embodiments, the hydrocarbons are moved from the first section to the second section through a third section. FIG. 106 depicts a side view representation of an embodiment using at least three treatment sections in a tar sands formation. Hydrocarbon layer 388 may be divide into three or more treatment sections. In certain embodiments, hydrocarbon layer 388 includes three different types of treatment sections: section 608A, section 608B, and section 608C. Section 608C and sections 608A are separated by sections 608B. Section 608C, sections 608A, and sections 608B may be horizontally displaced from each other in the formation. In some embodiments, one side of section 608C is adjacent to an edge of the treatment area of the formation or an untreated section of the formation is left on one side of section 608C before the same or a different pattern is formed on the opposite side of the untreated section.

[0912] In certain embodiments, sections 608A and 608C are heated at or near the same time to similar temperatures (for example, pyrolysis temperatures). Sections 608A and 608C may be heated to mobilize and/or pyrolyze hydrocarbons in the sections. The mobilized and/or pyrolyzed hydrocarbons may be produced (for example, through one or more production wells) from section 608A and/or section 608C. Section 608B may be heated to lower temperatures (for example, mobilization temperatures). Little or no production of hydrocarbons to the surface may take place through section 608B. For example, sections 608A and 608C may be heated to average temperatures of about 300° C. while section 608B is heated to an average temperature of about 100° C. and no production wells are operated in section 608B.

[0913] In certain embodiments, heating and producing hydrocarbons from section 608C creates fluid injectivity in the section. After fluid injectivity has been created in section 608C, a fluid such as a drive fluid (for example, steam, water, or hydrocarbons) and/or an oxidizing fluid (for example, air, oxygen, enriched air, or other oxidants) may be injected into the section. The fluid may be injected through heaters 412, a production well, and/or an injection well located in section 608C. In some embodiments, heaters 412 continue to provide heat while the fluid is being injected. In other embodiments, heaters 412 may be turned down or off before or during fluid injection.

[0914] In some embodiments, providing oxidizing fluid such as air to section 608C causes oxidation of hydrocarbons in the section. For example, coked hydrocarbons and/or heated hydrocarbons in section 608C may oxidize if the temperature of the hydrocarbons is above an oxidation ignition temperature. In some embodiments, treatment of section 608C with the heaters creates coked hydrocarbons with substantially uniform porosity and/or substantially uniform injectivity so that heating of the section is controllable when oxidizing fluid is introduced to the section. The oxidation of hydrocarbons in section 608C will maintain the average temperature of the section or increase the average temperature of the section to higher temperatures (for example, about 400° C. or above).

[0915] In some embodiments, injection of the oxidizing fluid is used to heat section 608C and a second fluid is introduced into the formation after or with the oxidizing fluid to create drive fluids in the section. During injection of oxidant, excess oxidant and/or oxidation products may be removed from section 608C through one or more production wells. After the formation is raised to a desired temperature, a second fluid may be introduced into section 608C to react with coke and/or hydrocarbons and generate drive fluid (for example, synthesis gas). In some embodiments, the second fluid includes water and/or steam. Reactions of the second fluid with carbon in the formation may be endothermic reactions that cool the formation. In some embodiments, oxidizing fluid is added with the second fluid so that some heating of section 608C occurs simultaneous with the endothermic reactions. In some embodiments, section 608C may be treated in alternating steps of adding oxidant to heat the formation, and then adding second fluid to generate drive fluids.

[0916] The generated drive fluids in section 608C may include steam, carbon dioxide, carbon monoxide, hydrogen, methane, and/or pyrolyzed hydrocarbons. The high temperature in section 608C and the generation of drive fluid in the section may increase the pressure of the section so the drive fluids move out of the section into adjacent sections. The increased temperature of section 608C may also provide heat to section 608B through conductive heat transfer and/or convective heat transfer from fluid flow (for example, hydrocarbons and/or drive fluid) to section 608B.

[0917] In some embodiments, hydrocarbons (for example, hydrocarbons produced from section 608C) are provided as a portion of the drive fluid. The injected hydrocarbons may include at least some pyrolyzed hydrocarbons such as pyrolyzed hydrocarbons produced from section 608C. In some embodiments, steam or water are provided as a portion of the drive fluid. Steam or water in the drive fluid may be used to control temperatures in the formation. For example, steam or water may be used to keep temperatures lower in the formation. In some embodiments, water injected as the drive fluid is turned into steam in the formation due to the higher temperatures in the formation. The conversion of water to steam may be used to reduce temperatures or maintain lower temperatures in the formation.

[0918] Fluids injected in section 608C may flow towards section 608B, as shown by the arrows in FIG. 106. Fluid movement through the formation transfers heat convectively through hydrocarbon layer 388 into sections 608B and/or 608A. In addition, some heat may transfer conductively through the hydrocarbon layer between the sections.

[0919] Low level heating of section 608B mobilizes hydrocarbons in the section. The mobilized hydrocarbons in section 608B may be moved by the injected fluid through the section towards section 608A, as shown by the arrows in FIG. 106. Thus, the injected fluid is pushing hydrocarbons from section 608C through section 608B to section 608A. Mobilized hydrocarbons may be upgraded in section 608A due to the higher temperatures in the section. Pyrolyzed hydrocarbons that move into section 608A may also be further upgraded in the section. The upgraded hydrocarbons may be produced through production wells located in section 608A.

[0920] In certain embodiments, at least some hydrocarbons in section 608B are mobilized and drained from the section prior to injecting the fluid into the formation. Some formations may have high oil saturation (for example, the Grosmont formation has high oil saturation). The high oil saturation corresponds to low gas permeability in the formation that may inhibit fluid flow through the formation. Thus, mobilizing and draining (removing) some oil (hydrocarbons) from the formation may create gas permeability for the injected fluids.

[0921] Fluids in hydrocarbon layer 388 may preferentially move horizontally within the hydrocarbon layer from the point of injection because tar sands tend to have a larger horizontal permeability than vertical permeability. The higher horizontal permeability allows the injected fluid to move hydrocarbons between sections preferentially versus fluids draining vertically due to gravity in the formation. Providing sufficient fluid pressure with the injected fluid may ensure that fluids are moved to section 608A for upgrading and/or production.

[0922] In certain embodiments, section 608B has a larger volume than section 608A and/or section 608C. Section 608B may be larger in volume than the other sections so that more hydrocarbons are produced for less energy input into the formation. Because less heat is provided to section 608B (the section is heated to lower temperatures), having a larger volume in section 608B reduces the total energy input to the formation per unit volume. The desired volume of section 608B may depend on factors such as, but not limited to, viscosity, oil saturation, and permeability. In addition, the degree of coking is much less in section 608B due to the lower temperature so less hydrocarbons are coked in the formation when section 608B has a larger volume. In some embodiments, the lower degree of heating in section 608B allows for cheaper capital costs as lower temperature materials (cheaper materials) may be used for heaters used in section 608B.

[0923] Certain types of formations have low initial permeabilities and high initial viscosities that inhibit these formations from being easily treated using conventional steam drive processes such as SAGD or CSS. For example, carbonate formations (such as the Grosmont reservoir in Alberta, Canada) have low permeabilities and high viscosities that make these formations unsuitable for conventional steam drive processes. Carbonate formations may also be highly heterogenous (for example, have highly different vertical and horizontal permeabilities), which makes it difficult to control flow of fluids (such as steam) through the formation. In addition, some carbonate formations are relatively shallow formations with low overburden fracture pressures that inhibit the use of high pressure steam injection because of the need to avoid breaking or fracturing the overburden.

[0924] In certain embodiments, formations with the above properties (such as the Grosmont reservoir or other carbonate formations) are treated using a combination of heating from heaters and steam drive processes. FIG. 107 depicts an embodiment for treating a formation with heaters in combination with one or more steam drive processes. Heater 412A is located in hydrocarbon containing layer 388 between injection well 602 and production well 206. Injection well 602 and/or production well 206 may be used to inject steam and produce hydrocarbons in a steam drive process, such as a SAGD (steam assisted gravity drainage) process. In certain embodiments, heater 412A is located substantially horizontally in layer 388. In some embodiments, injection well 602 and/or production well 206 are located substantially horizontally in layer 388.

[0925] In certain embodiments, heater 412A is located approximately vertically equidistant between injection well 602 and production well 206 (the heater is at or near the midpoint between the injection well and the production well). Heater 412A may provide heat to a portion of layer 388 surrounding the heater and proximate injection well 602 and production well 206. In some embodiments, heater 412A is an electric heater such as an insulated conductor heater or a conductor-in-conduit heater. In certain embodiments, heat provided by heater 412A increases the steam injectivity in the portion surrounding the heater. In certain embodiments, heater 412A provides heat at high heat injection rates such as those used for the in situ heat treatment process (for example, heat injection rates of at least about 1000 W/m).

[0926] As shown in FIG. 107, in certain embodiments, heater 412B is located below injection/production well 610. In certain embodiments, heater 412B is located substantially horizontally in layer 388. In some embodiments, injection/production well 610 is located substantially horizontally in layer 388. In some embodiments, injection/production well 610 is located substantially vertically in layer 388. In some embodiments, injection/production well 610 includes multiple wells located substantially vertically in layer 388.

[0927] In certain embodiments, injection/production well 610 is at least partially offset from heater 412B. Injection/production well 610 may be used to inject steam and produce hydrocarbons in a cyclic steam drive process, such as a CSS (cyclic steam injection) process. Heater 412B may provide heat to a portion of layer 388 surrounding the heater and proximate injection/production well 610. In some embodiments, heater 412B is an electric heater such as an insulated conductor heater or a conductor-in-conduit heater. In certain embodiments, heat provided by heater 412B increases the steam injectivity in the portion surrounding the heater. In certain embodiments, heater 412B provides heat at high heat injection rates such as those used for the in situ heat treatment process (for example, heat injection rates of at least about 1000 W/m).

[0928] In certain embodiments, layer 388 has different initial vertical and horizontal permeabilities (the initial permeability is heterogenous). In one embodiment, the initial vertical permeability in layer 388 is at most about 300 millidarcy and the initial horizontal permeability is at most about 1 darcy. Typically in carbonate formations, the initial vertical permeability is less than the initial horizontal permeability such as, for example, in the Grosmont reservoir in Alberta, Canada. The initial vertical and initial horizontal permeabilities may vary depending on the location in the formation and/or the type of formation. In one embodiment, layer 388 has an initial viscosity of at least about 1×10⁶ centipoise (cp). The initial viscosity may vary depending on the location or depth in the formation and/or the type of formation.

[0929] Typically, these initial permeabilities and initial viscosities are not favorable for steam injection into layer 388 because the steam injection pressure needed to get steam to move hydrocarbons through the formation is above the fracture pressure of overburden 400. Staying below the overburden fracture pressure may be especially difficult for shallower formations such as the Grosmont reservoir because the overburden fracture pressure is relatively small in such shallower formations. In certain embodiments, heater 412A and/or heater 412B are used to provide heat to layer 388 to increase the permeability and reduce the viscosity in the portion surrounding the heater such that steam injected into the layer at pressures below the overburden fracture pressure can move hydrocarbons in the layer. Thus, providing heat to the layer increases the steam injectivity in the layer.

[0930] In certain embodiments, a selected amount of heat, or selected amount of heating time, is provided from heater 412A and/or heater 412B to increase the permeability and reduce the viscosity in layer 388 before steam injection through injection well 602 or injection/production well 610 begins. In some embodiments, a simulation of reservoir conditions is used to assess or determine the selected amount of heat, or heating time, needed before steam injection into layer 388. For example, the selected amount of heating time for heater 412A may be about 1 year for layer 388 to have permeabilities and viscosities suitable for steam injection (sufficient steam injectivity is created in the layer) through injection well 602. The selected amount of heating time for heater 412B may be about 1 year for layer 388 to have permeabilities and viscosities suitable for steam injection (sufficient steam injectivity is created in the layer) through injection/production well 610.

[0931] In certain embodiments, heater 412A is turned off before steam injection begins. In other embodiments, heater 412A is turned off after steam injection begins. In some embodiments, heater 412A is turned off a selected amount of time after steam injection begins. The time the heater is turned off may be selected to provide, for example, desired properties in the hydrocarbons produced from the formation.

[0932] In certain embodiments, heater 412B remains on for a selected amount of time after steam injection/hydrocarbon production through injection/production well 610 begins. Heater 412B may remain on to maintain steam injectivity in the portion surrounding the heater and injection/production well 610. In some embodiments, heat provided from heater 412B increases the size of the portion with increased steam injectivity. After a period of time, heat provided from heater 412B may create steam injection interconnectivity between injection/production well 610 and production well 206. After interconnectivity between injection/production well 610 and production well 206 is achieved, heater 412B may be turned off.

[0933] Interconnectivity between injection/production well 610 and production well 206 allows steam injection from the injection/production well to move hydrocarbons to the production well. This hydrocarbon movement may increase the efficiency of steam injection and hydrocarbon production from the layer. The interconnectivity may also allow less injection wells and/or production wells to be used in treating the layer.

[0934] In certain embodiments, heating from heater 412A and/or heater 412B is controlled and/or turned off at a time to inhibit coke formation in the layer. Simulation of reservoir conditions may be used to determine when/if the onset of coking may occur in the layer. Additionally, steam injection into the formation may assist in inhibiting coke formation in the layer.

[0935] In certain embodiments, steam is injected through injection well 602 at or about the same pressure as steam is injected through injection/production well 610. In certain embodiments, steam is injected through injection well 602 and/or injection/production well 610 at a pressure that is above the formation fracturing pressure but below the overburden fracture pressure. Injecting steam above the formation fracturing pressure may increase the permeability and/or move steam or hydrocarbons through the formation at higher rates. Thus, injecting steam above the formation fracturing pressure may increase the rate of hydrocarbon production through production well 206 and/or injection/production well 610. Injecting steam below the overburden fracture pressure inhibits the steam from fracturing the overburden and allowing formation fluids to escape to the surface through the overburden (for example, maintains the integrity of the overburden).

[0936] In some embodiments, a pattern for treating a formation includes a repeating pattern of heaters 412A, 412B, injection well 602, production well 206, and injection/production well 610, as shown in FIG. 107. The pattern may be repeated horizontally and/or vertically in the formation. Using the repeating pattern to treat the formation may reduce the number of wells needed to treat the formation as compared to using typical steam drive processes or in situ heat treatment processes individually. In some embodiments, heaters 412A, 412B may be removed and reused in another portion of the formation, or another formation, after the heaters are turned off. The heaters may be allowed to cool down before being removed from the formation.

[0937] Using the embodiment depicted in FIG. 107 to treat the formation (for example, the Grosmont reservoir) may increase oil production and/or decrease the amount of steam needed for oil production as compared to using the SAGD process only. FIG. 108 depicts a comparison treating the formation using the embodiment depicted in FIG. 107 and treating the formation using the SAGD process. Cumulative oil production, cumulative steam-oil ratio, and top pressure for the formation are compared using the two techniques. Plot 612 depicts cumulative oil production for the embodiment depicted in FIG. 107. Plot 614 depicts cumulative oil production for the SAGD process. Plot 616 depicts cumulative steam-oil ratio for the embodiment depicted in FIG. 107. Plot 618 depicts cumulative steam-oil ratio for the SAGD process. Plot 620 depicts top pressure for the embodiment depicted in FIG. 107. Plot 622 depicts top pressure for the SAGD process. As shown in FIG. 108, cumulative oil production is significantly increased for the embodiment depicted in FIG. 107 while the steam-oil ratio is slightly decreased and the top pressure is substantially the same. Thus, the embodiment depicted in FIG. 107 is more efficient in producing oil than the SAGD process.

[0938] In some embodiments, karsted formations or karsted layers in formations have vugs in one or more layers of the formations. The vugs may be filled with viscous fluids such as bitumen or heavy oil. In some embodiments, the karsted layers have a porosity of at least about 20 porosity units, at least about 30 porosity units, or at least about 35 porosity units. The karsted formation may have a porosity of at most about 15 porosity units, at most about 10 porosity units, or at most about 5 porosity units. Vugs filled with viscous fluids may inhibit steam or other fluids from being injected into the formation or the layers. In certain embodiments, the karsted formation or karsted layers of the formation are treated using the in situ heat treatment process.

[0939] Heating of these formations or layers may decrease the viscosity of the viscous fluids in the vugs and allow the fluids to drain (for example, mobilize the fluids). Formations with karsted layers may have sufficient permeability so that when the viscosity of fluids (hydrocarbons) in the formation is reduced, the fluids drain and/or move through the formation relatively easily (for example, without a need for creating higher permeability in the formation).

[0940] In some embodiments, the relative amount (the degree) of karst in the formation is assessed using techniques known in the art (for example, 3D seismic imaging of the formation). The assessment may give a profile of the formation showing layers or portions with varying amounts of karst in the formation. In certain embodiments, more heat is provided to selected karsted portions of the formation than other karsted portions of the formation. In some embodiments, selective amounts of heat are provided to portions of the formation as a function of the degree of karst in the portions. Amounts of heat may be provided by varying the number and/or density of heaters in the portions with varying degrees of karst.

[0941] In certain embodiments, the hydrocarbon fluids in karsted portions have higher viscosities than hydrocarbons in other non-karsted portions of the formation. Thus, more heat may be provided to the karsted portions to reduce the viscosity of the hydrocarbons in the karsted portions.

[0942] In certain embodiments, only the karsted layers of the formation are treated using the in situ heat treatment process. Other non-karsted layers of the formation may be used as seals for the in situ heat treatment process. For example, karsted layers with different quantities of hydrocarbons in the layers may be treated while other layers are used as natural seals for the treatment process. In some embodiments, karsted layers with low quantities of hydrocarbons as compared to the other karsted and/or non-karsted layers are used as seals for the treatment process. The quantity of hydrocarbons in the Karsted layer may be determined using logging methods and/or Dean Stark distillation methods. The quantity of hydrocarbons may be reported as a volume percent of hydrocarbons per volume percent of rock, or as volume of hydrocarbons per mass of rock.

[0943] In some embodiments, karsted layers with fewer hydrocarbons are treated along with karsted layers with more hydrocarbons. In some embodiments, karsted layers with fewer hydrocarbons are above and below a karsted layer with more hydrocarbons (the middle karsted layer). Less heat may be provided to the upper and lower karsted layers than the middle karsted layer. Less heat may be provided in the upper and lower karsted layers by having greater heat spacing and/or less heaters in the upper and lower karsted layers as compared to the middle karsted layer. In some embodiments, less heating of the upper and lower karsted layers includes heating the layers to mobilization and/or visbreaking temperatures, but not to pyrolysis temperatures. In some embodiments, the upper and/or lower karsted layers are heated with heaters and the residual heat from the upper and/or lower layers transfers to the middle layer.

[0944] One or more production wells may be located in the middle karsted layer. Mobilized and/or visbroken hydrocarbons from the upper karsted layer may drain to the production wells in the middle karsted layer. Heat provided to the lower karsted layer may create a thermal expansion drive and/or a gas pressure drive in the lower karsted layer. The thermal expansion and/or gas pressure may drive fluids from the lower karsted layer to the middle karsted layer. These fluids may be produced through the production wells in the middle karsted layer. Providing some heat to the upper and lower karsted layers may increase the total recovery of fluids from the formation by, for example, 25% or more.

[0945] In some embodiments, the karsted layers with fewer hydrocarbons are further heated to pyrolysis temperatures after production from the karsted layer with more hydrocarbons is completed or almost completed. The karsted layers with fewer hydrocarbons may also be further treated by producing fluids through production wells located in the layers.

[0946] In some embodiments, a drive process, a solvent injection process and/or a pressurizing fluid process is used after the in situ heat treatment of the karsted formation or karsted layers. A drive process may include injection of a drive fluid such as steam. A drive process includes, but is not limited to, a steam injection process such as cyclic steam injection, a steam assisted gravity drainage process (SAGD), and a vapor solvent and SAGD process. A drive process may drive fluids from one portion of the formation towards a production well.

[0947] A solvent injection process may include injection of a solvating fluid. A solvating fluid includes, but is not limited to, water, emulsified water, hydrocarbons, surfactants, alkaline water solutions (for example, sodium carbonate solutions), caustic, polymers, carbon disulfide, carbon dioxide, or mixtures thereof. The solvation fluid may mix with, solvate and/or dilute the hydrocarbons to form a mixture of condensable hydrocarbons and solvation fluids. The mixture may have a reduced viscosity as compared to the initial viscosity of the fluids in the formation. The mixture may flow and/or be mobilized towards production wells in the formation.

[0948] A pressurizing process may include moving hydrocarbons in the formation by injection of a pressurized fluid. The pressurizing fluid may include, but is not limited to, carbon dioxide, nitrogen, steam, methane, and/or mixtures thereof.

[0949] In some embodiments, the drive process (for example, the steam injection process) is used to mobilize fluids before the in situ heat treatment process. Steam injection may be used to get hydrocarbons (oil) away from rock or other strata in the formation. The steam injection may mobilize the hydrocarbons without significantly heating the rock.

[0950] In some embodiments, fluid injected in the formation (for example, steam and/or carbon dioxide) may absorb heat from the formation and cool the formation depending on the pressure in the formation and the temperature of the injected fluid. In some embodiments, the injected fluid is used to recover heat from the formation. The recovered heat may be used in surface processing fluids and/or to preheat other portions of the formation using the drive process.

[0951] In some embodiments, heaters are used to preheat the karsted formation or karsted layers to create injectivity in the formation. In situ heat treatment of karsted formations and/or karsted layers may allow for drive fluid injection, solvent injection and/or pressurizing fluid injection where it was previously unfavorable or unmanageable. Typically, karsted formations were unfavorable for drive processes because channeling of the fluid injected in the formation inhibited pressure build-up in the formation. In situ heat treatment of karsted formations may allow for injection of a drive fluid, a solvent and/or a pressurizing fluid by reducing the viscosity of hydrocarbons in the formation and allowing pressure to build in the formations without significant bypass of the fluid through channels in the formations. For example, heating a section of the formation using in situ heat treatment may heat and mobilize heavy hydrocarbons (bitumen) by reducing the viscosity of the heavy hydrocarbons in the karsted layer. Some of the heated less viscous heavy hydrocarbons may flow from the karsted layer into other portions of the formation that are cooler than the heated karsted portion. The heated less viscous heavy hydrocarbons may flow through channels and/or fractures. The heated heavy hydrocarbons may cool and solidify in the channels, thus creating a temporary seal for the drive fluid, solvent, and/or pressurizing fluid.

[0952] In certain embodiments, the karsted formation or karsted layers are heated to temperatures below the decomposition temperature of minerals in the formation (for example, rock minerals such as dolomite and/or clay minerals such as kaolinite, illite, or smectite). In some embodiments, the karsted formation or karsted layers are heated to temperatures of at most 400° C., at most 450° C., or at most 500° C. (for example, to a temperature below a dolomite decomposition temperature at formation pressure). In some embodiments, the karsted formation or karsted layers are heated to temperatures below a decomposition temperature of clay minerals (such as kaolinite) at formation pressure.

[0953] In some embodiments, heat is preferentially provided to portions of the formation with low weight percentages of clay minerals (for example, kaolinite) as compared to the content of clay in other portions of the formation. For example, more heat may be provided to portions of the formation with at most 1% by weight clay minerals, at most 2% by weight clay minerals, or at most 3% by weight clay minerals than portions of the formation with higher weight percentages of clay minerals. In some embodiments, the rock and/or clay mineral distribution is assessed in the formation prior to designing a heater pattern and installing the heaters. The heaters may be arranged to preferentially provide heat to the portions of the formation that have been assessed to have lower weight percentages of clay minerals as compared to other portions of the formation. In certain embodiments, the heaters are placed substantially horizontally in layers with low weight percentages of clay minerals.

[0954] Providing heat to portions of the formation with low weight percentages of clay minerals may minimize changes in the chemical structure of the clays. For example, heating clays to high temperatures may drive water from the clays and change the structure of the clays. The change in structure of the clay may adversely affect the porosity and/or permeability of the formation. If the clays are heated in the presence of air, the clays may oxidize and the porosity and/or permeability of the formation may be adversely affected. Portions of the formation with a high weight percentage of clay minerals may be inhibited from reaching temperatures above temperatures that effect the chemical composition of the clay minerals at formation pressures. For example, portions of the formation with large amounts of kaolinite relative to other portions of the formation may be inhibited from reaching temperatures above 240° C. In some embodiments, portions of the formation with a high quantity of clay minerals relative to other portions of the formation may be inhibited from reaching temperatures above 200° C., above 220° C., above 240° C., or above 300° C.

[0955] In some embodiments, karsted formations may include water. Minerals (for example, carbonate minerals) in the formation may at least partially dissociate in the water to form carbonic acid. The concentration of carbonic acid in the water may be sufficient to make the water acidic. At pressure greater than ambient formation pressures, dissolution of minerals in the water may be enhanced, thus formation of acidic water is enhanced. Acidic water may react with other minerals in the formation such as dolomite (MgCa(CO₃)₂) and increase the solubility of the minerals. Water at lower pressures, or non-acidic water, may not solubilize the minerals in the formation. Dissolution of the minerals in the formation may form fractures in the formation. Thus, controlling the pressure and/or the acidity of water in the formation may control the solubilization of minerals in the formation. In some embodiments, other inorganic acids in the formation enhance the solubilization of minerals such as dolomite.

[0956] In some embodiments, the karsted formation or karsted layers are heated to temperatures above the decomposition temperature of minerals in the formation. At temperatures above the minerals decomposition temperature, the minerals may decompose to produce carbon dioxide or other products. The decomposition of the minerals and the carbon dioxide production may create permeability in the formation and mobilize viscous fluids in the formation. In some embodiments, the produced carbon dioxide is maintained in the formation to generate a gas cap in the formation. The carbon dioxide may be allowed to rise to the upper portions of the karsted layers to generate the gas cap.

[0957] In some embodiments, the production front of the drive process follows behind the heat front of the in situ heat treatment process. In some embodiments, areas behind the production front are further heated to produce more fluids from the formation. Further heating behind the production front may also maintain the gas cap behind the production front and/or maintain quality in the production front of the drive process.

[0958] In certain embodiments, the drive process is used before the in situ heat treatment of the formation. In some embodiments, the drive process is used to mobilize fluids in a first section of the formation. The mobilized fluids may then be pushed into a second section by heating the first section with heaters. Fluids may be produced from the second section. In some embodiments, the fluids in the second section are pyrolyzed and/or upgraded using the heaters.

[0959] In formations with low permeabilities, the drive process may be used to create a “gas cushion” or pressure sink before the in situ heat treatment process. The gas cushion may inhibit pressures from increasing quickly to fracture pressure during the in situ heat treatment process. The gas cushion may provide a path for gases to escape or travel during early stages of heating during the in situ heat treatment process.

[0960] In some embodiments, the drive process (for example, the steam injection process) is used to mobilize fluids before the in situ heat treatment process. Steam injection may be used to get hydrocarbons (oil) away from rock or other strata in the formation. The steam injection may mobilize the oil without significantly heating the rock.

[0961] In some embodiments, injection of a fluid (for example, steam or carbon dioxide) may consume heat in the formation and cool the formation depending on the pressure in the formation. In some embodiments, the injected fluid is used to recover heat from the formation. The recovered heat may be used in surface processing fluids and/or to preheat other portions of the formation using the drive process.

[0962] FIG. 109 depicts an embodiment for heating and producing from the formation with the temperature limited heater in a production wellbore. Production conduit 624 is located in wellbore 490. In certain embodiments, a portion of wellbore 490 is located substantially horizontally in formation 492. In some embodiments, the wellbore is located substantially vertically in the formation. In an embodiment, at least a portion of wellbore 490 is an open wellbore (an uncased wellbore). In some embodiments, the wellbore has a casing or liner with perforations or openings to allow fluid to flow into the wellbore.

[0963] Conduit 624 may be made from carbon steel or more corrosion resistant materials such as stainless steel. Conduit 624 may include apparatus and mechanisms for gas lifting or pumping produced oil to the surface. For example, conduit 624 includes gas lift valves used in a gas lift process. Examples of gas lift control systems and valves are disclosed in U.S. Pat. No. 6,715,550 to Vinegar et al. and U.S. Pat. No. 7,259,688 to Hirsch et al., and U.S. Patent Application Publication No. 2002-0036085 to Bass et al., each of which is incorporated by reference as if fully set forth herein. Conduit 624 may include one or more openings (perforations) to allow fluid to flow into the production conduit. In certain embodiments, the openings in conduit 624 are in a portion of the conduit that remains below the liquid level in wellbore 490. For example, the openings are in a horizontal portion of conduit 624.

[0964] Heater 412 is located in conduit 624. In some embodiments, heater 412 is located outside conduit 624, as shown in FIG. 110. The heater located outside the production conduit may be coupled (strapped) to the production conduit. In some embodiments, more than one heater (for example, two, three, or four heaters) are placed about conduit 624. The use of more than one heater may reduce bowing or flexing of the production conduit caused by heating on only one side of the production conduit. In an embodiment, heater 412 is a temperature limited heater. Heater 412 provides heat to reduce the viscosity of fluid (such as oil or hydrocarbons) in and near wellbore 490. In certain embodiments, heater 412 raises the temperature of the fluid in wellbore 490 up to a temperature of 250° C. or less (for example, 225° C., 200° C., or 150° C.). Heater 412 may be at higher temperatures (for example, 275° C., 300° C., or 325° C.) because the heater provides heat to conduit 624 and there is some temperature differential between the heater and the conduit. Thus, heat produced from the heater does not raise the temperature of fluids in the wellbore above 250° C.

[0965] In certain embodiments, heater 412 includes ferromagnetic materials such as Carpenter Temperature Compensator “32”, Alloy 42-6, Alloy 52, Invar 36, or other iron-nickel or iron-nickel-chromium alloys. In certain embodiments, nickel or nickel-chromium alloys are used in heater 412. In some embodiments, heater 412 includes a composite conductor with a more highly conductive material such as copper on the inside of the heater to improve the turndown ratio of the heater. Heat from heater 412 heats fluids in or near wellbore 490 to reduce the viscosity of the fluids and increase a production rate through conduit 624.

[0966] In certain embodiments, portions of heater 412 above the liquid level in wellbore 490 (such as the vertical portion of the wellbore depicted in FIGS. 109 and 110) have a lower maximum temperature than portions of the heater located below the liquid level. For example, portions of heater 412 above the liquid level in wellbore 490 may have a maximum temperature of 100° C. while portions of the heater located below the liquid level have a maximum temperature of 250° C. In certain embodiments, such a heater includes two or more ferromagnetic sections with different Curie temperatures and/or phase transformation temperature ranges to achieve the desired heating pattern. Providing less heat to portions of wellbore 490 above the liquid level and closer to the surface may save energy.

[0967] In certain embodiments, heater 412 is electrically isolated on the outside surface of the heater and allowed to move freely in conduit 624. In some embodiments, electrically insulating centralizers are placed on the outside of heater 412 to maintain a gap between conduit 624 and the heater.

[0968] In some embodiments, heater 412 is cycled (turned on and off) so that fluids produced through conduit 624 are not overheated. In an embodiment, heater 412 is turned on for a specified amount of time until a temperature of fluids in or near wellbore 490 reaches a desired temperature (for example, the maximum temperature of the heater). During the heating time (for example, 10 days, 20 days, or 30 days), production through conduit 624 may be stopped to allow fluids in the formation to “soak” and obtain a reduced viscosity. After heating is turned off or reduced, production through conduit 624 is started and fluids from the formation are produced without excess heat being provided to the fluids. During production, fluids in or near wellbore 490 will cool down without heat from heater 412 being provided. When the fluids reach a temperature at which production significantly slows down, production is stopped and heater 412 is turned back on to reheat the fluids. This process may be repeated until a desired amount of production is reached. In some embodiments, some heat at a lower temperature is provided to maintain a flow of the produced fluids. For example, low temperature heat (for example, 100° C., 125° C., or 150° C.) may be provided in the upper portions of wellbore 490 to keep fluids from cooling to a lower temperature.

[0969] In some embodiments, a temperature limited heater positioned in a wellbore heats steam that is provided to the wellbore. The heated steam may be introduced into a portion of the formation. In certain embodiments, the heated steam may be used as a heat transfer fluid to heat a portion of the formation. In some embodiments, the steam is used to solution mine desired minerals from the formation. In some embodiments, the temperature limited heater positioned in the wellbore heats liquid water that is introduced into a portion of the formation.

[0970] In an embodiment, the temperature limited heater includes ferromagnetic material with a selected Curie temperature and/or a selected phase transformation temperature range. The use of a temperature limited heater may inhibit a temperature of the heater from increasing beyond a maximum selected temperature (for example, a temperature at or about the Curie temperature and/or the phase transformation temperature range). Limiting the temperature of the heater may inhibit potential burnout of the heater. The maximum selected temperature may be a temperature selected to heat the steam to above or near 100% saturation conditions, superheated conditions, or supercritical conditions. Using a temperature limited heater to heat the steam may inhibit overheating of the steam in the wellbore. Steam introduced into a formation may be used for synthesis gas production, to heat the hydrocarbon containing formation, to carry chemicals into the formation, to extract chemicals or minerals from the formation, and/or to control heating of the formation.

[0971] A portion of the formation where steam is introduced or that is heated with steam may be at significant depths below the surface (for example, greater than about 1000 m, about 2500 m, or about 5000 m below the surface). If steam is heated at the surface of the formation and introduced to the formation through a wellbore, a quality of the heated steam provided to the wellbore at the surface may have to be relatively high to accommodate heat losses to the wellbore casing and/or the overburden as the steam travels down the wellbore. Heating the steam in the wellbore may allow the quality of the steam to be significantly improved before the steam is provided to the formation. A temperature limited heater positioned in a lower section of the overburden and/or adjacent to a target zone of the formation may be used to controllably heat steam to improve the quality of the steam injected into the formation and/or inhibit condensation along the length of the heater. In certain embodiments, the temperature limited heater improves the quality of the steam injected and/or inhibits condensation in the wellbore for long steam injection wellbores (especially for long horizontal steam injection wellbores).

[0972] A temperature limited heater positioned in a wellbore may be used to heat the steam to above or near 100% saturation conditions or superheated conditions. In some embodiments, a temperature limited heater may heat the steam so that the steam is above or near supercritical conditions. The static head of fluid above the temperature limited heater may facilitate producing 100% saturation, superheated, and/or supercritical conditions in the steam. Supercritical or near supercritical steam may be used to strip hydrocarbon material and/or other materials from the formation. In certain embodiments, steam introduced into the formation may have a high density (for example, a specific gravity of about 0.8 or above). Increasing the density of the steam may improve the ability of the steam to strip hydrocarbon material and/or other materials from the formation.

[0973] In some embodiments, the tar sands formation may be treated by the in situ heat treatment process to produce pyrolyzed product from the formation. A significant amount of carbon in the form of coke may remain in tar sands formation when production of pyrolysis product from the formation is complete. In some embodiments, the coke in the formation may be utilized to produce heat and/or additional products from the heated coke containing portions of the formation.

[0974] In some embodiments, air, oxygen enriched air, and/or other oxidants may be introduced into the treatment area that has been pyrolyzed to react with the coke in the treatment area. The temperature of the treatment area may be sufficiently hot to support burning of the coke without additional energy input from heaters. The oxidation of the coke may significantly heat the portion of the formation. Some of the heat may transfer to portions of the formation adjacent to the treatment area. The transferred heat may mobilize fluids in portions of the formation adjacent to the treatment area. The mobilized fluids may flow into and be produced from production wells near the perimeter of the treatment area.

[0975] Gases produced from the formation heated by combusting coke in the formation may be at high temperature. The hot gases may be utilized in an energy recovery cycle (for example, a Kalina cycle or a Rankine cycle) to produce electricity.

[0976] The air, oxygen enriched air and/or other oxidants may be introduced into the formation for a sufficiently long period of time to heat a portion of the treatment area to a desired temperature sufficient to allow for the production of synthesis gas of a desired composition. The temperature may be from 500° C. to about 1000° C. or higher. When the temperature of the portion is at or near the desired temperature, a synthesis gas generating fluid, such as water, may be introduced into the formation to result in the formation of synthesis gas. Synthesis gas produced from the formation may be sent to a treatment facility and/or be sent through a pipeline to a desired location. During introduction of the synthesis gas generating fluid, the introduction of air, oxygen enriched air, and/or other oxidants may be stopped, reduced, or maintained. If the temperature of the formation reduces so that the synthesis gas produced from the formation does not have the desired composition, introduction of the syntheses gas generating fluid may be stopped or reduced, and the introduction of air, enriched air and/or other oxidants may be started or increased so that oxidation of coke in the formation reheats portions of the treatment area. The introduction of oxidant to heat the formation and the introduction of synthesis gas generating fluid to produce synthesis gas may be cycled until all or a significant portion of the treatment area is treated.

[0977] In certain embodiments, a subsurface formation is treated in stages. The treatment may be initiated with electrical heating with further heating generated from oxidation of hydrocarbons and hot gas production from the formation. Hydrocarbons (e.g., heavy hydrocarbons and/or bitumen) may be moved from one portion of the formation to another where the hydrocarbons are produced from the formation. By using a combination of heaters, oxidizing fluid and/or drive fluid, the overall time necessary to initiate production from a formation may be decreased relative to times necessary to initiate production using heaters and/or drive processes alone. By controlling a rate of oxidizing fluid injection and/or drive fluid injection in conjunction with heating with heaters, a relatively uniform temperature distribution may be obtained in sections (portions) of the subsurface formation.

[0978] A method for treating a hydrocarbon containing formation with heaters in combination with an oxidizing fluid may include providing heat to a first portion of the formation from a plurality of heaters located in heater wells in the first portion. Fluids may be produced through one or more production wells in a second portion of the formation that is substantially adjacent to the first portion. The heat provided to the first portion may be reduced or turned off after a selected time. An oxidizing fluid may be provided through one or more of the heater wells in the first portion. Heat may be provided to the first portion and the second portion through oxidation of at least some hydrocarbons in the first portion. Fluids may be produced through at least one of the production wells in the second portion. The fluids may include at least some oxidized hydrocarbons. Transportation fuel may be produced from the hydrocarbons produced from the first and/or second of the formation.

[0979] FIG. 111 depicts a schematic of an embodiment of a first stage of treating the tar sands formation with electrical heaters. Hydrocarbon layer 388 may be separated into section 608A and section 608B. Heaters 412 may be located in section 608A. Production wells 206 may be located in section 608B. In some embodiments, production wells 206 extend into section 608A.

[0980] Heaters 412 may be used to heat and treat portions of section 608A through conductive, convective, and/or radiative heat transfer. For example, heaters 412 may mobilize, visbreak, and/or pyrolyze hydrocarbons in section 608A. Production wells 206 may be used to produce mobilized, visbroken, and/or pyrolyzed hydrocarbons from section 608A.

[0981] FIG. 112 depicts a schematic of an embodiment of a second stage of treating the tar sands formation with fluid injection and oxidation. After at least some hydrocarbons from section 608A have been produced (for example, a majority of hydrocarbons in the section or almost all producible hydrocarbons in the section), the heater wells in section 608A may be converted to injection wells 602. In some embodiments, the heater wells are open wellbores below the overburden. In some embodiments, the heater wells are initially installed into wellbores that include perforated casings. In some embodiments, the heater wells are perforated using perforation guns after heating from the heater wells is completed.

[0982] Injection wells 602 may be used to inject an oxidizing fluid (for example, air, oxygen, enriched air, or other oxidants) into the formation. In some embodiments, the oxidation includes liquid water and/or steam. The amount of oxidizing fluid may be controlled to adjust subsurface combustion patterns. In some embodiments, carbon dioxide or other fluids are injected into the formation to control heating/production in the formation. The oxidizing fluid may oxidize (combust) or otherwise react with hydrocarbons remaining in the formation (for example, coke). Water in the oxidizing fluid may react with coke and/or hydrocarbons in the hot formation to produce syngas in the formation. Production wells 206 in section 608B may be converted to heater/gas production wells 626. Heater/gas production wells 626 may be used to produce oxidation gases and/or syngas products from the formation. Producing the hot oxidation gases and/or syngas through heater/gas production wells 626 in section 608B may heat the section to higher temperatures so that hydrocarbons in the section are mobilized, visbroken, and/or pyrolyzed in the section. Production wells 206 in section 608C may be used to produce mobilized, visbroken, and/or pyrolyzed hydrocarbons from section 608B.

[0983] In certain embodiments, the pressure of the injected fluids and the pressure in formation are controlled to control the heating in the formation. The pressure in the formation may be controlled by controlling the production rate of fluids from the formation (for example, the production rate of oxidation gases and/or syngas products from heater/gas production wells 626). Heating in the formation may be controlled so that there is enough hydrocarbon volume in the formation to maintain the oxidation reactions in the formation. Heating may be controlled so that the formation near the injection wells is at a temperature that will generate desired synthesis gas if a synthesis gas generating fluid such as water is included in the oxidation fluid. Heating in the formation may also be controlled so that enough heat is generated to conductively heat the formation to mobilize, visbreak, and/or pyrolyze hydrocarbons in adjacent sections of the formation.

[0984] The process of injecting oxidizing fluid and/or water in one section, producing oxidation gases and/or syngas products in an adjacent section to heat the adjacent section, and producing upgraded hydrocarbons (mobilized, visbroken, and/or pyrolyzed hydrocarbons) from a subsequent section may be continued in further sections of the tar sands formation. For example, FIG. 113 depicts a schematic of an embodiment of a third stage of treating the tar sands formation with fluid injection and oxidation. The gas heater/producer wells in section 608B are converted to injection wells 602 to inject air and/or water. The producer wells in section 608C are converted to production wells (for example, heater/gas production wells 626) to produce oxidation gases and/or syngas products. Production wells 206 are formed in section 608D to produce upgraded hydrocarbons.

[0985] In some embodiments, significant amounts of residue and/or coke remain in a subsurface formation after heating the formation with heaters and producing formation fluids from the formation. In some embodiments, sections of the formation include heavy hydrocarbons such as bitumen that are difficult to heat to mobilization temperatures adjacent to sections of the formation that are being treated using an in situ heat treatment process. Heating of heavy hydrocarbons may require high energy input, a large number of heater wells and/or increase in capital costs (for example, materials for heater construction). It would be advantageous to produce formation fluids from subsurface formations with lower energy costs, fewer heater wells and/or heater cost with improved product quality and/or recovery efficiency.

[0986] In some embodiments, a method for treating a subsurface formation includes producing a at least a third hydrocarbons from a first portion by an in situ heat treatment process. An average temperature of the first portion is less than 350° C. An oxidizing fluid may be injected in the first portion to cause the average temperature in the first portion to increase sufficiently to oxidize hydrocarbon in the first portion and to raise the average temperature in the first portion to greater than 350° C. In some embodiments, the temperature of the first portion is raised to an average temperature ranging from 350° C. to 700° C. A heavy hydrocarbon fluid that includes one or more condensable hydrocarbons may be injected in the first portion to from a diluent and/or drive fluid. In some embodiments, a catalyst system is added to the first portion.

[0987] FIGS. 114, 115, and 116 depict side view representations of embodiments of treating a subsurface formation in stages with heaters, oxidizing fluid, catalyst, and/or drive fluid. Hydrocarbon layer 388 may be divided into three or more treatment sections. In certain embodiments, hydrocarbon layer 388 includes five treatment sections: section 608A, section 608B, section 608C, section 608D and section 608E. Sections 608A and section 608C are separated by section 608B. Sections 608C and section 608E are separated by section 608D. Section 608A through section 608E may be horizontally displaced from each other in the formation. In some embodiments, one side of section 608A is adjacent to an edge of the treatment area of the formation or an untreated section of the formation is left on one side of section 608A before the same or a different pattern is formed on the opposite side of the untreated section.

[0988] In certain embodiments, section 608A is heated to pyrolysis temperatures with heaters 412. Section 608A may be heated to mobilize and/or pyrolyze hydrocarbons in the section. In some embodiments, section 608A is heated to an average temperature of 250° C., 300° C., or up to 350° C. The mobilized and/or pyrolyzed hydrocarbons may be produced through one or more production wells 206. Once at least a third, a substantial portion, or all of the hydrocarbons have been produced from section 608A, the temperature in section 608A may be maintained at an average temperature that allows the section to be used as a reactor and/or reaction zone to treat formation fluid and/or hydrocarbons from surface facilities. Use of one or more heated portions of the formation to treat such hydrocarbons may reduce or eliminate the need for surface facilities that treat such fluids (for example, coking units and/or delayed coking units).

[0989] In certain embodiments, heating and producing hydrocarbons from sections 608A creates fluid injectivity in the sections. After fluid injectivity has been created in section 608A, an oxidizing fluid may be injected into the section. For example, oxidizing fluid may be injected in section 608A after at least a third or a majority of the hydrocarbons have been produced from the section. The fluid may be injected through heater wellbores, production wells 206, and/or injection wells located in section 608A. In some embodiments, heaters 412 continue to provide heat while the fluid is being injected. In certain embodiments, heaters 412 may be turned down or off before or during fluid injection.

[0990] During injection of oxidant, excess oxidant and/or oxidation products may be removed from section 608A through one or more production wells 206 and/or heater/gas production wells. In some embodiments, after the formation is raised to a desired temperature, a second fluid may be introduced into section 608A. The second fluid may be water and/or steam. Addition of the second fluid may cool the formation. For example, when the second fluid is steam and/or water, the reactions of the second fluid with coke and/or hydrocarbons are endothermic and produce synthesis gas. In some embodiments, oxidizing fluid is added with the second fluid so that some heating of section 608A occurs simultaneous with the endothermic reactions. In some embodiments, section 608A is treated in alternating steps of adding oxidant and second fluid to heat the formation for selected periods of time.

[0991] In certain embodiments, the pressure of the injected fluids and the pressure section 608A are controlled to control the heating in the formation. The pressure in section 608A may be controlled by controlling the production rate of fluids from the section (for example, the production rate of hydrocarbons, oxidation gases and/or syngas products). Heating in section 608A may be controlled so that section reaches a desired temperature (e.g., temperatures of at least 350° C., of at least about 400° C., or at least about 500° C., about 700° C., or higher). Injection of the oxidizing fluid may allow portions of the formation below the section heated by heaters to be heated, thus allowing heating of formation fluids in deeper and/or inaccessible portions of the formation. The control of heat and pressure in the section may improve efficiency and quality of products produced from the formation.

[0992] During heating and/or after heating of section 608A, heavy hydrocarbons with low economic value and/or waste hydrocarbon streams from surface facilities may be injected in the section. Low economic value hydrocarbons and/or waste hydrocarbon streams may include, but are not limited to, hydrocarbons produced during surface mining operations, residue, bitumen and/or bottom extracts from bitumen mining. In some embodiments, hydrocarbons produced from section 608A or other sections of the formation may be introduced into section 608A. In some embodiments, one or more of the heater wells in section 608A are converted to injection wells.

[0993] Heating of hydrocarbons and/or coke in section 608A may generate drive fluids. Generated drive fluids in section 608A may include air, steam, carbon dioxide, carbon monoxide, hydrogen, methane, pyrolyzed hydrocarbons and/or in situ diluent. In some embodiments, hydrocarbon fluids are introduced into section 608A prior to injecting an oxidizing fluid and/or the second fluid. Oxidation and/or thermal cracking of introduced hydrocarbon fluids may create the drive fluid.

[0994] In some embodiments, drive fluid may be injected into the formation. The addition of oxidizing fluid, steam, and/or water in the drive fluid may be used to control temperatures in section 608A. For example, the addition of hydrocarbons to section 608A may cool the average temperature in section 608A to a temperature below temperatures that allow for cracking of the introduced hydrocarbons. Oxidizing fluid may be injected to increase and/or maintain the average temperature between 250° C. and 700° C. or between 350° C. and 600° C. Maintaining the temperature between 250° C. and 700° C. may allow for the production of high quality hydrocarbons from the low value hydrocarbons and/or waste streams. Controlling the input of hydrocarbons, oxidizing fluid, and/or drive fluid into section 608A may allow for the production of condensable hydrocarbons with a minimal amount non-condensable gases. In some embodiments, controlling the input of hydrocarbons, oxidizing fluid, and/or drive fluid into section 608A may allow for the production of large amounts of non-condensable hydrocarbons and/or hydrogen with minimal amounts of condensable hydrocarbons.

[0995] In some embodiments, a catalyst system is introduced to section 608A when the section is at a desired temperature (for example, a temperature of at least 350° C., at least 400° C., or at least 500° C.). In some embodiments, the section is heated after and/or during introduction of the catalyst system. The catalyst system may be provided to the formation by injecting the catalyst system into one or more injection wells and/or production wells in section 608A. In some embodiments, the catalyst system is positioned in wellbores proximate the section of the formation to be treated. In some embodiments, the catalyst is introduced to one or more sections during in situ heat treatment of the sections. The catalyst may be provided to section 608A as a slurry and/or a solution in sufficient quantity to allow the catalyst to be dispersed in the section. For example, the catalyst system may be dissolved in water and/or slurried in an emulsion of water and hydrocarbons. At temperatures of at least 100° C., at least 200° C., or at least 250° C., vaporization of water from the solution allows the catalyst to be dispersed in the rock matrix of section 608A.

[0996] The catalyst system may include one or more catalysts. The catalysts may be supported or unsupported catalysts. Catalysts include, but are not limited to, alkali metal carbonates, alkali metal hydroxides, alkali metal hydrides, alkali metal amides, alkali metal sulfides, alkali metal acetates, alkali metal oxalates, alkali metal formates, alkali metal pyruvates, alkaline-earth metal carbonates, alkaline-earth metal hydroxides, alkaline-earth metal hydrides, alkaline-earth metal amides, alkaline-earth metal sulfides, alkaline-earth metal acetates, alkaline-earth metal oxalates, alkaline-earth metal formates, alkaline-earth metal pyruvates, or commercially available fluid catalytic cracking catalysts, dolomite, silicon-alumina catalyst fines, zeolites, zeolite catalyst fines any catalyst that promotes formation of aromatic hydrocarbons, or mixtures thereof.

[0997] In some embodiments, fractions from surface facilities include catalyst fines. Surface facilities may include catalytic cracking units and/or hydrotreating units. These fractions may be injected in section 608A to provide a source of catalyst for the section. Injection of the fractions in section 608A may provide an advantageous method for disposal and/or upgrading of the fractions as compared to conventional disposal methods for fractions containing catalyst fines.

[0998] After injecting catalyst in section 608A, the average temperature in section 608A may be increased or maintained in a range from about 250° C. to about 700° C., from about 300° C. to about 650° C., or from about 350° C. to about 600° C. by injection of reaction fluids (for example, oxidizing fluid, steam, water and/or combinations thereof). In some embodiments, heaters 412 are used to raise or maintain the temperature in section 608A in the desired range. In some embodiments, heaters 412 and the introduction of reaction fluids into section 608A are used to raise or maintain the temperature in the desired range. Hydrocarbon fluids may be introduced in section 608A once the desired temperature is obtained. In some embodiments, the catalyst system is slurried with a portion of the hydrocarbons, and the slurry is introduced to section 608A. In some embodiments, a portion of the hydrocarbon fluids are introduced to section 608A prior to introduction of the catalyst system. The introduced hydrocarbon fluids may be hydrocarbons in formation fluid from an adjacent portion of the formation, and/or low value hydrocarbons. The hydrocarbons may contact the catalyst system to produce desirable hydrocarbons (for example, visbroken hydrocarbons, cracked hydrocarbons, aromatic hydrocarbons, or mixtures thereof). The desired temperature in section 608A may be maintained by turning on heaters in the section and/or continuous injection of oxidizing fluid to cause exothermic reactions that heat the formation.

[0999] In some embodiments, hydrocarbons produced through thermal and/or catalytic treatment in section 608A may be used as a diluent and/or a solvent in the section. The produced hydrocarbons may include aromatic hydrocarbons. The aromatic enriched diluent may dilute or solubilize a portion of the heavy hydrocarbons in section 608A and/or other sections in the formation (for example, sections 608B and/or 608C) and form a mixture. The mixture may be produced from the formation (for example, produced from sections 608A and/or 608C). In some embodiments, the mixture is produced from section 608B. In some embodiments, the mixture drains to a bottom portion of the section and solubilizes additional hydrocarbons at the bottom of the section. Solubilized hydrocarbons may be produced or mobilized from the formation. In some embodiments, fluids produced in section 608A (for example, diluent, desirable products, oxidized products, and/or solubilized hydrocarbons) may be pushed towards section 608B as shown by the arrows in FIG. 114 by oxidizing fluid, drive fluid, and/or created drive fluid.

[1000] In some embodiments, the temperatures in section 608A and the generation of drive fluid in section 608A increases the pressure of section 608A so the drive fluid pushes fluids through section 608B into section 608C. Hot fluids flowing from section 608A into section 608B may melt, solubilize, visbreak and/or crack fluids in section 608B sufficiently to allow the fluids to move to section 608C. In section 608C, the fluids may be upgraded and/or produced through production wells 206.

[1001] In some embodiments, a portion of the catalyst system from section 608A enters section 608B and/or section 608C and contacts fluids in the sections. Contact of the catalyst with formation fluids in 608B and/or section 608C may result in the production of hydrocarbons having a lower API gravity than the mobilized fluids.

[1002] The fluid mixture formed from contact of hydrocarbons, formation fluid and/or mobilized fluids with the catalyst system may be produced from the formation. The liquid hydrocarbon portion of the fluid mixture may have an API gravity between 10° and 25°, between 12° and 23° or between 15° and 20°. In some embodiments, the produced mixture has at most 0.25 grams of aromatics per gram of total hydrocarbons. In some embodiments, the produced mixture includes some of the catalysts and/or used catalysts.

[1003] In some embodiments, contact of the hydrocarbon fluids with the catalyst system produces coke in 608A. Oxidizing fluid may be introduced into section 608A. The oxidizing fluid may react with the coke to generate heat that maintains the average temperature of section 608A in a desired range. For some time intervals, additional oxidizing fluid may be added to section 608A to increase the oxidation reactions to regenerate catalyst in the section. The reaction of the oxidizing fluid with the coke may reduce the amount of coke and heat formation and/or catalyst to temperatures sufficient to remove impurities on the catalyst. Coke, nitrogen containing compounds, sulfur containing compounds, and/or metals such as nickel and/or vanadium may be removed from the catalyst. Removing impurities from the catalyst in situ may enhance catalyst life. After catalyst regeneration, introduction of reaction fluids may be adjusted to allow section 608A to return to an average temperature in the desired temperature range. The average temperature in section 608A may the controlled to be in range from about 250° C. to about 700° C. Hydrocarbons may be introduced in section 608A to continue the cycle. Additional catalyst systems may be introduced into the formation as needed.

[1004] A method for treating a subsurface formation in stages may include using an in situ heat treatment process in combination with injection of an oxidizing fluid and/or drive fluid in one or more portions (sections) of the formation. In some embodiments, hydrocarbons are produced from a first portion and/or a third portion by an in situ heat treatment process. A second portion that separates the first and third portions may be heated with one or more heaters to an average temperature of at least about 100° C. The heat provided to the first portion may be reduced or turned off after a selected time. Oxidizing fluid may be injected in the first portion to oxidize hydrocarbons in the first portion and raise the temperature of the first portion. A drive fluid and/or additional oxidizing fluid may be injected and/or created in the third portion to cause at least some hydrocarbons to move from the third portion through the second portion to the first portion of the hydrocarbon layer. Injection of the oxidizing fluid in the first portion may be reduced or discontinued and additional hydrocarbons and/or syngas may be produced from the first portion of the formation. The additional hydrocarbons and/or syngas may include at least some hydrocarbons from the second and third portions of the formation. Transportation fuel may be produced from the hydrocarbons produced from the first, second and/or third portions of the formation. In some embodiments, a catalyst system is provided to the first portion and/or third portion.

[1005] In certain embodiments, sections 608A and 608C are heated at or near the same time to similar temperatures (for example, pyrolysis temperatures) with heaters 412. Sections 608A and 608C may be heated to mobilize and/or pyrolyze hydrocarbons in the sections. The mobilized and/or pyrolyzed hydrocarbons may be produced (for example, through one or more production wells 206) from section 608A and/or section 608C. Section 608B may be heated to lower temperatures (for example, mobilization temperatures) by heaters 412. Sections 608D and 608E may not be heated. Little or no production of hydrocarbons to the surface may take place through section 608B, section 608D and/or section 608E. For example, sections 608A and 608C may be heated to average temperatures of at least about 300° C. or at least about 330° C. while section 608B is heated to an average temperature of at least about 100° C., sections 608D and 608E are not heated and no production wells are operated in section 608B, section 608D, and/or section 608E. In some embodiments, heat from section 608A and/or section 608C transfers to sections section 608D and/or section 608E.

[1006] In some embodiments, heavy hydrocarbons in section 608B may be heated to mobilization temperatures and flow into sections 608A and 608C. The mobilized hydrocarbons may be produce from production wells 206 in sections 608A and 608C. After some or most of the fluids have been produced from sections 608A and 608C, production of formation fluids in the sections may be slowed and/or discontinued.

[1007] In certain embodiments, heating and producing hydrocarbons from sections 608A and 608C creates fluid injectivity in the sections. After fluid injectivity has been created in section 608C, an oxidizing fluid may be injected into the section. For example, oxidizing fluid may be injected in section 608C after a majority of the hydrocarbons have been produced from the section. The fluid may be injected through heaters 412, production wells 206, and/or injection wells located in section 608C. In some embodiments, heaters 412 continue to provide heat while the fluid is being injected. In certain embodiments, heaters 412 may be turned down or off before or during fluid injection.

[1008] During injection of oxidant, excess oxidant and/or oxidation products may be removed from section 608C through one or more production wells 206 and/or heater/gas production wells. In some embodiments, after the formation is raised to a desired temperature, a second fluid may be introduced into section 608C. The second fluid may be steam and/or water. Addition of the second fluid may cool the formation. For example, when the second fluid is steam and/or water, the reactions of the second fluid with coke and/or hydrocarbons are endothermic and produce synthesis gas. In some embodiments, oxidizing fluid is added with the second fluid so that some heating of section 608C occurs simultaneous with the endothermic reactions. In some embodiments, section 608C is treated in alternating steps of adding oxidant and second fluid to heat the formation for selected periods of time.

[1009] In certain embodiments, the pressure of the injected fluids and the pressure section 608C are controlled to control the heating in the formation. The pressure in section 608C may be controlled by controlling the production rate of fluids from the section (for example, the production rate of hydrocarbons, oxidation gases and/or syngas products). Heating in section 608C may be controlled so that there is enough hydrocarbon volume in the section to maintain the oxidation reactions in the formation. Heating and/or pressure in section 608C may also be controlled (for example, by producing a minimal amount of hydrocarbons, oxidation gases and/or syngas products) so that enough pressure is generated to create fractures in sections adjacent to the section (for example, creation of fractures in section 608B). Creation of fractures in adjacent sections may allow fluids from adjacent sections to flow into section 608C and cool the section. Injection of oxidizing fluid may allow portions of the formation below the section heated by heaters to be heated, thus allowing heating of formation fluids in deeper and/or inaccessible portions of the subsurface to be accessed. Section 608C may be cooled from temperatures that promote syngas production to temperatures that promote formation of visbroken and/or upgrade products. Such control of heat and pressure in the section may improve efficiency and quality of products produced from the formation.

[1010] During heating of section 608C or after the section has reached a desired temperature (e.g., temperatures of at least 300° C., at least about 400° C., or at least about 500° C.), an oxidizing fluid and/or a drive fluid may be injected and/or created in section 608A. The drive fluid includes, but is not limited to, steam, water, hydrocarbons, surfactants, polymers, carbon dioxide, air, or mixtures thereof. In some embodiments, the catalyst system described herein is injected in section 608A. In some embodiments, the catalyst system is injected prior to injecting the oxidizing fluid. In some embodiments, production of fluid from section 608A is discontinued prior to injecting fluids in the section. In some embodiments, heater wells in section 608A are converted to injection wells.

[1011] In some embodiments, drive fluids are created in section 608A. Created drive fluids may include air, steam, carbon dioxide, carbon monoxide, hydrogen, methane, pyrolyzed hydrocarbons and/or diluent. In some embodiments, hydrocarbons (for example, hydrocarbons produced from section 608A and/or section 608C, low value hydrocarbons and/or or waste hydrocarbon streams) are provided as a portion of the drive fluid. In some embodiments, hydrocarbons are introduced into section 608A prior to injecting an oxidizing fluid and/or the second fluid. Oxidation, catalytic cracking, and/or thermal cracking of introduced hydrocarbon fluids may create the drive fluid and/or a diluent.

[1012] In some embodiments, oxidizing fluid, steam or water are provided as a portion of the drive fluid. The addition of oxidizing fluid, steam, and/or water in the drive fluid may be used to control temperatures in the sections. For example, the addition of steam or water may be cool the section. In some embodiments, water injected as the drive fluid is turned into steam in the formation due to the higher temperatures in the formation. The conversion of water to steam may be used to reduce temperatures or maintain temperatures in the sections between 270° C. and 450° C. Maintaining the temperature between 270° C. and 450° C. may produce higher quality hydrocarbons and/or generate a minimal amount of non-condensable gases.

[1013] Residual hydrocarbons and/or coke in section 608A may be melted, visbroken, upgraded and/or oxidized to produce products that may be pushed towards section 608B as shown by the arrows in FIG. 114. In some embodiments, the temperature in section 608C and the generation of drive fluid in section 608A may increase the pressure of section 608A so the drive fluid pushes fluids through section 608B into section 608C. Hot fluids flowing from section 608A into section 608B may melt and/or visbreak fluids in section 608B sufficiently to allow the fluids to move to section 608C. In section 608C, the fluids may be upgraded and/or produced through production wells 206.

[1014] In some embodiments, oxidizing fluid injected in section 608A is controlled to raise the average temperature in the section to a desired temperature (for example, at least about 350° C., or at least about 450° C.). Injection of oxidizing fluid and/or drive fluid in section 608A may continue until most or a substantial portion of the fluids from section 608A are moved through section 608B to section 608C. After a period of time, injection of oxidant and/or drive fluid into 608A is slowed and/or discontinued.

[1015] Injection of oxidizing fluid into section 608C may be slowed or stopped during injection and/or creation of drive fluid and/or creation of diluent in section 608A. In some embodiments, injection of oxidizing fluid in section 608C is continued to maintain an average temperature in the section of about 500° C. during injection and/or creation of drive fluid and/or diluent in section 608A. In some embodiments, the catalyst system is injected in section 608C.

[1016] As section 608A and/or section 608C are treated with oxidizing fluid, heaters in sections 608D and 608E may be turned on. In some embodiments, section 608D is heated through conductive heat transfer from section 608C and/or convective heat transfer. Section 608E may be heated with heaters. For example, an average temperature in section 608E may be raised to above 300° C. while an average temperature in section 608D is maintained between 80° C. and 120° C. (for example, at about 100° C.).

[1017] As temperatures in section 608E reach a desired temperature (for example, above 300° C.), production of formation fluids from section 608E through production wells 206 may be started. The temperature may be reached before, during or after oxidizing fluid and/or drive fluid is injected and/or drive fluid and/or diluent is created in section 608A.

[1018] Once the desired temperature in section 608E has been obtained (for example, above 300° C., or above 400° C.), production may be slowed and/or stopped in section 608C and oxidation fluid and/or drive fluid is injected and/or created in section 608C to move fluids from section 608C through cooler section 608D towards section 608E as shown by the arrows in FIG. 115. Injection and/or creation of additional oxidation fluid and/or drive fluid in section 608C may upgrade hydrocarbons from section 608B that are in section 608C and/or may move fluids towards section 608E.

[1019] In some embodiments, heaters in combination with heating produced by oxidizing hydrocarbons in sections 608A, 608C and/or section 608E allows for a reduction in the number of heaters to be used in the sections and/or less capital costs as heaters made of less expensive materials may be used. The heating pattern may be repeated through the formation.

[1020] In some embodiments, fluids in hydrocarbon layer 388 (for example, layers in a tar sands formation) may preferentially move horizontally within the hydrocarbon layer from the point of injection because the layers tend to have a larger horizontal permeability than vertical permeability. The higher horizontal permeability allows the injected fluid to move hydrocarbons between sections preferentially versus fluids draining vertically due to gravity in the formation. Providing sufficient fluid pressure with the injected fluid may ensure that fluids are moved from section 608A through section 608B into section 608C for upgrading and/or production or from section 608C through section 608D into section 608E for upgrading and/or production. Increased heating in sections 608A, 608C, and 608E may mobilize fluids from sections 608B and 608D into adjacent sections. Increased heating may also mobilize fluids below section 608A through 608E and the fluid may flow from the colder sections into the heated sections for upgrading and/or production due to pressure gradients established by producing fluid from the formation. In some embodiments, one or more production wells are placed in the formation below sections 608A through 608E to facilitate production of additional hydrocarbons.

[1021] In some embodiments, after sections 608A and 608C are heated to desired temperatures, the oxidizing fluid is injected into section 608C to increase the temperature in the section. The fluids in section 608C may move through section 608B into section 608A as indicated by the arrows in FIG. 116. The fluids may be produced from section 608A. Once a majority of the fluids have been produced from section 608A, the treatment process described in FIG. 114 and FIG. 115 may be repeated.

[1022] In some embodiments, treating a formation in stages includes heating a first portion from one or more heaters located in the first portion. Hydrocarbons may be produced from the first portion. Heat provided to the first portion may be reduced or turned off after a selected time. A second portion may be substantially adjacent to the first portion. An oxidizing fluid may be injected in the first portion to cause a temperature of the first portion to increase sufficiently to oxidize hydrocarbons in the first portion and a third portion, the third portion being substantially below the first portion. The second portion may be heated from heat provided from the first portion and/or third portion and/or one or more heaters located in the second portion such that an average temperature in the second portion is at least about 100° C. Hydrocarbons may flow from the second portion into the first portion and/or third portion. Injection of the oxidizing fluid may be reduced or discontinued in the first portion. The temperature of the first portion may cool to below 600° C. to 700° C. and additional hydrocarbons may be produced from the first portion of the formation. The additional hydrocarbons may include oxidized hydrocarbons from the first portion, at least some hydrocarbons from the second portion, at least some hydrocarbons from the third portion of the formation, or mixtures thereof. Transportation fuel may be produced from the hydrocarbons produced from the first, second and/or third portions of the formation.

[1023] In some embodiments, in situ heat treatment followed by oxidation and/or catalyst addition as described for horizontal sections is performed in vertical sections of the formation. Heating a bottom vertical layer followed by oxidation may create microfractures in middle sections thus allowing heavy hydrocarbons to flow from the “cold” middle section to the warmer bottom section. Lighter fluids may flow into the top section and continue to be upgraded and/or produced through production wells. In some embodiments, two vertical sections are treated with heaters followed by oxidizing fluid.

[1024] In some embodiments, heaters in combination with an oxidizing fluid and/or drive fluid are used in various patterns. For example, cylindrical patterns, square patterns, or hexagonal patterns may be used to heat and produce fluids from a subsurface formation. FIG. 117 and FIG. 118, depict various patterns for treatment of a subsurface formation. FIG. 117 depicts an embodiment of treating a subsurface formation using a cylindrical pattern. FIG. 118 depicts an embodiment of treating multiple sections of a subsurface formation in a rectangular pattern. FIG. 119 is a schematic top view of the pattern depicted in FIG. 118.

[1025] Hydrocarbon layer 388 may be separated into section 608A and section 608B. Section 608A represents a section of the subsurface formation that is to be produced using an in situ heat treatment process. Section 608B represents a section of formation that surrounds section 608A and is not heated during the in situ heat treatment process. In certain embodiments, section 608B has a larger volume than section 608A and/or section 608C. Section 608A may be heated using heaters 412 to mobilize and/or pyrolyze hydrocarbons in the section. The mobilized and/or pyrolyzed hydrocarbons may be produced (for example, through one or more production wells 206) from section 608A. After some or all of the hydrocarbons in section 608A have been produced, an oxidizing fluid may be injected into the section. The fluid may be injected through heaters 412, a production well, and/or an injection well located in section 608A. In some embodiments, at least a portion of heaters 412 are used and/or converted to injection wells. In some embodiments, heaters 412 continue to provide heat while the fluid is being injected. In other embodiments, heaters 412 may be turned down or off before or during fluid injection.

[1026] In some embodiments, providing oxidizing fluid such as air to section 608A causes oxidation of hydrocarbons in the section and in portions of section 608C. In some embodiments, treatment of section 608A with the heaters creates coked hydrocarbons and formation with substantially uniform porosity and/or substantially uniform injectivity so that heating of the section is controllable when oxidizing fluid is introduced to the section. The oxidation of hydrocarbons in section 608A will maintain the average temperature of the section or increase the average temperature of the section to higher temperatures (for example, above 400° C., above 500° C., above 600° C., or higher).

[1027] In some embodiments, an average temperature of section 608C that is located below section 608A increases due to heat generated through oxidation of hydrocarbons and/or coke in section 608A. For example, an average temperature in section 608C may increase from formation temperature to above 500° C. As the average temperature in section 608A and/or section 608C increases through oxidation reactions, the temperature in section 608B increases and fluids may be mobilized towards section 608A as shown by the arrows in FIG. 117 and FIG. 118. In some embodiments, section 608B is heated by heaters to an average temperature of at least about 100° C.

[1028] In section 608A, mobilized hydrocarbons are oxidized and/or pyrolyzed to produce visbroken, oxidized, pyrolyzed products. For example, cold bitumen in section 608B may be heated to mobilization temperature of at least about 100° C. so that it flows into section 608A and/or section 608C. In section 608A and/or section 608C, the bitumen is pyrolyzed to produce formation fluids. Fluids may be produced through production wells 206 and/or heater/gas production wells in section 608A. In some embodiments, no fluids are produced from section 608A during oxidation. Injection of oxidizing fluid may be reduced or discontinued in section 608A once a desired temperature is reached (for example, a temperature of at least 350° C., at least 300° C., or above 450° C.). Once oxidizing fluid is slowed and/or discontinued in sections 608A, 608C, the sections may cool (e.g. to temperatures below about 700° C., about 600° C., below 500° C. or below 400° C.) and remain at upgrading and/or pyrolysis temperatures for a period of time. Fluids may continue to be upgraded and may be produced from section 608A through production wells.

[1029] In certain embodiments, section 608B and/or section 608D as described in reference to FIGS. 111-119 has a larger volume than section 608A, section 608C, and/or section 608E. Section 608B and/or section 608D may be larger in volume than the other sections so that more hydrocarbons are produced for less energy input into the formation. Because less heat is provided to section 608B and/or section 608D (the section is heated to lower temperatures), having a larger volume in section 608B and/or section 608D reduces the total energy input to the formation per unit volume. The desired volume of section 608B and/or section 608D may depend on factors such as, but not limited to, viscosity, oil saturation, and permeability. In addition, the degree of coking is much less in section 608B and/or section 608D due to the lower temperature so less hydrocarbons are coked in the formation when section 608B and/or section 608D has a larger volume. In some embodiments, the lower degree of heating in section 608B and/or section 608D allows for cheaper capital costs as lower temperature materials (cheaper materials) may be used for heaters used in section 608B and/or section 608D.

[1030] Using the remaining hydrocarbons for heat generation and only using electrical heating for the initial heating stage may improve the overall energy use efficiency of treating the formation. Using electrical heating only in the initial step may decrease the electrical power needs for treating the formation. In addition, forming wells that are used for the combination of production, injection, and heating/gas production may decrease well construction costs. In some embodiments, hot gases produced from the formation are provided to turbines. Providing the hot gases to turbines may recover some energy and improve the overall energy use efficiency of the process used to treat the formation.

[1031] Treating the subsurface formation, as shown by the embodiments of FIGS. 111-117 may utilize carbon remaining after production of mobilized, visbroken, and/or pyrolyzed hydrocarbons for heat generation in the formation. In some embodiment, treating hydrocarbons in the subsurface formation, as shown in by the embodiments in FIGS. 111-117 creates products having economic value from hydrocarbons having low economic value and/or from waste hydrocarbon streams from surface facilities.

[1032] Treating hydrocarbon containing formations in order to convert, upgrade, and/or extract the hydrocarbons is an expensive and time consuming process. Any process and/or system which might increase the efficiency of the treatment of the formation is highly desirable. Increasing the efficiency of the treatment of the formation may include optimizing heat source locations and the spacing between the heat sources in a pattern of heat sources. Increasing the efficiency of the treatment of the formation may include optimizing the heating schedule of the formation. Repositioning the location of a producer wells (e.g., vertically within the formation) may increase the efficiency of the treatment of the formation. Adjusting the initial bottom-hole pressure of one or more producer well in the formation may increase the efficiency of the formation treatment process. Adjusting the blowdown time of one or more producer wells may increase the efficiency of the formation treatment process. Optimizing one or more of the mentioned variables alone, or in combination, may increase the efficiency of the formation treatment process resulting in reduced costs and/or increased production. Even a relatively small increase of efficiency may result in billions of dollars of additional revenue due to the scale of such treatment processes in the form of reduced operating costs, increased quality of the hydrocarbon product produced, and/or increased quantity of the hydrocarbon product produced from the formation.

[1033] Many different types of wells or wellbores may be used to treat the hydrocarbon containing formation using the in situ heat treatment process. In some embodiments, vertical and/or substantially vertical wells are used to treat the formation. In some embodiments, horizontal (such as J-shaped wells and/or L-shaped wells), and/or u-shaped wells are used to treat the formation. In some embodiments, combinations of horizontal wells, vertical wells, and/or other combinations are used to treat the formation. In certain embodiments, wells extend through the overburden of the formation to a hydrocarbon containing layer of the formation. Heat in the wells may be lost to the overburden. In certain embodiments, surface and/or overburden infrastructures used to support heaters and/or production equipment in horizontal wellbores and/or u-shaped wellbores are large in size and/or numerous.

[1034] In certain embodiments, heaters, heater power sources, production equipment, supply lines, and/or other heater or production support equipment are positioned in substantially horizontal and/or inclined tunnels. Positioning these structures in tunnels may allow smaller sized heaters and/or other equipment to be used to treat the formation. Positioning these structures in tunnels may also reduce energy costs for treating the formation, reduce emissions from the treatment process, facilitate heating system installation, and/or reduce heat loss to the overburden, as compared to conventional hydrocarbon recovery processes that utilize surface based equipment. U.S. Published Patent Application Nos. 2007-0044957 to Watson et al.; 2008-0017416 to Watson et al.; and 2008-0078552 to Donnelly et al., all of which are incorporated herein by reference, describe methods of drilling from a shaft for underground recovery of hydrocarbons and methods of underground recovery of hydrocarbons.

[1035] In some embodiments, increasing the efficiency of the treatment of the formation may include optimizing heat source locations and the spacing between the heat sources in a pattern of heat sources. In certain embodiments, heat sources (for example, heaters) have uneven or irregular spacing in a heater pattern. For example, the space between heat sources in the heater pattern varies or the heat sources are not evenly distributed in the heater pattern. In certain embodiments, the space between heat sources in the heater pattern decreases as the distance from the production well at the center of the pattern increases. Thus, the density of heat sources (number of heat sources per square area) increases as the heat sources get more distant from the production well.

[1036] In some embodiments, heat sources are evenly spaced in the heater pattern but have varying heat outputs such that the heat sources provide an uneven or varying heat distribution in the heater pattern. Varying the heat output of the heat sources may be used to, for example, effectively mimic having heat sources with varying spacing in the heater pattern. For example, heat sources closer to the production well at the center of the heater pattern may provide lower heat outputs than heat sources at further distances from the production well. The heater outputs may be varied such that the heater outputs gradually increase as the heat sources increase in distance from the production well.

[1037] Heat sources may be positioned in an irregular pattern in a horizontally oriented heating zone of the formation in relation to, for example, a producer well. Heat sources may be positioned in an irregular pattern in a vertically oriented heating zone of the formation in relation to, for example, a producer well. Irregular patterns may have advantages over previous equivalently spaced patterns relative to a producer well. For example, irregular patterns of heat sources may create channels within the formation to assist in directing hydrocarbons through the channels more efficiently to producer wells. In some embodiments, patterns of heat sources may be based on the distribution and/or type of hydrocarbons in the formation. The portion of the formation may be divided into different heating zones. Different zones within the same formation may have different patterns of heaters within each zone, for example, depending upon the particular type of hydrocarbon within the particular heating zone.

[1038] Using irregular patterns for positioning heat sources in the formation may reduce the number of heat sources needed in the formation. The installation and maintenance of heat sources in a formation accounts for a significant percentage of the operating costs associated with the treatment of the formation. In some instances, installation and maintenance of heat sources in the formation may account for as much as 60% or more of the operating costs of treating the formation. Reducing the number of heaters used to treat the formation has significant economic benefits. Reducing the time that heaters are used to heat the portion of the formation will reduce costs associated with treating the portion.

[1039] In certain embodiments, the uneven or irregular spacing of heat sources is based on regular geometric patterns. For example, the irregular spacing of heat sources may be based on a hexagonal, triangular, square, octagonal, other geometric combinations, and/or combinations thereof. In some embodiments, heat sources are placed at irregular intervals along one or more of the geometric patterns to provide the irregular spacing. In some embodiments, the heat sources are placed in an irregular geometric pattern. In some embodiments, the geometric pattern has irregular spacing between rows in the pattern to provide the irregular spacing of heat sources.

[1040] Increasing the efficiency of the treatment of the formation may include optimizing the heating schedule of the formation. As previously mentioned, the installation and maintenance of heat sources in a formation accounts for a significant percentage of the operating costs associated with the treatment of the formation. Maintenance may include the energy required by the heat sources to heat the formation. Previously, treatment of a formation included heating the formation with heat sources, the majority of which were typically turned on at the same time or at least within a relatively short time frame. In some embodiments, implementing a heating schedule may include heating the portion of the formation in phases. Different horizontal zones within the portion of the formation may be controlled independently and may be heated at different times during the treatment process. Different vertical zones within the portion of the formation may be controlled independently and may be heated at different times during the treatment process. Heat sources within different zones within a portion may start initiate their heating cycle at different times.

[1041] Heating in a first zone of the formation may be initiated using a first set of heat sources positioned in the first zone. Heating in a second zone of the formation may be initiated using a second set of heat sources positioned in the second zone. Heating may be initiated in the second zone after the first set of heat sources in the first zone have commenced heating the first zone. Heating in the first zone may continue after heating in the second zone initiates. In some embodiments, heating in the first zone may discontinue when, or at some point after, heating in the second zone initiates. When referring to the first zone or the second zone herein, this nomenclature should not be seen as limiting and these terms do not refer to the physical relation of the different zones to each other within the portion of the formation. In some embodiments, the portion of the formation may include two or more heating zones. For example, the portion of the formation may include 3, 4, 5, or 6 heating zones per portion of the formation. In certain embodiments, the portion of the formation includes 4 heating zones per portion of the formation. The heating zone may include one or more rows of heat sources. In some embodiments, heat produced by heat sources within different heating zones overlaps providing a cumulative heating effect upon the portion of the formation where the overlap occurs. Different portions of the formation may have different heat source patterns and/or numbers of heat sources within each zone.

[1042] In some embodiments, heater sequencing is used to increase efficiency by heating a bottom portion of the formation before heating an upper portion of the formation. Heating the bottom portion of the formation first may allow some in situ conversion of any hydrocarbons (for example, bitumen) in the bottom portion. As hydrocarbons products are produced from the bottom portion using productions wells positioned in the formation, hydrocarbons from the upper portion of the formation may be conveyed towards the bottom portion. In some embodiments, hydrocarbons from the upper portion that have been conveyed to the lower portion have not been heated by heat sources positioned in the upper portion.

[1043] In some embodiments, the lower portion of the formation includes approximately the lower third of the formation (not including the overburden). The upper portion may include approximately the upper two thirds of the formation (not including the overburden). In certain embodiments, about 20% or more heat flux per volume is injected into the lower portion than the upper portion over the first five years of treatment of the formation. For the entire formation, such injection may equate into about 15% less heat flux per volume for the first five years as compared to turning on all of the heaters at the same time using heaters with consistent heater spacing.

[1044] Greater heat flux per volume may be provided to one portion (for example, the lower portion) relative to another portion (for example, the upper portion) of the formation using several different methods. In some embodiments, the lower portion includes more heat sources than the upper portion. In some embodiments, heat sources in the lower portion provide heat for a longer period of time than heat sources in the upper portion of the formation. In some embodiments, heat sources in the lower portion provide more energy per heat source than heat sources in the upper portion. Any combination of the mentioned methods may be used to ensure greater heat flux to one portion of the formation relative to another portion of the formation.

[1045] Producing hydrocarbons from the lower portion first may create space in the lower formation for hydrocarbons from the upper portion to be conveyed by gravity to the lower portion. Not heating hydrocarbons in the upper portion of the formation may reduce over cracking or over pyrolyzing of these hydrocarbons, which may result in a better quality of produced hydrocarbons for the formation. Using such a strategy may result in a lower gas to oil ratio. In some embodiments, a greater reduction in the percentage of gas produced relative to the increase in the percentage of oil produced may result, but the overall total market value of the products may be greater.

[1046] In certain embodiments, hydrocarbons in the lower portion are pyrolyzed and produced first, and any pyrolyzation products (for example, gas products) resulting from the pyrolyzation process in the lower portion may move out of the lower portion into the upper portion. Products moving from the lower portion to the upper portion of the formation may result in pressure increasing in the upper portion. Pressure increases in the upper portion may result in increased permeability in the upper portion resulting in easier movement of hydrocarbons in the upper portion to the lower portion for pyrolyzation and/or production. Pyrolyzation products moving to the upper portion may heat the upper portion of the formation.

[1047] In certain embodiments, production wells are positioned in and/or substantially adjacent a lower portion of the formation. Positioning production wells in and/or substantially adjacent a lower portion of the formation facilitates production of hydrocarbons from the lower portion of the formation. Heat sources adjacent to the production well may be horizontally and/or vertically offset from the production well. In some embodiments, a horizontal row of heat sources is positioned at a depth equivalent to the depth of the production well. A row of multiple heat sources may also be positioned at a greater or lesser depth than the depth of the production well. Such an arrangement of heat sources relative to the production well may create channels within the formation for movement of mobilized and/or pyrolyzed hydrocarbons toward the production well.

[1048] FIG. 120 depicts a cross-sectional representation of substantially horizontal heaters 412 positioned in a pattern with consistent spacing in a hydrocarbon layer in the Grosmont formation. Horizontal heaters 412 are positioned in a consistently spaced pattern around and in relation to producer wells 206 in hydrocarbon layer 388 beneath overburden 400. Patterns with consistent spacing, typically horizontally and vertically, as depicted in FIG. 120 have been discussed previously. FIG. 121 depicts a cross-sectional representation of substantially horizontal heaters 412 positioned in a pattern with irregular spacing in hydrocarbon layer 388 in the Grosmont formation. Horizontal heaters 412 are positioned in an irregularly spaced pattern around and in relation to producer wells 206 in hydrocarbon layer 388 beneath overburden 400. In the embodiment depicted in FIG. 120, there are 16 horizontal heaters 412 per producer well 206. The pattern depicted in FIG. 121 includes four rows of heaters in four heating zones 628A-D. In the embodiment depicted in FIG. 121, vertical spacing between the different rows of heaters in heating zones 628A-D is irregular. There may be at least some to significant overlap of the heat between the rows of heaters. For example, heaters 412 in zones 628C-D may both heat the area of the formation positioned substantially between the two rows of heaters. In the embodiment depicted in FIG. 121, there are 18 horizontal heaters 412 per producer well 206.

[1049] Heaters 412 in the FIG. 120 embodiment may initiate heating the formation substantially within the same time frame. Heaters 412 in the FIG. 121 embodiment may employ a phased heating process for heating the formation. Heaters 412 in zones 628C-D may initiate first, heating the formation at the same time. Heaters 412 in zone 628B may initiate at a later date (for example, ˜104 days after the heaters in zones 628C-D), and finally followed by heaters 412 in zone 628A (for example, ˜593 days after the heaters in zones 628C-D).

[1050] FIG. 122 depicts a graphical representation of a comparison of the temperature and the pressure over time for two different portions of the formation using the different heating patterns. Curve 630 depicts the average temperature and curve 632 the average pressure during the treatment process using the consistently spaced heater pattern depicted in FIG. 120. Curve 634 depicts the average temperature and curve 636 the average pressure during the treatment process using the optimized heater pattern depicted in FIG. 121. FIG. 122 shows that average temperature and pressure are lower for the portion of the formation using the optimized heater pattern. The lower average temperature and pressure for the portion of the formation using the optimized heater pattern may explain the increased quality of oil produced by this portion.

[1051] FIG. 123 depicts a graphical representation of a comparison of the average temperature over time for different treatment areas for two different portions of the formation using the different heating patterns. Curves 638, 642, and 646 show the average temperature over time for the Upper Grosmont 3, the Upper Ireton, and Nisku areas, respectively, of the portion of the formation during the treatment process using the consistently spaced heater pattern depicted in FIG. 120. Curves 640, 644, and 648 show the average temperature over time for the Upper Grosmont 3, the Upper Ireton, and Nisku areas, respectively, of the portion of the formation during the treatment process using the optimized heater pattern depicted in FIG. 121. A lower average temperature is seen in FIG. 123 for the optimized heater pattern for the deeper Upper Grosmont 3 and Upper Ireton; however, the Nisku which is heated directly in the optimized heater pattern has a higher average temperature.

[1052] In the embodiment depicted in FIG. 120, the bottom-hole pressure was overall kept at a relatively high pressure, which varied greatly over the course of the treatment process. Additionally, the blowdown time was at greater than 2000 days and the upper layer of the hydrocarbon containing portion below the overburden was not heated for the embodiment depicted in FIG. 120. However, for the embodiment depicted in FIG. 121, the bottom-hole pressure was overall kept at a relatively low pressure which varied little for long periods of time over the course of the treatment process. The blowdown time was at ˜400 days and the upper layer of the hydrocarbon containing portion below the overburden was heated (see the heaters in zone 628A) for the embodiment depicted in FIG. 121. In some embodiments, the pressure in the formation is increased to between about 300 psi (about 2070 kPa) and about 500 psi (3450 kPa) for a period of time. The period of time may be 200 days to 600 days, 300 days to 500 days, or 350 days to 450 days. After the period of time has expired, the pressure in the formation may be decreased to between about 75 psi (about 515 kPa) and about 150 psi (about 1030 kPa). FIG. 124 depicts a graphical representation of the bottom-hole pressures over time for two producer wells (curves 650 and 652) associated with the heater pattern in FIG. 120 and for two producer wells (curves 654 and 656) associated with the heater pattern in FIG. 121. Some of the differences between the two treatment processes are summarized in TABLE 2.

[1053]

	TABLE 2
	Heater Pattern	Heater Pattern
	in FIG. 120	in FIG. 121
Number of Heaters/Producer	16	18
Heating Schedule	Constant heating	Phased heating
	of entire portion
	of formation
Blowdown Time	Late (>2000 days)
Bottom-Hole Pressure	High and variable	Low and steady
Heater Spacing	Consistent spacing	Variable horizontal
		and vertical spacing
Upper Area of Treated Portion	No direct heat	Directly heated with
		installed heaters

[1054] The differences between the heating process depicted in FIG. 120 and in FIG. 121 resulted in significant differences in the results of the treatment processes. In the optimized heating treatment process, depicted in FIG. 121, a preferably much lower gas-to-oil ratio (GOR) resulted relative to the treatment process depicted in FIG. 120. Heating in zone 628A increased liquid hydrocarbon production by ˜38% in the zone relative to a similar area in the treatment process depicted in FIG. 120. In addition, overall oil production was increased and the bitumen fraction decreased for the optimized heating treatment process FIG. 121 relative to the FIG. 120 treatment process.

[1055] FIG. 125 depicts a graphical representation of a comparison of the cumulative oil and gas products extracted over time from two different portions of the formation using the different heating patterns. Curves 658 and 662 show the cumulative oil and gas products, respectively, extracted over time for the portion of the formation using the consistently spaced heater pattern depicted in FIG. 120. Curves 660 and 664 show the cumulative oil and gas products, respectively, extracted over time for the portion of the formation using the optimized heater pattern depicted in FIG. 121. The optimized heater pattern produced significantly more oil, but less gas, due to the lower operating temperatures and less pyrolyzation of the hydrocarbons. Some of the differences between the results of using the two treatment processes are summarized in TABLE 3.

[1056]

	TABLE 3
	Heater Pattern	Heater Pattern	Percent
	in FIG. 120	in FIG. 121	Change
Cumulative Oil (bbl)	58,891	78,746	33.7%
Cumulative TB (bbl)	16,802	17,771	5.8%
Cumulative HO (bbl)	22,051	32,577	47.7%
Cumulative LO (bbl)	19,263	27,879	44.7%
Cumulative Gas	104.0	69.5	−33.2%
(MMscf)
Cumulative Heat	80,715	77,577	−3.9%
(MMBTU)
Heat Efficiency	0.73	1.02	39.7%
(bbl/MMBTU)
API	22.9	24.6	7.4%
NPV ($MM)	1.54	2.17	40.9%
NPV/Capital Expenses	4.47	5.64	26.2%
NPV/(Capital Expenses +	1.18	1.64	39.0%
Operating Expenses)

[1057] The increases in quantity and quality in liquid hydrocarbons for the optimized heating treatment process resulted in an increase of ˜$1 billion in net present value (NPV). Net present value may be roughly calculated using EQN. 10:
NPV=Σ{Annually Discounted(oil revenue−operating expenses−energy expenses)}−wellbore capital expenses.  EQN. (10)

[1058] FIG. 126 depicts a cross-sectional representation of another embodiment of substantially horizontal heaters 412 positioned in a pattern with irregular spacing in hydrocarbon layer 388 in the Grosmont formation. Horizontal heaters 412 are positioned in an irregularly spaced pattern around and in relation to producer wells 206 beneath overburden 400. The pattern depicted in FIG. 126 includes five rows of heaters in five heating zones 628A-E. In the embodiment depicted in FIG. 126, vertical spacing between the different rows of heaters in heating zones 628A-E is irregular. There may be at least some to significant overlap of the heat between the rows of heaters. For example, heaters 412 in zones 628C-E may both heat the area of the formation positioned substantially between the three rows of heaters. In the embodiment depicted in FIG. 126, there are 18 horizontal heaters 412 per producer well 206 as in the irregularly spaced four row heater pattern depicted in FIG. 121.

[1059] Heaters 412 in the FIG. 126 embodiment may employ a phased heating process for heating the formation similar to the embodiment depicted in FIG. 121. Heaters 412 in zone 628E may initiate first. Heaters 412 in zone 628D may initiate at a later date (for example, ˜5 days after the heaters in zone 628E), followed by heaters 412 in zone 628C (for example, ˜57 days after the heaters in zone 628E). Heaters 412 in zone 628B may initiate at a later date (for example, ˜391 days after the heaters in zone 628E), finally followed by heaters 412 in zone 628A (for example, ˜547 days after the heaters in zone 628E).

[1060] FIG. 127 depicts a cross-sectional representation of yet another embodiment substantially horizontal heaters 412 positioned in a pattern with irregular spacing in hydrocarbon layer 388 in an hydrocarbon layer. In an embodiment, the hydrocarbon layer is a portion of the Grosmont formation. The pattern depicted in FIG. 127 includes four rows of heaters in four heating zones 628A-D. In the embodiment depicted in FIG. 127, vertical spacing between the different rows of heaters in heating zones 628A-D is irregular. In the embodiment depicted in FIG. 127, there are 17 horizontal heaters 412 per producer well 206.

[1061] Heaters 412 in the FIG. 127 embodiment may employ a phased heating process for heating the formation similar to the embodiment depicted in FIG. 121. Heaters 412 in zones 628C-D may initiate first. Heaters 412 in zone 628B may initiate at a later date (for example, ˜17 days after the heaters in zones 628C-D), followed by heaters 412 in zone 628A (for example, ˜411 days after the heaters in zones 628C-D).

[1062] FIG. 128 depicts a cross-sectional representation of another additional embodiment of substantially horizontal heaters 412 positioned in a pattern with irregular spacing in hydrocarbon layer 388 in the Grosmont formation. The pattern depicted in FIG. 128 includes four rows of heaters in four heating zones 628A-D. In the embodiment depicted in FIG. 128, vertical spacing between the different rows of heaters in heating zones 628A-D is irregular. In the embodiment depicted in FIG. 128, there are 15 horizontal heaters 412 per producer well 206.

[1063] Heaters 412 in the FIG. 128 embodiment may employ a phased heating process for heating the formation, similar to the embodiment depicted in FIG. 121. Heaters 412 in zones 628C-D may initiate first. Heaters 412 in zone 628B may initiate at a later date (for example, ˜46 days after the heaters in zones 628C-D), followed by heaters 412 in zone 628A (for example, ˜291 days after the heaters in zones 628C-D). A comparison of some of the results of the different optimized heating patterns are summarized in TABLE 4. TABLE 4 shows that different patterns of heaters have real impact on the overall efficiency and profitability of the treatment process for subsurface hydrocarbon containing formations. As shown in TABLE 4, using fewer heaters does not necessarily lead to the most desirable result (for example, higher NPV values). In certain embodiments, the most efficient heater pattern for certain formations appear to be the heater pattern depicted in FIG. 121.

[1064]

	TABLE 4
	Heater Pattern	Heater Pattern	Heater Pattern	Heater Pattern
	in FIG. 121	in FIG. 126	in FIG. 127	in FIG. 128
No. of Heaters/Producer	18	18	17	15
Capital Expenses	384,000	384,000	364000	324,000
NPV ($MM)	2.17	1.98	1.90	1.68
NPV/Capital Expenses	5.64	5.15	5.30	5.18
IRR	0.67	0.60	0.63	0.67
Max. Pressure	471.3	608.69	686.3	572.2
Cum. Oil (bbl)	78,745.9	71,107.9	67,551.48	60,132.5
API	24.6	27.94	23.16	21.6
NPV/(Capital Expenses +	1.64	1.50	1.54	1.50
Operating Expenses)

[1065] FIG. 129 depicts a cross-sectional representation of another embodiment of substantially horizontal heaters 412 positioned in a pattern with consistent spacing in hydrocarbon layer 388 (similar to the heater pattern in 120) in the Peace River formation. In the embodiment depicted in FIG. 129, there are 9 horizontal heaters 412 per producer well 206. FIG. 130 depicts a cross-sectional representation of an embodiment of substantially horizontal heaters 412 positioned in a pattern with irregular spacing in hydrocarbon layer 388, with three rows of heaters in three heating zones 628A-C. In the embodiment depicted in FIG. 130, vertical spacing between the different rows of heaters in heating zones 628A-C is irregular. In the embodiment depicted in FIG. 130, there are 13 horizontal heaters 412 per producer well 206.

[1066] Heaters 412 in the FIG. 130 embodiment may employ a phased heating process for heating the formation similar to the embodiment depicted in FIG. 121 in the Peace River formation. Heaters 412 in zone 628C may initiate first. Heaters 412 in zone 628A may initiate at a later date (for example, ˜53 days after the heaters in zone 628C), followed by heaters 412 in zone 628B (for example, ˜93 days after the heaters in zone 628C). The optimized heating pattern depicted in FIG. 130 (NPV was 5.57) demonstrated greater efficiency than the heating pattern depicted in FIG. 129 (NPV was 1.05).

[1067] In some embodiments, when optimizing the heating of the portion of the formation, certain limiting variables are taken into consideration. The pressure in the upper area of the portion of the formation may be limited. Imposing limits on the pressure in the upper portion of the formation may inhibit the overburden from pyrolyzation and allowing products from the treatment process to escape in an uncontrolled manner. Pressure in the upper area of the portion limited to less than or equal to about 1500 psi (about 10 MPa), about 1250 psi (about 8.6 MPa), about 1000 psi (about 6.9 MPa), about 750 psi (about 5.2 MPa), or about 500 psi (about 3.4 MPa). In some embodiments, pressure in the upper area of the portion of the formation may be maintained at about 750 psi (about 5.2 MPa) or less.

[1068] In some embodiments, bottom-hole pressure may need to be maintained greater than or equal to a particular pressure. Bottom-hole pressure, in some examples, may need to be maintained during production at or above about 250 psi (about 1.7 MPa), about 170 psi (about 1.2 MPa), about 115 psi (about 800 kPa), or about 70 psi (about 480 kPa). In some embodiments, a desired bottom-hole pressure may be maintained at or above about 115 psi (about 800 kPa). The minimum bottom-hole pressure required may be dependent on a number of factors, for example, type of formation or the type of hydrocarbons contained in the formation.

[1069] A downhole heater assembly may include 5, 10, 20, 40, or more heaters coupled together. For example, a heater assembly may include between 10 and 40 heaters. Heaters in a downhole heater assembly may be coupled in series. In some embodiments, heaters in a heater assembly may be spaced from about 8 meters (about 25 feet) to about 60 meters (about 195 feet) apart. For example, heaters in a heater assembly may be spaced about 15 meters (about 50 feet) apart. Spacing between heaters in a heater assembly may be a function of heat transfer from the heaters to the formation. Spacing between heaters may be chosen to limit temperature variation along a length of a heater assembly to acceptable limits. Heaters in a heater assembly may include, but are not limited to, electrical heaters, flameless distributed combustors, natural distributed combustors, and/or oxidizers. In some embodiments, heaters in a downhole heater assembly may include only oxidizers.

[1070] Fuel may be supplied to oxidizers a fuel conduit. In some embodiments, the fuel for the oxidizers includes synthesis gas, non-condensable gases produced from treatment area of in situ heat treatment processes, air, enriched air, or mixtures thereof. In some embodiments, the fuel includes synthesis gas (for example, a mixture that includes hydrogen and carbon monoxide) that was produced using an in situ heat treatment process. In certain embodiments, the fuel may include natural gas mixed with heavier components such as ethane, propane, butane, or carbon monoxide. In some embodiments, the fuel and/or synthesis gas may include non-combustible gases such as nitrogen. In some embodiments, the fuel contains products from a coal or heavy oil gasification process. The coal or heavy oil gasification process may be an in situ process or an ex situ process. After initiation of combustion of fuel and oxidant mixture in oxidizers, composition of the fuel may be varied to enhance operational stability of the oxidizers.

[1071] The non-condensable gases may include combustible gases (for example, hydrogen, hydrogen sulfide, methane and other hydrocarbon gases) and noncombustible gases (for example, carbon dioxide). The presence of noncombustible gases may inhibit coking of the fuel and/or may reduce the flame zone temperature of oxidizers when the fuel is used as fuel for oxidizers of downhole oxidizer assemblies. The reduced flame zone temperature may inhibit formation of NOx compounds and/or other undesired combustion products by the oxidizers. Other components such as water may be included in the fuel supplied to the burners. Combustion of in situ heat treatment process gas may reduce and/or eliminate the need for gas treatment facilities and/or the need to treat the non-condensable portion of formation fluid produced using the in situ heat treatment process to obtain pipeline gas and/or other gas products. Combustion of in situ heat treatment process gas in burners may create concentrated carbon dioxide and/or SO_x effluents that may be used in other processes, sequestered and/or treated to remove undesired components.

[1072] In certain embodiments, fuel used to initiate combustion may be enriched to decrease the temperature required for ignition or otherwise facilitate startup of oxidizers. In some embodiments, hydrogen or other hydrogen rich fluids may be used to enrich fuel initially supplied to the oxidizers. After ignition of the oxidizers, enrichment of the fuel may be stopped. In some embodiments, a portion or portions of a fuel conduit may include a catalytic surface (for example, a catalytic outer surface) to decrease an ignition temperature of fuel.

[1073] In some embodiments, oxygen is produced through the decomposition of water. For example, electrolysis of water produces oxygen and hydrogen. Using water as a source of oxygen provides a source of oxidant with minimal or no carbon dioxide emissions. The produced hydrogen may be used as a hydrogenation fluid for treating hydrocarbon fluids in situ or ex situ, a fuel source and/or for other purposes. FIG. 131 depicts a schematic representation of an embodiment of a system for producing oxygen using electrolysis of water for use in an oxidizing fluid provided to burners that heat treatment area 666. Water stream 668 enters electrolysis unit 670. In electrolysis unit 670, current is applied to water stream 668 and produces oxygen stream 672 and hydrogen stream 674. In some embodiments, electrolysis of water stream 668 is performed at temperatures ranging from about 600° C. to about 1000° C., from about 700° C. to about 950° C., or from 800° C. to about 900° C. In some embodiments, electrolysis unit 670 is powered by nuclear energy and/or a solid oxide fuel cell and/or a molten salt fuel cell. The use of nuclear energy and/or a solid oxide fuel cell and/or a molten salt fuel cell provides a heat source with minimal and/or no carbon dioxide emissions. High temperature electrolysis may generate hydrogen and oxygen more efficiently than conventional electrolysis because energy losses resulting from the conversion of heat to electricity and electricity to heat are avoided by directly utilizing the heat produced from the nuclear reactions without producing electricity. Oxygen stream 672 mixes with mixed oxidizing fluid 676 and/or is mixed with oxidizing fluid 678. A portion or all of hydrogen stream 674 may be recycled to electrolysis unit 670 and used as an energy source. A portion or all of hydrogen stream 674 may be used for other purposes such as, but not limited to, a fuel for burners and/or a hydrogen source for in situ or ex situ hydrogenation of hydrocarbons.

[1074] Exhaust gas 680 from burners used to heat treatment area 666 may be directed to exhaust treatment unit 682. Exhaust gas 680 may include, but is not limited to, carbon dioxide and/or SO_x. In exhaust separation unit 682, carbon dioxide stream 684 is separated from SO_x stream 686. Separated carbon dioxide stream 684 may be mixed with diluent fluid 688, may be used as a carrier fluid for oxidizing fluid 678, may be used as a drive fluid for producing hydrocarbons, and/or may be sequestered. SO_x stream 686 may be treated using known SO_x treatment methods (for example, sent to a Claus plant). Formation fluid 212′ produced from heat treatment area 666 may be mixed with formation fluid 212 from other treatment areas and/or formation fluid 212′ may enter separation unit 214. Separation unit 214 may separate the formation fluid into in situ heat treatment process liquid stream 216, in situ heat treatment process gas 218, and aqueous stream 220. Gas separation unit 222 may remove one or more components from in situ heat treatment process gas 218 to produce fuel 690 and one or more other streams 692. Fuel 690 may include, but is not limited to, hydrogen, sulfur compounds, hydrocarbons having a carbon number of at most 5, carbon oxides, nitrogen compounds, or mixtures thereof. In some embodiments, gas separation unit 222 uses chemical and/or physical treatment systems to remove or reduce the amount of carbon dioxide in fuel 690. Fuel 690 may enter fuel conduit 520 that provides fuel to oxidizers of oxidizer assemblies that heat treatment area 666.

[1075] In some embodiments, electrolysis unit 670 is powered by nuclear energy. Nuclear energy may be provided by a number of different types of available nuclear reactors and nuclear reactors currently under development (for example, generation IV reactors). In some embodiments, nuclear reactors may include a self-regulating nuclear reactor. Self-regulating nuclear reactors may include a fissile metal hydride which functions as both fuel for the nuclear reaction as well as a moderator for the nuclear reaction. The nuclear reaction may be moderated by the temperature driven mobility of the hydrogen isotope contained in the hydride. Self-regulating nuclear reactors may produce thermal power on the order of tens of megawatts per unit. Self-regulating nuclear reactors may operate at a maximum fuel temperature ranging from about 400° C. to about 900° C., from about 450° C. to about 800° C., and from about 500° C. to about 600° C. Self-regulating nuclear reactors have several advantages including, but not limited to, a compact/modular design, ease of transport, and a simple cost effective design.

[1076] In some embodiments, nuclear reactors may include one or more very high temperature reactors (VHTRs). VHTRs may use helium as a coolant to drive a gas turbine for treating hydrocarbon fluids in situ, powering electrolysis unit 670 and/or for other purposes. VHTRs may produce heat for electrolysis units up to about 950° C. or more. In some embodiments, nuclear reactors may include a sodium-cooled fast reactor (SFR). SFRs may be designed on a smaller scale (for example, 50 MWe), and therefore are more cost effective to manufacture on site for treating hydrocarbon fluids in situ, powering electrolysis units and/or for other purposes. SFRs may be of a modular design and potentially portable. SFRs may produce heat for electrolysis units ranging from about 500° C. to about 600° C., from about 525° C. to about 575° C., or from 540° C. to about 560° C.

[1077] In some embodiments, pebble bed reactors may be employed to provide heat for electrolysis. Pebble bed reactors may produce up to about 165 MWe. Pebble bed reactors may produce heat for electrolysis units ranging from about 500° C. to about 1100° C., from about 800° C. to about 1000° C., or from about 900° C. to about 950° C. In some embodiments, nuclear reactors may include supercritical-water-cooled reactors (SCWRs) based at least in part on previous light water reactors (LWR) and supercritical fossil-fired boilers. In some embodiments, SCWRs may be employed to provide heat for electrolysis. SCWRs may produce heat for electrolysis units ranging from about 400° C. to about 650° C., from about 450° C. to about 550° C., or from about 500° C. to about 550° C.

[1078] In some embodiments, nuclear reactors may include lead-cooled fast reactors (LFRs). In some embodiments, LFRs may be employed to provide heat for electrolysis. LFRs may be manufactured in a range of sizes, from modular systems to several hundred megawatt or more sized systems. LFRs may produce heat for electrolysis units ranging from about 400° C. to about 900° C., from about 500° C. to about 850° C., or from about 550° C. to about 800° C.

[1079] In some embodiments, nuclear reactors may include molten salt reactors (MSRs). In some embodiments, MSRs may be employed to provide heat for electrolysis. MSRs may include fissile, fertile, and fission isotopes dissolved in a molten fluoride salt with a boiling point of about 1,400° C. which function as both the reactor fuel and the coolant. MSRs may produce heat for electrolysis units ranging from about 400° C. to about 900° C., from about 500° C. to about 850° C., or from about 600° to about 800° C.

[1080] In some embodiments, pulverized coal is the fuel used to heat the subsurface formation. The pulverized coal may be carried into the wellbores with a non-oxidizing fluid (for example, carbon dioxide and/or nitrogen). An oxidant may be mixed with the pulverized coal at several locations in the wellbore. The oxidant may be air, oxygen enriched air and/or other types of oxidizing fluids. Igniters located at or near the mixing locations initiate oxidation of the coal and oxidant. The igniters may be catalytic igniters, glow plugs, spark plugs, and/or electrical heaters (for example, an insulated conductor temperature limited heater with heating sections located at mixing locations of pulverized coal and oxidant) that are able to initiate oxidation of the oxidant with the pulverized coal.

[1081] The particles of the pulverized coal may be small enough to pass through flow orifices and achieve rapid combustion in the oxidant. The pulverized coal may have a particle size distribution from about 1 micron to about 300 microns, from about 5 microns to about 150 microns, or from about 10 microns to about 100 microns. Other pulverized coal particle size distributions may also be used. At 600° C., the time to burn the volatiles in pulverized coal with a particle size distribution from about 10 microns to about 100 microns may be about one second.

[1082] In certain embodiments, a heater is located in a u-shaped wellbore or an L-shaped wellbore. The heater may include a heating section that is moved during treatment of the formation. Moving the heating section during treatment of the formation allows the heating section to be used over a wide area of the formation. Using the movable heating section may allow the heating section (and/or heater) to be significantly shorter in length than the length of the wellbore. The shorter heating section may reduce equipment costs and/or operating costs of the heater as compared to a longer heating section (for example, a heating section that has a length nearly as long as the length of the wellbore).

[1083] FIG. 132 depicts an embodiment of heater 412 with heating section 694 located in a u-shaped wellbore. Heater 412 is located in opening 386. In certain embodiments, opening 386 is a u-shaped opening with a substantially horizontal or inclined section in hydrocarbon layer 388 below overburden 400. Heater 412 may be a u-shaped heater with ends that extend out of both legs of the wellbore. In certain embodiments, heater 412 is an electrical resistance heater (a heater that provides heat by electrical resistance heating when energized with electrical current). In some embodiments, heater 412 is an oxidation heater (for example, a heater that oxidizes (combusts) fluids to produce heat). In certain embodiments, heater 412 is a circulating fluid heater such as a molten salt circulating heater.

[1084] In certain embodiments, heater 412 includes heating section 694. Heating section 694 may be the portion of heater 412 that provides heat to hydrocarbon layer 388. In certain embodiments, heating section 694 is the portion of heater 412 that has a higher electrical resistance than the rest of the heater such that the heating section is the only portion of the heater that provides substantial heat output to hydrocarbon layer 388. In some embodiments, heating section 694 is the portion of the heater that includes a downhole oxidizer (for example, downhole burner) or a plurality of downhole oxidizers. Other portions of heater 412 may be non-heating portions of the heater (for example, lead-in or lead-out sections of the heater) or portions of the heater that provide negligible heat output.

[1085] In certain embodiments, heater 412 is similar in length to the horizontal portion of opening 386 and heating section 694 is the portion of heater 412 shown in FIG. 132. Thus, heating section 694 is short in length compared to the horizontal portion of opening 386. In some embodiments, heating section 694 extends along the entire horizontal portion of heater 412 (or nearly the entire horizontal portion of the heater) and the heater is short in length compared to the horizontal portion of opening 386 such that the heating section is shorter in length than the horizontal portion of the opening.

[1086] In some embodiments, heating section 694 is at most ½ the length of the horizontal portion of opening 386, at most ¼ the length of the horizontal portion of opening 386, or at most ⅕ the length of the horizontal portion of opening 386. For example, the horizontal portion of opening 386 in hydrocarbon layer 388 may be between about 1500 m and about 3000 m in length and heating section 694 may be between about 300 m and about 500 m in length.

[1087] Having shorter heating section 694 allows heat to be provided to a small portion of hydrocarbon layer 388. The portion of hydrocarbon layer 388 heated by heating section 694 may be first volume 696. First volume 696 may be created around heater 412 proximate heating section 694.

[1088] In certain embodiments, heater 412 and heating section 694 are moved to provide heat to another portion of the formation. FIG. 133 depicts heater 412 with heating section 694 moved to heat second volume 698. In some embodiments, heating section 694 is moved by pulling heater 412 from one end of opening 386 (for example, pulling the heater from the left end of the opening, as shown in FIG. 133). In certain embodiments, heater 412 and heating section 694 are moved further to provide heat to third volume 700, as shown in FIG. 134.

[1089] In certain embodiments, first volume 696, second volume 698, and third volume 700 are heated sequentially from the first volume to the third volume. In some embodiments, portions of the volumes may overlap depending on the moving rate (movement speed) of heater 412 and heating section 694. In certain embodiments, heater 412 and heating section 694 are moved at a controlled rate. For example, heater 412 and heating section 694 may be moved after treating first volume 696 for a selected period of time or after a selected temperature is reached in the first volume.

[1090] Moving heater 412 and heating section 694 at the controlled rate may provide controlled heating in hydrocarbon layer 388. In some embodiments, the moving rate is controlled to control the amount of mobilization in hydrocarbon layer 388, first volume 696, second volume 698, and/or third volume 700. In some embodiments, the moving rate is controlled to control the amount of pyrolyzation in hydrocarbon layer 388, first volume 696, second volume 698, and/or third volume 700. The moving rate when mobilizing may be faster than the moving rate when pyrolyzing as more heat needs to be provided in a selected volume of the formation to result in pyrolyzation of hydrocarbons in the selected volume. In general, the moving rate of heater 412 and heating section 694 is controlled to achieve desired heating results for treatment of hydrocarbon layer 388. The moving rate may be determined, for example, by assessing treatment of hydrocarbon layer 388 using simulations and/or other calculations.

[1091] In certain embodiments, heater 412 is a u-shaped heater that is moved (for example, pulled) through u-shaped opening 386, as shown in FIGS. 132-134. In some embodiments, heater 412 is an L-shaped or J-shaped heater that is moved through a u-shaped opening (for example, the heater may be shaped like the heater depicted in FIG. 134). The L-shaped or J-shaped heater may be moved by either pulling or pushing the heater from either end of the u-shaped opening.

[1092] In some embodiments, heater 412 is an L-shaped or J-shaped heater that is moved through an L-shaped or J-shaped opening. FIGS. 135-137 depict movement of L-shaped or J-shaped heater 412 as the heater is moved through opening 386 to heat first volume 696, second volume 698, and third volume 700.

[1093] FIG. 138 depicts an embodiment with two heaters 412A, 412B located in u-shaped opening 386. Heaters 412A, 412B may have heating sections 694A, 694B, respectively. Heaters 412A, 412B and heating sections 694A, 694B may be moved (pulled) away from each other, as shown by the arrows in FIG. 138. Moving heating sections 694A, 694B in opposite directions may create heated volumes in hydrocarbon layer 388 on each side of the middle of opening 386. In some embodiments, the heated volumes created by heating section 694A may substantially mirror the heated volumes created by heating section 694B. Thus, mirrored heated volumes may be sequentially created going in opposite directions from the middle of opening 386 by moving heating sections 694A, 694B away from each other at a controlled rate.

[1094] In certain embodiments, movable heaters allow for closer spacing between heaters during early phases of in situ heat treatment without increasing the number of wellbores in the formation. The closer spacing is possible because of overlapping heating sections during the early phases of treatment. FIG. 139 depicts a top view of treatment area 666 treated using non-overlapping heating sections 694A, 694B in heaters 412A, 412B. As shown in FIG. 139, heaters 412A, 412B are L-shaped or J-shaped heaters located substantially horizontal or at an incline in the formation. Heaters 412A, 412B extend from build sections 702A, 702B, respectively.

[1095] In an embodiment, heating sections 694A, 694B heat in two phases. The solid sections of heaters 412A, 412B, shown as heating sections 694A, 694B in FIG. 139, are the first phase of heating. The solid sections provide heat in the center portion of treatment area 666. Heating sections 694A, 694B in the first phase are located end-to-end (the ends of the heating sections abut but do not touch) and do not overlap, as shown in FIG. 139. The cross-hatched sections of heaters 412A, 412B are the second phase of heating. In the second phase of heating, heating sections 694A, 694B move into the cross-hatched sections of heaters 412A, 412B to heat the edge portions of treatment area 666. In the embodiment depicted in FIG. 139, 18 heaters 412A, 412B are used to heat treatment area 666.

[1096] FIG. 140 depicts a top view of treatment area 666 treated using overlapping heating sections 694A, 694B in the first phase of heating using heaters 412A, 412B. In the embodiment depicted in FIG. 140, heaters 412A, 412B heat treatment area 666 in two phases such as in the embodiment depicted in FIG. 139. In the first phase, however, heating sections 694A, 694B overlap and are located adjacent to each other, as shown in FIG. 140. Thus, heating sections 694A, 694B (and heaters 412A, 412B) have closer spacing during the first phase in the embodiment depicted in FIG. 140 than the embodiment depicted in FIG. 139. For example, heating sections 694A, 694B shown in FIG. 140 have half the spacing of the heating sections shown in FIG. 139. In addition, heat provided by heating sections 694A during the first phase in the embodiment depicted in FIG. 140 overlaps with heat provided by heating sections 694B, which also increases the heat provided to the center portion of treatment area 666. The closer spacing may accelerate heating of the center portion of treatment area 666 without increasing the number of heaters 412A, 412B in the treatment area (there are still 18 heaters in the embodiment depicted in FIG. 140). In addition, heat provided by heating sections 694A during the first phase in the embodiment depicted in FIG. 140 overlaps with heat provided by heating sections 694B, which increases the heat provided to the center portion of treatment area 666. During the second phase of heating, heating sections 694A, 694B (the cross-hatched sections) in the embodiment depicted in FIG. 140 may have similar spacing as the second phase heating sections in the embodiment depicted in FIG. 139.

[1097] As shown in the embodiment depicted in FIG. 140, build section 702B may be moved closer to build section 702A in order to achieve the closer heater spacing in the first phase of heating. Thus, the volume of treatment area 666 heated during the two phases of heating may be smaller than the volume heated in the embodiment depicted in FIG. 139. In certain embodiments, additional heaters may be placed in remaining volume 704 of treatment area 666. These additional heaters may heat remaining volume 704 such that a similar volume of treatment area 666 is heated in the embodiment depicted in FIG. 140 as the volume heated in the embodiment depicted in FIG. 139. The additional heaters used to heat remaining volume 704, depicted in FIG. 140, may be placed in the formation at later times during treatment of the formation. The additional heaters may have a discounted cost compared to heaters placed in the formation at earlier times.

[1098] In some embodiments, fast fluidized transport line systems may be used for subsurface heating. Fast fluidized transport line systems may have significantly higher overall energy efficiency as compared to using electrical heating. The systems may have high heat transfer efficiency. Low value fuel (for example, bitumen or pulverized coal) may be used as the heat source. Solid transport line circulation is commercially proven technology having relatively reliable operation.

[1099] Fast fluidized transport systems may include one or more combustion units, wellbores, a treatment area, and piping to transport fluidized material from the combustion units through the wellbores to heat the treatment area. In some embodiments, one or more of combustion units used to heat the formation are furnaces, nuclear reactors, or other high temperature heat sources. Such combustion units heat fluidized material that passes through the combustion units. Each combustion unit may provide hot fluidized material to a large number of u-shaped wellbores. For example, one combustion unit may supply hot fluidized material to 20 or more u-shaped wellbores. In some embodiments, the u-shaped wellbores are formed so that the surface footprint has long rows of inlet and exit legs of u-shaped wellbores. The exit legs and inlet legs of these u-shaped wellbores are located in adjacent rows. Additional fluidized transport systems would be located on the same row to supply all of the u-shaped wellbores on the row. Also, additional fluidized transport systems would be positioned on adjacent rows to supply inlet legs and outlet legs of the adjacent rows.

[1100] Fluidized material may include coal particles (for example, pulverized coal), other hydrocarbon or carbon containing material (for example, bitumen and coke), and heat carrier particles. The heat carrier particles may include, but are not limited to, sand, silica, ceramic particles, waste fluidized catalytic cracking catalyst, other particles used for heat transfer, or mixtures thereof. In some embodiments, the particle range distribution of the fluidized material may span from between about 5 and 200 microns.

[1101] A portion of the hydrocarbon content in fluidized material may combust and/or pyrolyze in the combustion units. Fluidized material may still have a significant carbon (coke) and/or hydrocarbon content after passing through the combustion unit. The oxidant may react with the carbon and/or hydrocarbons in the fluidized material in the u-shaped conduits. The combustion of hydrocarbons and carbon in the fluidized material may maintain a high temperature of the fluidized material and/or generate heat that transfers to the formation.

[1102] Gas lifting may facilitate transport of the fluidized material in the u-shaped conduits. Multiple valves in the outlet legs may allow entry of lift gas into the outlet legs to transport the fluidized material to the treatment area. In some embodiments, the lift gas is air. Other gases may be used as the lift gas.

[1103] In some in situ heat treatment process embodiments, a circulation system is used to heat the formation. Using the circulation system for in situ heat treatment of a hydrocarbon containing formation may reduce energy costs for treating the formation, reduce emissions from the treatment process, and/or facilitate heating system installation. In certain embodiments, the circulation system is a closed loop circulation system. FIG. 141 depicts a schematic representation of a system for heating a formation using a circulation system. The system may be used to heat hydrocarbons that are relatively deep in the ground and that are in formations that are relatively large in extent. In some embodiments, the hydrocarbons may be 100 m, 200 m, 300 m or more below the surface. The circulation system may also be used to heat hydrocarbons that are shallower in the ground. The hydrocarbons may be in formations that extend lengthwise up to 1000 m, 3000 m, 5000 m, or more. The heaters of the circulation system may be positioned relative to adjacent heaters such that superposition of heat between heaters of the circulation system allows the temperature of the formation to be raised at least above the boiling point of aqueous formation fluid in the formation.

[1104] In some embodiments, heaters 412 are formed in the formation by drilling a first wellbore and then drilling a second wellbore that connects with the first wellbore. Piping may be positioned in the u-shaped wellbore to form u-shaped heater 412. Heaters 412 are connected to heat transfer fluid circulation system 706 by piping. In some embodiments, the heaters are positioned in triangular patterns. In some embodiments, other regular or irregular patterns are used. Production wells and/or injection wells may also be located in the formation. The production wells and/or the injection wells may have long, substantially horizontal sections similar to the heating portions of heaters 412, or the production wells and/or injection wells may be otherwise oriented (for example, the wells may be vertically oriented wells, or wells that include one or more slanted portions).

[1105] As depicted in FIG. 141, heat transfer fluid circulation system 706 may include heat supply 708, first heat exchanger 710, second heat exchanger 712, and fluid movers 714. Heat supply 708 heats the heat transfer fluid to a high temperature. Heat supply 708 may be a furnace, solar collector, chemical reactor, nuclear reactor, fuel cell, and/or other high temperature source able to supply heat to the heat transfer fluid. If the heat transfer fluid is a gas, fluid movers 714 may be compressors. If the heat transfer fluid is a liquid, fluid movers 714 may be pumps. After exiting formation 492, the heat transfer fluid passes through first heat exchanger 710 and second heat exchanger 712 to fluid movers 714. First heat exchanger 710 transfers heat between heat transfer fluid exiting formation 492 and heat transfer fluid exiting fluid movers 714 to raise the temperature of the heat transfer fluid that enters heat supply 708 and reduce the temperature of the fluid exiting formation 492. Second heat exchanger 712 further reduces the temperature of the heat transfer fluid. In some embodiments, second heat exchanger 712 includes or is a storage tank for the heat transfer fluid.

[1106] Heat transfer fluid passes from second heat exchanger 712 to fluid movers 714. Fluid movers 714 may be located before heat supply 708 so that the fluid movers do not have to operate at a high temperature.

[1107] In an embodiment, the heat transfer fluid is carbon dioxide. Heat supply 708 is a furnace that heats the heat transfer fluid to a temperature in a range from about 700° C. to about 920° C., from about 770° C. to about 870° C., or from about 800° C. to about 850° C. In an embodiment, heat supply the heat transfer fluid to a temperature of about 820° C. The heat transfer fluid flows from heat supply 708 to heaters 412. Heat transfers from heaters 412 to formation 492 adjacent to the heaters. The temperature of the heat transfer fluid exiting formation 492 may be in a range from about 350° C. to about 580° C., from about 400° C. to about 530° C., or from about 450° C. to about 500° C. to about the temperature of the heat transfer fluid exiting formation 492 is about 480° C. The metallurgy of the piping used to form heat transfer fluid circulation system 706 may be varied to significantly reduce costs of the piping. High temperature steel may be used from heat supply 708 to a point where the temperature is sufficiently low so that less expensive steel can be used from that point to first heat exchanger 710. Several different steel grades may be used to form the piping of heat transfer fluid circulation system 706.

[1108] In some embodiments, solar salt (for example, a salt containing 60 wt % NaNO₃ and 40 wt % KNO₃) is used as the heat transfer fluid in the circulated fluid system. Solar salt may have a melting point of about 230° C. and an upper working temperature limit of about 565° C. In some embodiments, LiNO₃ (for example, between about 10% by weight and about 30% by weight LiNO₃) may be added to the solar salt to produce tertiary salt mixtures with wider operating temperature ranges and lower melting temperatures with only a slight decrease in the maximum working temperature as compared to solar salt. The lower melting temperature of the tertiary salt mixtures may decrease the preheating requirements and allow the use of pressurized water and/or pressurized brine as a heat transfer fluid for preheating the piping of the circulation system. The corrosion rates of the metal of the heaters due to the tertiary salt compositions at 550° C. is comparable to the corrosion rate of the metal of the heaters due to solar salt at 565° C. TABLE 5 shows melting points and upper limits for solar salt and tertiary salt mixtures. Aqueous solutions of tertiary salt mixtures may transition into a molten salt upon removal of water without solidification, thus allowing the molten salt to be provided and/or stored as aqueous solutions.

[1109]

TABLE 5
			Upper working
	Composition of NO3	Melting Point	temperature limit
NO3 Salt	Salt (weight %)	(° C.) of NO3 salt	(° C.) of NO3 salt
Na:K	60:40	230	600
Li:Na:K	12:18:70	200	550
Li:Na:K	20:28:52	150	550
Li:Na:K	27:33:40	160	550
Li:Na:K	30:18:52	120	550