Mines and tunnels for use in treating subsurface hydrocarbon containing formations

[0001] This patent application is a continuation of U.S. patent application Ser. No. 12/422,094 filed Apr. 10, 2009, now abandoned which claims priority to U.S. Provisional Patent No. 61/046,329 entitled “METHODS, SYSTEMS AND PROCESSES FOR USE IN TREATING SUBSURFACE FORMATIONS” to Vinegar et al. filed on Apr. 18, 2008 and 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.

[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.; and U.S. Pat. No. 7,320,364 to Fairbanks. This patent application incorporates by reference in its entirety each of U.S. Patent Application Publication Nos. 2007-0133960 to Vinegar et al.; 2007-0221377 to Vinegar et al.; 2008-0017380 to Vinegar et al.; 2008-0217015 to Vinegar et al.; and 2009-0071652 to Vinegar et al. This patent application incorporates by reference in its entirety U.S. patent application Ser. No. 12/250,352 to Vinegar 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. 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. 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. 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.

[0007] During some in situ processes, wax may be used to reduce vapors and/or to encapsulate contaminants in the ground. Wax may be used during remediation of wastes to encapsulate contaminated material. U.S. Pat. No. 7,114,880 to Carter, and U.S. Pat. No. 5,879,110 to Carter, each of which is incorporated herein by reference, describe methods for treatment of contaminants using wax during the remediation procedures.

[0008] In some embodiments, a casing or other pipe system may be placed or formed in a wellbore. U.S. Pat. No. 4,572,299 issued to Van Egmond et al., which is incorporated by reference as if fully set forth herein, describes spooling an electric heater into a well. In some embodiments, components of a piping system may be welded together. Quality of formed wells may be monitored by various techniques. In some embodiments, quality of welds may be inspected by a hybrid electromagnetic acoustic transmission technique known as EMAT. EMAT is described in U.S. Pat. No. 5,652,389 to Schaps et al.; U.S. Pat. No. 5,760,307 to Latimer et al.; U.S. Pat. No. 5,777,229 to Geier et al.; and U.S. Pat. No. 6,155,117 to Stevens et al., each of which is incorporated by reference as if fully set forth herein.

[0009] In some embodiments, an expandable tubular may be used in a wellbore. Expandable tubulars are described in U.S. Pat. No. 5,366,012 to Lohbeck, and U.S. Pat. No. 6,354,373 to Vercaemer et al., each of which is incorporated by reference as if fully set forth herein.

[0010] Heaters may be placed in wellbores to heat a formation during an in situ process. 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; and U.S. Pat. No. 4,886,118 to Van Meurs et al.; each of which is incorporated by reference as if fully set forth herein.

[0011] Application of heat to oil shale formations is described in U.S. Pat. No. 2,923,535 to Ljungstrom and U.S. Pat. No. 4,886,118 to Van Meurs et al. Heat may be applied to the oil shale formation to pyrolyze kerogen in the oil shale formation. The heat may also fracture the formation to increase permeability of the formation. The increased permeability may allow formation fluid to travel to a production well where the fluid is removed from the oil shale formation. In some processes disclosed by Ljungstrom, for example, an oxygen containing gaseous medium is introduced to a permeable stratum, preferably while still hot from a preheating step, to initiate combustion.

[0012] A heat source may be used to heat a subterranean formation. Electric heaters may be used to heat the subterranean formation by radiation and/or conduction. An electric heater may resistively heat an element. U.S. Pat. No. 2,548,360 to Germain, which is incorporated by reference as if fully set forth herein, describes an electric heating element placed in a viscous oil in a wellbore. The heater element heats and thins the oil to allow the oil to be pumped from the wellbore. U.S. Pat. No. 4,716,960 to Eastlund et al., which is incorporated by reference as if fully set forth herein, describes electrically heating tubing of a petroleum well by passing a relatively low voltage current through the tubing to prevent formation of solids. U.S. Pat. No. 5,065,818 to Van Egmond, which is incorporated by reference as if fully set forth herein, describes an electric heating element that is cemented into a well borehole without a casing surrounding the heating element.

[0013] U.S. Pat. No. 6,023,554 to Vinegar et al., which is incorporated by reference as if fully set forth herein, describes an electric heating element that is positioned in a casing. The heating element generates radiant energy that heats the casing. A granular solid fill material may be placed between the casing and the formation. The casing may conductively heat the fill material, which in turn conductively heats the formation.

[0014] U.S. Pat. No. 4,570,715 to Van Meurs et al., which is incorporated by reference as if fully set forth herein, describes an electric heating element. The heating element has an electrically conductive core, a surrounding layer of insulating material, and a surrounding metallic sheath. The conductive core may have a relatively low resistance at high temperatures. The insulating material may have electrical resistance, compressive strength, and heat conductivity properties that are relatively high at high temperatures. The insulating layer may inhibit arcing from the core to the metallic sheath. The metallic sheath may have tensile strength and creep resistance properties that are relatively high at high temperatures.

[0015] U.S. Pat. No. 5,060,287 to Van Egmond, which is incorporated by reference as if fully set forth herein, describes an electrical heating element having a copper-nickel alloy core.

[0016] Obtaining permeability in an oil shale formation between injection and production wells tends to be difficult because oil shale is often substantially impermeable. Many methods have attempted to link injection and production wells. These methods include: hydraulic fracturing such as methods investigated by Dow Chemical and Laramie Energy Research Center; electrical fracturing by methods investigated by Laramie Energy Research Center; acid leaching of limestone cavities by methods investigated by Dow Chemical; steam injection into permeable nahcolite zones to dissolve the nahcolite by methods investigated by Shell Oil and Equity Oil; fracturing with chemical explosives by methods investigated by Talley Energy Systems; fracturing with nuclear explosives by methods investigated by Project Bronco; and combinations of these methods. Many of these methods, however, have relatively high operating costs and lack sufficient injection capacity.

[0017] 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.

[0018] In situ production of hydrocarbons from tar sand may be accomplished by heating and/or injecting a gas into the formation. U.S. Pat. No. 5,211,230 to Ostapovich et al. and U.S. Pat. No. 5,339,897 to Leaute, which are incorporated by reference as if fully set forth herein, describe a horizontal production well located in an oil-bearing reservoir. A vertical conduit may be used to inject an oxidant gas into the reservoir for in situ combustion.

[0019] U.S. Pat. No. 2,780,450 to Ljungstrom describes heating bituminous geological formations in situ to convert or crack a liquid tar-like substance into oils and gases.

[0020] U.S. Pat. No. 4,597,441 to Ware et al., which is incorporated by reference as if fully set forth herein, describes contacting oil, heat, and hydrogen simultaneously in a reservoir. Hydrogenation may enhance recovery of oil from the reservoir.

[0021] U.S. Pat. No. 5,046,559 to Glandt and U.S. Pat. No. 5,060,726 to Glandt et al., which are incorporated by reference as if fully set forth herein, describe preheating a portion of a tar sand formation between an injector well and a producer well. Steam may be injected from the injector well into the formation to produce hydrocarbons at the producer well.

[0022] As outlined 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 still a need for improved methods and systems for production of hydrocarbons, hydrogen, and/or other products from various hydrocarbon containing formations.

[0023] 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.

[0024] 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.

[0025] In certain embodiments, a system for treating a subsurface hydrocarbon containing formation includes one or more substantially horizontal or inclined tunnels extending from at least one shaft; and one or more heater sources located in one or more heater wellbores coupled to at least one of the substantially horizontal or inclined tunnels.

[0026] In certain embodiments, a method of treating a subsurface hydrocarbon containing formation includes providing one or more shafts; providing one or more substantially horizontal or inclined tunnels extending from at least one of the shafts; providing one or more wellbores from at least one of the tunnels; and providing one or more heat sources to at least one of the wellbores.

[0027] 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.

[0028] In further embodiments, treating a subsurface formation is performed using any of the methods, systems, or heaters described herein.

[0029] In further embodiments, additional features may be added to the specific embodiments described herein.

[0334] 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.

[0335] 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.

[0336] “Alternating current (AC)” refers to a time-varying current that reverses direction substantially sinusoidally. AC produces skin effect electricity flow in a ferromagnetic conductor.

[0337] “Annular region” is the region between an outer conduit and an inner conduit positioned in the outer conduit.

[0338] “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.

[0339] “ASTM” refers to American Standard Testing and Materials.

[0340] 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).

[0341] “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.

[0342] “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.

[0343] 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.

[0344] “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.

[0345] “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.

[0346] “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.

[0347] “Clogging” refers to impeding and/or inhibiting flow of one or more compositions through a process vessel or a conduit.

[0348] “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.

[0349] “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.

[0350] “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.

[0351] “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.

[0352] “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.

[0353] “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.

[0354] “Cycle oil” refers to a mixture of light cycle oil and heavy cycle oil. “Light cycle oil” refers to hydrocarbons having a boiling range distribution between 430° F. (221° C.) and 650° F. (343° C.) that are produced from a fluidized catalytic cracking system. Light cycle oil content is determined by ASTM Method D5307. “Heavy cycle oil” refers to hydrocarbons having a boiling range distribution between 650° F. (343° C.) and 800° F. (427° C.) that are produced from a fluidized catalytic cracking system. Heavy cycle oil content is determined by ASTM Method D5307.

[0355] “Diad” refers to a group of two items (for example, heaters, wellbores, or other objects) coupled together.

[0356] “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.

[0357] “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.

[0358] “Fluid injectivity” is the flow rate of fluids injected per unit of pressure differential between a first location and a second location.

[0359] “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.

[0360] 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.

[0361] “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.

[0362] “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.

[0363] “Gasoline hydrocarbons” refer to hydrocarbons having a boiling point range from 32° C. (90° F.) to about 204° C. (400° F.). Gasoline hydrocarbons include, but are not limited to, straight run gasoline, naphtha, fluidized or thermally catalytically cracked gasoline, VB gasoline, and coker gasoline. Gasoline hydrocarbons content is determined by ASTM Method D2887.

[0364] “Heat flux” is a flow of energy per unit of area per unit of time (for example, Watts/meter²).

[0365] 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 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 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 heater that provides heat to a zone proximate and/or surrounding a heating location such as a heater well.

[0366] 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.

[0367] “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.

[0368] 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.

[0369] 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.

[0370] “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.

[0371] 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.

[0372] 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.

[0373] “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.

[0374] “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.

[0375] “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.

[0376] “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.

[0377] “Modulated direct current (DC)” refers to any substantially non-sinusoidal time-varying current that produces skin effect electricity flow in a ferromagnetic conductor.

[0378] “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.

[0379] “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.

[0380] “Nitrogen compound content” refers to an amount of nitrogen in an organic compound. Nitrogen content is as determined by ASTM Method D5762.

[0381] “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.

[0382] “Olefins” are molecules that include unsaturated hydrocarbons having one or more non-aromatic carbon-carbon double bonds.

[0383] “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.

[0384] “Organonitrogen compounds” refers 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.

[0385] “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.

[0386] “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.

[0387] “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.

[0388] “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.

[0389] “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.

[0390] “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.

[0391] “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.

[0392] “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.

[0393] “Residue” refers to hydrocarbons that have a boiling point above 537° C. (1000° F.).

[0394] “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.

[0395] “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).

[0396] “Subsidence” is a downward movement of a portion of a formation relative to an initial elevation of the surface.

[0397] “Sulfur compound content” refers to an amount of sulfur in an organic compound. Sulfur content is as determined by ASTM Method D4294.

[0398] “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.

[0399] “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.

[0400] “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.

[0401] “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°.

[0402] 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.

[0403] “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.

[0404] “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).

[0405] “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.

[0406] “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.

[0407] “Thermal oxidation stability” refers to thermal oxidation stability of a liquid. Thermal oxidation stability is as determined by ASTM Method D3241.

[0408] “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.

[0409] “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).

[0410] “Triad” refers to a group of three items (for example, heaters, wellbores, or other objects) coupled together.

[0411] “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.

[0412] 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”.

[0413] “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.

[0414] “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.

[0415] “Viscosity” refers to kinematic viscosity at 40° C. unless otherwise specified. Viscosity is as determined by ASTM Method D445.

[0416] “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.

[0417] A “vug” is a cavity, void or large pore in a rock that is commonly lined with mineral precipitates.

[0418] “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.

[0419] 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.”

[0420] 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.

[0421] 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.

[0422] 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.).

[0423] 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.).

[0424] 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.

[0425] 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.

[0426] 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.

[0427] 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.

[0428] 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.

[0429] 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.

[0430] 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.

[0431] 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.

[0432] 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.

[0433] 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.

[0434] 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.

[0435] 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.

[0436] 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.

[0437] 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.

[0438] 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.

[0439] 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.

[0440] 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.

[0441] 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.

[0442] 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.

[0443] 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.

[0444] 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.

[0445] 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.

[0446] 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.

[0447] 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.

[0448] 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.

[0449] 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.

[0450] 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.

[0451] 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.

[0452] 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.

[0453] 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.

[0454] 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.

[0455] 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.

[0456] 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.

[0457] 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.

[0458] 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.

[0459] 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.

[0460] 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.

[0461] 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.

[0462] 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.

[0463] 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.

[0464] 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.

[0465] 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.

[0466] 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, polyvinylidenedifluoroide, polytetrafluoroethylene, or combinations thereof.

[0467] 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.

[0468] 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.

[0469] 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.

[0470] 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 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.

[0471] 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.

[0472] 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.

[0473] 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.

[0474] 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.

[0475] 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.

[0476] 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).

[0477] 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 268 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.

[0478] 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.

[0479] In some embodiments, liquid stream 236 includes organonitrogen compounds. 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 an aqueous acid solution 264 to form an aqueous stream 266 and product hydrocarbon stream 268. Hydrocarbon stream 268 may include at most 0.01% by weight nitrogen compounds. Hydrocarbon stream 268 may enter hydrotreating unit 244.

[0480] Aqueous acid solution 264 includes water and acids suitable to complex with nitrogen compounds (for example, sulfuric acid, phosphoric acid, acetic acid, formic acid and/or other suitable acidic compounds). Aqueous stream 266 includes salts of the organonitrogen compounds and 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.

[0481] In some embodiments, at least a portion of aqueous stream 266 and/or concentrated organonitrogen stream 272 are introduced in a hydrocarbon portion or layer of 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, the aqueous stream 266 and/or concentrated organonitrogen stream 272 is used a part of the water source for solution mining nahcolite from the formation. In some embodiments, the 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, the 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. In the heated formation, the organonitrogen compounds may form hydrocarbons, amines, and/or ammonia and at least some of such hydrocarbons, amines and/or ammonia may be produced. In some embodiments, at least some of the acid used in the extraction process is produced.

[0482] In some embodiments, streams 242, 248, 270, 268 entering hydrotreating unit 244 are contacted with hydrogen in the presence of one or more catalysts to produce hydrotreated liquid streams 274, 276. Hydrotreating 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.

[0483] In some embodiments, hydrocarbon 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, at most 110 ppm, at most 50 ppm, or at most 10 ppm of nitrogen compounds. The separated liquid stream may have a sulfur compound content of at most 1000 ppm, at most 500 ppm, at most 300 ppm, at most 100 ppm, or at most 10 ppm by weight of sulfur compounds.

[0484] 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.

[0485] 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.

[0486] 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.

[0487] 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.

[0488] 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.

[0489] 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.

[0490] 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.

[0491] 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.

[0492] 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.

[0493] 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 300 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 300 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.

[0494] Tube manufacturing facilities 300 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.

[0495] Tube manufacturing facilities 300 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.

[0496] In some embodiments, tube manufacturing facilities 300 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.

[0497] 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 300 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.

[0498] The coiled tubing may be moved from the tubing manufacturing facility to the well site using gantries 302. Drilling gantry 304 may be used at the well site. Several drilling gantries 304 may be used to form wellbores at different locations. Supply systems for drilling fluid or other needs may be coupled to drilling gantries 304 from central facilities 306.

[0499] Drilling gantry 304 or other equipment may be used to set the conductor for the well. Drilling gantry 304 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 300. 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.

[0500] In some embodiments, drilling gantry 304 takes the reel of coiled tubing from gantry 302. In some embodiments, gantry 302 is coupled to drilling gantry 304 during the formation of the wellbore. For example, the coiled tubing may be fed from gantry 302 to drilling gantry 304, or the drilling gantry lifts the gantry to a feed position and the tubing is fed from the gantry to the drilling gantry.

[0501] 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.

[0502] 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.

[0503] When drilling gantry 304 is finished at well site 308, the drilling gantry may release gantry 302 with the empty reel or return the empty reel to the gantry. Gantry 302 may take the empty reel back to tube manufacturing facility 300 to be loaded with another coiled tube. Gantries 302 may move on looped path 310 from tube manufacturing facility 300 to well sites 308 and back to the tube manufacturing facility.

[0504] Drilling gantry 304 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.

[0505] In some embodiments, positioning and/or tracking system may be utilized to track gantries 302, drilling gantries 304, 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.

[0506] 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.

[0507] 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.

[0508] 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.

[0509] 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.

[0510] 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.

[0511] FIG. 5 depicts a schematic representation of an embodiment of drilling string 312 with dual motors in bottom hole assembly 314. Drilling string 312 is coupled to bottom hole assembly 314. Bottom hole assembly 314 includes motor 316A and motor 316B. Motor 316A may be a bent sub and/or bent housing steerable mud motor. Motor 316A may drive drill bit 318. Motor 316B may operate in a rotation direction that is opposite to the rotation of drilling string 312 and/or motor 316A. Motor 316B may operate at a relatively low rotary speed and have high torque capacity as compared to motor 316A. Bottom hole assembly 314 may include sensing array 320 between motors 316A, motor 316B. Sensing array 320 may include a collar with various directional sensors and telemetry.

[0512] As noted above, motor 316B may rotate in a direction opposite to the rotation of drilling string 312. In this manner, portions of bottom hole assembly 314 beyond motor 316B may have less rotation in the direction of rotation of drilling string 312. In some embodiments, motor 316B is a reverse-rotation low speed motor. The revolutions per minute (rpm) versus differential pressure relationship for bottom hole assembly 314 may be assessed prior to running drilling string 312 and the bottom hole assembly 314 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 316B). 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.

[0513] In some embodiments, motor 316B is operated at a substantially fixed speed. For example, motor 316B may be operated at a speed of 30 rpm. Other speeds may be used as desired.

[0514] 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 316B, shown in FIG. 5, may be installed in an inverted orientation to produce a relative counter-clockwise rotation in portions of bottom hole assembly 314 distal to motor 316B (see counterclockwise arrow).

[0515] FIG. 6 depicts a schematic representation of an embodiment of drilling string 312 including motor 332 in bottom hole assembly 314. Motor 332 may be a low rpm, high torque motor that includes stator 322, rotor 324, and motor shaft 326. Motor shaft 326 couples to driveshaft 330 of drilling string 312 at connection 328. A bit box may be provided at the end of motor shaft 326. Motor shaft 326 and the bit box may face up-hole. The bit box may be fixed relative to drilling string 312. Stator 322 may rotate counter-clockwise relative to drilling string 312.

[0516] 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 316A is transferred to motor 332. In some embodiments, a threading kit is used (for example, at connection 328) 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 328). In some embodiments, the connection includes profile-matched sleeve and/or backoff-protected connection.

[0517] 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.

[0518] In some embodiments, the rotation speed of drilling string 312 is used to control the trajectory of the wellbore being formed. For example, drilling string 312 may initially be rotating at 40 rpm, and motor 316B rotates at 30 rpm. The counter-rotation of motor 316B and drilling string 312 results in a forward rotation speed (for example, an absolute forward rotation speed) of 10 rpm in the lower portion of bottom hole assembly 314 (the portion of the bottom hole assembly below motor 316B). When a directional course correction is to be made, the speed of drilling string 312 is changed to the neutral drilling speed. Because drilling string 312 is rotating, there is no need to lift drill bit 318 off the bottom of the borehole. Operating at neutral drilling speed may effectively cancel the torque of the drilling string so that drill bit 318 is subjected to torque induced by motor 316A and the formation.

[0519] 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 312 keeps windup of the drilling string consistent and stabilizes drill bit 318. Directional changes of drill bit 318 may be made by changing the speed of drilling string 312. 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 312.

[0520] 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.

[0521] The connection of bottom hole assembly 314 to drilling string 312 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 318 during wellbore formation). Threaded connections along drilling string 312 may include profile-matched sleeves such as those known in the art for utilities drilling systems.

[0522] In some embodiments, a control system used to control wellbore formation includes a system that sets a desired rotation speed of drilling string 312 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.

[0523] In certain embodiments, drilling string 312 is integrated with position measurement and downhole tools (for example, sensing array 320) to autonomously control the hole path along a designed geometry. An autonomous control system for controlling the path of drilling string 312 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.

[0524] 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).

[0525] 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.

[0526] In some embodiments, differential pressure or torque across motor 316A and/or motor 316B 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 312. Measurements of torque and the rate of penetration/weight-on-bit/torque relationship may be used to control the feed rate of drilling string 312 into the formation.

[0527] 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.

[0528] 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 312 with non-rotating sensor 344. Non-rotating sensor 344 is located behind motor 316. Motor 316 may be a steerable motor. Motor 316 is located behind drill bit 318. In certain embodiments, sensor 344 is located between non-magnetic components in drilling string 312.

[0529] In some embodiments, non-rotating sensor 344 is located in a sleeve over motor 316. In some embodiments, non-rotating sensor 344 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.

[0530] In certain embodiments, non-rotating sensor 344 includes one or more transceivers for communicating data either into drilling string 312 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 344 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.

[0531] FIG. 10 depicts an embodiment for assessing a position of a first wellbore relative to a second wellbore using multiple magnets. First wellbore 340A is formed in a subsurface formation. Wellbore 340A may be formed by directionally drilling in the formation along a desired path. For example, wellbore 340A may be horizontally or vertically drilled, or drilled at an inclined angle, in the subsurface formation.

[0532] Second wellbore 340B may be formed in the subsurface formation with drill bit 318 on drilling string 312. In certain embodiments, drilling string 312 includes one or more magnets 342. Wellbore 340B may be formed in a selected relationship to wellbore 340A. In certain embodiments, wellbore 340B is formed substantially parallel to wellbore 340A. In other embodiments, wellbore 340B is formed at other angles relative to wellbore 340A. In some embodiments, wellbore 340B is formed perpendicular to wellbore 340A.

[0533] In certain embodiments, wellbore 340A includes sensing array 320. Sensing array 320 may include two or more sensors 344. Sensors 344 may sense magnetic fields produced by magnets 342 in wellbore 340B. The sensed magnetic fields may be used to assess a position of wellbore 340A relative to wellbore 340B. In some embodiments, sensors 344 measure two or more magnetic fields provided by magnets 342.

[0534] Two or more sensors 344 in wellbore 340A may allow for continuous assessment of the relative position of wellbore 340A versus wellbore 340B. Using two or more sensors 344 in wellbore 340A may also allow the sensors to be used as gradiometers. In some embodiments, sensors 344 are positioned in advance (ahead of) magnets 342. Positioning sensors 344 in advance of magnets 342 allows the magnets to traverse past the sensors so that the magnet's position (the position of wellbore 340B) is measurable continuously or “live” during drilling of wellbore 340B. Sensing array 320 may be moved intermittently (at selected intervals) to move sensors 344 ahead of magnets 342. Positioning sensors 344 in advance of magnets 342 also allows the sensors to measure, store, and zero the Earth's field before sensing the magnetic fields of the magnets. The Earth's field may be zeroed by, for example, using a null function before arrival of the magnets, calculating background components from a known sensor attitude, or using paired sensors that function as gradiometers.

[0535] The relative position of wellbore 340B versus wellbore 340A may be used to adjust the drilling of wellbore 340B using drilling string 312. For example, the direction of drilling for wellbore 340B may be adjusted so that wellbore 340B remains a set distance away from wellbore 340A and the wellbores remain substantially parallel. In certain embodiments, the drilling of wellbore 340B is continuously adjusted based on continuous position assessments made by sensors 344. Data from drilling string 312 (for example, orientation, attitude, and/or gravitational data) may be combined or synchronized with data from sensors 344 to continuously assess the relative positions of the wellbores and adjust the drilling of wellbore 340B accordingly. Continuously assessing the relative positions of the wellbores may allow for coiled tubing drilling of wellbore 340B.

[0536] In some embodiments, drilling string 312 may include two or more sensing arrays. The sensing arrays may include two or more sensors. Using two or more sensing arrays in drilling string 312 may allow for direct measurement of magnetic interference of magnets 342 on the measurement of the Earth's magnetic field. Directly measuring any magnetic interference of magnets 342 on the measurement of the Earth's magnetic field may reduce errors in readings (for example, error to pointing azimuth). The direct measurement of the field gradient from the magnets from within drill string 312 also provides confirmation of reference field strength of the field to be measured from within wellbore 340A.

[0537] FIG. 11 depicts an embodiment for assessing a position of a first wellbore relative to a second wellbore using a continuous pulsed signal. Signal wire 346 may be placed in wellbore 340A. Sensor 344 may be located in drilling string 312 in wellbore 340B. In certain embodiments, wire 346 provides a current path and/or reference voltage signal (for example, a pulsed DC reference signal) into wellbore 340A. In one embodiment, the reference voltage signal is a 10 Hz pulsed DC signal. In one embodiment, the reference voltage signal is a 5 Hz pulsed DC signal. In some embodiments, the reference voltage signal is between 0.5 Hz pulsed DC signal and 0.75 Hz pulsed DC signal. Providing the current path and reference voltage signal may generate a known and, in some embodiments, fixed current in wellbore 340A. In some embodiments, the voltage signal is automatically varied on the surface to generate a uniform fixed current in the wellbore. Automatically varying the voltage signal on the surface may minimize bandwidth needs by reducing or eliminating the need to send current downhole and/or sensor raw data uphole.

[0538] In some embodiments, wire 346 carries current into and out of wellbore 340A (the forward and return conductors are both on the wire). In some embodiments, wire 346 carries current into wellbore 340A and the current is returned on a casing in the wellbore (for example, the casing of a heater or production conduit in the wellbore). In some embodiments, wire 346 carries current into wellbore 340A and the current is returned on another conductor located in the formation. For example, current flows from wire 346 in wellbore 340A through the formation to an electrode (current return) in the formation. In certain embodiments, current flows out an end of wellbore 340A. The electrode may be, for example, an electrode in another wellbore in the formation or a bare electrode extending from another wellbore in the formation. The electrode may be the casing in another wellbore in the formation. In some embodiments, wellbore 340A is substantially horizontal in the formation and current flows from wire 346 in the wellbore to a bare electrode extending from a substantially vertical wellbore in the formation.

[0539] The electromagnetic field provided by the voltage signal may be sensed by sensor 344. The sensed signal may be used to assess a position of wellbore 340B relative to wellbore 340A.

[0540] In some embodiments, wire 346 is a ranging wire located in wellbore 340A. In some embodiments, the voltage signal is provided by an electrical conductor that will be used as part of a heater in wellbore 340A. In some embodiments, the voltage signal is provided by an electrical conductor that is part of a heater or production equipment located in wellbore 340A. Wire 346, or other electrical conductors used to provide the voltage signal, may be grounded so that there is no current return along the wire or in the wellbore. Return current may cancel the electromagnetic field produced by the wire.

[0541] Where return current exists, the current may be measured and modeled to generate a “net current” from which a resultant electromagnetic field may be resolved. For example, in some areas, a 600 A signal current may only yield a 3-6 A net current. In some embodiments where it is not feasible to eliminate sufficient return current along the wellbore containing the conductor, two conductors may be installed in separate wellbores. In this method, signal wires from each of the existing wellbores are connected to opposite voltage terminals of the signal generator. The return current path is in this way guided through the earth from the contactor region of one conductor to the other. In certain embodiments, calculations are used to assess (determine) the amount of voltage needed to conduct current through the formation.

[0542] In certain embodiments, the reference voltage signal is turned on and off (pulsed) so that multiple measurements are taken by sensor 344 over a selected time period. The multiple measurements may be averaged to reduce or eliminate resolution error in sensing the reference voltage signal. In some embodiments, providing the reference voltage signal, sensing the signal, and adjusting the drilling based on the sensed signals are performed continuously without providing any data to the surface or any surface operator input to the downhole equipment. For example, an automated system located downhole may be used to perform all the downhole sensing and adjustment operations. In some embodiments, an iterative process is used to perform calculations used in the automated downhole sensing and adjustment operations. In certain embodiments, distance and direction are calculated continuously downhole, filtered and averaged. A best estimate final distance and direction may be output to the surface and combined with known along hole depth and source location to determine three-axis position data.

[0543] The signal field generated by the net current passing through the conductors may be resolved from the general background field existing when the signal field is “off”. A method for resolving the signal field from the general background field on a continuous basis may include: 1.) calculating background components based on the known attitude of the sensors and the known value background field strength and dip; 2.) a synchronized “null” function to be applied immediately before the reference field is switched “on”; 3.) synchronized sampling of forward and reversed DC polarities (the subtraction of these sampled values may effectively remove the background field yielding the reference total current field); and/or 4.) sampling values of background magnetic field at one or more fixed sampling frequencies and storing them for subtraction from the reference signal “on” data.

[0544] In some embodiments, slight changes in the sensor roll position and/or movement of the sensor between sampling steps (for example, between samples of signal off and signal on data) is compensated or counteracted by rotating the sensor data coordinate system to a reference attitude (for example, a “zero”) after each sample is taken or after a set of data is taken. For example, the sensor data coordinate system may be rotated to a tensor coordinate system. Parameters such as position, inclination, roll, and/or azimuth of the sensor may be calculated using sensor data rotated to the tensor coordinate system. In some embodiments, adjustments in calculations and/or data gathering are made to adjust for sensing and ranging at low wellbore inclination angles (for example, angles near vertical).

[0545] FIG. 12 depicts an embodiment for assessing a position of a first wellbore relative to a second wellbore using a radio ranging signal. Sensor 344 may be placed in wellbore 340A. Source 348 may be located in drilling string 312 in wellbore 340B. In some embodiments, source 348 is located in wellbore 340A and sensor 344 is located in wellbore 340B. In certain embodiments, source 348 is an electromagnetic wave producing source. For example, source 348 may be an electromagnetic sonde. Sensor 344 may be an antenna (for example, an electromagnetic or radio antenna). In some embodiments sensor 344 is located in part of a heater in wellbore 340A.

[0546] The signal provided by source 348 may be sensed by sensor 344. The sensed signal may be used to assess a position of wellbore 340B relative to wellbore 340A. In certain embodiments, the signal is continuously sensed using sensor 344. “Continuous” or “continuously” in the context of sensing signals (such as magnetic, electromagnetic, voltage, or other electrical or magnetic signals) includes sensing continuous signals and sensing pulsed signals repeatedly over a selected period time. The continuously sensed signal may be used to continuously and/or automatically adjust the drilling of wellbore 340B by drillbit 318. The continuous sensing of the electromagnetic signal may be dual directional so as to create a data link between transceivers. The antenna/sensor 344 may be directly connected to a surface interface allowing a data link between surface and subsurface to be established.

[0547] In some embodiments, source 348 and/or sensor 344 are sources and sensors used in a walkover radio locater system. Walkover radio locater systems are, for example, used in telecommunications to locate underground lines and to communicate the location to drilling tools used for utilities installation. Radio locater systems may be available, for example, from Digital Control Incorporated (Kent, Wash., U.S.A.). In some embodiments, the walkover radio located system components may be modified to be located in wellbore 340A and wellbore 340B so that the relative positions of the wellbores are assessable using the walkover radio located system components.

[0548] In certain embodiments, multiple sources and multiple sensors may be used to assess and adjust the drilling of one or more wellbores. FIG. 13 depicts an embodiment for assessing a position of a plurality of first wellbores relative to a plurality of second wellbores using radio ranging signals. Sources 348 may be located in a plurality of wellbores 340A. Sensors 344 may be located in one or more wellbores 340B. In some embodiments, sources 348 are located in wellbores 340B and sensors 344 are located in wellbores 340A.

[0549] In one embodiment, wellbores 340A are drilled substantially vertically in the formation and wellbores 340B are drilled substantially horizontally in the formation. Thus, wellbores 340B are substantially perpendicular to wellbores 340A. Sensors 344 in wellbores 340B may detect signals from one or more of sources 348. Detecting signals from more than one source may allow for more accurate measurement of the relative positions of the wellbores in the formation. In some embodiments, electromagnetic attenuation and phase shift detected from multiple sources is used to define the position of a sensor (and the wellbore). The paths of the electromagnetic radio waves may be predicted to allow detection and use of the electromagnetic attenuation and the phase shift to define the sensor position.

[0550] In certain embodiments, continuous pulsed signals and/or radio ranging signals are used to form a plurality of wellbores in a formation. FIG. 14 depicts a top view representation of an embodiment for forming a plurality of wellbores in a formation. Treatment area 350 may include clusters of heaters 352 on opposite sides of the treatment area. Control wellbore 340A may be located at or near the center line of treatment area 350. In certain embodiments, control wellbore 340A is located in a barrier area between heater corridors 354A, 354B. Control wellbore 340A may be a horizontal, substantially horizontal, or slightly inclined wellbore. Control wellbore 340A may have a length between about 250 m and about 3000 m, between about 500 m and about 2500 m, or between about 1000 m and about 2000 m.

[0551] In certain embodiments, the position (lateral and/or vertical position) of control wellbore 340A in treatment area 350 is assessed relative to vertical wellbores 340B, 340C, of which the position is known. The relative position to vertical wellbores 340B, 340C of control wellbore 340A may be assessed using, for example, continuous pulsed signals and/or radio ranging signals as described herein. In certain embodiments, vertical wellbores 340B, 340C are located within about 10 m, within about 5 m, or within about 3 m of control wellbore 340A.

[0552] Heater wellbores 340D may be the first heater wellbores deployed in either corridor 354A or corridor 354B. Ranging sources (for example, wire 346, depicted in FIG. 11, or source 348, depicted in FIGS. 12 and 13) and/or sensors (for example, sensors 344, depicted in FIGS. 11-13) located in either heater wellbores 340D and/or control wellbore 340A may be used to assess the positions (lateral and/or vertical) of the heater wellbores relative to the control wellbore. In some embodiments, the ranging systems are deployed inside a conduit provided into control wellbore 340A. In some embodiments, control wellbore 340A acts as a current return for electrical current flowing from heater wellbores 340D. Control wellbore 340A may include a steel casing or other metal element that allows current to flow into the wellbore. The current may be returned to the surface through control wellbore 340A to complete the electrical circuit used for ranging (as shown by the dotted lines in FIG. 14).

[0553] In certain embodiments, the position of heater wellbores 340D are further assessed using ranging from vertical wellbores 340E. Assessing the position of heater wellbores 340D relative to vertical wellbores 340E may be used to verify position data from ranging from control wellbore 340A. Vertical wellbores 340B, 340C, 340E may have depths that are at least the depth of heater wellbores 340D and/or control wellbore 340A. In certain embodiments, vertical wellbores 340E are located within about 10 m, within about 5 m, or within about 3 m of heater wellbores 340D.

[0554] After heater wellbores 340D are formed in treatment area 350, additional heater wellbores may be formed in corridor 354A and/or corridor 354B. The additional heater wellbores may be formed using heater wellbores 340D and/or control wellbore 340A as guides. For example, ranging systems may be located in heater wellbores 340D and/or control wellbore 340A to assess and/or adjust the relative position of the additional heater wellbores while the additional heater wellbores are being formed.

[0555] In some embodiments, central monitoring system 356 is coupled to control wellbore 340A. In certain embodiments, central monitoring system 356 includes a geomagnetic monitoring system. Central monitoring system 356 may be located at a known location relative to control wellbore 340A and heater wellbores 340D. The known location may include known alignment azimuths from control wellbore 340A. For example, the known location may include north-south alignment azimuths, east-west alignment azimuths, and any heater wellbore alignment azimuth that is intended for corridor 354A and/or corridor 354B (for example, azimuths off the 90° angle depicted in FIG. 14). The geomagnetic monitoring system, along with the known location, may be used to calibrate individual tools used during formation of wellbores and ranging operations and/or to assess the properties of components in bottom hole assemblies or other downhole assemblies.

[0556] FIGS. 15 and 16 depict an embodiment for assessing a position of a first wellbore relative to a second wellbore using a heater assembly as a current conductor. In some embodiments, a heater may be used as a long conductor for a reference current (pulsed DC or AC) to be injected for assessing a position of a first wellbore relative to a second wellbore. If a current is injected onto an insulated internal heater element, the current may pass to the end of heater element 352 where it makes contact with heater casing 358. This is the same current path when the heater is in heating mode. Once the current passes across to bottom hole assembly 314B, at least some of the current is generally absorbed by the earth on the current's return trip back to the surface, resulting in a net current (difference in Amps in (A_i) versus Amps out (A_o)).

[0557] Resulting electromagnetic field 360 is measured by sensor 344 (for example, a transceiving antenna) in bottom hole assembly 314A of first wellbore 340A being drilled in proximity to the location of heater 352. A predetermined “known” net current in the formation may be relied upon to provide a reference magnetic field.

[0558] The injection of the reference current may be rapidly pulsed and synchronized with the receiving antenna and/or sensor data. Access to a high data rate signal from the magnetometers can be used to filter the effects of sensor movement during drilling. The measurement of the reference magnetic field may provide a distance and direction to the heater. Averaging many of these results will provide the position of the actively drilled hole. The known position of the heater and known depth of the active sensors may be used to assess position coordinates of easting, northing, and elevation.

[0559] The quality of data generated with such a method may depend on the accuracy of the net current prediction along the length of the heater. Using formation resistivity data, a model may be used to predict the losses to earth along the length of the heater canister and/or wellbore casing or wellbore liner.

[0560] The current may be measured on both the element and the bottom hole assembly at the surface. The difference in values is the overall current loss to the formation. It is anticipated that the net field strength will vary along the length of the heater. The field is expected to be greater at the surface when the positive voltage applies to the bottom hole assembly.

[0561] If there are minimal losses to earth in the formation, the net field may not be strong enough to provide a useful detection range. In some embodiments, a net current in the range of about 2 A to about 50 A, about 5 A to about 40 A, or about 10 A to about 30 A, may be employed.

[0562] In some embodiments, two or more heaters are used as a long conductor for a reference current (pulsed DC or AC) to be injected for assessing a position of a first wellbore relative to a second wellbore. Utilizing two or more separate heater elements may result in relatively better control of return current path and therefore better control of reference current strength.

[0563] A two or more heater method may not rely on the accuracy of a “model of current loss to formation”, as current is contained in the heater element along the full length of the heaters. Current may be rapidly pulsed and synchronized with the transceiving antenna and/or sensor data to resolve distance and direction to the heater. FIGS. 17 and 18 depict an embodiment for assessing a position of first wellbore 340A relative to second wellbore 340B using two heater assemblies 352A and 352B as current conductors. Resulting electromagnetic field 360 is measured by sensor 344 (for example, a transceiving antenna) in bottom hole assembly 314A of first wellbore 340A being drilled in proximity to the location of heaters 352A in second wellbores 340B.

[0564] In some embodiments, parallel well tracking (PWT) may be used for assessing a position of a first wellbore relative to a second wellbore. Parallel well tracking may utilize magnets of a known strength and a known length positioned in the pre-drilled second wellbore. Magnetic sensors positioned in the active first wellbore may be used to measure the field from the magnets in the second wellbore. Measuring the generated magnetic field in the second wellbore with sensors in the first wellbore may assess distance and direction of the active first wellbore. In some embodiments, magnets positioned in the second wellbore may be carefully positioned and multiple static measurements taken to resolve any general “background” magnetic field. Background magnetic fields may be resolved through use of a null function before positioning the magnets in the second wellbore, calculating background components from known sensor attitudes, and/or a gradiometer setup.

[0565] In some embodiments, reference magnets may be positioned in the drilling bottom hole assembly of the first wellbore. Sensors may be positioned in the passive second wellbore. The prepositioned sensors may be nulled prior to the arrival of the magnets in the detectable range to eliminate Earth's background field. Nulling the sensors may significantly reduce the time required to assess the position and direction of the first wellbore during drilling as the bottom hole assembly continues drilling with no stoppages. The commercial availability of low cost sensors such as Terrella6™ (available from Clymer Technologies (Mystic, Conn., U.S.A.)) (utilizing magnetoresistives rather than fluxgates) may be incorporated into the wall of a deployment coil at useful separations.

[0566] In some embodiments, multiple types of sources may be used in combination with two or more sensors to assess and adjust the drilling of one or more wellbores. A method of assessing a position of a first wellbore relative to a second wellbore may include a combination of angle sensors, telemetry, and/or ranging systems. Such a method may be referred to as umbilical position control.

[0567] Angle sensors may assess an attitude (i.e., the azimuth, inclination, and roll) of a bottom hole assembly. Assessing the attitude of a bottom hole assembly may include measuring, for example, azimuth, inclination, and/or roll. Telemetry may transmit data (for example, measurements) between the surface and, for example, sensors positioned in a wellbore. Ranging may assess the position of a bottom hole assembly in a first wellbore relative to a second wellbore. In some embodiments, the second wellbore may include an existing, previously drilled wellbore.

[0568] FIG. 19 depicts an embodiment of an umbilical positioning control system employing a magnetic gradiometer system and wellbore to wellbore wireless telemetry system. The magnetic gradiometer system may be used to resolve bottom hole assembly interference. Second transceiver 362B may be deployed from the surface down second wellbore 340B, which effectively functions as a telemetry system for first wellbore 340A. A transceiver may communicate with the surface via wire or fiber optics (for example, wire 364) coupled to the transceiver.

[0569] In first wellbore 340A, sensor 344A may be coupled to first transceiving antenna 362A. First transceiving antenna 362A may communicate with second transceiving antenna 362B in second wellbore 340B. The first transceiving antenna may be positioned on bottom hole assembly 314. Sensors coupled to the first transceiving antenna may include, for example, magnetometers and/or accelerometers. In certain embodiments, sensors coupled to the first transceiving antenna may include dual magnetometer/accelerometer sets.

[0570] To accomplish data transfer, first transceiving antenna 362A transmits (“short hops”) measured data through the ground to second transceiving antenna 362B located in the second wellbore. The data may then be transmitted to the surface via embedded wires 364 in the deployment tubular. In some embodiments, data transmission to/from the surface is provided through one or more data lines (wires) that previously exist in the deployment tubular wellbore.

[0571] Two redundant ranging systems may be utilized for umbilical control systems. A first ranging system may include a version of parallel well tracking (PWT). FIG. 20 depicts an embodiment of an umbilical positioning control system employing a magnetic gradiometer system in an existing wellbore. A PWT may include a pair of sensors 344B (for example, magnetometer/accelerometer sets) embedded in the wall of second wellbore deployment coil (the umbilical) or within a nonmagnetic section of jointed tubular string. These sensors act as a magnetic gradiometer to detect the magnetic field from reference magnet 342 installed in bottom hole assembly 314 of first wellbore 340A. In a horizontal section of the second wellbore, a relative position of the umbilical to the first wellbore reference magnet(s) may be determined by the gradient. Data may be sent to the surface through fiber optic cables or wires 364 positioned in second wellbore 340B.

[0572] FIGS. 21 and 22 depict an embodiment of umbilical positioning control system employing a combination of systems being used in a first stage of deployment and a second stage of deployment, respectively. A third set of sensors 344C (for example, magnetometers) may be located on the leading end of wire 364 in second wellbore 340B. Sensors 344B, 344C may detect magnetic fields produced by reference magnets 342 in bottom hole assembly 314 of first wellbore 340A. The role of sensors 344C may include mapping the Earth's magnetic field ahead of the arrival of the gradient sensors and confirming that the angle of the deployment tubular matches that of the originally defined hole geometry. Since the attitude of the magnetic field sensors are known based on the original survey of the hole and the checks of sensors 344B, 344C, the values for the Earth's field can be calculated based on current sensor orientation (inclinometers measure the roll and inclination and the model defines azimuth, Mag total, and Mag dip). Using this method, an estimation of the field vector due to reference magnets 342 can be calculated allowing distance and direction to be resolved.

[0573] A second ranging system may be based on using the signal strength and phase of the “through the earth” wireless link (for example, radio) established between first transceiving antenna 362A in first wellbore 340A and second transceiving antenna 362B in second wellbore 340B. Sensor 344A may be coupled to first transceiving antenna 362A. Given the close spacing of wellbores 340A, 340B and the variability in electrical properties of the formation, the attenuation rates for the electromagnetic signal may be predictable. Predictable attenuation rates for the electromagnetic signal allow the signal strength to be used as a measure of separation between first and second transceiver pairs 362A, 362B. The vector direction of the magnetic field induced by the electromagnetic transmissions from the first wellbore may provide the direction. A transceiver may communicate with the surface via wire or fiber optics (for example, wire 364) coupled to the transceiver.

[0574] With a known resistivity of the formation and operating frequency, the distance between the source and point of measurement may be calculated. FIG. 23 depicts two examples of the relationship between power received and distance based upon two different formations with different resistivities 366 and 368. If 10 W is transmitted at a 12 Hz frequency in 20 ohm-m formation 366, the power received amounts to approximately 9.10 W at 30 m distance. The resistivity was chosen at random and may vary depending on where you are in the ground. If a higher resistivity was chosen at the given frequency, such as 100 ohm-m formation 368, a lower attenuation is observed, and a low characterization occurs whereupon it receives 9.58 W at 30 m distance. Thus, high resistivity, although transmitting power desirably, shows a negative affect in electromagnetic ranging possibilities. Since the main influence in attenuation is the distance itself, calculations may be made solving for the distance between a source and a point of measurement.

[0575] The frequency of the electromagnetic source operates on is another factor that affects attenuation. Typically, the higher the frequency, the higher the attenuation and vice versa. A strategy for choosing between various frequencies may depend on the formation chosen. For example, while the attenuation at a resistivity of 100 ohm-m may be good for data communications, it may not be sufficient for distance calculations. Thus, a higher frequency may be chosen to increase attenuation. Alternatively, a lower frequency may be chosen for the opposite purpose. In some embodiments, a combination of different frequencies is used in sequence to optimize for both low and high frequency functions.

[0576] Wireless data communications in ground may allow an opportunity for electromagnetic ranging and the variable frequency it operates on must be observed to balance out benefits for both functionalities. Benefits of wireless data communication may include, but are not be limited to: 1) automatic depth sync through the use of ranging and telemetry; 2) fast communications with a dedicated coil for a transceiving antenna running in the second wellbore that is hardwired (for example, with optic fiber); 3) functioning as an alternative method for fast communication when hardwire in the first wellbore is not available; 4) functioning in under balanced and over balanced drilling; 5) providing a similar method for transmitting control commands to a bottom hole assembly; 6) reusing sensors to reduce costs and waste; 7) decreasing noise measurement functions split between the first wellbore and the second wellbore; and/or 8) using simultaneous multiple position measurement techniques to provide real time best estimates of position and attitude.

[0577] In some embodiments, it may be advisable to employ sensors able to compensate for magnetic fields produced internally by carbon steel casing built in the vertical section of a reference hole (for example, high range magnetometers). In some embodiments, modification may be made to account for problems with wireless antenna communications between wellbores penetrating through wellbore casings.

[0578] Pieces of formation or rock may protrude or fall into the wellbore due to various failures including rock breakage or plastic deformation during and/or after wellbore formation. Protrusions may interfere with drilling string movement and/or the flow of drilling fluids. Protrusions may prevent running tubulars into the wellbore after the drilling string has been removed from the wellbore. Significant amounts of material entering or protruding into the wellbore may cause wellbore integrity failure and/or lead to the drilling string becoming stuck in the wellbore. Some causes of wellbore integrity failure may be in situ stresses and high pore pressures. Mud weight may be increased to hold back the formation and inhibit wellbore integrity failure during wellbore formation. When increasing the mud weight is not practical, the wellbore may be reamed.

[0579] Reaming the wellbore may be accomplished by moving the drilling string up and down one joint while rotating and circulating. Picking the drilling string up can be difficult because of material protruding into the borehole above the bit or BHA (bottom hole assembly). Picking up the drilling string may be facilitated by placing upward facing cutting structures on the drill bit. Without upward facing cutting structures on the drill bit, the rock protruding into the borehole above the drill bit must be broken by grinding or crushing rather than by cutting. Grinding or crushing may induce additional wellbore failure.

[0580] Moving the drilling string up and down may induce surging or pressure pulses that contribute to wellbore failure. Pressure surging or fluctuations may be aggravated or made worse by blockage of normal drilling fluid flow by protrusions into the wellbore. Thus, attempts to clear the borehole of debris may cause even more debris to enter the wellbore.

[0581] When the wellbore fails further up the drilling string than one joint from the drill bit, the drilling string must be raised more than one joint. Lifting more than one joint in length may require that joints be removed from the drilling string during lifting and placed back on the drilling string when lowered. Removing and adding joints requires additional time and labor, and increases the risk of surging as circulation is stopped and started for each joint connection.

[0582] In some embodiments, cutting structures may be positioned at various points along the drilling string. Cutting structures may be positioned on the drilling string at selected locations, for example, where the diameter of the drilling string or BHA changes. FIG. 24A and FIG. 24B depict cutting structures 370 located at or near diameter changes in drilling string 312 near to drill bit 318 and/or BHA 314. As depicted in FIG. 24C, cutting structures 370 may be positioned at selected locations along the length of BHA 314 and/or drilling string 312 that has a substantially uniform diameter. Cutting structures 370 may remove formation that extends into the wellbore as the drilling string is rotated. Cuttings formed by the cutting structures 370 may be removed from the wellbore by the normal circulation used during the formation of the wellbore.

[0583] FIG. 25 depicts an embodiment of drill bit 318 including cutting structures 370. Drill bit 318 includes downward facing cutting structures 370b for forming the wellbore. Cutting structures 370a are upwardly facing cutting structures for reaming out the wellbore to remove protrusions from the wellbore.

[0584] In some embodiments, some cutting structures may be upwardly facing, some cutting structures may be downwardly facing, and/or some cutting structures may be oriented substantially perpendicular to the drilling string. FIG. 26 depicts an embodiment of a portion of drilling string 312 including upward facing cutting structures 370a, downward facing cutting structures 370b, and cutting structures 370c that are substantially perpendicular to the drilling string. Cutting structures 370a may remove protrusions extending into wellbore 340 that would inhibit upward movement of drilling string 312. Cutting structures 370a may facilitate reaming of wellbore 340 and/or removal of drilling string 312 from the wellbore for drill bit change, BHA maintenance and/or when total depth has been reached. Cutting structures 370b may remove protrusions extending into wellbore 340 that would inhibit downward movement of drilling string 312. Cutting structures 370c may ensure that enlarged diameter portions of drilling string 312 do not become stuck in wellbore 340.

[0585] Positioning downward facing cutting structures 370b at various locations along a length of the drilling string may allow for reaming of the wellbore while the drill bit forms additional borehole at the bottom of the wellbore. The ability to ream while drilling may avoid pressure surges in the wellbore caused by lifting the drilling string. Reaming while drilling allows the wellbore to be reamed without interrupting normal drilling operation. Reaming while drilling allows the wellbore to be formed in less time because a separate reaming operation is avoided. Upward facing cutting structures 370a allow for easy removal of the drilling string from the wellbore.

[0586] In some embodiments, the drilling string includes a plurality of cutting structures positioned along the length of the drilling string, but not necessarily along the entire length of the drilling string. The cutting structures may be positioned at regular or irregular intervals along the length of the drilling string. Positioning cutting structures along the length of the drilling string allows the entire wellbore to be reamed without the need to remove the entire drilling string from the wellbore.

[0587] Cutting structures may be coupled or attached to the drilling string using techniques known in the art (for example, by welding). In some embodiments, cutting structures are formed as part of a hinged ring or multi-piece ring that may be bolted, welded, or otherwise attached to the drilling string. In some embodiments, the distance that the cutting structures extend beyond the drilling string may be adjustable. For example, the cutting element of the cutting structure may include threading and a locking ring that allows for positioning and setting of the cutting element.

[0588] In some wellbores, a wash over or over-coring operation may be needed to free or recover an object in the wellbore that is stuck in the wellbore due to caving, closing, or squeezing of the formation around the object. The object may be a canister, tool, drilling string, or other item. A wash-over pipe with downward facing cutting structures at the bottom of the pipe may be used. The wash over pipe may also include upward facing cutting structures and downward facing cutting structures at locations near the end of the wash-over pipe. The additional upward facing cutting structures and downward facing cutting structures may facilitate freeing and/or recovery of the object stuck in the wellbore. The formation holding the object may be cut away rather than broken by relying on hydraulics and force to break the portion of the formation holding the stuck object.

[0589] A problem in some formations is that the formed borehole begins to close soon after the drilling string is removed from the borehole. Boreholes which close up soon after being formed make it difficult to insert objects such as tubulars, canisters, tools, or other equipment into the wellbore. In some embodiments, reaming while drilling applied to the core drilling string allows for emplacement of the objects in the center of the core drill pipe. The core drill pipe includes one or more upward facing cutting structures in addition to cutting structures located at the end of the core drill pipe. The core drill pipe may be used to form the wellbore for the object to be inserted in the formation. The object may be positioned in the core of the core drill pipe. Then, the core drill pipe may be removed from the formation. Any parts of the formation that may inhibit removal of the core drill pipe are cut by the upward facing cutting structures as the core drill pipe is removed from the formation.

[0590] Replacement canisters may be positioned in the formation using over core drill pipe. First, the existing canister to be replaced is over cored. The existing canister is then pulled from within the core drill pipe without removing the core drill pipe from the borehole. The replacement canister is then run inside of the core drill pipe. Then, the core drill pipe is removed from the borehole. Upward facing cutting structures positioned along the length of the core drill pipe cut portions of the formation that may inhibit removal of the core drill pipe.

[0591] During some in situ heat treatment processes, wellbores may need to be formed in heated formations. Wellbores may also need to be formed in hot portions of geothermally heated or other high temperature formations. Certain formations may be heated by heat sources (for example, heaters) to temperatures above ambient temperatures of the formations. In some embodiments, formations are heated to temperatures significantly above ambient temperatures of the formations. For example, a formation may be heated to a temperature at least about 50° C. above ambient temperature, at least about 100° C. above ambient temperature, at least about 200° C. above ambient temperature, or at least about 500° C. above ambient temperature. Wellbores drilled into hot formation may be additional or replacement heater wells, additional or replacement production wells, and/or monitor wells.

[0592] Cooling while drilling may enhance wellbore stability, safety, and longevity of drilling tools. When the drilling fluid is liquid, significant wellbore cooling can occur due to the circulation of the drilling fluid. Downhole cooling does not have to be applied all the way to the bottom of the wellbore to have beneficial effects. Applying cooling to only part of the drilling string and/or downhole equipment may be a trade off between benefit and the effort involved to apply the cooling to the drilling string and downhole equipment. The target of the cooling may be the formation, the drill bit, and/or the bottom hole assembly. In some embodiments, cooling of the formation is inhibited to promote wellbore stability. Cooling of the formation may be inhibited by using insulation to inhibit heat transfer from the formation to the drilling string, bottom hole assembly, and/or the drill bit. In some embodiments, insulation is used to inhibit heat transfer and/or phase changes of drilling fluid and/or cooling fluid in portions of the drilling string, bottom hole assembly, and/or the drill bit.

[0593] In some in situ heat treatment process embodiments, a barrier formed around all or a portion of the in situ heat treatment process is formed by freeze wells that form a low temperature zone around the freeze wells. A portion of the cooling capacity of the freeze well equipment may be utilized to cool the equipment needed to drill into the hot formation. A closed loop circulation system may be used to cool drilling bits and/or other downhole equipment. Drilling bits may be advanced slowly in hot sections to ensure that the formed wellbore cools sufficiently to preclude drilling problems and/or to enhance borehole stability.

[0594] When using conventional circulation, drilling fluid flows down the inside of the drilling string and back up the outside of the drilling string. Other circulation systems, such as reverse circulation, may also be used. In some embodiments, the drill pipe may be positioned in a pipe-in-pipe configuration, or a pipe-in-pipe-in-pipe configuration (for example, when a closed loop circulation system is used to cool downhole equipment).

[0595] The drilling string used to form the wellbore may function as a counter-flow heat exchanger. The deeper the well, the more the drilling fluid heats up on the way down to the drill bit as the drilling string passes through heated portions of the formation. When normal circulation does not deliver low enough temperatures drilling fluid to the drill bit to provide adequate cooling, two options may be employed to enhance cooling: mud coolers on the surface can be used to reduce the inlet temperature of the drilling fluid being pumped downhole; and, if cooling is still inadequate, an at least partially insulated drilling string can be used to reduce the counter-flow heat exchanger effect.

[0596] For various reasons including, but not limited to, lost circulation, wells are frequently drilled with gas (for example, air, nitrogen, carbon dioxide, methane, ethane, and other light hydrocarbon gases) or gas/liquid mixtures. Gas/liquid mixtures are used as the drilling fluid primarily to maintain a low equivalent circulating density (low downhole pressure gradient). Gas has low potential for cooling the wellbore because mass flow rates of gas drilling are much lower than when liquid drilling fluid is used. Also, gas has a low heat capacity compared to liquid. As a result of heat flow from the outside to the inside of the drilling string, the gas arrives at the drill bit at close to formation temperature. Controlling the inlet temperature of the gas (analogous to using mud coolers when drilling with liquid) or using insulated drilling string may marginally reduce the counter-flow heat exchanger effect when gas drilling. Some gases are more effective than others at transferring heat, but the use of gasses with better heat transfer properties may not significantly improve wellbore cooling while gas drilling.

[0597] Gas drilling may deliver the drilling fluid to the drill bit at close to the formation temperature. The gas may have little capacity to absorb heat. A feature of gas drilling is the low density column in the annulus. The benefits of gas drilling can be accomplished if the drilling fluid or a cooling fluid is liquid while flowing down the drilling string and gas while flowing back up the annulus. The heat of vaporization may be used to cool the drill bit and the formation rather than using the sensible heat of the drilling fluid to cool.

[0598] An advantage of this approach may be that even though the liquid arrives at the bit at close to formation temperature, the liquid can absorb heat by vaporizing. The heat of vaporization is typically larger than the heat that can be absorbed by a temperature rise. As a comparison, a 7⅞″ wellbore is drilled with a 3½″ drilling string circulating low density mud at about 203 gpm with about a 100 ft/min typical annular velocity. Drilling through a 450° F. zone at 1000 feet will result in a mud exit temperature about 8° F. hotter than the inlet temperature. This results in the removal of about 14,000 Btu/min. The removal of this heat lowers the bit temperature from about 450° F. to about 285° F. If liquid water is injected down the drilling string and allowed to boil at the bit and steam is produced up the annulus, the mass flow required to remove ½″ cuttings is about 34 lb_m/min assuming the back pressure is about 100 psia. At 34 lb_m/min, the heat removed from the wellbore would be about 34 lb_m/min×(1187-180) Btu/lb_m, or about 34,000 Btu/min. This heat removal amount is about 2.4 times the liquid cooling case. Thus, at reasonable annular steam flow rates, a significant amount of heat may be removed by vaporization.

[0599] The high velocities required for gas drilling may be achieved by the expansion that occurs during vaporization rather than by employing compressors on the surface. Eliminating or minimizing the need for compressors may simplify the drilling process, eliminate or lower compression costs, and eliminate or reduce a source of heat applied to the drilling fluid on the way to the drill bit.

[0600] In some embodiments, it is helpful to inhibit vaporization within the drilling string. If the drilling fluid flowing downwards vaporizes before reaching the drill bit, the heat of vaporization tends to extract heat from the drilling fluid flowing up the annulus. The heat transferred from the annulus (outside the drilling string) to inside the drilling string is heat that is not rejected from the well when drilling fluid reaches the surface. Vaporization that occurs inside of the drilling string before the drilling fluid reaches the bottom of the hole is less beneficial to drill bit and/or wellbore cooling. FIG. 27 depicts drilling fluid flow in drilling string 312 in wellbore 340 with no control of vaporization of the fluid. Liquid drilling fluid flows down drilling string 312 as indicated by arrow 372. Liquid changes to vapor at interface 374. Vapor flows down drilling string 312 below interface 374 as indicated by arrow 376. In certain embodiments, interface 374 is a region instead of an abrupt change from liquid to vapor. Vapor and cuttings may flow up the annular region between drilling string 312 and formation 380 in the directions indicated by arrows 378. Heat transfers from formation 380 to the vapor moving up drilling string 312 and to the drilling string. Heat from drilling string 312 transfers to liquid and vapor flowing down the drilling string.

[0601] If the pressure in the drilling string is maintained above the boiling pressure for a given temperature by use of a back pressure device, then the transfer of heat from outside the drilling string to fluid on the inside of the drilling string can be limited so that the fluid on the inside of the drilling string does not change phases. Fluid downstream of the back pressure device may be allowed to change phase. The fluid downstream the back pressure device may be partially or totally vaporized. Vaporization may result in the drilling fluid absorbing the heat of vaporization from the drill bit and formation. For example, if the back pressure device is set to allow flow only when the back pressure is above a selected pressure (for example, 250 psi for water or another pressure depending on the fluid), the fluid within the drilling string may not vaporize unless the temperature is above a selected temperature (for example, 400° F. for water or another temperature depending on the fluid). If the temperature of the formation is above the selected temperature (for example, the temperature is about 500° F.), steps may be taken to inhibit vaporization of the fluid on the way down to the drill bit. In an embodiment, the back pressure device is set to maintain a back pressure that inhibits vaporization of the drilling fluid at the temperature of the formation (for example, 580 psi to inhibit vaporization up to a temperature of 500° F. for water). In another embodiment, the drilling pipe is insulated and/or the drilling fluid is cooled so that the back pressure device is able to maintain any drilling fluid that reaches the drill bit as a liquid.

[0602] Examples of two back pressure devices that may be used to maintain elevated pressure within the drilling string are a choke and a pressure activated valve. Other types of back pressure devices may also be used. Chokes have a restriction in the flow area that creates back pressure by resisting flow. Resisting the flow results in increased upstream pressure to force the fluid through the restriction. Pressure activated valves may not open until a minimum upstream pressure is obtained. The pressure difference across a pressure activated valve may determine if the pressure activated valve is open to allow flow or the valve is closed.

[0603] In some embodiments, both a choke and a pressure activated valve may be used. A choke can be the bit nozzles allowing the liquid to be jetted toward the drill bit and the bottom of the hole. The bit nozzles may enhance drill bit cleaning and help inhibit fouling of the drill bit and pressure activated valve. Fouling may occur if boiling in the drill bit or pressure activated valve causes solids to precipitate. The pressure activated valve may inhibit premature vaporization at low flow rates such as flow rates below which the chokes are effective.

[0604] In some embodiments, additives are added to the cooling fluid or the drilling fluid. The additives may modify the properties of the fluids in the liquid phase and/or the gas phase. Additives may include, but are not limited to, surfactants to foam the fluid, additives to chemically alter the interaction of the fluid with the formations (for example, to stabilize the formation), additives to control corrosion, and additives for other benefits.

[0605] In some embodiments, a non-condensable gas is added to the cooling fluid or the drilling fluid pumped down the drilling string. The non-condensable gas may be, but is not limited to, nitrogen, carbon dioxide, air, and mixtures thereof. Adding the non-condensable gas results in pumping a two phase mixture down the drilling string. One reason for adding the non-condensable gas may be to enhance the flow of the fluid out of the formation. The presence of the non-condensable gas may inhibit condensation of the vaporized cooling or drilling fluid and/or help to carry cuttings out of the formation. In some embodiments, one or more heaters are present at one or more locations in the wellbore to provide heat that inhibits condensation and reflux of cooling or drilling fluid leaving the formation.

[0606] In certain embodiments, managed pressure drilling and/or managed volumetric drilling is used during the formation of wellbores. The back pressure on the wellbore may be held to a prescribed value to control the downhole pressure. Similarly, the volume of fluid entering and exiting the wellbore may be balanced such that there is no or minimally controlled net influx or out-flux of drilling fluid into the formation.

[0607] FIG. 28 depicts a representation of a system for forming wellbore 340 in heated formation 380. Liquid drilling fluid flows down the drilling string to bottom hole assembly 314 in the direction indicated by arrow 372. Bottom hole assembly 314 may include back pressure device 382. Back pressure device 382 may include pressure activated valves and/or chokes. In some embodiments, back pressure device 382 is adjustable. Back pressure device 382 may be electrically coupled to bottom hole assembly 314. The control system for bottom hole assembly 314 may control the inlet flow of cooling or drilling fluid and may adjust the amount of flow through back pressure device 382 to maintain the pressure of cooling or drilling fluid located above the back pressure device above a desired pressure. Thus, back pressure device 382 may be operated to control vaporization of the cooling fluid. In certain embodiments, back pressure device 382 includes a control volume. In some embodiments, the control volume is a conduit that carries the cooling fluid to bottom hole assembly 314.

[0608] The desired pressure may be a pressure sufficient to maintain cooling or drilling fluid as a liquid phase to cool drill bit 318 when the liquid phase of the cooling or drilling fluid is vaporized. At least a portion of the liquid phase of the cooling or drilling fluid may vaporize and absorb heat from drill bit 318. In certain embodiments, vaporization of the cooling fluid is controlled to control a temperature at or near bottom hole assembly 314. In some embodiments, bottom hole assembly 314 includes insulation to inhibit heat transfer from the formation to the bottom hole assembly. In some embodiments, drill bit 318 includes a conduit for flow of the cooling fluid. Vapor phase cooling or drilling fluid and cuttings may flow upwards to the surface in the direction indicated by arrow 378.

[0609] In some embodiments, cooling fluid in a closed loop is circulated into and out of the wellbore to provide cooling to the formation, drilling string, and/or downhole equipment. In some embodiments, phase change of the cooling fluid is not utilized during cooling. In some embodiments, the cooling fluid is subjected to a phase change to cool the formation, drilling string, and/or downhole equipment.

[0610] In an embodiment, cooling fluid in a closed loop system is passed through a back pressure device and allowed to vaporize to provide cooling to a selected region. FIG. 29 depicts a partial cross-sectional representation of a system that uses phase change of a cooling fluid to provide downhole cooling. Drilling fluid may flow down the center drilling string to drill bit 318 in the direction indicated by arrow 372. Return drilling fluid and cuttings may flow to the surface in the direction indicated by arrows 378. Cooling fluid may flow down the annular region between center drilling string and the middle drilling string in the direction indicated by arrows 388. The cooling fluid may pass through back pressure device 382 to a vaporization chamber. The vaporization chamber may be located above the bottom hole assembly. Back pressure device 382 may maintain a significant portion of cooling fluid in a liquid phase above the back pressure device. Cooling fluid is allowed to vaporize below back pressure device 382 in the vaporization chamber. In certain embodiments, at least a majority of the cooling fluid is vaporized. Return vaporized cooling fluid may flow back to a cooling system that reliquefies the cooling fluid for subsequent usage in the drilling string and/or another drilling string. The vaporized cooling fluid may flow to the surface in the annular region between the middle drilling string and the outer drilling string in the direction indicated by arrows 390. Liquid cooling fluid may maintain the drilling fluid flowing through the center drilling string at a temperature below the boiling temperature of the cooling fluid.

[0611] FIG. 30 depicts a representation of a system for forming wellbore 340 in heated formation 380 using reverse circulation. Drilling fluid flows down the annular region between formation 380 and outer drilling string 312 in the direction indicated by arrows 384. Drilling fluid and cuttings pass through drill bit 318 and up center drilling string 312′ in the direction indicated by arrow 386. Cooling fluid may flow down the annular region between outer drilling string 312 and center drilling string 312′ in the direction indicated by arrows 388. The cooling fluid may be water or another type of cooling fluid that is able to change from a liquid phase to a vapor phase and absorb heat. The cooling fluid may flow to back pressure device 382. Back pressure device 382 may maintain the pressure of the cooling fluid located above the back pressure device above a pressure sufficient to maintain the cooling fluid as a liquid phase to cool drill bit 318 when the liquid phase of the drilling fluid is vaporized. Cooling fluid may pass through back pressure device 382 into vaporization chamber 392. Vaporization of cooling fluid may absorb heat from drill bit 318 and/or from formation 380. Vaporized cooling fluid may pass through one or more lift valves into center drilling string 312′ to help transport drilling fluid and cuttings to the surface.

[0612] In some embodiments, an auto-positioning control system in combination with a rack and pinion drilling system may be used for forming wellbores in a formation. Use of an auto-positioning 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 auto-positioning system may be continuously and/or semi-continuously calibrated during drilling. FIG. 31 depicts a schematic of a portion of a system including a rack and pinion drive system. Rack and pinion drive system 400 includes, but is not limited to, rack 404, carriage 406, chuck drive system 408, and circulating sleeve 424. Chuck drive system 408 may hold tubular 410. 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 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.

[0613] Rack and pinion drive system 400 may be coupled to auto-positioning control system 412. Auto-positioning control system 412 may include, but is not limited to, rotary steerable systems, dual motor rotary steerable systems, and/or hole measurement systems. In some embodiments, heaters are included in tubular 410. In some embodiments, auto-positioning measurement tools are positioned in the heaters. In some embodiments, a measurement system includes magnetic ranging and/or a non-rotating sensor.

[0614] In some embodiments, a hole measuring system includes 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 with the surface as reference of string 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. 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 surface, the known along hole depth and source location may be combined with the calculated distance and direction to calculate X, Y & Z position data.

[0615] During drilling with jointed pipes, the time taken to shut down circulation, add the next pipe, re-establish circulation, and hole making may require a substantial amount of time, particularly when using two-phase circulation. Handling tubulars (for example, pipes) has historically been a key safety risk area 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.

[0616] In some embodiments, a drilling sequence is used in which tubulars are added to a string without interrupting the drilling process. Such a sequence may allow continuous rotary drilling with large diameter tubulars. 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 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. 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.

[0617] FIGS. 32A-32D depict a schematic of an illustrative continuous drilling sequence. The system used to carry out the continuous drilling sequence includes bottom drive system 414, tubular handling system 416, and top drive system 418. Top drive system 418 includes circulating sleeve 420 and drive sub 422. Top drive system 416 may be, for example, a rotary drive system or a rack and pinion drive system. Bottom drive system 414 includes circulating sleeve 424 and chuck 426. For example, bottom drive system 414 may be a rack and pinion type system such as depicted in FIG. 31. In some embodiments, the chuck may be on a separate carriage system. During the sequence, new tubulars (for example, new tubular 428) may be coupled successively, one after another, to an existing tubular (for example, existing tubular 410). Bottom drive system 414 and top drive system 418 may alternate control of the drilling operation.

[0618] As the sequence commences, existing tubular 410 is coupled to chuck 426, and bottom drive system 414 controls drilling. Fluid may flow through port 430 into circulating sleeve 424 of bottom drive system 414. Top drive system 418 is at reference line Y and bottom drive system 414 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. 32A-32D. As shown in FIG. 32A, new tubular 428 may be aligned with bottom drive system 414 using tubular handling system 416. Once in position, top drive system 418 may be connected to a top end (for example, a box end) of new tubular 428.

[0619] As shown in FIG. 32B, as chuck 426 of bottom drive system 414 continues to control drilling, top drive system 418 lowers and positions or drops a bottom end of new tubular 428 in circulating sleeve 424 (see arrows). Once new tubular 428 is in the chamber of circulating sleeve 424, circulation changes to top drive system 418 and a connection is made between new tubular 428 and existing tubular 410. After the connection between existing tubular 410 and new tubular 428 is made, bottom drive system 414 may relinquish control of the drilling process to top drive system 418. Fluid flows through port 432 into circulating sleeve 420 of top drive system 418.

[0620] As shown in FIG. 32C, with top drive system 418 controlling the drilling process, bottom drive system 414 may be actuated to travel upward (see arrow) toward top drive system 418 along the length of new tubular 428. When bottom drive system 414 reaches the top of new tubular 428, the new tubular may be engaged with chuck 426 of bottom drive system 414. Top drive system 418 may relinquish control of the drilling process to bottom drive system 414. Bottom drive system 414 may resume control of the drilling operation while top drive system 418 disconnects from the new tubular 428. Chuck 426 may transfer force to new tubular 428 to continue drilling. Top drive system 418 may be raised relative to bottom drive system 414 (see arrow) (for example, until top drive system 418 reaches reference line Y). As shown in FIG. 32D, bottom drive system 414 may be lowered to push new tubular 428 and existing tubular 410 downward into the formation (see arrows). Bottom drive system 414 may continue to be lowered (for example, until bottom drive system 414 has returned to reference line Z). The sequence described above may be repeated any number of times so as to maintain continuous drilling operations.

[0621] FIG. 33 depicts a schematic of an embodiment of circulating sleeve 424. Fluid may enter circulating sleeve 424 through port 430 and flow around existing tubular 410. Fluid may remove heat away from chuck 426 and/or tubulars. Circulating sleeve 424 includes opening 434. Opening 434 allows new tubular 428 to enter circulating sleeve 424 so that the new tubular may be coupled to existing tubular 410. In some embodiments, a valve is provided at opening 434. For example, the valve may be a UBD circulation valve. Opening 434 may include one or more tooljoints 436. Tooljoints 436 may guide entry of new tubular 428 in an inner section of circulating sleeve. As new tubular 428 enters opening 434 of circulating sleeve 424, fluid flow through the circulating sleeve may be under pressure. For example, fluid through the circulating sleeve may be at pressures of up to about 13.8 MPa (up to about 2000 psi).

[0622] In some embodiments, circulating sleeve 424 may include, and/or operate in conjunction with, one or more valves. FIG. 34 depicts a schematic of system including circulating sleeve 424, side valve 438, and top valve 440. Side valve 438 may be a check valve incorporated into a side entry flow and check valve port. Top entry valve 440 may be a check valve. Use of check valves may facilitate change of circulation entry points and creation of a seal.

[0623] As circulating system sleeve 424 comes into proximity with drive sub 422 (as described in FIG. 32D), fluid from top drive system 418 may be flowing from circulating sleeve 420 of top drive system 418 through top valve 440. Circulating sleeve 424 may be pressurized and side valve 438 may open to provide flow. Top valve 440 may shut and/or partially close as side valve 438 opens to provide flow to circulating sleeve 420. Circulation may be slowed or discontinued through top drive system 418. As circulation is stopped through top drive system 418, top valve 440 may close completely and all fluid may be furnished through side valve 438 from port 430.

[0624] In some embodiments, one piece of equipment may be used to drill multiple wellbores in a single day. The wellbores may be formed at penetration rates that are many times faster than the penetration rates using conventional drilling with drilling bits. The high penetration rate allows separate equipment to accomplish drilling and casing operations in a more efficient manner than using a one-rig approach. The high penetration rate requires accurate, near real time directional drilling control in three dimensions.

[0625] In some embodiments, high penetration rates may be attained using composite coiled tubing in combination with particle jet drilling. Particle jet drilling forms an opening in a formation by impacting the formation with high velocity fluid containing particles to remove material from the formation. The particles may function as abrasives. In addition to composite coiled tubing and particle jet drilling, a downhole electric orienter, bubble entrained mud, downhole inertial navigation, and a computer control system may be needed. Other types of drilling fluid and drilling fluid systems may be used instead of using bubble entrained mud. Such drilling fluid systems may include, but are not limited to, straight liquid circulation systems, multiphase circulation systems using liquid and gas, and/or foam circulation systems.

[0626] Composite coiled tubing has a fatigue life that is significantly greater than the fatigue life of steel coiled tubing. Composite coiled tubing is available from Airborne Composites BV (The Hague, The Netherlands). Composite coiled tubing can be used to form many boreholes in a formation. The composite coiled tubing may include integral power lines for providing electricity to downhole tools. The composite coiled tubing may include integral data lines for providing real time information regarding downhole conditions to the computer control system and for sending real time control information from the computer control system to the downhole equipment. The primary computer control system may be downhole or may be at surface.

[0627] The coiled tubing may include an abrasion resistant outer sheath. The outer sheath may inhibit damage to the coiled tubing due to sliding experienced by the coiled tubing during deployment and retrieval. In some embodiments, the coiled tubing may be rotated during use in lieu of or in addition to having an abrasion resistant outer sheath to minimize uneven wear of the composite coiled tubing.

[0628] Particle jet drilling may advantageously allow for stepped changes in the drilling rate. Drill bits are no longer needed and downhole motors are eliminated. Particle jet drilling may decouple cutting formation to form the borehole from the bottom hole assembly (BHA). Decoupling cutting formation to form the borehole from the BHA reduces the impact that variable formation properties (for example, formation dip, vugs, fractures and transition zones) have on wellbore trajectory. The decoupling lowers the required torque and thrust that would normally be required if conventional drilling bits were used to form a borehole in the formation. By decoupling cutting formation to form the borehole from the BHA, directional drilling may be reduced to orienting one or more particle jet nozzles in appropriate directions. The orientation of the BHA becomes easier with the reduced torque on the assembly from the hole making process. Additionally, particle jet drilling may be used to under ream one or more portions of a wellbore to form a larger diameter opening.

[0629] Particles may be introduced into a pressurized injection stream during particle jet drilling. The ability to achieve and circulate high particle laden fluid under pressure may facilitate the successful use of particle jet drilling. Traditional oilfield drilling and/or servicing pumps are not designed to handle the abrasive nature of the particles used for particle jet drilling for extended periods of time. Wear on the pump components may be high resulting in impractical maintenance and repairs. One type of pump that may be used for particle jet drilling is a heavy duty piston membrane pump. Heavy duty piston membrane pumps may be available from ABEL GmbH & Co. KG (Buchen, Germany). Piston membrane pumps have been used for long term, continuous pumping of slurries containing high total solids in the mining and power industries. Piston membrane pumps are similar to triplex pumps used for drilling operations in the oil and gas industry except heavy duty preformed membranes separate the slurry from the hydraulic side of the pump. In this fashion, the solids laden fluid is brought up to pressure in the injection line in one step and circulated downhole without damaging the internal mechanisms of the pump.

[0630] Another type of pump that may be used for particle jet drilling is an annular pressure exchange pump. Annular pressure exchange pumps may be available from Macmahon Mining Services Pty Ltd (Lonsdale, Australia). Annular pressure exchange pumps have been used for long term, continuous pumping of slurries containing high total solids in the mining industry. Annular pressure exchange pumps use hydraulic oil to compress a hose inside a high-strength pressure chamber in a peristaltic like way to displace the contents of the hose. Annular pressure exchange pumps may obtain continuous flow by having twin chambers. One chamber fills while the other chamber is purged.

[0631] The BHA may include a downhole electric orienter. The downhole electric orienter may allow for directional drilling by directing one or more jets or particle jet drilling nozzles in an appropriate fashion to facilitate forward hole making progress in the desired direction. The downhole electric orienter may be coupled to a computer control system through one or more integral data lines of the composite coiled tubing. Power for the downhole electric orienter may be supplied through an integral power line of the composite coiled tubing or through a battery system in the BHA.

[0632] Bubble entrained mud may be used as the drilling fluid. Bubble entrained mud may allow for particle jet drilling without raising the equivalent circulating density to unacceptable levels. A form of managed pressure drilling may be affected by varying the density of bubble entrainment. In some embodiments, particles in the drilling fluid may be separated from the drilling fluid using magnetic recovery when the particles include iron or alloys that may be influenced by magnetic fields. Bubble entrained mud may be used because using air or other gas as the drilling fluid may result in excessive wear of components from high velocity particles in the return stream. The density of the bubble entrained mud going downhole as a function of real time gains and losses of fluid may be automated using the computer control system.

[0633] In some embodiments, multiphase systems are used. For example, if gas injection rates are low enough that wear rates are acceptable, a gas-liquid circulating system may be used. Bottom hole circulating pressures may be adjusted by the computer control system. The computer control system may adjust the gas and/or liquid injection rates.

[0634] In some embodiments, pipe-in-pipe drilling is used. Pipe-in-pipe drilling may include circulating fluid through the space between the outer pipe and the inner pipe instead of between the wellbore and the drill string. Pipe-in-pipe drilling may be used if contact of the drilling fluid with one or more fresh water aquifers is not acceptable. Pipe-in-pipe drilling may be used if the density of the drilling fluid cannot be adjusted low enough to effectively reduce potential lost circulation issues.

[0635] Downhole inertial navigation may be part of the BHA. The use of downhole inertial navigation allows for determination of the position (including depth, azimuth and inclination) without magnetic sensors. Magnetic interference from casings and/or emissions from the high density of wells in the formation may interfere with a system that determines the position of the BHA based on magnet sensors.

[0636] The computer control system may receive information from the BHA. The computer control system may process the information to determine the position of the BHA. The computer control system may control drilling fluid rate, drilling fluid density, drilling fluid pressure, particle density, other variables, and/or the downhole electric orienter to control the rate of penetration and/or the direction of borehole formation.

[0637] FIG. 35 depicts a representation of an embodiment of bottom hole assembly 314 used to form an opening in the formation. Composite coiled tubing 442 may be secured to connector 444 of BHA 314. Connector 444 may be coupled to combination circulation and disconnect sub 446. Sub 446 may include ports 448. Sub 446 may be coupled to tractor system 450. Tractor system 450 may include a plurality of grippers 452 and ram 454. Tractor system 450 may be coupled to sensor sub 456 that includes inertial navigation sensors, pressure sensors, temperature sensors and/or other sensors. Sensor sub 456 may be coupled to orienter 458. Orienter 458 may be coupled to jet head 460. Jet head 460 may include centralizers 462. Other BHA embodiments may include other components and/or the same components in a different order.

[0638] In some embodiments, the jet head is rotated during use. The BHA may include a motor for rotating the jet head. FIG. 36 depicts an embodiment of jet head 460 with multiple nozzles 464. The motor in the BHA may rotate jet head 460 in the direction indicated by the arrow. Nozzles 464 may direct particle jet streams 466 against the formation. FIG. 37 depicts an embodiment of jet head 460 with single nozzles 464. Nozzle 464 may direct particle jet stream 466 against the formation.

[0639] In some embodiments, the jet head is not rotated during use. FIG. 38 depicts an embodiment of non-rotational jet head 460. Jet head 460 may include one or more nozzles 464 that direct particle jet streams 466 against the formation.

[0640] Direction change of the wellbore formed by the BHA may be controlled in a number of ways. FIG. 39 depicts a representation wherein the BHA includes an electrical orienter 458. Electrical orienter 458 adjusts angle θ between a back portion of the BHA and jet head 460 that allows the BHA to form the opening in the direction indicated by arrow 468. FIG. 40 depicts a representation wherein jet head 460 includes directional jets 470 around the circumference of the jet head. Directing fluid through one or more of the directional jets 470 applies a force in the direction indicated by arrow 472 to jet head 460 that moves the jet head so that one or more jets of the jet head form the wellbore in the direction indicated by arrow 468.

[0641] In some embodiments, the tractor system of the BHA may be used to change the direction of wellbore formation. FIG. 41 depicts tractor system 450 in use to change the direction of wellbore formation to the direction indicated by arrow 468. One or more grippers of the rear gripper assembly may be extended to contact the formation and establish a desired angle of jet head. Ram 454 may be extended to move jet head forward. When ram 454 is fully extended, grippers of the front gripper assembly may be extended to contact the formation, and grippers of the read gripper assembly may be retracted to allow the ram to be compressed. Force may be applied to the coiled tubing to compress ram 454. When the ram is compressed, grippers of the front gripper assembly may be retracted, and grippers of the rear gripper assembly may be extended to contact the formation and set the jet head in the desired direction. Additional wellbore may be formed by directing particle jets through the jet head while extending ram 454.

[0642] In some embodiments, robots are used to perform a task in a wellbore formed or being formed using composite coiled tubing. The task may be, but is not limited to, providing traction to move the coiled tubing, surveying, removing cuttings, logging, and/or freeing pipe. For example, a robot may be used when drilling a horizontal opening if enough weight cannot be applied to the BHA to advance the coiled tubing and BHA in the formed borehole. The robot may be sent down the borehole. The robot may clamp to the composite coiled tubing or BHA. Portions of the robot may extend to engage the formation. Traction between the robot and the formation may be used to advance the robot forward so that the composite coiled tubing and the BHA advance forward. The displacement data from the forward advancement of the BHA using the robot may be supplied directly to the inertial navigation system to improve accuracy of the opening being formed.

[0643] The robots may be battery powered. To use the robot, drilling could be stopped, and the robot could be connected to the outside of the composite coiled tubing. The robot would run along the outside of the composite coiled tubing to the bottom of the hole. If needed, the robot could electrically couple to the BHA. The robot could couple to a contact plate on the BHA. The BHA may include a step-down transformer that brings the high voltage, low current electricity supplied to the BHA to a lower voltage and higher current (for example, one third the voltage and three times the amperage supplied to the BHA). The lower voltage, higher current electricity supplied from the step-down transformer may be used to recharge the batteries of the robot. In some embodiments, the robot may function while coupled to the BHA. The batteries may supply sufficient energy for the robot to travel to the drill bit and back to the surface.

[0644] A robot may be run integral to the BHA on the end of the composite coiled tubing. Portions of the robot may extend to engage the formation. Traction between the robot and the formation may be used to advance the robot forward so that the composite coiled tubing and the BHA advance forward. The integral robot could be battery powered, could be powered by the composite coiled tubing power lines or could be hydraulically powered by flow through the BHA.

[0645] FIG. 42 depicts a perspective representation of opened robot 474. Robot 474 may be used for propelling the BHA forward in the wellbore. Robot 474 may include electronics, a battery, and a drive mechanism such as wheels, chains, treads, or other mechanism for advancing the robot forward. The battery and the electronics may be power the drive mechanism. Robot 474 may be placed around composite coiled tubing and closed. Robot 474 may travel down the composite coiled tubing but cannot pass over the BHA. FIG. 43 depicts a representation of robot attached to composite coiled tubing 442 and abutting BHA 314. When robot 474 reaches BHA 314, the robot may electrically couple to the BHA. BHA 314 may supply power to the robot to power the drive mechanism and/or recharge the battery of the robot. BHA 314 may send control signals to the electronics of robot 474 that control the operation of the robot when the robot is coupled to the BHA. The control signals provided by BHA 314 may instruct robot 474 to move forward to move the BHA forward.

[0646] 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.

[0647] 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.

[0648] 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.

[0649] 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.

[0650] 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.

[0651] 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.

[0652] 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.

[0653] 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.

[0654] 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.

[0655] 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.

[0656] 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.

[0657] 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.

[0658] 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).

[0659] 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.

[0660] 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.

[0661] FIG. 44 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 476. 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)

[0662] 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.

[0663] 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 476). 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.

[0664] 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.

[0665] 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.

[0666] 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.

[0667] 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.

[0668] 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.

[0669] 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.

[0670] 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.

[0671] 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.

[0672] 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.

[0673] 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.

[0674] 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.

[0675] 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.

[0676] 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.

[0677] 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.

[0678] 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.

[0679] 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.

[0680] 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.

[0681] 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.

[0682] 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.

[0683] 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.

[0684] 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.

[0685] 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.

[0686] 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.

[0687] 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.

[0688] 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.

[0689] 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.

[0690] 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.

[0691] 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.

[0692] 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.

[0693] 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.

[0694] 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.

[0695] 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.

[0696] 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.

[0697] 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.

[0698] 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.

[0699] 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.

[0700] 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.

[0701] 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.

[0702] 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.

[0703] 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.

[0704] 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.

[0705] 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.

[0706] 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, δ, is:
δ=1981.5*(ρ/(μ*f))^1/2;  (EQN. 2)
in which:

• • [0707] δ=skin depth in inches;
  • [0708] ρ=resistivity at operating temperature (ohm-cm);
  • [0709] μ=relative magnetic permeability; and
  • [0710] f=frequency (Hz).
    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.

[0711] 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).

[0712] 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.

[0713] 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.

[0714] 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).

[0715] 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.

[0716] 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.

[0717] 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.

[0718] 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.

[0719] 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.

[0720] 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.

[0721] 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.

[0722] 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.

[0723] 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.

[0724] 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.

[0725] 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 θ 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.

[0726] 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.

[0727] The insulation layer may comprise 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.

[0728] 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.

[0729] 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.

[0730] 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.

[0731] 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.

[0732] 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.

[0733] 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.

[0734] 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.

[0735] 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.

[0736] 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.).

[0737] 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.

[0738] FIGS. 45-62 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.

[0739] 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.

[0740] FIG. 45 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. 46 and 47 depict transverse cross-sectional views of the embodiment shown in FIG. 45. In one embodiment, ferromagnetic section 480 is used to provide heat to hydrocarbon layers in the formation. Non-ferromagnetic section 482 is used in the overburden of the formation. Non-ferromagnetic section 482 provides little or no heat to the overburden, thus inhibiting heat losses in the overburden and improving heater efficiency. Ferromagnetic section 480 includes a ferromagnetic material such as 409 stainless steel or 410 stainless steel. Ferromagnetic section 480 has a thickness of 0.3 cm. Non-ferromagnetic section 482 is copper with a thickness of 0.3 cm. Inner conductor 484 is copper. Inner conductor 484 has a diameter of 0.9 cm. Electrical insulator 486 is silicon nitride, boron nitride, magnesium oxide powder, or another suitable insulator material. Electrical insulator 486 has a thickness of 0.1 cm to 0.3 cm.

[0741] FIG. 48 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. 49, 50, and 51 depict transverse cross-sectional views of the embodiment shown in FIG. 48. Ferromagnetic section 480 is 410 stainless steel with a thickness of 0.6 cm. Non-ferromagnetic section 482 is copper with a thickness of 0.6 cm. Inner conductor 484 is copper with a diameter of 0.9 cm. Outer conductor 488 includes ferromagnetic material. Outer conductor 488 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 488 is 409, 410, or 446 stainless steel with an outer diameter of 3.0 cm and a thickness of 0.6 cm. Electrical insulator 486 includes compacted magnesium oxide powder with a thickness of 0.3 cm. In some embodiments, electrical insulator 486 includes silicon nitride, boron nitride, or hexagonal type boron nitride. Conductive section 490 may couple inner conductor 484 with ferromagnetic section 480 and/or outer conductor 488.

[0742] FIG. 52A and FIG. 52B 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 484 is copper. Electrical insulator 486 is silicon nitride, boron nitride, or magnesium oxide. Outer conductor 488 is a 1″ Schedule 80 446 stainless steel pipe. Outer conductor 488 is coupled to jacket 492. Jacket 492 is made from corrosion resistant material such as 347H stainless steel. In an embodiment, conductive layer 494 is placed between outer conductor 488 and jacket 492. Conductive layer 494 is a copper layer. Heat is produced primarily in outer conductor 488, resulting in a small temperature differential across electrical insulator 486. Conductive layer 494 allows a sharp decrease in the resistance of outer conductor 488 as the outer conductor approaches the Curie temperature and/or the phase transformation temperature range. Jacket 492 provides protection from corrosive fluids in the wellbore.

[0743] In certain embodiments, inner conductor 484 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 486 may be magnesium oxide with an outer diameter between about 1″ and about 1.2″ (for example, about 1.11″). Outer conductor 488 may include non-ferromagnetic electrically conductive material with high mechanical strength such as 825 stainless steel. Outer conductor 488 may have an outer diameter between about 1.2″ and about 1.5″ (for example, about 1.33″). In certain embodiments, inner conductor 484 is a forward current path and outer conductor 488 is a return current path. Conductive layer 494 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 494 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 494 may reduce the thickness needed in outer conductor 488 and/or jacket 492, which may allow a lower cost heater to be made. Jacket 492 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 492 may have a thickness of at least 2 times the skin depth of the ferromagnetic material in the jacket. Jacket 492 may provide protection from corrosive fluids in the wellbore. In some embodiments, inner conductor 484, electrical insulator 486, and outer conductor 488 are formed as composite heater (for example, an insulated conductor heater) and conductive layer 494 and jacket 492 are formed around (for example, wrapped) the composite heater and welded together to form the larger heater embodiment described herein.

[0744] In certain embodiments, jacket 492 includes ferromagnetic material that has a higher Curie temperature than ferromagnetic material in inner conductor 484. 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 484 until the Curie temperature of the ferromagnetic material in the inner conductor is reached. After the Curie temperature of ferromagnetic material in inner conductor 484 is reached, a majority of the current flows through the core of copper in the inner conductor. The ferromagnetic properties of jacket 492 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.

[0745] 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.

[0746] 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.

[0747] FIG. 53 depicts a cross-sectional representation of an embodiment of the composite conductor with the support member. Core 496 is surrounded by ferromagnetic conductor 498 and support member 500. In some embodiments, core 496, ferromagnetic conductor 498, and support member 500 are directly coupled (for example, brazed together or metallurgically bonded together). In one embodiment, core 496 is copper, ferromagnetic conductor 498 is 446 stainless steel, and support member 500 is 347H alloy. In certain embodiments, support member 500 is a Schedule 80 pipe. Support member 500 surrounds the composite conductor having ferromagnetic conductor 498 and core 496. Ferromagnetic conductor 498 and core 496 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.

[0748] In certain embodiments, the diameter of core 496 is adjusted relative to a constant outside diameter of ferromagnetic conductor 498 to adjust the turndown ratio of the temperature limited heater. For example, the diameter of core 496 may be increased to 1.14 cm while maintaining the outside diameter of ferromagnetic conductor 498 at 1.9 cm to increase the turndown ratio of the heater.

[0749] FIG. 54 depicts a cross-sectional representation of an embodiment of the composite conductor with support member 500 separating the conductors. In one embodiment, core 496 is copper with a diameter of 0.95 cm, support member 500 is 347H alloy with an outside diameter of 1.9 cm, and ferromagnetic conductor 498 is 446 stainless steel with an outside diameter of 2.7 cm. The support member depicted in FIG. 54 has a lower creep strength relative to the support members depicted in FIG. 53.

[0750] In certain embodiments, support member 500 is located inside the composite conductor. FIG. 55 depicts a cross-sectional representation of an embodiment of the composite conductor surrounding support member 500. Support member 500 is made of 347H alloy. Inner conductor 484 is copper. Ferromagnetic conductor 498 is 446 stainless steel. In one embodiment, support member 500 is 1.25 cm diameter 347H alloy, inner conductor 484 is 1.9 cm outside diameter copper, and ferromagnetic conductor 498 is 2.7 cm outside diameter 446 stainless steel. The turndown ratio is higher than the turndown ratio for the embodiments depicted in FIGS. 53, 54, and 56 for the same outside diameter, but the creep strength is lower.

[0751] In some embodiments, the thickness of inner conductor 484, which is copper, is reduced and the thickness of support member 500 is increased to increase the creep strength at the expense of reduced turndown ratio. For example, the diameter of support member 500 is increased to 1.6 cm while maintaining the outside diameter of inner conductor 484 at 1.9 cm to reduce the thickness of the conduit. This reduction in thickness of inner conductor 484 results in a decreased turndown ratio relative to the thicker inner conductor embodiment but an increased creep strength.

[0752] FIG. 56 depicts a cross-sectional representation of an embodiment of the composite conductor surrounding support member 500. In one embodiment, support member 500 is 347H alloy with a 0.63 cm diameter center hole. In some embodiments, support member 500 is a preformed conduit. In certain embodiments, support member 500 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 500 is 347H alloy with an inside diameter of 0.63 cm and an outside diameter of 1.6 cm, inner conductor 484 is copper with an outside diameter of 1.8 cm, and ferromagnetic conductor 498 is 446 stainless steel with an outside diameter of 2.7 cm.

[0753] 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 502 in FIG. 57.

[0754] FIG. 57 depicts a cross-sectional representation of an embodiment of the conductor-in-conduit heater. Conductor 502 is disposed in conduit 504. Conductor 502 is a rod or conduit of electrically conductive material. Low resistance sections 506 are present at both ends of conductor 502 to generate less heating in these sections. Low resistance section 506 is formed by having a greater cross-sectional area of conductor 502 in that section, or the sections are made of material having less resistance. In certain embodiments, low resistance section 506 includes a low resistance conductor coupled to conductor 502.

[0755] Conduit 504 is made of an electrically conductive material. Conduit 504 is disposed in opening 508 in hydrocarbon layer 510. Opening 508 has a diameter that accommodates conduit 504.

[0756] Conductor 502 may be centered in conduit 504 by centralizers 512. Centralizers 512 electrically isolate conductor 502 from conduit 504. Centralizers 512 inhibit movement and properly locate conductor 502 in conduit 504. Centralizers 512 are made of ceramic material or a combination of ceramic and metallic materials. Centralizers 512 inhibit deformation of conductor 502 in conduit 504. Centralizers 512 are touching or spaced at intervals between approximately 0.1 m (meters) and approximately 3 m or more along conductor 502.

[0757] A second low resistance section 506 of conductor 502 may couple conductor 502 to wellhead 478. Electrical current may be applied to conductor 502 from power cable 514 through low resistance section 506 of conductor 502. Electrical current passes from conductor 502 through sliding connector 516 to conduit 504. Conduit 504 may be electrically insulated from overburden casing 518 and from wellhead 478 to return electrical current to power cable 514. Heat may be generated in conductor 502 and conduit 504. The generated heat may radiate in conduit 504 and opening 508 to heat at least a portion of hydrocarbon layer 510.

[0758] Overburden casing 518 may be disposed in overburden 520. In some embodiments, overburden casing 518 is surrounded by materials (for example, reinforcing material and/or cement) that inhibit heating of overburden 520. Low resistance section 506 of conductor 502 may be placed in overburden casing 518. Low resistance section 506 of conductor 502 is made of, for example, carbon steel. Low resistance section 506 of conductor 502 may be centralized in overburden casing 518 using centralizers 512. Centralizers 512 are spaced at intervals of approximately 6 m to approximately 12 m or, for example, approximately 9 m along low resistance section 506 of conductor 502. In a heater embodiment, low resistance sections 506 are coupled to conductor 502 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 506 generates little or no heat in overburden casing 518. Packing 522 may be placed between overburden casing 518 and opening 508. Packing 522 may be used as a cap at the junction of overburden 520 and hydrocarbon layer 510 to allow filling of materials in the annulus between overburden casing 518 and opening 508. In some embodiments, packing 522 inhibits fluid from flowing from opening 508 to surface 524.

[0759] FIG. 58 depicts a cross-sectional representation of an embodiment of a removable conductor-in-conduit heat source. Conduit 504 may be placed in opening 508 through overburden 520 such that a gap remains between the conduit and overburden casing 518. Fluids may be removed from opening 508 through the gap between conduit 504 and overburden casing 518. Fluids may be removed from the gap through conduit 526. Conduit 504 and components of the heat source included in the conduit that are coupled to wellhead 478 may be removed from opening 508 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.

[0760] 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.

[0761] 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.

[0762] 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.

[0763] 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.

[0764] 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.

[0765] 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.

[0766] 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.

[0767] 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.

[0768] 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.

[0769] 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.

[0770] 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.

[0771] 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.

[0772] 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.

[0773] 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.

[0774] 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.

[0775] 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.

[0776] 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).

[0777] FIG. 59 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 496 is an inner conductor of the temperature limited heater. In certain embodiments, core 496 is a highly electrically conductive material such as copper or aluminum. In some embodiments, core 496 is a copper alloy that provides mechanical strength and good electrically conductivity such as a dispersion strengthened copper. In one embodiment, core 496 is Glidcop® (SCM Metal Products, Inc., Research Triangle Park, N.C., U.S.A.). Ferromagnetic conductor 498 is a thin layer of ferromagnetic material between electrical conductor 528 and core 496. In certain embodiments, electrical conductor 528 is also support member 500. In certain embodiments, ferromagnetic conductor 498 is iron or an iron alloy. In some embodiments, ferromagnetic conductor 498 includes ferromagnetic material with a high relative magnetic permeability. For example, ferromagnetic conductor 498 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 498 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.

[0778] In certain embodiments, electrical conductor 528 provides support for ferromagnetic conductor 498 and the temperature limited heater. Electrical conductor 528 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 498. In certain embodiments, electrical conductor 528 is a corrosion resistant member. Electrical conductor 528 (support member 500) may provide support for ferromagnetic conductor 498 and corrosion resistance. Electrical conductor 528 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 498.

[0779] In an embodiment, electrical conductor 528 is 347H stainless steel. In some embodiments, electrical conductor 528 is another electrically conductive, good mechanical strength, corrosion resistant material. For example, electrical conductor 528 may be 304H, 316H, 347HH, NF709, Incoloy® 800H alloy (Inco Alloys International, Huntington, W. Va., U.S.A.), Haynes® HR120® alloy, or Inconel® 617 alloy.

[0780] In some embodiments, electrical conductor 528 (support member 500) includes different alloys in different portions of the temperature limited heater. For example, a lower portion of electrical conductor 528 (support member 500) 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.

[0781] In some embodiments, ferromagnetic conductor 498 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.

[0782] In the embodiment depicted in FIG. 59, ferromagnetic conductor 498, electrical conductor 528, and core 496 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 528 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 498. In certain embodiments, the temperature limited heater depicted in FIG. 59 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 528 to provide the majority of electrically resistive heat output. The temperature limited heater depicted in FIG. 59 may be smaller because ferromagnetic conductor 498 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.

[0783] In some embodiments, the support member and the corrosion resistant member are different members in the temperature limited heater. FIGS. 60 and 61 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 528 is jacket 492. Electrical conductor 528, ferromagnetic conductor 498, support member 500, and core 496 (in FIG. 60) or inner conductor 484 (in FIG. 61) 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 528 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 498. For example, electrical conductor 528 is 825 stainless steel or 347H stainless steel. In some embodiments, electrical conductor 528 has a small thickness (for example, on the order of 0.5 mm).

[0784] In FIG. 60, core 496 is highly electrically conductive material such as copper or aluminum. Support member 500 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 498.

[0785] In FIG. 61, support member 500 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 498. Inner conductor 484 is highly electrically conductive material such as copper or aluminum.

[0786] 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.

[0787] FIGS. 62A and 62B depict cross-sectional representations of an embodiment of the insulated conductor heater with the temperature limited heater as the heating member. Insulated conductor 530 includes core 496, ferromagnetic conductor 498, inner conductor 484, electrical insulator 486, and jacket 492. Core 496 is a copper core. Ferromagnetic conductor 498 is, for example, iron or an iron alloy.

[0788] Inner conductor 484 is a relatively thin conductive layer of non-ferromagnetic material with a higher electrical conductivity than ferromagnetic conductor 498. In certain embodiments, inner conductor 484 is copper. Inner conductor 484 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 484 is copper with 6% by weight nickel (for example, CuNi6 or LOHM™). In some embodiments, inner conductor 484 is CuNi10Fe1Mn alloy. Below the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 498, the magnetic properties of the ferromagnetic conductor confine the majority of the flow of electrical current to inner conductor 484. Thus, inner conductor 484 provides the majority of the resistive heat output of insulated conductor 530 below the Curie temperature and/or the phase transformation temperature range.

[0789] In certain embodiments, inner conductor 484 is dimensioned, along with core 496 and ferromagnetic conductor 498, so that the inner conductor provides a desired amount of heat output and a desired turndown ratio. For example, inner conductor 484 may have a cross-sectional area that is around 2 or 3 times less than the cross-sectional area of core 496. Typically, inner conductor 484 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 484, core 496 has a diameter of 0.66 cm, ferromagnetic conductor 498 has an outside diameter of 0.91 cm, inner conductor 484 has an outside diameter of 1.03 cm, electrical insulator 486 has an outside diameter of 1.53 cm, and jacket 492 has an outside diameter of 1.79 cm. In an embodiment with a CuNi6 inner conductor 484, core 496 has a diameter of 0.66 cm, ferromagnetic conductor 498 has an outside diameter of 0.91 cm, inner conductor 484 has an outside diameter of 1.12 cm, electrical insulator 486 has an outside diameter of 1.63 cm, and jacket 492 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.

[0790] Electrical insulator 486 may be magnesium oxide, aluminum oxide, silicon dioxide, beryllium oxide, boron nitride, silicon nitride, or combinations thereof. In certain embodiments, electrical insulator 486 is a compacted powder of magnesium oxide. In some embodiments, electrical insulator 486 includes beads of silicon nitride.

[0791] In certain embodiments, a small layer of material is placed between electrical insulator 486 and inner conductor 484 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 486 and inner conductor 484.

[0792] Jacket 492 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 492 provides some mechanical strength for insulated conductor 530 at or above the Curie temperature and/or the phase transformation temperature range of ferromagnetic conductor 498. In certain embodiments, jacket 492 is not used to conduct electrical current.

[0793] 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.

[0794] 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.

[0795] 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.

[0796] 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.

[0797] 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.

[0798] 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.

[0799] FIG. 63 depicts a top view representation of heater 352 with three insulated conductors 530 in conduit 526. Heater 352 may be located in a heater well in the subsurface formation. Conduit 526 may be a sheath, jacket, or other enclosure around insulated conductors 530. Each insulated conductor 530 includes core 496, electrical insulator 486, and jacket 492. Insulated conductors 530 may be mineral insulated conductors with core 496 being a copper alloy (for example, a copper-nickel alloy such as Alloy 180), electrical insulator 486 being magnesium oxide, and jacket 492 being Incoloy® 825, copper, or stainless steel (for example 347H stainless steel). In some embodiments, jacket 492 includes non-work hardenable metals so that the jacket is annealable.

[0800] In some embodiments, core 496 and/or jacket 492 include ferromagnetic materials. In some embodiments, one or more insulated conductors 530 are temperature limited heaters. In certain embodiments, the overburden portion of insulated conductors 530 include high electrical conductivity materials in core 496 (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 526 includes non-corrosive materials and/or high strength materials such as stainless steel. In one embodiment, conduit 526 is 347H stainless steel.

[0801] Insulated conductors 530 may be coupled to the single transformer in a three-phase configuration (for example, a three-phase wye configuration). Each insulated conductor 530 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 352 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.

[0802] Electrical insulator 486′ may be located inside conduit 526 to electrically insulate insulated conductors 530 from the conduit. In certain embodiments, electrical insulator 486′ is magnesium oxide (for example, compacted magnesium oxide). In some embodiments, electrical insulator 486′ is silicon nitride (for example, silicon nitride blocks). Electrical insulator 486′ electrically insulates insulated conductors 530 from conduit 526 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 486′ inside conduit 526 has at least the thickness of electrical insulators 486 in insulated conductors 530. The increased thickness of insulation in heater 352 (from electrical insulators 486 and/or electrical insulator 486′) inhibits and may prevent current leakage into the formation from the heater. In some embodiments, electrical insulator 486′ spatially locates insulated conductors 530 inside conduit 526.

[0803] FIG. 64 depicts an embodiment of three-phase wye transformer 532 coupled to a plurality of heaters 352. For simplicity in the drawing, only four heaters 352 are shown in FIG. 64. It is to be understood that several more heaters may be coupled to the transformer 532. As shown in FIG. 64, each leg (each insulated conductor) of each heater is coupled to one phase of transformer 532 and current is returned to the neutral or ground of the transformer (for example, returned through conductor 534 depicted in FIGS. 63 and 65).

[0804] Return conductor 534 may be electrically coupled to the ends of insulated conductors 530 (as shown in FIG. 65) current returns from the ends of the insulated conductors to the transformer on the surface of the formation. Return conductor 534 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 534 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 534 is a small insulated conductor (for example, small mineral insulated conductor). Return conductor 534 may be coupled to the neutral or ground leg of the transformer in a three-phase wye configuration. Thus, insulated conductors 530 are electrically isolated from conduit 526 and the formation. Using return conductor 534 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 492, depicted in FIG. 63. One or more jackets 492 may be coupled to cores 496 at the end of the heaters and return current to the neutral of the three-phase wye transformer.

[0805] FIG. 65 depicts a side view representation of the end section of three insulated conductors 530 in conduit 526. 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 536 coupled to conduit 526. In some embodiments, contactor section 536 is welded or brazed to conduit 526. Termination 538 is located in contactor section 536. Termination 538 is electrically coupled to insulated conductors 530 and return conductor 534. Termination 538 electrically couples the cores of insulated conductors 530 to the return conductor 534 at the ends of the heaters.

[0806] In certain embodiments, heater 352, depicted in FIGS. 63 and 65, 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.

[0807] Heaters that include three insulated conductors 530 in conduit 526, as depicted in FIGS. 63 and 65, 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.

[0808] Insulated conductors 530 may be pre-assembled prior to the bundling either on site or at a remote location. Insulated conductors 530 and return conductor 534 may be positioned on spools. A machine may draw insulated conductors 530 and return conductor 534 from the spools at a selected rate. Preformed blocks of insulation material may be positioned around return conductor 534 and insulated conductors 530. In an embodiment, two blocks are positioned around return conductor 534 and three blocks are positioned around insulated conductors 530 to form electrical insulator 486′. 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 526 around electrical insulator 486′, the bundle of insulated conductors 530, and return conductor 534, the conduit may be compacted against the electrical insulator 534 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.

[0809] In some embodiments, heater 352 (which includes conduit 526 around electrical insulator 486′ and the bundle of insulated conductors 530 and return conductor 534) 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 352 into the wellbore.

[0810] In some embodiments, one or more components of heater 352 are varied (for example, removed, moved, or replaced) while the operation of the heater remains substantially identical. FIG. 66 depicts an embodiment of heater 352 with three insulated cores 496 in conduit 526. In this embodiment, electrical insulator 486′ surrounds cores 496 and return conductor 534 in conduit 526. Cores 496 are located in conduit 526 without an electrical insulator and jacket surrounding the cores. Cores 496 are coupled to the single transformer in a three-phase wye configuration with each core 496 coupled to one phase of the transformer. Return conductor 534 is electrically coupled to the ends of cores 496 and returns current from the ends of the cores to the transformer on the surface of the formation.

[0811] FIG. 67 depicts an embodiment of heater 352 with three insulated conductors 530 and insulated return conductor in conduit 526. In this embodiment, return conductor 534 is an insulated conductor with core 496, electrical insulator 486, and jacket 492. Return conductor 534 and insulated conductors 530 are located in conduit 526 surrounded by electrical insulator 486′. Return conductor 534 and insulated conductors 530 may be the same size or different sizes. Return conductor 534 and insulated conductors 530 operate substantially the same as in the embodiment depicted in FIGS. 63 and 65.

[0812] 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.

[0813] 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 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.

[0814] 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.

[0815] 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).

[0816] 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. 68 depicts outer tubing 540 partially unspooled from coiled tubing rig 542. Outer tubing 540 may be made of metal or polymeric material. Outer tubing 540 may be a flexible conduit such as, for example, a tubing guide string or other coiled tubing string. Heater 352 may be pushed into outer tubing 540, as shown in FIG. 69. In certain embodiments, heater 352 is pushed into outer tubing 540 by pumping the heater into the outer tubing.

[0817] In certain embodiments, one or more flexible cups 544 are coupled to the outside of heater 352. Flexible cups 544 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 540 as heater 352 is pushed or pumped into the outer tubing. For example, flexible cups 544 may have flexible edges that provide limited mechanical resistance as heater 352 is pushed into outer tubing 540 but remain in contact with the inner walls of outer tubing 540 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 540 between flexible cups 544 allows heater 352 to be continuously pushed into the outer tubing with lower pump pressures. Without flexible cups 544, higher pressures may be needed to push heater 352 into outer tubing 540. In some embodiments, cups 544 allow some pressure to be released while maintaining some pressure in outer tubing 540. In certain embodiments, flexible cups 544 are spaced to distribute pumping forces optimally along heater 352 inside outer tubing 540.

[0818] Heater 352 is pushed into outer tubing 540 until the heater is fully inserted into the outer tubing, as shown in FIG. 70. Drilling guide 546 may be coupled to the end of heater 352. Heater 352, outer tubing 540, and drilling guide 546 may be spooled onto coiled tubing rig 542, as shown in FIG. 71. After heater 352, outer tubing 540, and drilling guide 546 are spooled onto coiled tubing rig 542, 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).

[0819] FIG. 72 depicts coiled tubing rig 542 being used to install heater 352 and outer tubing 540 into opening 508 using drilling guide 546. In certain embodiments, opening 508 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 352 has a heating section that is placed in the substantially horizontally or inclined portion of opening 508 to be used to heat the hydrocarbon containing layer. In some embodiments, opening 508 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 518 may be located around the outer walls of opening 508 in an overburden section of the formation. In some embodiments, drilling fluid is left in opening 508 after the opening has been completed (the opening has been drilled).

[0820] FIG. 73 depicts heater 352 and outer tubing 540 installed in opening 508. Gap 548 may be left at or near the far end of heater 352 and outer tubing 540. Gap 548 may allow for some heater expansion in opening 508 after the heater is energized.

[0821] After heater 352 and outer tubing 540 are installed in opening 508, the outer tubing may be removed from the opening to leave the heater in place in the opening. FIG. 74 depicts outer tubing 540 being removed from opening 508 while leaving heater 352 installed in the opening. Outer tubing 540 is spooled back onto coiled tubing rig 542 as the outer tubing is pulled off heater 352. In some embodiments, outer tubing 540 is pumped down to allow the outer tubing to be pulled off heater 352.

[0822] FIG. 75 depicts outer tubing 540 used to provide packing material 522 into opening 508. As outer tubing 540 reaches the “shoe” or bend in opening 508, the outer tubing may be used to provide packing material into the opening. The shoe of opening 508 may be located at or near the bottom of overburden casing 518. Packing material 522 may be provided (for example, pumped) through outer tubing 540 and out the end of the outer tubing at the shoe of opening 508. Packing material 522 is provided into opening 508 to seal off the opening around heater 352. Packing material 522 provides a barrier between the overburden section and heating section of opening 508. In certain embodiments, packing material 522 is cement or another suitable plugging material. In some embodiments, outer tubing 540 is continuously spooled while packing material 522 is provided into opening 508. Outer tubing 540 may be spooled slowly while packing material 522 is provided into opening 508 to allow the packing material to settle into the opening properly.

[0823] After packing material 522 is provided into opening 508, outer tubing 540 is spooled further onto coiled tubing rig 542, as shown in FIG. 76. FIG. 77 depicts outer tubing 540 spooled onto coiled tubing rig 542 with heater 352 installed in opening 508. In certain embodiments, flexible cups 544 are spaced in the portion of opening 508 with overburden casing 518 to facilitate adequate stand-off of heater 352 in the overburden portion of the opening. Flexible cups 544 may electrically insulate heater 352 from overburden casing 518. For example, flexible cups 544 may space apart heater 352 and overburden casing 518 such that they are not in physical contact with each other.

[0824] After outer tubing 540 is removed from opening 508, wellhead 478 and/or other completions may be installed at the surface of the opening, as shown in FIG. 78. When heater 352 is energized to begin heating, flexible cups 544 may begin to burn or melt off. Flexible cups 544 may begin to burn or melt off at relatively low temperatures during the heating process.

[0825] FIG. 79 depicts an embodiment of a heater in wellbore 550 in formation 380. The heater includes insulated conductor 530 in conduit 504 with material 552 between the insulated conductor and the conduit. In some embodiments, insulated conductor 530 is a mineral insulated conductor. Electricity supplied to insulated conductor 530 resistively heats the insulated conductor. Insulated conductor conductively transfers heat to material 552. Heat may transfer within material 552 by heat conduction and/or by heat convection. Radiant heat from insulated conductor 530 and/or heat from material 552 transfers to conduit 504. Heat may transfer to the formation from the heater by conductive or radiative heat transfer from conduit 504. Material 552 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 504 above material 552. The gas may inhibit oxidation or other chemical changes of material 552. The gas may inhibit vaporization of material 552. 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.

[0826] Insulated conductor 530 and conduit 504 may be placed in an opening in a subsurface formation. Insulated conductor 530 and conduit 504 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 530 includes core 496, electrical insulator 486, and jacket 492. In some embodiments, core 496 is a copper core. In some embodiments, core 496 includes other electrical conductors or alloys (for example, copper alloys). In some embodiments, core 496 includes a ferromagnetic conductor so that insulated conductor 530 operates as a temperature limited heater. In some embodiments, core 496 does not include a ferromagnetic conductor.

[0827] In some embodiments, core 496 of insulated conductor 530 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 530 may be sized and/or made of material that resistively heats to a high temperature. These portions of core 530 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.

[0828] In some embodiments, core 496 of insulated conductor 530 is a highly conductive material such as copper. Core 496 may be electrically coupled to jacket 492 at or near the end of the insulated conductor. In some embodiments, insulated conductor 530 is electrically coupled to conduit 504. Electrical current supplied to insulated conductor 530 may resistively heat core 496, jacket 492, material 552, and/or conduit 504. Resistive heating of core 496, jacket 492, material 552, and/or conduit 504 generates heat that may transfer to the formation.

[0829] Electrical insulator 486 may be magnesium oxide, aluminum oxide, silicon dioxide, beryllium oxide, boron nitride, silicon nitride, or combinations thereof. In certain embodiments, electrical insulator 486 is a compacted powder of magnesium oxide. In some embodiments, electrical insulator 486 includes beads of silicon nitride. In certain embodiments, a thin layer of material clad over core 496 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 496.

[0830] In some embodiments, material 552 may be relatively corrosive. Jacket 492 and/or at least the inside surface of conduit 504 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 504 may be plated or lined with nickel. In some embodiments, material 552 may be relatively non-corrosive. Jacket 492 and/or at least the inside surface of conduit 504 may be made of a material such as carbon steel.

[0831] In some embodiments, jacket 492 of insulated conductor 530 is not used as the main return of electrical current for the insulated conductor. In embodiments where material 552 is a good electrical conductor such as a molten metal, current returns through the molten metal in the conduit and/or through the conduit 504. In some embodiments, conduit 504 is made of a ferromagnetic material, (for example 410 stainless steel). Conduit 504 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.

[0832] In some embodiments, material 552 returns electrical current to the surface from insulated conductor 530 (i.e., the material acts as the return or ground conductor for the insulated conductor). Material 552 may provide a current path with low resistance so that a long insulated conductor 530 is useable in conduit 504. The long heater may operate at low voltages for the length of the heater due to the presence of material 552 that is conductive.

[0833] FIG. 80 depicts an embodiment of a portion of insulated conductor 530 in conduit 504 wherein material 552 is a good conductor (for example, a liquid metal) and current flow is indicated by the arrows. Current flows down core 496 and returns through jacket 492, material 552, and conduit 504. Jacket 492 and conduit 504 may be at approximately constant potential. Current flows radially from jacket 492 to conduit 504 through material 552. Material 552 may resistively heat. Heat from material 552 may transfer through conduit 504 into the formation.

[0834] In embodiments where material 552 is partially electrically conductive (for example, the material is a molten salt), current returns mainly through jacket 492. All or a portion of the current that passes through partially conductive material 552 may pass to ground through conduit 504.

[0835] In the embodiment depicted in FIG. 79, core 496 of insulated conductor 530 has a diameter of about 1 cm, electrical insulator 486 has an outside diameter of about 1.6 cm, and jacket 492 has an outside diameter of about 1.8 cm. In other embodiments, the insulated conductor is smaller. For example, core 496 has a diameter of about 0.5 cm, electrical insulator 486 has an outside diameter of about 0.8 cm, and jacket 492 has an outside diameter of about 0.9 cm. Other insulated conductor geometries may be used. For the same size conduit 504, the smaller geometry of insulated conductor 530 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.

[0836] Material 552 may be placed between the outside surface of insulated conductor 530 and the inside surface of conduit 504. In certain embodiments, material 552 is placed in the conduit in a solid form as balls or pellets. Material 552 may melt below the operating temperatures of insulated conductor 530. Material may melt above ambient subsurface formation temperatures. Material 552 may be placed in conduit 504 after insulated conductor 530 is placed in the conduit. In certain embodiments, material 552 is placed in conduit 530 as a liquid. The liquid may be placed in conduit 504 before or after insulated conductor 530 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 552 may be placed in conduit 504 before or after insulated conductor 530 is energized (i.e., supplied with electricity). Material 552 may be added to conduit 504 or removed from the conduit after operation of the heater is initialized. Material 552 may be added to or removed from conduit 504 to maintain a desired head of fluid in the conduit. In some embodiments, the amount of material 552 in conduit 504 may be adjusted (i.e., added to or depleted) to adjust or balance the stresses on the conduit. Material 552 may inhibit deformation of conduit 504. The head of material 552 in conduit 504 may inhibit the formation from crushing or otherwise deforming the conduit should the formation expand against the conduit. The head of fluid in conduit 504 allows the wall of the conduit to be relatively thin. Having thin conduits 504 may increase the economic viability of using multiple heaters of this type to heat portions of the formation.

[0837] Material 552 may support insulated conductor 530 in conduit 504. The support provided by material 552 of insulated conductor 530 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 530 is buoyant in material 552 in conduit 504. For example, insulated conductor may be buoyant in molten metal. The buoyancy of insulated conductor 530 reduces creep associated problems in long, substantially vertical heaters. A bottom weight or tie down may be coupled to the bottom of insulated conductor 530 to inhibit the insulated conductor from floating in material 552.

[0838] Material 552 may remain a liquid at operating temperatures of insulated conductor 530. In some embodiments, material 552 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 552 provides enhanced heat transfer from insulated conductor 530 to conduit 504 at or near the operating temperatures of the insulated conductor.

[0839] Material 552 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 552 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.

[0840] Material 552 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 weight 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.).

[0841]

	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

[0842] 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.

[0843] 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.

[0844] Conduit 504 may be a carbon steel or stainless steel canister. In some embodiments, conduit 504 may include cladding on the outer surface to inhibit corrosion of the conduit by formation fluid. Conduit 504 may include cladding on an inner surface of the conduit that is corrosion resistant to material 552 in the conduit. Cladding applied to conduit 504 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 504 is a canister of 410 stainless steel with an outside diameter of about 6 cm. Conduit 504 may not need a thick wall because material 552 may provide internal pressure that inhibits deformation or crushing of the conduit due to external stresses.

[0845] FIG. 81 depicts an embodiment of the heater positioned in wellbore 550 of formation 380 with a portion of insulated conductor 530 and conduit 504 oriented substantially horizontally in the formation. Material 552 may provide a head in conduit 504 due to the pressure of the material. The pressure head may keep material 552 in conduit 504. The pressure head may also provide internal pressure that inhibits deformation or collapse of conduit 504 due to external stresses.

[0846] 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.

[0847] 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. 82 depicts a cross-sectional representation of ribbed conduit 554. FIG. 83 depicts a perspective view of a portion of ribbed conduit 554. Ribbed conduit 554 may include rings 556 and ribs 558. Rings 556 and ribs 558 may improve the heat transfer characteristics of ribbed conduit 554. 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 556 may be spaced about every 3.8 cm. Rings 556 may have a height of about 1.9 cm and a thickness of about 0.5 cm. Six ribs 558 may be spaced evenly about conduit 504. Ribs 558 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 554 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.

[0848] 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.

[0849] 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.

[0850] The material to be heated by the insulated conductor may be placed in an open wellbore. FIG. 84 depicts material 552 in open wellbore 550 in formation 380 with insulated conductor 530 in the wellbore. In some embodiments, a gas (for example, nitrogen, carbon dioxide, and/or helium) is placed in wellbore 550 above material 552. The gas may inhibit oxidation or other chemical changes of material 552. The gas may inhibit vaporization of material 552.

[0851] Material 552 may have a melting point that is above the pyrolysis temperature of hydrocarbons in the formation. The melting point of material 552 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 530 may result in coking that reduces the permeability and plugs the formation near wellbore 550. The plugged formation inhibits material 552 from leaking from wellbore 550 into formation 380 when the material is a liquid. In some embodiments, material 552 is a salt.

[0852] In some embodiments, material 552 leaking from wellbore 550 into formation 380 may be self-healing and/or self-sealing. Material 552 flowing away from wellbore 550 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 552 in a liquid state. The rapid drop off in temperature may result in migrating material 552 solidifying close to wellbore 550. Solidified material 552 may inhibit migration of additional material from wellbore 550, and thus self-heal and/or self-seal the wellbore.

[0853] Return electrical current for insulated conductor 530 may return through jacket 492 of the insulated conductor. Any current that passes through material 552 may pass to ground. Above the level of material 552, any remaining return electrical current may be confined to jacket 492 of insulated conductor 530.

[0854] 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.

[0855] 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.

[0856] 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.

[0857] 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.

[0858] 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.

[0859] FIG. 85 depicts schematic cross-sectional representation of a portion of a formation with heat pipes 560 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 560 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 560 mechanically attach to heat source 202. Heat pipes 560 may be spaced a desired distance apart. In an embodiment, heat pipes 560 are spaced apart by about 40 feet. In other embodiments, large or smaller spacings are used. Heat pipes 560 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 560 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 560 may be vertically positioned in the formation. In some embodiments, heat pipes 560 are placed at an angle in the formation.

[0860] Heat pipes 560 may include sealed conduit 562, seal 564, liquid heat transfer fluid 566 and vaporized heat transfer fluid 568. In some embodiments, heat pipes 560 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 562 may have first portion 570 and second portion 572. Liquid heat transfer fluid 566 may be in first portion 570. Heat source 202 external to heat pipe 560 supplies heat that vaporizes liquid heat transfer fluid 566. Vaporized heat transfer fluid 568 diffuses into second portion 572. Vaporized heat transfer fluid 568 condenses in second portion and transfers heat to conduit 562, which in turn transfers heat to the formation. The condensed liquid heat transfer fluid 566 flows by gravity to first portion 570.

[0861] Position of seal 564 is a factor in determining the effective length of heat pipe 560. The effective length of heat pipe 560 may also depend on the physical properties of the heat transfer fluid and the cross-sectional area of conduit 562. Enough heat transfer fluid may be placed in conduit 562 so that some liquid heat transfer fluid 566 is present in first portion 570 at all times.

[0862] Seal 564 may provide a top seal for conduit 562. In some embodiments, conduit 562 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 562 to evacuate the conduit before the conduit is sealed. Drawing a vacuum on conduit 562 before sealing the conduit may enhance vapor diffusion throughout the conduit. In some embodiments, an oxygen getter may be introduced in conduit 562 to react with any oxygen present in the conduit.

[0863] FIG. 86 depicts a perspective cut-out representation of a portion of a heat pipe embodiment with heat pipe 560 located radially around oxidizer assembly 574. Oxidizers 576 of oxidizer assembly 574 are positioned adjacent to first portion 570 of heat pipe 560. Fuel may be supplied to oxidizers 576 through fuel conduit 578. Oxidant may be supplied to oxidizers 576 through oxidant conduit 580. Exhaust gas may flow through the space between outer conduit 582 and oxidant conduit 580. Oxidizers 576 combust fuel to provide heat that vaporizes liquid heat transfer fluid 566. Vaporized heat transfer fluid 568 rises in heat pipe 560 and condenses on walls of the heat pipe to transfer heat to sealed conduit 562. Exhaust gas from oxidizers 576 provides heat along the length of sealed conduit 562. The heat provided by the exhaust gas along the effective length of heat pipe 560 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.

[0864] FIG. 87 depicts a cross-sectional representation of an angled heat pipe embodiment with oxidizer assembly 574 located near a lowermost portion of heat pipe 560. Fuel may be supplied to oxidizers 576 through fuel conduit 578. Oxidant may be supplied to oxidizers 576 through oxidant conduit 580. Exhaust gas may flow through the space between outer conduit 582 and oxidant conduit 580.

[0865] FIG. 88 depicts a perspective cut-out representation of a portion of a heat pipe embodiment with oxidizer 576 located at the bottom of heat pipe 560. Fuel may be supplied to oxidizer 576 through fuel conduit 578. Oxidant may be supplied to oxidizer 576 through oxidant conduit 580. Exhaust gas may flow through the space between the outer wall of heat pipe 560 and outer conduit 582. Oxidizer 576 combusts fuel to provide heat that vaporizers liquid heat transfer fluid 566. Vaporized heat transfer fluid 568 rises in heat pipe 560 and condenses on walls of the heat pipe to transfer heat to sealed conduit 562. Exhaust gas from oxidizers 576 provides heat along the length of sealed conduit 562 and to outer conduit 582. The heat provided by the exhaust gas along the effective length of heat pipe 560 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. 89 depicts a similar embodiment with heat pipe 560 positioned at an angle in the formation.

[0866] FIG. 90 depicts a perspective cut-out representation of a portion of a heat pipe embodiment with oxidizer 576 that produces flame zone adjacent to liquid heat transfer fluid 566 in the bottom of heat pipe 560. Fuel may be supplied to oxidizer 576 through fuel conduit 578. Oxidant may be supplied to oxidizer 576 through oxidant conduit 580. Oxidant and fuel are mixed and combusted to produce flame zone 584. Flame zone 584 provides heat that vaporizes liquid heat transfer fluid 566. Exhaust gases from oxidizer 576 may flow through the space between oxidant conduit 580 and the inner surface of heat pipe 560, and through the space between the outer surface of the heat pipe and outer conduit 582. The heat provided by the exhaust gas along the effective length of heat pipe 560 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.

[0867] FIG. 91 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 574 may provide high heat input to liquid heat transfer fluid 566 of heat pipe 560. A portion of oxidizer assembly with the oxidizers may be helically wound around a tapered portion of heat pipe 560. The tapered portion may have a large surface area to accommodate the oxidizers. Fuel may be supplied to the oxidizers of oxidizer assembly 574 through fuel conduit 578. Oxidant may be supplied to oxidizer 576 through oxidant conduit 580. Exhaust gas may flow through the space between the outer wall of heat pipe 560 and outer conduit 582. Exhaust gas from oxidizers 576 provides heat along the length of sealed conduit 562 and to outer conduit 582. The heat provided by the exhaust gas along the effective length of heat pipe 560 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.

[0868] FIG. 92 depicts a cross-sectional representation of a heat pipe embodiment that is angled within the formation. First wellbore 586 and second wellbore 588 are drilled in the formation using magnetic ranging or techniques so that the first wellbore intersects the second wellbore. Heat pipe 560 may be positioned in first wellbore 586. First wellbore 586 may be sloped so that liquid heat transfer fluid 566 within heat pipe 560 is positioned near the intersection of the first wellbore and second wellbore 588. Oxidizer assembly 574 may be positioned in second wellbore 588. Oxidizer assembly 574 provides heat to heat pipe 560 that vaporizes liquid heat transfer fluid in the heat pipe. Packer or seal 590 may direct exhaust gas from oxidizer assembly 574 through first wellbore 586 to provide additional heat to the formation from the exhaust gas.

[0869] 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.

[0870] 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.

[0871] 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.

[0872] FIG. 93 depicts an embodiment of three heaters coupled in a three-phase configuration. Conductor “legs” 592, 594, 596 are coupled to three-phase transformer 598. Transformer 598 may be an isolated three-phase transformer. In certain embodiments, transformer 598 provides three-phase output in a wye configuration. Input to transformer 598 may be made in any input configuration, such as the shown delta configuration. Legs 592, 594, 596 each include lead-in conductors 600 in the overburden of the formation coupled to heating elements 602 in hydrocarbon layer 510. Lead-in conductors 600 include copper with an insulation layer. For example, lead-in conductors 600 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 600 are located in an overburden portion of the formation. The overburden portion may include overburden casings 518. Heating elements 602 may be temperature limited heater heating elements. In an embodiment, heating elements 602 are 410 stainless steel rods (for example, 3.1 cm diameter 410 stainless steel rods). In some embodiments, heating elements 602 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 602 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.

[0873] In certain embodiments, heating elements 602 are exposed to hydrocarbon layer 510 and fluids from the hydrocarbon layer. Thus, heating elements 602 are “bare metal” or “exposed metal” heating elements. Heating elements 602 may be made from a material that has an acceptable sulfidation rate at high temperatures used for pyrolyzing hydrocarbons. In certain embodiments, heating elements 602 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° C.). 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 602 are made from material that has a sulfidation rate below a selected value in a temperature range. In some embodiments, heating elements 602 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 602 may also be substantially inert to galvanic corrosion.

[0874] In some embodiments, heating elements 602 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.

[0875] 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.

[0876] Heating elements 602 may be coupled to contacting elements 604 at or near the underburden of the formation. Contacting elements 604 are copper or aluminum rods or other highly conductive materials. In certain embodiments, transition sections 606 are located between lead-in conductors 600 and heating elements 602, and/or between heating elements 602 and contacting elements 604. Transition sections 606 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 606 are made of materials that electrically couple lead-in conductors 600 and heating elements 602 while providing little or no heat output. Thus, transition sections 606 help to inhibit overheating of conductors and insulation used in lead-in conductors 600 by spacing the lead-in conductors from heating elements 602. Transition section 606 may have a length of between about 3 m and about 9 m (for example, about 6 m).

[0877] Contacting elements 604 are coupled to contactor 608 in contacting section 610 to electrically couple legs 592, 594, 596 to each other. In some embodiments, contact solution 612 (for example, conductive cement) is placed in contacting section 610 to electrically couple contacting elements 604 in the contacting section. In certain embodiments, legs 592, 594, 596 are substantially parallel in hydrocarbon layer 510 and leg 592 continues substantially vertically into contacting section 610. The other two legs 594, 596 are directed (for example, by directionally drilling the wellbores for the legs) to intercept leg 592 in contacting section 610.

[0878] Each leg 592, 594, 596 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 592, 594, 596 may be arranged in a triangular pattern so that the three legs form a triangular shaped three-phase heater. In an embodiment, legs 592, 594, 596 are arranged in a triangular pattern with 12 m spacing between the legs (each side of the triangle has a length of 12 m).

[0879] FIG. 94 depicts a side view cross-sectional representation of an embodiment of centralizer 512 on heater 352. FIG. 95 depicts an end view cross-sectional representation of the embodiment of centralizer 512 on heater 352 depicted in FIG. 94. In certain embodiments, centralizers 512 are made of three or more parts coupled to heater 352 so that the parts are spaced around the outside diameter of the heater. Having spaces between the parts of a centralizer allows debris to fall along the heater (when the heater is vertical or substantially vertical) and inhibit debris from collecting at the centralizer. In certain embodiments, the centralizer is installed on a long heater without inserting a ring. In certain embodiments, heater 352, as depicted in FIGS. 94 and 95, is an electrical conductor used as part of a heater (for example, the electrical conductor of a conductor-in-conduit heater). In certain embodiments, centralizer 512 includes three centralizer parts 512A, 512B, and 512C. In other embodiments, centralizer 512 includes four or more centralizer parts. Centralizer parts 512A, 512B, 512C may be evenly distributed around the outside diameter of heater 352. Centralizer parts 512A, 512B, 512C may have shapes that inhibit collection of material and/or gouging of the canister that surrounds heater 352, even when the centralizer parts are rotated in the canister. In some embodiments, upper portions of centralizer parts 512A, 512B, 512C may taper and/or be rounded to inhibit accumulation of material on top of the centralizer parts.

[0880] In certain embodiments, centralizer parts 512A, 512B, 512C include insulators 614 and weld bases 616. Insulators 614 may be made of electrically insulating material such as, but not limited to, ceramic (for example, magnesium oxide) or silicon nitride. Weld bases 616 may be made of weldable metal such as, but not limited to, Alloy 625, the same metal used for heater 352, or another metal that may be brazed or solid state welded to insulators 614 and welded to a metal used for heater 352.

[0881] Weld bases 616 may be brazed or brazed to heater 352. In certain embodiments, insulators 614 are brazed, or otherwise coupled, to weld bases 616 to form centralizer parts 512A, 512B, 512C. Point load transfer between insulators 614 and weld bases 616 may be minimized by the coupling. In some embodiments, weld bases 616 are coupled to heater 352 first and then insulators 614 are coupled to the weld bases to form centralizer parts 512A, 512B, 512C. Insulators 614 may be coupled to weld bases 616 as the heater is being installed into the formation. In some embodiments, the bottoms of insulators 614 conform to the shape of heater 352. In other embodiments, the bottoms of insulators 614 are flat or have other geometries.

[0882] In certain embodiments, centralizer parts 512A, 512B, 512C are spaced evenly around the outside diameter of heater 352, as shown in FIGS. 94 and 95. In other embodiments, centralizer parts 512A, 512B, 512C have other spacings around the outside diameter of heater 352.

[0883] Having space between centralizer parts 512A, 512B, 512C allows installation of the heaters and centralizers from a spool or coiled tubing installation of the heaters and centralizers. Centralizer parts 512A, 512B, 512C also allow debris (for example, metal dust or pieces of formation) to fall along heater 352 through the area of the centralizer. Thus, debris is inhibited from collecting at or near centralizer 512. In addition, centralizer parts 512A, 512B, 512C may be inexpensive to manufacture and install and easy to replace if broken.

[0884] FIG. 96 depicts a side view representation of an embodiment of a substantially u-shaped three-phase heater. First ends of legs 592, 594, 596 are coupled to transformer 598 at first location 618. In an embodiment, transformer 598 is a three-phase AC transformer. Ends of legs 592, 594, 596 are electrically coupled together with connector 620 at second location 622. Connector 620 electrically couples the ends of legs 592, 594, 596 so that the legs can be operated in a three-phase configuration. In certain embodiments, legs 592, 594, 596 are coupled to operate in a three-phase wye configuration. In certain embodiments, legs 592, 594, 596 are substantially parallel in hydrocarbon layer 510. In certain embodiments, legs 592, 594, 596 are arranged in a triangular pattern in hydrocarbon layer 510. In certain embodiments, heating elements 602 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 592, 594, 596 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.

[0885] In certain embodiments, overburden casings (for example, overburden casings 518, depicted in FIGS. 93 and 96) in overburden 520 include materials that inhibit ferromagnetic effects in the casings. Inhibiting ferromagnetic effects in casings 518 reduces heat losses to the overburden. In some embodiments, casings 518 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 520 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 518 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.

[0886] In certain embodiments, one or more non-ferromagnetic materials used in casings 518 are used in a wellhead coupled to the casings and legs 592, 594, 596. 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 518 to inhibit reflux of heated gases into the wellhead and/or the casings.

[0887] In certain embodiments, one or more of legs 592, 594, 596 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.

[0888] FIG. 97 depicts a top view representation of an embodiment of a plurality of triads of three-phase heaters in the formation. Each triad 624 includes legs A, B, C (which may correspond to legs 592, 594, 596 depicted in FIGS. 93 and 96) that are electrically coupled by linkages 626. Each triad 624 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.

[0889] The phases of each triad 624 may be arranged so that legs A, B, C correspond between triads as shown in FIG. 97. 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 624 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.

[0890] In the early stages of heating, an exposed heating element (for example, heating element 602 depicted in FIGS. 93 and 96) 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.

[0891] 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.

[0892] 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. 98 depicts a top view representation of the embodiment depicted in FIG. 97 with production wells 206. In certain embodiments, production wells 206 are located at or near center of triad 624. 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. 98.

[0893] Certain embodiments of heaters include single-phase conductors in a single wellbore. For example, FIGS. 93 and 96 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.

[0894] FIG. 99 depicts a schematic of an embodiment of a heat treatment system including heater 352 and production wells 206. In certain embodiments, heater 352 is a three-phase heater that includes legs 592, 594, 596 coupled to transformer 598 and terminal connector 620. Legs 592, 594, 596 may each include single-phase conductors. Legs 592, 594, 596 may be coupled together to form a triad heater. In certain embodiments, legs 592, 594, 596 are relatively long heater sections. For example, legs 592, 594, 596 may be about 3000 m or longer in length.

[0895] In some embodiments, as shown in FIG. 99, production wells 206 are located substantially horizontally in the formation and below legs 592, 594, 596 of heater 352. In some embodiments, production wells 206 are located at an incline or vertically in the formation. As shown in FIG. 99, production wells 206 may include two production wells that extend from each side of heater 352 towards the center of the heater substantially lengthwise along the heated sections of legs 592, 594, 596. In some embodiments, one production well 206 extends substantially lengthwise along the heated sections of the legs.

[0896] FIG. 100 depicts a side-view representation of one leg of heater 352 in the subsurface formation. Leg 592 is shown as representative of any leg in of heater 352 in the formation. Leg 592 may include heating element 602 in hydrocarbon layer 510 below overburden 520. In certain embodiments, heating element 602 is located substantially horizontal in hydrocarbon layer 510. Transition section 606 may couple heating element 602 to lead-in cable 600. Lead-in cable 600 may be an overburden section or overburden element of heater 352. Lead-in cable 600 couples heating element 602 and transition section 606 to electrical components at the surface (for example, transformer 598 and/or terminal connector 620 depicted in FIG. 99).

[0897] As shown in FIG. 100, heater casing 358 extends from the surface to at or near end of transition section 606. Overburden casing 518 substantially surrounds heater casing 358 in overburden 520. Surface conductor 628 substantially surrounds overburden casing 518 at or near the surface of the formation.

[0898] In certain embodiments, heating element 602 is an exposed metal or bare metal heating element. For example, heating element 602 may be an exposed ferromagnetic metal heating element such as 410 stainless steel. Lead-in cable 600 includes low resistance electrical conductors such as copper or copper-cladded steel. Lead-in cable 600 may include electrical insulation or otherwise be electrically insulated from overburden 520 (for example, overburden casing 518 may include electrical insulation on an inside surface of the casing). Transition section 606 may include a combination of stainless steel and copper suitable for transition between heating element 602 and lead-in cable 600.

[0899] In some embodiments, heater casing 358 includes non-ferromagnetic stainless steel or another suitable material that has high hanging strength and is non-ferromagnetic. Overburden casing 518 and/or surface conductor 628 may include carbon steel or other suitable materials.

[0900] FIG. 101 depicts a schematic representation of a surface cabling configuration with a ground loop used for heater 352 and production well 206. In certain embodiments, ground loop 630 substantially surrounds legs 592, 594, 596 of heater 352, production well 206, and transformer 598. Power cable 514 may couple transformer 598 to legs 592, 594, 596 of heater 352. The center portion of power cable 514 coupled to center leg 594 may be put into loop 632. Loop 632 extends the center portion of power cable 514 to have approximately the same length as the portions of power cable 514 coupled to side legs 592, 596. Having each portion of power cable 514 approximately the same length inhibits creation of phase differences between the legs.

[0901] In certain embodiments, transformer 598 is coupled to ground loop 630 to ground the transformer and heater 352. In some embodiments, production well 206 is coupled to ground loop 630 to ground the production well.

[0902] FIG. 102 depicts a side view of an overburden portion of leg 592. Lead-in cable 600 is substantially surrounded by heater casing 358 and overburden casing 518 (“casing 358/518”) in the overburden of the formation. Current flow in lead-in cable 600 (represented by +/− symbols at ends the lead-in cable) induces current flow with opposite polarity on casing 358/518 (represented by +/− symbols on line 634). This induced voltage on casing 358/518 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)

[0903] Because typically high power is provided through lead-in cable 600 in order to provide power to long heater elements, the induced voltages and currents on casing 358/518 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.

[0904] 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.

[0905] FIG. 103 depicts a side view of overburden portions of legs 592, 594 grounded to ground loop 630. Legs 592, 594 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 636. Because legs 592, 594 are grounded to ground loop 630, the magnitude of circular current flow (curve 636) (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.

[0906] FIG. 104 depicts a side view of overburden portions of legs 592, 594 with the legs grounded to a ground loop. Ungrounding legs 592, 594 reduces the magnitude of the circular current flow between the legs (current density on the casings), as shown by curve 636. 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 592, 594 such as, but not limited to, increasing wellhead electrical fields above safe levels.

[0907] FIG. 105 depicts a side view of overburden portions of legs 592, 594 with the electrically conductive portions of casings 358/518 lowered selected depth 638 below the surface. As shown by curve 636, lowering the conductive portion of casings 358/518 selected depth 638 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 638 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.

[0908] In certain embodiments, the conductive portions of casings 358/518 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 358/518 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.

[0909] The non-conductive portion of casing 358/518 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.

[0910] In certain embodiments, the non-conductive portion of casing 358/518 extends to a depth that is below the surface moisture zone in the formation. Keeping the conductive portion of casing 358/518 below the surface moisture zone inhibits induced currents from reaching the surface.

[0911] In some embodiments, the non-conductive portion of casing 358/518 extends to a depth that is at least the distance between legs 592, 594. For example, for a 40′ (about 12 m) spacing between legs, the non-conductive portion of casing 358/518 may extend at least about 100′ (about 30 m) below the surface. In some embodiments, the non-conductive portion of casing 358/518 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 358/518 may extend to a depth of at most about 150 m, about 300 m, or about 500 m from the surface.

[0912] The non-conductive portion of casing 358/518 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 358/518 may extend to a depth that is slightly above or near the beginning of the bend in a u-shaped heater.

[0913] The desired depth of non-conductive portion of casing 358/518 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.

[0914] In some embodiments, the overburden portions of legs 592, 594 are moved closer together so that the non-conductive portion of casing 358/518 can be moved to a shallower depth. For example, the overburden portions of legs 592, 594 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.

[0915] In certain embodiments, as depicted in FIG. 105, legs 592, 594 are ungrounded with the casings lowered the selected distance. In some embodiments, however, legs 592, 594 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.

[0916] FIG. 106 depicts an embodiment of three u-shaped heaters with common overburden sections coupled to a single three-phase transformer. In certain embodiments, heaters 352A, 352B, 352C are exposed metal heaters. In some embodiments, heaters 352A, 352B, 352C are exposed metal heaters with a thin, electrically insulating coating on the heaters. For example, heaters 352A, 352B, 352C may be 410 stainless steel, carbon steel, 347H stainless steel, or other corrosion resistant stainless steel rods or tubulars (such as 2.5 cm or 3.2 cm diameter rods). The rods or tubulars may have porcelain enamel coatings on the exterior of the rods to electrically insulate the rods.

[0917] In some embodiments, heaters 352A, 352B, 352C are insulated conductor heaters. In some embodiments, heaters 352A, 352B, 352C are conductor-in-conduit heaters. Heaters 352A, 352B, 352C may have substantially parallel heating sections in hydrocarbon layer 510. Heaters 352A, 352B, 352C may be substantially horizontal or at an incline in hydrocarbon layer 510. In some embodiments, heaters 352A, 352B, 352C enter the formation through common wellbore 340A. Heaters 352A, 352B, 352C may exit the formation through common wellbore 340B. In certain embodiments, wellbores 340A, 340B are uncased (for example, open wellbores) in hydrocarbon layer 510.

[0918] Openings 508A, 508B, 508C span between wellbore 340A and wellbore 340B. Openings 508A, 508B, 508C may be uncased openings in hydrocarbon layer 510. In certain embodiments, openings 508A, 508B, 508C are formed by drilling from wellbore 340A and/or wellbore 340B. In some embodiments, openings 508A, 508B, 508C are formed by drilling from each wellbore 340A and 340B and connecting at or near the middle of the openings. Drilling from both sides towards the middle of hydrocarbon layer 510 allows longer openings to be formed in the hydrocarbon layer. Thus, longer heaters may be installed in hydrocarbon layer 510. For example, heaters 352A, 352B, 352C may have lengths of at least about 1500 m, at least about 3000 m, or at least about 4500 m.

[0919] Having multiple long, substantially horizontal or inclined heaters extending from only two wellbores in hydrocarbon layer 510 reduces the footprint of wells on the surface needed for heating the formation. The number of overburden wellbores that need to be drilled in the formation is reduced, which reduces capital costs per heater in the formation. Heating the formation with long, substantially horizontal or inclined heaters also reduces overall heat losses in overburden 520 when heating the formation because of the reduced number of overburden sections used to treat the formation (for example, losses in overburden 520 are a smaller fraction of total power supplied to the formation).

[0920] In some embodiments, heaters 352A, 352B, 352C are installed in wellbores 340A, 340B and openings 508A, 508B, 508C by pulling the heaters through the wellbores and the openings from one end to the other. For example, an installation tool may be pushed through the openings and coupled to a heater in wellbore 340A. The heater may then be pulled through the openings towards wellbore 340B using the installation tool. The heater may be coupled to the installation tool using a connector such as a claw, a catcher, or other devices known in the art.

[0921] In some embodiments, the first half of an opening is drilled from wellbore 340A and then the second half of the opening is drilled from wellbore 340B through the first half of the opening. The drill bit may be pushed through to wellbore 340A and a first heater may be coupled to the drill bit to pull the first heater back through the opening and install the first heater in the opening. The first heater may be coupled to the drill bit using a connector such as a claw, a catcher, or other devices known in the art.

[0922] After the first heater is installed, a tube or other guide may be placed in wellbore 340A and/or wellbore 340B to guide drilling of a second opening. FIG. 107 depicts a top view of an embodiment of heater 352A and drilling guide 546 in wellbore 340. Drilling guide 546 may be used to guide the drilling of the second opening in the formation and the installation of a second heater in the second opening. Insulator 486A may electrically and mechanically insulate heater 352A from drilling guide 546. Drilling guide 546 and insulator 486A may protect heater 352A from being damaged while the second opening is being drilled and the second heater is being installed.

[0923] After the second heater is installed, drilling guide 546 may be placed in wellbore 340 to guide drilling of a third opening, as shown in FIG. 108. Drilling guide 546 may be used to guide the drilling of the third opening in the formation and the installation of a third heater in the third opening. Insulators 486A and 486B may electrically and mechanically insulate heaters 352A and 352B, respectively, from drilling guide 546. Drilling guide 546 and insulators 486A and 486B may protect heaters 352A and 352B from being damaged while the third opening is being drilled and the third heater is being installed. After the third heater is installed, insulators 486A and 486B may be removed and a centralizer may be placed in wellbore 340 to separate and space heaters 352A, 352B, 352C. FIG. 109 depicts heaters 352A, 352B, 352C in wellbore 340 separated by centralizer 512.

[0924] In some embodiments, all the openings are formed in the formation and then the heaters are installed in the formation. In certain embodiments, one of the openings is formed and one of the heaters is installed in the formation before the other openings are formed and the other heaters are installed. The first installed heater may be used as a guide during the formation of additional openings. The first installed heater may be energized to produce an electromagnetic field that is used to guide the formation of the other openings. For example, the first installed heater may be energized with a bipolar DC current to magnetically guide drilling of the other openings.

[0925] In certain embodiments, heaters 352A, 352B, 352C are coupled to a single three-phase transformer 532 at one end of the heaters, as shown in FIG. 106. Heaters 352A, 352B, 352C may be electrically coupled in a triad configuration. In some embodiments, two heaters are coupled together in a diad configuration. Transformer 532 may be a three-phase wye transformer. The heaters may each be coupled to one phase of transformer 532. Using three-phase power to power the heaters may be more efficient than using single-phase power. Using three-phase connections for the heaters allows the magnetic fields of the heaters in wellbore 340A to cancel each other. The cancelled magnetic fields may allow overburden casing 518A to be ferromagnetic (for example, carbon steel). Using ferromagnetic casings in the wellbores may be less expensive and/or easier to install than non-ferromagnetic casings (such as fiberglass casings).

[0926] In some embodiments, the overburden section of heaters 352A, 352B, 352C are coated with an insulator, such as a polymer or an enamel coating, to inhibit shorting between the overburden sections of the heaters. In some embodiments, only the overburden sections of the heaters in wellbore 340A are coated with the insulator as the heater sections in wellbore 340B may not have significant electrical losses. In some embodiments, ends or end portions (portions at, near, or in the vicinity of the ends) of heaters 352A, 352B, 352C in wellbore 340A are at least one diameter of the heaters away from overburden casing 518A so that no insulator is needed. The ends or end portions of heaters 352A, 352B, 352C may be, for example, centralized in wellbore 340A using a centralizer to keep the heaters the desired distance away from overburden casing 518A.

[0927] In some embodiments, the ends or end portions of heaters 352A, 352B, 352C passing through wellbore 340B are electrically coupled together and grounded outside of the wellbore, as shown in FIG. 106. The magnetic fields of the heaters may cancel each other in wellbore 340B. Thus, overburden casing 518B may be ferromagnetic (for example, carbon steel). In certain embodiments, the overburden section of heaters 352A, 352B, 352C are copper rods or tubulars. The build sections of the heaters (the transition sections between the overburden sections and the heating sections) may also be made of copper or similar electrically conductive material.

[0928] In some embodiments, the ends or end portions of heaters 352A, 352B, 352C passing through wellbore 340B are electrically coupled together inside the wellbore. The ends or end portions of the heaters may be coupled inside the wellbore at or near the bottom of overburden 520. Coupling the heaters together at or near overburden 520 reduces electrical losses in the overburden section of the wellbore.

[0929] FIG. 110 depicts an embodiment for coupling ends or end portions of heaters 352A, 352B, 352C in wellbore 340B. Plate 640 may be located at or near the bottom of the overburden section of wellbore 340B. Plate 640 may have openings sized to allow heaters 352A, 352B, 352C to be inserted through the plate. Plate 640 may be slid down heaters 352A, 352B, 352C into position in wellbore 340B. Plate 640 may be made of copper or another electrically conductive material.

[0930] Balls 642 may be placed into the overburden section of wellbore 340B. Plate 640 may allow balls 642 to settle in the overburden section of wellbore 340B around heaters 352A, 352B, 352C. Balls 642 may be made of electrically conductive material such as copper or nickel-plated copper. Balls 642 and plate 640 may electrically couple heaters 352A, 352B, 352C to each other so that the heaters are grounded. In some embodiments, portions of the heaters above plate 640 (the overburden sections of the heaters) are made of carbon steel while portions of the heaters below the plate (build sections of the heaters) are made of copper.

[0931] In some embodiments, heaters 352A, 352B, 352C, as depicted in FIG. 106, provide varying heat outputs along the lengths of the heaters. For example, heaters 352A, 352B, 352C may have varying dimensions (for example, thicknesses or diameters) along the lengths of the heater. The varying thicknesses may provide different electrical resistances along the length of the heater and, thus, different heat outputs along the length of the heaters.

[0932] In some embodiments, heaters 352A, 352B, 352C are divided into two or more sections of heating. In some embodiments, the heaters are divided into repeating sections of different heat outputs (for example, alternating sections of two different heat outputs that are repeated). In some embodiments, the repeating sections of different heat outputs may be used to heat the formation in stages. In one embodiment, the halves of the heaters closest to wellbore 340A may provide heat in a first section of hydrocarbon layer 510 and the halves of the heaters closest to wellbore 340B may provide heat in a second section of hydrocarbon layer 510. Hydrocarbons in the formation may be mobilized by the heat provided in the first section. Hydrocarbons in the second section may be heated to higher temperatures than the first section to upgrade the hydrocarbons in the second section (for example, the hydrocarbons may be further mobilized and/or pyrolyzed). Hydrocarbons from the first section may move, or be moved, into the second section for the upgrading. For example, a drive fluid may be provided through wellbore 340A to move the first section mobilized hydrocarbons to the second section.

[0933] In some embodiments, more than three heaters extend from wellbore 340A and/or 340B. If multiples of three heaters extend from the wellbores and are coupled to transformer 532, the magnetic fields may cancel in the overburden sections of the wellbores as in the case of three heaters in the wellbores. For example, six heaters may be coupled to transformer 532 with two heaters coupled to each phase of the transformer to cancel the magnetic fields in the wellbores.

[0934] In some embodiments, multiple heaters extend from one wellbore in different directions. FIG. 111 depicts a schematic of an embodiment of multiple heaters extending in different directions from wellbore 340A. Heaters 352A, 352B, 352C may extend to wellbore 340B. Heaters 352D, 352E, 352F may extend to wellbore 340C in the opposite direction of heaters 352A, 352B, 352C. Heaters 352A, 352B, 352C and heaters 352D, 352E, 352F may be coupled to a single, three-phase transformer so that magnetic fields are cancelled in wellbore 340A.

[0935] In some embodiments, heaters 352A, 352B, 352C may have different heat outputs from heaters 352D, 352E, 352F so that hydrocarbon layer 510 is divided into two heating sections with different heating rates and/or temperatures (for example, a mobilization and a pyrolyzation section). In some embodiments, heaters 352A, 352B, 352C and/or heaters 352D, 352E, 352F may have heat outputs that vary along the lengths of the heaters to further divide hydrocarbon layer 510 into more heating sections. In some embodiments, additional heaters may extend from wellbore 340B and/or wellbore 340C to other wellbores in the formation as shown by the dashed lines in FIG. 111.

[0936] In some embodiments, multiple levels of heaters extend between two wellbores. FIG. 112 depicts a schematic of an embodiment of multiple levels of heaters extending between wellbore 340A and wellbore 340B. Heaters 352A, 352B, 352C may provide heat to a first level of hydrocarbon layer 510. Heaters 352D, 352E, 352F may branch off and provide heat to a second level of hydrocarbon layer 510. Heaters 352G, 352H, 3521 may further branch off and provide heat to a third level of hydrocarbon layer 510. In some embodiments, heaters 352A, 352B, 352C, heaters 352D, 352E, 352F, and heaters 352G, 352H, 352I provide heat to levels in the formation with different properties. For example, the different groups of heaters may provide different heat outputs to levels with different properties in the formation so that the levels are heated at or about the same rate.

[0937] In some embodiments, the levels are heated at different rates to create different heating zones in the formation. For example, the first level (heated by heaters 352A, 352B, 352C) may be heated so that hydrocarbons are mobilized, the second level (heated by heaters 352D, 352E, 352F) may be heated so that hydrocarbons are somewhat upgraded from the first level, and the third level (heated by heaters 352G, 352H, 352I) may be heated to pyrolyze hydrocarbons. As another example, the first level may be heated to create gases and/or drive fluid in the first level and either the second level or the third level may be heated to mobilize and/or pyrolyze fluids or just to a level to allow production in the level. In addition, heaters 352A, 352B, 352C, heaters 352D, 352E, 352F, and/or heaters 352G, 352H, 352I may have heat outputs that vary along the lengths of the heaters to further divide hydrocarbon layer 510 into more heating sections.

[0938] FIG. 113 depicts a schematic of an embodiment of a u-shaped heater that has an inductively energized tubular. Heater 352 includes electrical conductor 528 and tubular 644 in an opening that spans between wellbore 340A and wellbore 340B. In certain embodiments, electrical conductor 528 and/or the current carrying portion of the electrical conductor is electrically insulated from tubular 644. Electrical conductor 528 and/or the current carrying portion of the electrical conductor is electrically insulated from tubular 644 such that electrical current does not flow from the electrical conductor to the tubular, or vice versa (for example, the tubular is not directly connected electrically to the electrical conductor).

[0939] In some embodiments, electrical conductor 528 is centralized inside tubular 644 (for example, using centralizers 512 or other support structures, as shown in FIG. 114). Centralizers 512 may electrically insulate electrical conductor 528 from tubular 644. In some embodiments, tubular 644 contacts electrical conductor 528. For example, tubular 644 may hang, drape, or otherwise touch electrical conductor 528. In some embodiments, electrical conductor 528 includes electrical insulation (for example, magnesium oxide or porcelain enamel) that insulates the current carrying portion of the electrical conductor from tubular 644. The electrical insulation inhibits current from flowing between the current carrying portion of electrical conductor 528 and tubular 644 if the electrical conductor and the tubular are in physical contact with each other.

[0940] In some embodiments, electrical conductor 528 is an exposed metal conductor heater or a conductor-in-conduit heater. In certain embodiments, electrical conductor 528 is an insulated conductor such as a mineral insulated conductor. The insulated conductor may have a copper core, copper alloy core, or a similar electrically conductive, low resistance core that has low electrical losses. In some embodiments, the core is a copper core with a diameter between about 0.5″ (1.27 cm) and about 1″ (2.54 cm). The sheath or jacket of the insulated conductor may be a non-ferromagnetic, corrosion resistant steel such as 347 stainless steel, 625 stainless steel, 825 stainless steel, 304 stainless steel, or copper with a protective layer (for example, a protective cladding). The sheath may have an outer diameter of between about 1″ (2.54 cm) and about 1.25″ (3.18 cm).

[0941] In some embodiments, the sheath or jacket of the insulated conductor is in physical contact with the tubular 644 (for example, the tubular is in physical contact with the sheath along the length of the tubular) or the sheath is electrically connected to the tubular. In such embodiments, the electrical insulation of the insulated conductor electrically insulates the core of the insulated conductor from the jacket and the tubular. FIG. 115 depicts an embodiment of an induction heater with the sheath of an insulated conductor in electrical contact with tubular 644. Electrical conductor 528 is the insulated conductor. The sheath of the insulated conductor is electrically connected to tubular 644 using electrical contactors 646. In some embodiments, electrical contactors 646 are sliding contactors. In certain embodiments, electrical contactors 646 electrically connect the sheath of the insulated conductor to tubular 644 at or near the ends of the tubular. Electrically connecting at or near the ends of tubular 644 substantially equalizes the voltage along the tubular with the voltage along the sheath of the insulated conductor. Equalizing the voltages along tubular 644 and along the sheath may inhibit arcing between the tubular and the sheath.

[0942] Tubular 644, such as the tubular shown in FIGS. 113, 114, and 115, may be ferromagnetic or include ferromagnetic materials. Tubular 644 may have a thickness such that when electrical conductor 528 induces electrical current flow on the surfaces of tubular 644 when the electrical conductor is energized with time-varying current. The electrical conductor induces electrical current flow due to the ferromagnetic properties of the tubular. Current flow is induced on both the inside surface of the tubular and the outside surface of tubular 644. Tubular 644 may operate as a skin effect heater when current flow is induced in the skin depth of one or more of the tubular surfaces. In certain embodiments, the induced current circulates axially (longitudinally) on the inside and/or outside surfaces of tubular 644. Longitudinal flow of current through electrical conductor 528 induces primarily longitudinal current flow in tubular 644 (the majority of the induced current flow is in the longitudinal direction in the tubular). Having primarily longitudinal induced current flow in tubular 644 may provide a higher resistance per foot than if the induced current flow is primarily angular current flow.

[0943] In certain embodiments, current flow in tubular 644 is induced with low frequency current in electrical conductor 528 (for example, from 50 Hz or 60 Hz up to about 1000 Hz). In some embodiments, induced currents on the inside and outside surfaces of tubular 644 are substantially equal.

[0944] In certain embodiments, tubular 644 has a thickness that is greater than the skin depth of the ferromagnetic material in the tubular at or near the Curie temperature of the ferromagnetic material or at or near the phase transformation temperature of the ferromagnetic material. For example, tubular 644 may have a thickness of at least 2.1, at least 2.5 times, at least 3 times, or at least 4 times the skin depth of the ferromagnetic material in the tubular near the Curie temperature or the phase transformation temperature of the ferromagnetic material. In certain embodiments, tubular 644 has a thickness of at least 2.1 times, at least 2.5 times, at least 3 times, or at least 4 times the skin depth of the ferromagnetic material in the tubular at about 50° C. below the Curie temperature or the phase transformation temperature of the ferromagnetic material.

[0945] In certain embodiments, tubular 644 is carbon steel. In some embodiments, tubular 644 is coated with a corrosion resistant coating (for example, porcelain or ceramic coating) and/or an electrically insulating coating. In some embodiments, electrical conductor 528 has an electrically insulating coating. Examples of the electrically insulating coating on tubular 644 and/or electrical conductor 528 include, but are not limited to, a porcelain enamel coating, an alumina coating, or an alumina-titania coating.

[0946] In some embodiments, tubular 644 and/or electrical conductor 528 are coated with a coating such as polyethylene or another suitable low friction coefficient coating that may melt or decompose when the heater is energized. The coating may facilitate placement of the tubular and/or the electrical conductor in the formation.

[0947] In some embodiments, tubular 644 includes corrosion resistant ferromagnetic material such as, but not limited to, 410 stainless steel, 446 stainless steel, T/P91 stainless steel, T/P92 stainless steel, alloy 52, alloy 42, and Invar 36. In some embodiments, tubular 644 is a stainless steel tubular with cobalt added (for example, between about 3% by weight and about 10% by weight cobalt added) and/or molybdenum (for example, about 0.5% molybdenum by weight).

[0948] At or near the Curie temperature or the phase transformation temperature of the ferromagnetic material in tubular 644, the magnetic permeability of the ferromagnetic material decreases rapidly. When the magnetic permeability of tubular 644 decreases at or near the Curie temperature or the phase transformation temperature, there is little or no current flow in the tubular because, at these temperatures, the tubular is essentially non-ferromagnetic and electrical conductor 528 is unable to induce current flow in the tubular. With little or no current flow in tubular 644, the temperature of the tubular will drop to lower temperatures until the magnetic permeability increases and the tubular becomes ferromagnetic. Thus, tubular 644 self-limits at or near the Curie temperature or the phase transformation temperature and operates as a temperature limited heater due to the ferromagnetic properties of the ferromagnetic material in the tubular. Because current is induced in tubular 644, the turndown ratio may be higher and the drop in current sharper for the tubular than for temperature limited heaters that apply current directly to the ferromagnetic material. For example, heaters with current induced in tubular 644 may have turndown ratios of at least about 5, at least about 10, or at least about 20 while temperature limited heaters that apply current directly to the ferromagnetic material may have turndown ratios that are at most about 5.

[0949] When current is induced in tubular 644, the tubular provides heat to hydrocarbon layer 510 and defines the heating zone in the hydrocarbon layer. In certain embodiments, tubular 644 heats to temperatures of at least about 300° C., at least about 500° C., or at least about 700° C. Because current is induced on both the inside and outside surfaces of tubular 644, the heat generation of the tubular is increased as compared to temperature limited heaters that have current directly applied to the ferromagnetic material and current flow is limited to one surface. Thus, less current may be provided to electrical conductor 528 to generate the same heat as heaters that apply current directly to the ferromagnetic material. Using less current in electrical conductor 528 decreases power consumption and reduces power losses in the overburden of the formation.

[0950] In certain embodiments, tubulars 644 have large diameters. The large diameters may be used to equalize or substantially equalize high pressures on the tubular from either the inside or the outside of the tubular. In some embodiments, tubular 644 has a diameter in a range between about 1.5″ (about 3.8 cm) and about 6″ (about 15.2 cm). In some embodiments, tubular 644 has a diameter in a range between about 3 cm and about 13 cm, between about 4 cm and about 12 cm, or between about 5 cm and about 11 cm. Increasing the diameter of tubular 644 may provide more heat output to the formation by increasing the heat transfer surface area of the tubular.

[0951] In certain embodiments, tubular 644 has surfaces that are shaped to increase the resistance of the tubular. FIG. 116 depicts an embodiment of a heater with tubular 644 having radial grooved surfaces. Heater 352 may include electrical conductors 528A,B coupled to tubular 644. Electrical conductors 528A,B may be insulated conductors. Electrical contactors may electrically and physically couple electrical conductors 528A,B to tubular 644. In certain embodiments, the electrical contactors are attached to ends of electrical conductors 528A,B. The electrical contactors have a shape such that when the ends of electrical conductors 528A,B are pushed into the ends of tubular 644, the electrical contactors physically and electrically couple the electrical conductors to the tubular. For example, the electrical contactors may be cone shaped. Heater 352 generates heat when current is applied directly to tubular 644. Current is provided to tubular 644 using electrical conductors 528A,B. Grooves 648 may increase the heat transfer surface area of tubular 644.

[0952] In some embodiments, one or more surfaces of the tubular of an induction heater may be textured to increase the resistance of the heater and increase the heat transfer surface area of the tubular. FIG. 117 depicts heater 352 that is an induction heater. Electrical conductor 528 extends through tubular 644.

[0953] Tubular 644 may include grooves 648. In some embodiments, grooves 648 are cut in tubular 644. In some embodiments, fins are coupled to tubular to form ridges and grooves 648. The fins may be welded or otherwise attached to the tubular. In an embodiment, the fins are coupled to a tubular sheath that is placed over the tubular. The sheath is physically and electrically coupled to the tubular to form tubular 644.

[0954] In certain embodiments, grooves 648 are on the outer surface of tubular 644. In some embodiments, the grooves are on the inner surface of the tubular. In some embodiments, the grooves are on both the inner and outer surfaces of the tubular.

[0955] In certain embodiments, grooves 648 are radial grooves (grooves that wrap around the circumference of tubular 644). In certain embodiments, grooves 648 are straight, angled, or spiral grooves or protrusions. In some embodiments, grooves 648 are evenly spaced grooves along the surface of tubular 644. In some embodiments, grooves 648 are part of a threaded surface on tubular 644 (the grooves are formed as a winding thread on the surface). Grooves 648 may have a variety of shapes as desired. For example, grooves 648 may have square edges, rectangular edges, v-shaped edges, u-shaped edges, or have rounded edges.

[0956] Grooves 648 increase the effective resistance of tubular 644 by increasing the path length of induced current on the surface of the tubular. Grooves 648 increase the effective resistance of tubular 644 as compared to a tubular with the same inside and outside diameters with smooth surfaces. Because induced current travels axially, the induced current has to travel up and down the grooves along the surface of the tubular. Thus, the depth of grooves 648 may be varied to provide a selected resistance in tubular 644. For example, increasing the grooves depth increases the path length and the resistance.

[0957] Increasing the resistance of tubular 644 with grooves 648 increases the heat generation of the tubular as compared to a tubular with smooth surfaces. Thus, the same electrical current in electrical conductor 528 will provide more heat output in the radial grooved surface tubular than the smooth surface tubular. Therefore, to provide the same heat output with the radial grooved surface tubular as the smooth surface tubular, less current is needed in electrical conductor 528 with the radial grooved surface tubular.

[0958] In some embodiments, grooves 648 are filled with materials that decompose at lower temperatures to protect the grooves during installation of tubular 644. For example, grooves 648 may be filled with polyethylene or asphalt. The polyethylene or asphalt may melt and/or desorb when heater 352 reaches normal operating temperatures of the heater.

[0959] It is to be understood that grooves 648 may be used in other embodiments of tubulars 644 described herein to increase the resistance of such tubulars. For example, grooves 648 may be used in embodiments of tubulars 644 depicted in FIGS. 113, 114, and 115.

[0960] FIG. 118 depicts an embodiment of heater 352 divided into tubular sections to provide varying heat outputs along the length of the heater. Heater 352 may include tubular sections 644A, 644B, 644C, 644D that have different properties to provide different heat outputs in each tubular section. Heat output from tubular sections 644D may be less than the heat output from grooved sections 644A, 644B, 644C. Examples of properties that may be varied include, but are not limited to, thicknesses, diameters, cross-sectional areas, resistances, materials, number of grooves, depth of grooves. The different properties in tubular sections 644A, 644B, and 644C may provide different maximum operating temperatures (for example, different Curie temperatures or phase transformation temperatures) along the length of heater 352. The different maximum temperatures of the tubular sections provides different heat outputs from the tubular sections. Sections such as grooved section 644A may be separate sections that are placed down the wellbore in separation installation procedures. Some sections, such as grooved section 644B and 644C may be connected together by non-grooved section 644D, and may be placed down the wellbore together.

[0961] Providing different heat outputs along heater 352 may provide different heating in one or more hydrocarbon layers. For example, heater 352 may be divided into two or more sections of heating to provide different heat outputs to different sections of a hydrocarbon layer and/or different hydrocarbon layers.

[0962] In one embodiment, a first portion of heater 352 may provide heat to a first section of the hydrocarbon layer and a second portion of the heater may provide heat to a second section of the hydrocarbon layer. Hydrocarbons in the first section may be mobilized by the heat provided by the first portion of the heater. Hydrocarbons in the second section may be heated by the second portion of the heater to a higher temperature than the first section. The higher temperature in the second section may upgrade hydrocarbons in the second section relative to the first section. For example, the hydrocarbons may be mobilized, visbroken, and/or pyrolyzed in the second section. Hydrocarbons from the first section may be moved into the second section by, for example, a drive fluid provided to the first section. As another example, heater 352 may have end sections that provide higher heat outputs to counteract heat losses at the ends of the heater to maintain a more constant temperature in the heated portion of the formation.

[0963] In certain embodiments, three, or multiples of three, electrical conductors enter and exit the formation through common wellbores with tubulars surrounding the electrical conductors in the portion of the formation to be heated. FIG. 119 depicts an embodiment of three electrical conductors 528A,B,C entering the formation through first common wellbore 340A and exiting the formation through second common wellbore 340C with three tubulars 644A,B,C surrounding the electrical conductors in hydrocarbon layer 510. In some embodiments, electrical conductors 528A,B,C are powered by a single, three-phase wye transformer. Tubulars 644A,B,C and portions of electrical conductors 528A,B,C may be in three separate wellbores in hydrocarbon layer 510. The three separate wellbores may be formed by drilling the wellbores from first common wellbore 340A to second common wellbore 340B, vice versa, or drilling from both common wellbores and connecting the drilled openings in the hydrocarbon layer.

[0964] Having multiple induction heaters extending from only two wellbores in hydrocarbon layer 510 reduces the footprint of wells on the surface needed for heating the formation. The number of overburden wellbores drilled in the formation is reduced, which reduces capital costs per heater in the formation. Power losses in the overburden may be a smaller fraction of total power supplied to the formation because of the reduced number of wells through the overburden used to treat the formation. In addition, power losses in the overburden may be smaller because the three phases in the common wellbores substantially cancel each other and inhibit induced currents in the casings or other structures of the wellbores.

[0965] In some embodiments, three, or multiples of three, electrical conductors and tubulars are located in separate wellbores in the formation. FIG. 120 depicts an embodiment of three electrical conductors 528A,B,C and three tubulars 644A,B,C in separate wellbores in the formation. Electrical conductors 528A,B,C may be powered by single, three-phase wye transformer 532 with each electrical conductor coupled to one phase of the transformer. In some embodiments, the single, three-phase wye transformer is used to power 6, 9, 12, or other multiples of three electrical conductors. Connecting multiples of three electrical conductors to the single, three-phase wye transformer may reduce equipment costs for providing power to the induction heaters.

[0966] In some embodiments, two, or multiples of two, electrical conductors enter the formation from a first common wellbore and exit the formation from a second common wellbore with tubulars surrounding each electrical conductor in the hydrocarbon layer. The multiples of two electrical conductors may be powered by a single, two-phase transformer. In such embodiments, the electrical conductors may be homogenous electrical conductors (for example, insulated conductors using the same materials throughout) in the overburden sections and heating sections of the insulated conductor. The reverse flow of current in the overburden sections may reduce power losses in the overburden sections of the wellbores because the currents reduce or cancel inductive effects in the overburden sections.

[0967] In certain embodiments, tubulars 644 depicted in FIGS. 113-119 include multiple layers of ferromagnetic materials separated by electrical insulators. FIG. 121 depicts an embodiment of a multilayered induction tubular. Tubular 644 includes ferromagnetic layers 650A,B,C separated by electrical insulators 486A,B. Three ferromagnetic layers and two layers of electrical insulators are shown in FIG. 121. Tubular 644 may include additional ferromagnetic layers and/or electrical insulators as desired. For example, the number of layers may be chosen to provide a desired heat output from the tubular.

[0968] Ferromagnetic layers 650A,B,C are electrically insulated from electrical conductor 528 by, for example, an air gap. Ferromagnetic layers 650A,B,C are electrically insulated from each other by electrical insulator 486A and electrical insulator 486B. Thus, direct flow of current is inhibited between ferromagnetic layers 650A,B,C and electrical conductor 528. When current is applied to electrical conductor 528, electrical current flow is induced in ferromagnetic layers 650A,B,C because of the ferromagnetic properties of the layers. Having two or more electrically insulated ferromagnetic layers provides multiple current induction loops for the induced current. The multiple current induction loops may effectively appear as electrical loads in series to a power source for electrical conductor 528. The multiple current induction loops may increase the heat generation per unit length of tubular 644 as compared to a tubular with only one current induction loop. For the same heat output, the tubular with multiple layers may have a higher voltage and lower current as compared to the single layer tubular.

[0969] In certain embodiments, ferromagnetic layers 650A,B,C include the same ferromagnetic material. In some embodiments, ferromagnetic layers 650A,B,C include different ferromagnetic materials. Properties of ferromagnetic layers 650A,B,C may be varied to provide different heat outputs from the different layers. Examples of properties of ferromagnetic layers 650A,B,C that may be varied include, but are not limited to, ferromagnetic material and thicknesses of the layers.

[0970] Electrical insulators 486A and 486B may be magnesium oxide, porcelain enamel, and/or another suitable electrical insulator. The thicknesses and/or materials of electrical insulators 486A and 486B may be varied to provide different operating parameters for tubular 644.

[0971] In some embodiments, fluids are circulated through tubulars 644 depicted in FIGS. 113-119. In some embodiments, fluids are circulated through the tubulars to add heat to the formation. For example, fluids may be circulated through the tubulars to preheat the formation prior to energizing the tubulars (providing current to the heating system). In some embodiments, fluids are circulated through the tubulars to recover heat from the formation. The recovered heat may be used to provide heat to other portions of the formation and/or surface processes used to treat fluids produced from the formation. In some embodiments, the fluids are used to cool down the heater.

[0972] In certain embodiments, insulated conductors are operated as induction heaters. FIG. 122 depicts a cross-sectional end view of an embodiment of insulated conductor 530 that is used as an induction heater. FIG. 123 depicts a cross-sectional side view of the embodiment depicted in FIG. 122. Insulated conductor 530 includes core 496, electrical insulator 486, and jacket 492. Core 496 may be copper or another non-ferromagnetic electrical conductor with low resistance that provides little or no heat output. In some embodiments, core may be clad with a thin layer of material such as nickel to inhibit migration of portions of the core into electrical insulator 486. Electrical insulator 486 may be magnesium oxide or another suitable electrical insulator that inhibits arcing at high voltages.

[0973] Jacket 492 includes at least one ferromagnetic material. In certain embodiments, jacket 492 includes carbon steel or another ferromagnetic steel (for example, 410 stainless steel, 446 stainless steel, T/P91 stainless steel, T/P92 stainless steel, alloy 52, alloy 42, and Invar 36). In some embodiments, jacket 492 includes an outer layer of corrosion resistant material (for example, stainless steel such as 347H stainless steel or 304 stainless steel). The outer layer may be clad to the ferromagnetic material or otherwise coupled to the ferromagnetic material using methods known in the art.

[0974] In certain embodiments, jacket 492 has a thickness of at least about 2 skin depths of the ferromagnetic material in the jacket. In some embodiments, jacket 492 has a thickness of at least about 3 skin depths, at least about 4 skin depths, or at least about 5 skin depths. Increasing the thickness of jacket 492 may increase the heat output from insulated conductor 530.

[0975] In one embodiment, core 496 is copper with a diameter of about 0.5″ (1.27 cm), electrical insulator 486 is magnesium oxide with a thickness of about 0.20″ (0.5 cm) (the outside diameter is about 0.9″ (2.3 cm)), and jacket 492 is carbon steel with an outside diameter of about 1.6″ (4.1 cm) (the thickness is about 0.35″ (0.88 cm)). A thin layer (about 0.1″ (0.25 cm) thickness (outside diameter of about 1.7″ (4.3 cm)) of corrosion resistant material 347H stainless steel may be clad on the outside of jacket 492.

[0976] In another embodiment, core 496 is copper with a diameter of about 0.338″ (0.86 cm), electrical insulator 486 is magnesium oxide with a thickness of about 0.096″ (0.24 cm) (the outside diameter is about 0.53″ (1.3 cm)), and jacket 492 is carbon steel with an outside diameter of about 1.13″ (2.9 cm) (the thickness is about 0.30″ (0.76 cm)). A thin layer (about 0.065″ (0.17 cm) thickness (outside diameter of about 1.26″ (3.2 cm)) of corrosion resistant material 347H stainless steel may be clad on the outside of jacket 492.

[0977] In another embodiment, core 496 is copper, electrical insulator 486 is magnesium oxide, and jacket 492 is a thin layer of copper surrounded by carbon steel. Core 496, electrical insulator 486, and the thin copper layer of jacket 492 may be obtained as a single piece of insulated conductor. Such insulated conductors may be obtained as long pieces of insulated conductors (for example, lengths of about 500′ (about 150 m) or more). The carbon steel layer of jacket 492 may be added by drawing down the carbon steel over the long insulated conductor. Such an insulated conductor may only generate heat on the outside of jacket 492 as the thin copper layer in the jacket shorts to the inside surface of the jacket.

[0978] In some embodiments, jacket 492 is made of multiple layers of ferromagnetic material. The multiple layers may be the same ferromagnetic material or different ferromagnetic materials. For example, in one embodiment, jacket 492 is a 0.35″ (0.88 cm) thick carbon steel jacket made from three layers of carbon steel. The first and second layers are 0.10″ (0.25 cm) thick and the third layer is 0.15″ (0.38 cm) thick. In another embodiment, jacket 492 is a 0.3″ (0.76 cm) thick carbon steel jacket made from three 0.10″ (0.25 cm) thick layers of carbon steel.

[0979] In certain embodiments, jacket 492 and core 496 are electrically insulated such that there is no direct electrical connection between the jacket and the core. Core 496 may be electrically coupled to a single power source with each end of the core being coupled to one pole of the power source. For example, insulated conductor 530 may be a u-shaped heater located in a u-shaped wellbore with each end of core 496 being coupled to one pole of the power source.

[0980] When core 496 is energized with time-varying current, the core induces electrical current flow on the surfaces of jacket 492 (as shown by the arrows in FIG. 123) due to the ferromagnetic properties of the ferromagnetic material in the jacket. In certain embodiments, current flow is induced on both the inside and outside surfaces of jacket 492. In these induction heater embodiments, jacket 492 operates as the heating element of insulated conductor 530.

[0981] At or near the Curie temperature or the phase transformation temperature of the ferromagnetic material in jacket 492, the magnetic permeability of the ferromagnetic material decreases rapidly. When the magnetic permeability of jacket 492 decreases at or near the Curie temperature or the phase transformation temperature, there is little or no current flow in the jacket because, at these temperatures, the jacket is essentially non-ferromagnetic and core 496 is unable to induce current flow in the jacket. With little or no current flow in jacket 492, the temperature of the jacket will drop to lower temperatures until the magnetic permeability increases and the jacket becomes ferromagnetic. Thus, jacket 492 self-limits at or near the Curie temperature or the phase transformation temperature and insulated conductor 530 operates as a temperature limited heater due to the ferromagnetic properties of the jacket. Because current is induced in jacket 492, the turndown ratio may be higher and the drop in current sharper for the jacket than if current is directly applied to the jacket.

[0982] In certain embodiments, portions of jacket 492 in the overburden of the formation do not include ferromagnetic material (for example, are non-ferromagnetic). Having the overburden portions of jacket 492 made of non-ferromagnetic material inhibits current induction in the overburden portions of the jackets. Power losses in the overburden are inhibited or reduced by inhibiting current induction in the overburden portions.

[0983] FIG. 124 depicts a cross-sectional view of an embodiment of two-leg insulated conductor 530 that is used as an induction heater. FIG. 125 depicts a longitudinal cross-sectional view of the embodiment depicted in FIG. 124. Insulated conductor 530 is a two-leg insulated conductor that includes two cores 496A,B; two electrical insulators 486A,B; and two jackets 492A,B. The two legs of insulated conductor 530 may be in physical contact with each other such that jacket 492A contacts jacket 492B along their lengths. Cores 496A,B; electrical insulators 486A,B; and jackets 492A,B may include materials such as those used in the embodiment of insulated conductor 530 depicted in FIGS. 122 and 123.

[0984] As shown in FIG. 125, core 496A and core 496B are coupled to transformer 532 and terminal block 652. Thus, core 496A and core 496B are electrically coupled in series such that current in core 496A flows in an opposite direction from current in core 496B, as shown by the arrows in FIG. 125. Current flow in cores 496A,B induces current flow in jackets 492A,B, respectively, as shown by the arrows in FIG. 125.

[0985] In certain embodiments, portions of jacket 492A and/or jacket 492B are coated with an electrically insulating coating (for example, a porcelain enamel coating, alumina coating, and/or alumina-titania coating). The electrically insulating coating may inhibit the currents in one jacket from affecting current in the other jacket or vice versa (for example, current in one jacket cancelling out current in the other jacket). Electrically insulating the jackets from each other may inhibit the turndown ratio of the heater from being reduced by the interaction of induced currents in the jackets.

[0986] Because core 496A and core 496B are electrically coupled in series to a single transformer (transformer 532), insulated conductor 530 may be located in a wellbore that terminates in the formation (for example, a wellbore with a single surface opening such as an L-shaped or J-shaped wellbore). Insulated conductor 530, as depicted in FIG. 125, may be operated as a subsurface termination induction heater with electrical connections between the heater and the power source (the transformer) being made through one surface opening.

[0987] Portions of jackets 492A,B in the overburden and/or adjacent to portions of the formation that are not to be significantly heated (for example, thick shale breaks between two hydrocarbon layers) may be non-ferromagnetic to inhibit induction currents in such portions. The jacket may include one or more sections that are electrically insulating to restrict induced current flow to heater portions of the insulated conductor. Inhibiting induction currents in the overburden portion of the jackets inhibits inductive heating and/or power losses in the overburden. Induction effects in other structures in the overburden that surround insulated conductor 530 (for example, overburden casings) may be inhibited because the current in core 496A flows in an opposite direction from the current in core 496B.

[0988] FIG. 126 depicts a cross-sectional view of an embodiment of a multilayered insulated conductor that is used as an induction heater. Insulated conductor 530 includes core 496 surrounded by electrical insulator 486A and jacket 492A. Electrical insulator 486A and jacket 492A comprise a first layer of insulated conductor 530. The first layer is surrounded by a second layer that includes electrical insulator 486B and jacket 492B. Two layers of electrical insulators and jackets are shown in FIG. 126. The insulated conductor may include additional layers as desired. For example, the number of layers may be chosen to provide a desired heat output from the insulated conductor.

[0989] Jacket 492A and jacket 492B are electrically insulated from core 496 and each other by electrical insulator 486A and electrical insulator 486B. Thus, direct flow of current is inhibited between jacket 492A and jacket 492B and core 496. When current is applied to core 496, electrical current flow is induced in both jacket 492A and jacket 492B because of the ferromagnetic properties of the jackets. Having two or more layers of electrical insulators and jackets provides multiple current induction loops. The multiple current induction loops may effectively appear as electrical loads in series to a power source for insulated conductor 530. The multiple current induction loops may increase the heat generation per unit length of insulated conductor 530 as compared to an insulated conductor with only one current induction loop. For the same heat output, the insulated conductor with multiple layers may have a higher voltage and lower current as compared to the single layer insulated conductor.

[0990] In certain embodiments, jacket 492A and jacket 492B include the same ferromagnetic material. In some embodiments, jacket 492A and jacket 492B include different ferromagnetic materials. Properties of jacket 492A and jacket 492B may be varied to provide different heat outputs from the different layers. Examples of properties of jacket 492A and jacket 492B that may be varied include, but are not limited to, ferromagnetic material and thicknesses of the layers.

[0991] Electrical insulators 486A and 486B may be magnesium oxide, porcelain enamel, and/or another suitable electrical insulator. The thicknesses and/or materials of electrical insulators 486A and 486B may be varied to provide different operating parameters for insulated conductor 530.

[0992] FIG. 127 depicts an end view of an embodiment of three insulated conductors 530 located in a coiled tubing conduit and used as induction heaters. Insulated conductors 530 may each be, for example, the insulated conductor depicted in FIGS. 122, 123, and 126. The cores of insulated conductors 530 may be coupled to each other such that the insulated conductors are electrically coupled in a three-phase wye configuration. FIG. 128 depicts a representation of cores 496 of insulated conductors 530 coupled together at their ends.

[0993] As shown in FIG. 127, insulated conductors 530 are located in tubular 644. Tubular 644 may be a coiled tubing conduit or other coiled tubing tubular or casing. Insulated conductors 530 may be in a spiral or helix formation inside tubular 644 to reduce stresses on the insulated conductors when the insulated conductors are coiled, for example, on a coiled tubing reel. Tubular 644 allows the insulated conductors to be installed in the formation using a coiled tubing rig and protects the insulated conductors during installation into the formation.

[0994] FIG. 129 depicts an end view of an embodiment of three insulated conductors 530 located on a support member and used as induction heaters. Insulated conductors 530 may each be, for example, the insulated conductor depicted in FIGS. 122, 123, and 126. The cores of insulated conductors 530 may be coupled to each other such that the insulated conductors are electrically coupled in a three-phase wye configuration. For example, the cores may be coupled together as shown in FIG. 128.

[0995] As shown in FIG. 129, insulated conductors 530 are coupled to support member 500. Support member 500 provides support for insulated conductors 530. Insulated conductors 530 may be wrapped around support member 500 in a spiral or helix formation. In some embodiments, support member 500 includes ferromagnetic material. Current flow may be induced in the ferromagnetic material of support member 500. Thus, support member 500 may generate some heat in addition to the heat generated in the jackets of insulated conductors 530.

[0996] In certain embodiments, insulated conductors 530 are held together on support member 500 with band 654. Band 654 may be stainless steel or another non-corrosive material. In some embodiments, band 654 includes a plurality of bands that hold together insulated conductors 530. The bands may be periodically placed around insulated conductors 530 to hold the conductors together.

[0997] In some embodiments, jacket 492, depicted in FIGS. 122 and 123, or jackets 492A,B, depicted in FIG. 125, include grooves or other structures on the outer surface and/or the inner surface of the jacket to increase the effective resistance of the jacket. Increasing the resistance of jacket 492 and/or jackets 492A,B with grooves increases the heat generation of the jackets as compared to jackets with smooth surfaces. Thus, the same electrical current in core 496 and/or cores 496A,B will provide more heat output in the grooved surface jackets than the smooth surface jackets.

[0998] In some embodiments, jacket 492 (such as the jackets depicted in FIGS. 122 and 123, or jackets 492A,B depicted in FIG. 125) are divided into sections to provide varying heat outputs along the length of the heaters. For example, jacket 492 and/or jackets 492A,B may be divided into sections such as tubular sections 644A, 644B, and 644C, depicted in FIG. 118. The sections of the jackets 492 depicted in FIGS. 122, 123, and 125 may have different properties to provide different heat outputs in each section. Examples of properties that may be varied include, but are not limited to, thicknesses, diameters, resistances, materials, number of grooves, depth of grooves. The different properties in the sections may provide different maximum operating temperatures (for example, different Curie temperatures or phase transformation temperatures) along the length of insulated conductor 530. The different maximum temperatures of the sections provides different heat outputs from the sections.

[0999] In certain embodiments, induction heaters include insulated electrical conductors surrounded by spiral wound ferromagnetic materials. For example, the spiral wound ferromagnetic materials may operate as inductive heating elements similarly to tubulars 644, depicted in FIGS. 113-119. FIG. 130 depicts a representation of an embodiment of an induction heater with core 496 and electrical insulator 486 surrounded by ferromagnetic layer 650. Core 496 may be copper or another non-ferromagnetic electrical conductor with low resistance that provides little or no heat output. Electrical insulator 486 may be a polymeric electrical insulator such as Teflon®, XPLE (cross-linked polyethylene), or EPDM (ethylene-propylene diene monomer). In some embodiments, core 496 and electrical insulator 486 are obtained together as a polymer (insulator) coated cable. In some embodiments, electrical insulator 486 is magnesium oxide or another suitable electrical insulator that inhibits arcing at high voltages and/or at high temperatures.

[1000] In certain embodiments, ferromagnetic layer 650 is spirally wound onto core 496 and electrical insulator 486. Ferromagnetic layer 650 may include carbon steel or another ferromagnetic steel (for example, 410 stainless steel, 446 stainless steel, T/P91 stainless steel, T/P92 stainless steel, alloy 52, alloy 42, and Invar 36).

[1001] In some embodiments, ferromagnetic layer 650 is spirally wound onto an insulated conductor. In some embodiments, ferromagnetic layer 650 includes an outer layer of corrosion resistant material. In some embodiments, ferromagnetic layer is bar stock. FIG. 131 depicts a representation of an embodiment of insulated conductor 530 surrounded by ferromagnetic layer 650. Insulated conductor 530 includes core 496, electrical insulator 486, and jacket 492. Core 496 is copper or another non-ferromagnetic electrical conductor with low resistance that provides little or no heat output. Electrical insulator 486 is magnesium oxide or another suitable electrical insulator. Ferromagnetic layer 650 is spirally wound onto insulated conductor 530.

[1002] Spirally winding ferromagnetic layer 650 onto the heater may increase control over the thickness of the ferromagnetic layer as compared to other construction methods for induction heaters. For example, more than one ferromagnetic layer 650 may be wound onto the heater to vary the output of the heater. The number of ferromagnetic layers 650 may be chosen to provide desired output from the heater. FIG. 132 depicts a representation of an embodiment of an induction heater with two ferromagnetic layers 650A,B spirally wound onto core 496 and electrical insulator 486. In some embodiments, ferromagnetic layer 650A is counter-wound relative to ferromagnetic layer 650B to provide neutral torque on the heater. Neutral torque may be useful when the heater is suspended or allowed to hang freely in an opening in the formation.

[1003] The number of spiral windings (for example, the number of ferromagnetic layers) may be varied to alter the heat output of the induction heater. In addition, other parameters may be varied to alter the heat output of the induction heater. Examples of other varied parameters include, but are not limited to, applied current, applied frequency, geometry, ferromagnetic materials, and thickness and/or number of spiral windings.

[1004] Use of spiral wound ferromagnetic layers may allow induction heaters to be manufactured in continuous long lengths by spiral winding the ferromagnetic material onto long lengths of conventional or easily manufactured insulated cable. Thus, spiral wound induction heaters may have reduced manufacturing costs as compared to other induction heaters. The spiral wound ferromagnetic layers may increase the mechanical flexibility of the induction heater as compared to solid ferromagnetic tubular induction heaters. The increased flexibility may allow spiral wound induction heaters to be bent over surface protrusions such as hanger joints.

[1005] FIG. 133 depicts an embodiment for assembling ferromagnetic layer 650 onto insulated conductor 530. Insulated conductor 530 may be an insulated conductor cable (for example, mineral insulated conductor cable or polymer insulated conductor cable) or other suitable electrical conductor core covered by insulation.

[1006] In certain embodiments, ferromagnetic layer 650 is made of ferromagnetic material 656 fed from reel 658 and wound onto insulated conductor 530. Reel 658 may be a coiled tubing rig or other rotatable feed rig. Reel 658 may rotate around insulated conductor 530 as ferromagnetic material 656 is wound onto the insulated conductor to form ferromagnetic layer 650. Insulated conductor 530 may be fed from a reel or from a mill as reel 658 rotates around the insulated conductor.

[1007] In some embodiments, ferromagnetic material 656 is heated prior to winding the material onto insulated conductor 530. For example, ferromagnetic material 656 may be heated using inductive heater 660. Pre-heating ferromagnetic material 656 prior to winding the ferromagnetic material may allow the ferromagnetic material to contract and grip onto insulated conductor 530 when the ferromagnetic material cools.

[1008] In some embodiments, portions of casings in the overburden sections of heater wellbores have surfaces that are shaped to increase the effective diameter of the casing. Casings in the overburden sections of heater wellbores may include, but are not limited to, overburden casings, heater casings, heater tubulars, and/or jackets of insulated conductors. Increasing the effective diameter of the casing may reduce inductive effects in the casing when current used to power a heater or heaters below the overburden is transmitted through the casing (for example, when one phase of power is being transmitted through the overburden section). When current is transmitted in only one direction through the overburden, the current may induce other currents in ferromagnetic or other electrically conductive materials such as those found in overburden casings. These induced currents may provide undesired power losses and/or undesired heating in the overburden of the formation.

[1009] FIG. 134 depicts an embodiment of casing 662 having a grooved or corrugated surface. In certain embodiments, casing 662 includes grooves 664. In some embodiments, grooves 664 are corrugations or include corrugations. Grooves 664 may be formed as a part of the surface of casing 662 (for example, the casing is formed with grooved surfaces) or the grooves may be formed by adding or removing (for example, milling) material on the surface of the casing. For example, grooves 664 may be located on a long piece of tubular that is welded to casing 662.

[1010] In certain embodiments, grooves 664 are on the outer surface of casing 662. In some embodiments, grooves 664 are on the inner surface of casing 662. In some embodiments, grooves 664 are on both the inner and outer surfaces of casing 662.

[1011] In certain embodiments, grooves 664 are axial grooves (grooves that go longitudinally along the length of casing 662). In certain embodiments, grooves 664 are straight, angled, or longitudinally spiral. In some embodiments, grooves 664 are substantially axial grooves or spiral grooves with a significant longitudinal component (i.e., the spiral angle is less than 10°, less than 5°, or less than 1°). In some embodiments, grooves 664 extend substantially axially along the length of casing 662. In some embodiments, grooves 664 are evenly spaced grooves along the surface of casing 662. Grooves 664 may have a variety of shapes as desired. For example, grooves 664 may have square edges, v-shaped edges, u-shaped edges, rectangular edges, or have rounded edges.

[1012] Grooves 664 increase the effective circumference of casing 662. Grooves 664 increase the effective circumference of casing 662 as compared to the circumference of a casing with the same inside and outside diameters and smooth surfaces. The depth of grooves 664 may be varied to provide a selected effective circumference of casing 662. For example, axial grooves that are ¼″ (0.63 cm) wide and ¼″ (0.63 cm) deep, and spaced ¼″ (0.63 cm) apart may increase the effective circumference of a 6″ (15.24 cm) diameter pipe from 18.84″ (47.85 cm) to 37.68″ (95.71 cm) (or the circumference of a 12″ (30.48 cm) diameter pipe).

[1013] In certain embodiments, grooves 664 increase the effective circumference of casing 662 by a factor of at least about 2 as compared to a casing with the same inside and outside diameters and smooth surfaces. In some embodiments, grooves 664 increase the effective circumference of casing 662 by a factor of at least about 3, at least about 4, or at least about 6 as compared to a casing with the same inside and outside diameters and smooth surfaces.

[1014] Increasing the effective circumference of casing 662 with grooves 664 increases the surface area of the casing. Increasing the surface area of casing 662 reduces the induced current in the casing for a given current flux. Power losses associated with inductive heating in casing 662 are reduced as compared to a casing with smooth surfaces because of the reduced induced current. Thus, the same electrical current will provide less heat output from inductive heating in the axial grooved surface casing than the smooth surface casing. Reducing the heat output in the overburden section of the heater will increase the efficiency of, and reduce the costs associated with, operating the heater. Increasing the effective circumference of casing 662 and reducing inductive effects in the casing allows the casing to be made with less expensive materials such as carbon steel.

[1015] In some embodiments, an electrically insulating coating (for example, a porcelain enamel coating) is placed on one or more surfaces of casing 662 to inhibit current and/or power losses from the casing. In some embodiments, casing 662 is formed from two or more longitudinal sections of casing (for example, longitudinal sections welded or threaded together end to end). The longitudinal sections may be aligned so that the grooves on the sections are aligned. Aligning the sections may allow for cement or other material to flow along the grooves.

[1016] In some embodiments, an insulated conductor heater is placed in the formation by itself and the outside of the insulated conductor heater is electrically isolated from the formation because the heater has little or no voltage potential on the outside of the heater. FIG. 135 depicts an embodiment of a single-ended, substantially horizontal insulated conductor heater that electrically isolates itself from the formation. In such an embodiment, heater 352 is insulated conductor 530. Insulated conductor 530 may be a mineral insulated conductor heater (for example, insulated conductor 530 depicted in FIGS. 136A and 136B). Insulated conductor 530 is located in opening 508 in hydrocarbon layer 510. In certain embodiments, opening 508 is an uncased or open wellbore. In some embodiments, opening 508 is a cased or lined wellbore. In some embodiments, insulated conductor heater 530 is a substantially u-shaped heater and is located in a substantially u-shaped opening.

[1017] Insulated conductor 530 has little or no current flowing along the outside surface of the insulated conductor so that the insulated conductor is electrically isolated from the formation and leaks little or no current into the formation. The outside surface (or jacket) of insulated conductor 530 is a metal or thermal radiating body so that heat is radiated from the insulated conductor to the formation.

[1018] FIGS. 136A and 136B depict cross-sectional representations of an embodiment of insulated conductor 530 that is electrically isolated on the outside of jacket 492. In certain embodiments, jacket 492 is made of ferromagnetic materials. In one embodiment, jacket 492 is made of 410 stainless steel. In other embodiments, jacket 492 is made of T/P91 or T/P92 stainless steel. In some embodiments, jacket 492 may include carbon steel. Core 496 is made of a highly conductive material such as copper or a copper alloy. Electrical insulator 486 is an electrically insulating material such as magnesium oxide. Insulated conductor 530 may be an inexpensive and easy to manufacture heater.

[1019] In the embodiment depicted in FIGS. 136A and 136B, core 496 brings current into the formation, as shown by the arrow. Core 496 and jacket 492 are electrically coupled at the distal end (bottom) of the heater. Current returns to the surface of the formation through jacket 492. The ferromagnetic properties of jacket 492 confine the current to the skin depth along the inside diameter of the jacket, as shown by arrows 666 in FIG. 136A. Jacket 492 has a thickness at least 2 or 3 times the skin depth of the ferromagnetic material used in the jacket at 25° C. and at the design current frequency so that most of the current is confined to the inside surface of the jacket and little or no current flows on the outside diameter of the jacket. Thus, there is little or no voltage potential on the outside of jacket 492. Having little or no voltage potential on the outside surface of insulated conductor 530 does not expose the formation to any high voltages, inhibits current leakage to the formation, and reduces or eliminates the need for isolation transformers, which decrease energy efficiency.

[1020] Because core 496 is made of a highly conductive material such as copper and jacket 492 is made of more resistive ferromagnetic material, a majority of the heat generated by insulated conductor 530 is generated in the jacket. Generating the majority of the heat in jacket 492 increases the efficiency of heat transfer from insulated conductor 530 to the formation over an insulated conductor (or other heater) that uses a core or a center conductor to generate the majority of the heat.

[1021] In certain embodiments, core 496 is made of copper. Using copper in core 496 allows the heating section of the heater and the overburden section to have identical core materials. Thus, the heater may be made from one long core assembly. The long single core assembly reduces or eliminates the need for welding joints in the core, which can be unreliable and susceptible to failure. Additionally, the long, single core assembly heater may be manufactured remote from the installation site and transported in a final assembly (ready to install assembly) to the installation site. The single core assembly also allows for long heater lengths (for example, about 1000 m or longer) depending on the breakdown voltage of the electrical insulator.

[1022] In certain embodiments, jacket 492 is made from two or more layers of the same materials and/or different materials. Jacket 492 may be formed from two or more layers to achieve thicknesses needed for the jacket (for example, to have a thickness at least 3 times the skin depth of the ferromagnetic material used in the jacket at 25° C. and at the design current frequency). Manufacturing and/or material limitations may limit the thickness of a single layer of jacket material. For example, the amount each layer can be strained during manufacturing (forming) the layer on the heater may limit the thickness of each layer. Thus, to reach jacket thicknesses needed for certain embodiments of insulated conductor 530, jacket 492 may be formed from several layers of jacket material. For example, three layers of T/P92 stainless steel may be used to form jacket 492 with a thickness of about 3 times the skin depth of the T/P92 stainless steel at 25° C. and at the design current frequency.

[1023] In some embodiments, jacket 492 includes two or more different materials. In some embodiments, jacket 492 includes different materials in different layers of the jacket. For example, jacket 492 may have one or more inner layers of ferromagnetic material chosen for their electrical and/or electromagnetic properties and one or more outer layers chosen for its non-corrosive properties.

[1024] In some embodiments, the thickness of jacket 492 and/or the material of the jacket are varied along the heater length. The thickness and/or material of jacket 492 may be varied to vary electrical properties and/or mechanical properties along the length of the heater. For example, the thickness and/or material of jacket 492 may be varied to vary the turndown ratio or the Curie temperature along the length of the heater. In some embodiments, the inner layer of jacket 492 includes copper or other highly conductive metals in the overburden section of the heater. The inner layer of copper limits heat losses in the overburden section of the heater.

[1025] FIGS. 137 and 138 depict an embodiment of insulated conductor 530 inside tubular 644. Insulated conductor 530 may include core 496, electrical insulator 486, and jacket 492. Core 496 and jacket 492 may be electrically coupled (shorted) at a distal end of the insulated conductor. FIG. 139 depicts a cross-sectional representation of an embodiment of the distal end of insulated conductor 530 inside tubular 644. Endcap 668 may electrically couple core 496 and jacket 492 to tubular 644 at the distal end of insulated conductor 530 and the tubular. Endcap 668 may include electrical conducting materials such as copper or steel.

[1026] In certain embodiments, core 496 is copper, electrical insulator 486 is magnesium oxide, and jacket 492 is non-ferromagnetic stainless steel (for example, 316H stainless steel, 347H stainless steel, 204-Cu stainless steel, 201Ln stainless steel, or 204 M stainless steel). Insulated conductor 530 may be placed in tubular 644 to protect the insulated conductor, increase heat transfer to the formation, and/or allow for coiled tubing or continuous installation of the insulated conductor. Tubular 644 may be made of ferromagnetic material such as 410 stainless steel, T/P 9 alloy steel, T/P91 alloy steel, low alloy steel, or carbon steel. In certain embodiments, tubular 644 is made of corrosion resistant materials. In some embodiments, tubular 644 is made of non-ferromagnetic materials.

[1027] In certain embodiments, jacket 492 of insulated conductor 530 is longitudinally welded to tubular 644 along weld joint 670, as shown in FIG. 138. The longitudinal weld may be a laser weld, a tandem GTAW (gas tungsten arc welding) weld, or an electron beam weld that welds the surface of jacket 492 to tubular 644. In some embodiments, tubular 644 is made from a longitudinal strip of metal. Tubular 644 may be made by rolling the longitudinal strip to form a cylindrical tube and then welding the longitudinal ends of the strip together to make the tubular.

[1028] In certain embodiments, insulated conductor 530 is welded to tubular 644 as the longitudinal ends of the strip are welded together (in the same welding process). For example, insulated conductor 530 is placed along one of the longitudinal ends of the strip so that jacket 492 is welded to tubular 644 at the location where the ends are welded together. In some embodiments, insulated conductor 530 is welded to one of the longitudinal ends of the strip before the strip is rolled to form the cylindrical tube. The ends of the strip may then be welded to form tubular 644.

[1029] In some embodiments, insulated conductor 530 is welded to tubular 644 at another location (for example, at a circumferential location away from the weld joining the ends of the strip used to form the tubular). For example, jacket 492 of insulated conductor 530 may be welded to tubular 644 diametrically opposite from where the longitudinal ends of the strip used to form the tubular are welded. In some embodiments, tubular 644 is made of multiple strips of material that are rolled together and coupled (for example, welded) to form the tubular with a desired thickness. Using more than one strip of metal may be easier to roll into the cylindrical tube used to form the tubular.

[1030] Jacket 492 and tubular 644 may be electrically and mechanically coupled at weld joint 670. Longitudinally welding jacket 492 to tubular 644 inhibits arcing between insulated conductor 530 and the tubular. Tubular 644 may return electrical current from core 496 along the inside of the tubular if the tubular is ferromagnetic. If tubular 644 is non-ferromagnetic, a thin electrically insulating layer such as a porcelain enamel coating or a spray coated ceramic may be put on the outside of the tubular to inhibit current leakage from the tubular into the formation. In some embodiments, a fluid is placed in tubular 644 to increase heat transfer between insulated conductor 530 and the tubular and/or to inhibit arcing between the insulated conductor and the tubular. Examples of fluids include, but are not limited to, thermally conductive gases such as helium, carbon dioxide, or steam. Fluids may also include fluids such as oil, molten metals, or molten salts (for example, solar salt (60% NaNO₃/40% KNO₃)). In some embodiments, heat transfer fluids are transported inside tubular 644 and heated inside the tubular (in the space between the tubular and insulated conductor 530). In some embodiments, an optical fiber, thermocouple, or other temperature sensor is placed inside tubular 644.

[1031] In certain embodiments, the heater depicted in FIGS. 137, 138, and 139 is energized with AC current (or time-varying electrical current). A majority of the heat is generated in tubular 644 when the heater is energized with AC current. If tubular 644 is ferromagnetic and the wall thickness of the tubular is at least about twice the skin depth at 25° C. and at the design current frequency, then the heater will operate as a temperature limited heater. Generating the majority of the heat in tubular 644 improves heat transfer to the formation as compared to a heater that generates a majority of the heat in the insulated conductor.

[1032] 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.

[1033] 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.

[1034] 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.

[1035] In some embodiments, at least a portion of the flexible cables are in contact with the outer second tubular. FIG. 140 depicts a cross-sectional representation of heater 352 including nine single-phase flexible cable conductors 502 positioned between first tubular 644a and second tubular 644b. Forming the heater such that the flexible cable conductors are in contact with the second tubular 644b results in the flexible cables providing conductive heat transfer between the first tubular 644a and the second tubular. In such embodiments, conductive heat transfer functions as the primary method of heat transfer to second tubular 644b.

[1036] In some embodiments, the flexible cables are inhibited from contacting the outer second tubular. FIG. 141 depicts a cross-sectional representation of heater 352 including nine single-phase flexible cable conductors 502 positioned between first tubular 644a and second tubular 644b with spacers 672. Spacers 672 may be positioned between first tubular 644a and second tubular 644b. The spacers may function to maintain separation between the tubulars and inhibit the flexible cables from contacting second tubular 644b. In such embodiments, radiative heat transfer functions as the primary method of heat transfer to second tubular 644b.

[1037] In some embodiments, spacers 672 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.

[1038] In some embodiments, heat transfer material (for example, heat transfer fluid) is located in the annulus between first tubular 644a and second tubular 644b. 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.

[1039] 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.

[1040] 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).

[1041] 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. 140 and 141). 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.

[1042] 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.

[1043] 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. 142 depicts a cross-sectional representation of heater 352 including nine multiple (in FIG. 142, each flexible cable includes three conductors) flexible cable conductors 502 positioned between first tubular 644a and second tubular 644b. FIG. 143 depicts a cross-sectional representation of heater 352 including nine multiple (in FIG. 143, each flexible cable includes three conductors) flexible cable conductors 502 positioned between first tubular 644a and second tubular 644b with spacers 672. Heater 352 depicted in FIG. 143 includes spacers 672. The multiple conductor flexible cables depicted in FIGS. 142 and 143 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.

[1044] The liner heaters, depicted in FIGS. 140, 141, 142, and 143, 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.

[1045] 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.

[1046] 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.

[1047] 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).

[1048] In certain embodiments, wellheads for the wellbores may be made of one or more non-ferromagnetic materials. FIG. 144 depicts an embodiment of wellhead 674. 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.

[1049] 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. 144, non-ferromagnetic material 676 is coupled (and electrically coupled) to the inside walls of conduit 504 and wellhead walls 678. 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.

[1050] 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.

[1051] In certain embodiments, a horizontal heater, or a heater at an incline is installed in more than one part. FIG. 145 depicts an embodiment of heater 352 that has been installed in two parts. Heater 352 includes heating section 352A and lead-in section 352B. Heating section 352A may be located horizontally or at an incline in a hydrocarbon layer in the formation. Lead-in section 352B may be the overburden section or low resistance section of the heater (for example, the section of the heater with little or no electrical heat output).

[1052] During installation of heater 352, heating section 352A may be installed first into the formation. Heating section 352A may be installed by pushing the heating section into the opening in the formation using a drill pipe or other installation tool that pushes the heating section into the opening. After installation of heating section 352A, the installation tool may be removed from the opening in the formation. Installing only heating section 352A with the installation tool at this time may allow the heating section to be installed further into the formation than if the heating section and the lead-in section are installed together because a higher compressive strength may be applied to the heating section alone (for example, the installation tool only has to push in the horizontal or inclined direction).

[1053] In some embodiments, heating section 352A is coupled to mechanical connector 680. Connector 680 may be used to hold heating section 352A in the opening. In some embodiments, connector 680 includes copper or other electrically conductive materials so that the connector is used as an electrical connector (for example, as an electrical ground). In some embodiments, connector 680 is used to couple heating section 352A to a bus bar or electrical return rod located in an opening perpendicular to the opening of the heating section.

[1054] Lead-in section 352B may be installed after installation of heating section 352A. Lead-in section 352B may be installed with a drill pipe or other installation tool. In some embodiments, the installation tool may be the same tool used to install heating section 352A.

[1055] Lead-in section 352B may couple to heating section 352A as the lead-in section is installed into the opening. In certain embodiments, coupling joint 682 is used to couple lead-in section 352B to heating section 352A. Coupling joint 682 may be located on either lead-in section 352B or heating section 352A. In some embodiments, coupling joint 682 includes portions located on both sections. Coupling joint 682 may be a coupler such as, but not limited to, a wet connect or wet stab. In some embodiments, heating section 352A includes a catcher or other tool that guides an end of lead-in section 352B to form coupling joint 682.

[1056] In some embodiments, coupling joint 682 includes a container (for example, a can) located on heating section 352A that accepts the end of lead-in section 352B. Electrically conductive beads (for example, balls, spheres, or pebbles) may be located in the container. The beads may move around as the end of lead-in section 352B is pushed into the container to make electrical contact between the lead-in section and heating section 352A. The beads may be made of, for example, copper or aluminum. The beads may be coated or covered with a corrosion inhibitor such as nickel. In some embodiments, the beads are coated with a solder material that melts at lower temperatures (for example, below the boiling point of water in the formation). A high electrical current may be applied to the container to melt the solder. The melted solder may flow and fill void spaces in the container and be allowed to solidify before energizing the heater. In some embodiments, sacrificial beads are put in the container. The sacrificial beads may corrode first so that copper or aluminum beads in the container are less likely to be corroded during operation of the heater.

[1057] Modern utility voltage regulators have microprocessor controllers that monitor output voltage and adjust taps up or down to match a desired setting. Typical controllers include current monitoring and may be equipped with remote communications capabilities. The controller firmware may be modified for current based control (for example, control desired for maintaining constant wattage as heater resistances vary with temperature). Load resistance monitoring as well as other electrical analysis based evaluation and control are a possibility because of the availability of both current and voltage sensing by the controller. In addition to current, sensed electrical properties including, but not limited to power, voltage, power factor, resistance or harmonics may be used as control parameters. Typical tap changers have a 200% of nominal, short time current rating. Thus, the regulator controller may be programmed to respond to overload currents by means of tap changer operation.

[1058] Electronic heater controls such as silicon-controlled rectifiers (SCRs) may be used to provide power to and control subsurface heaters. SCRs may be expensive to use and may waste electrical energy in the power circuit. SCRs may also produce harmonic distortions during power control of the subsurface heaters. Harmonic distortion may put noise on the power line and stress heaters. In addition, SCRs may overly stress heaters by switching the power between being full on and full off rather than regulating the power at or near the ideal current setting. Thus, there may be significant overshooting and/or undershooting at the target current for temperature limited heaters (for example, heaters using ferromagnetic materials for self-limiting temperature control).

[1059] A variable voltage, load tap changing transformer, which is based on a load tap changing regulator design, may be used to provide power to and control subsurface heaters more simply and without the harmonic distortion associated with electronic heater control. The variable voltage transformer may be connected to power distribution systems by simple, inexpensive fused cutouts. The variable voltage transformer may provide a cost effective, stand alone, full function heater controller and isolation transformer.

[1060] FIG. 146 depicts a schematic for a conventional design of tap changing voltage regulator 684. Regulator 684 provides plus or minus 10% adjustment of the input or line voltage. Regulator 684 includes primary winding 686 and tap changer section 688, which includes the secondary winding of the regulator. Primary winding 686 is a series winding electrically coupled to the secondary winding of tap changer section 688. Tap changer section 688 includes eight taps 690A-H that separate the voltage on the secondary winding into voltage steps. Moveable tap changer 692 is a moveable preventive autotransformer with a balance winding. Tap changer 692 may be a sliding tap changer that moves between taps 690A-H in tap changer section 688. Tap changer 692 may be capable of carrying high currents up to, for example, 668 A or more.

[1061] Tap changer 692 contacts either one tap 690 or bridges between two taps to provide a midpoint between the two tap voltages. Thus, 16 equivalent voltage steps are created for tap changer 692 to couple to in tap changer section 688. The voltage steps divide the 10% range of regulation equally (⅝% per step). Switch 694 changes the voltage adjustment between plus and minus adjustment. Thus, voltage can be regulated plus 10% or minus 10% from the input voltage.

[1062] Voltage transformer 696 senses the potential at bushing 698. The potential at bushing 698 may be used for evaluation by a microprocessor controller. The controller adjusts the tap position to match a preset value. Control power transformer 700 provides power to operate the controller and the tap changer motor. Current transformer 702 is used to sense current in the regulator.

[1063] FIG. 147 depicts a schematic for variable voltage, load tap changing transformer 704. The schematic for transformer 704 is based on the load tap changing regulator schematic depicted in FIG. 146. Primary winding 686 is isolated from the secondary winding of tap changer section 688 to create distinct primary and secondary windings. Primary winding 686 may be coupled to a voltage source using bushings 706, 708. The voltage source may provide a first voltage across primary winding 686. The first voltage may be a high voltage such as voltages of at least 5 kV, at least 10 kV, at least 25 kV, or at least 35 kV up to about 50 kV. The secondary winding in tap changer section 688 may be coupled to an electrical load (for example, one or more subsurface heaters) using bushings 710, 712. The electrical load may include, but not be limited to, an insulated conductor heater (for example, mineral insulated conductor heater), a conductor-in-conduit heater, a temperature limited heater, a dual leg heater, or one heater leg of a three-phase heater configuration. The electrical load may be other than a heater (for example, a bottom hole assembly for forming a wellbore).

[1064] The secondary winding in tap changer section 688 steps down the first voltage across primary winding 686 to a second voltage (for example, voltage lower than the first voltage or a second voltage). In certain embodiments, the secondary winding in tap changer section 688 steps down the voltage from primary winding 686 to the second voltage that is between 5% and 20% of the first voltage across the primary winding. In some embodiments, the secondary winding in tap changer section 688 steps down the voltage from primary winding 686 to the second voltage that is between 1% and 30% or between 3% and 25% of the first voltage across the primary winding. In one embodiment, the secondary winding in tap changer section 688 steps down the voltage from primary winding 686 to the second voltage that is 10% of the first voltage across the primary winding. For example, a first voltage of 7200 V across the primary winding may be stepped down to a second voltage of 720 V across the secondary winding in tap changer section 688.

[1065] In some embodiments, the step down percentage in tap changer section 688 is preset. In some embodiments, the step down percentage in tap changer section 688 may be adjusted as needed for desired operation of a load coupled to transformer 704.

[1066] Taps 690A-H (or any other number of taps) divide the second voltage on the secondary winding in tap changer section 688 into voltage steps. The second voltage is divided into voltage steps from a selected minimum percentage of the second voltage up to the full value of the second voltage. In certain embodiments, the second voltage is divided into equivalent voltage steps between the selected minimum percentage and the full second voltage value. In some embodiments, the selected minimum percentage is 0% of the second voltage. For example, the second voltage may be equally divided by the taps in voltage steps ranging between 0 V and 720 V. In some embodiments, the selected minimum percentage is 25% or 50% of the second voltage.

[1067] Transformer 704 includes tap changer 692 that contacts either one tap 690 or bridges between two taps to provide a midpoint between the two tap voltages. The position of tap changer 692 on the taps determines the voltage provided to an electrical load coupled to bushings 710, 712. As an example, an arrangement with 8 taps in tap changer section 688 provides 16 voltage steps for tap changer 692 to couple to in tap changer section 688. Thus, the electrical load may be provided with 16 different voltages varying between the selected minimum percentage and the second voltage.

[1068] In certain embodiments of transformer 704, the voltage steps divide the range between the selected minimum percentage and the second voltage equally (the voltage steps are equivalent). For example, eight taps may divide a second voltage of 720 V into 16 voltage steps between 0 V and 720 V so that each tap increments the voltage provided to the electrical load by 45V. In some embodiments, the voltage steps divide the range between the selected minimum percentage and the second voltage in non-equal increments (the voltage steps are not equivalent).

[1069] Switch 694 may be used to electrically disconnect bushing 712 from the secondary winding and taps 690. Electrically isolating bushing 712 from the secondary winding turns off the power (voltage) provided to the electrical load coupled to bushings 710, 712. Thus, switch 694 provides an internal disconnect in transformer 704 to electrically isolate and turn off power (voltage) to the electrical load coupled to the transformer.

[1070] In transformer 704, voltage transformer 696, control power transformer 700, and current transformer 702 are electrically isolated from primary winding 686. Electrical isolation protects voltage transformer 696, control power transformer 700, and current transformer 702 from current and/or voltage overloads caused by primary winding 686.

[1071] In certain embodiments, transformer 704 is used to provide power to a variable electrical load (for example, a subsurface heater such as, but not limited to, a temperature limited heater using ferromagnetic material that self-limits at the Curie temperature or a phase transition temperature range). Transformer 704 allows power to the electrical load to be adjusted in small voltage increments (voltage steps) by moving tap changer 692 between taps 690. Thus, the voltage supplied to the electrical load may be adjusted incrementally to provide constant current to the electrical load in response to changes in the electrical load (for example, changes in resistance of the electrical load). Voltage to the electrical load may be controlled from a minimum voltage (the selected minimum percentage) up to full potential (the second voltage) in increments. The increments may be equal increments or non-equal increments. Thus, power to the electrical load does not have to be turned full on or off to control the electrical load such as is done with a SCR controller. Using small increments may reduce cycling stress on the electrical load and may increase the lifetime of the device that is the electrical load. Transformer 704 changes the voltage using mechanical operation instead of the electrical switching used in SCRs. Electrical switching can add harmonics and/or noise to the voltage signal provided to the electrical load. The mechanical switching of transformer 704 provides clean, noise free, incrementally adjustable control of the voltage provided to the electrical load.

[1072] Transformer 704 may be controlled by controller 714. Controller 714 may be a microprocessor controller. Controller 714 may be powered by control power transformer 700. Controller 714 may assess properties of transformer 704, including tap changer section 688, and/or the electrical load coupled to the transformer. Examples of properties that may be assessed by controller 714 include, but are not limited to, voltage, current, power, power factor, harmonics, tap change operation count, maximum and minimum value recordings, wear of the tap changer contacts, and electrical load resistance.

[1073] In certain embodiments, controller 714 is coupled to the electrical load to assess properties of the electrical load. For example, controller 714 may be coupled to the electrical load using an optical fiber. The optical fiber allows measurement of properties of the electrical load such as, but not limited to, electrical resistance, impedance, capacitance, and/or temperature. In some embodiments, controller 714 is coupled to voltage transformer 696 and/or current transformer 702 to assess the voltage and/or current output of transformer 704. In some embodiments, the voltage and current are used to assess a resistance of the electrical load over one or more selected time periods. In some embodiments, the voltage and current are used to assess or diagnose other properties of the electrical load (for example, temperature).

[1074] In certain embodiments, controller 714 adjusts the voltage output of transformer 704 in response to changes in the electrical load coupled to the transformer or other changes in the power distribution system such as, but not limited to, input voltage to the primary winding or other power supply changes. For example, controller 714 may adjust the voltage output of transformer 704 in response to changes in the electrical resistance of the electrical load. Controller 714 may adjust the output voltage by controlling the movement of control tap changer 692 between taps 690 to adjust the voltage output of transformer 704. In some embodiments, controller 714 adjusts the voltage output of transformer 704 so that the electrical load (for example, a subsurface heater) is operated at a relatively constant current. In some embodiments, controller 714 may adjust the voltage output of transformer 704 by moving tap changer 692 to a new tap, assess the resistance and/or power at the new tap, and move the tap changer to another new tap if needed.

[1075] In some embodiments, controller 714 assesses the electrical resistance of the load (for example, by measuring the voltage and current using the voltage and current transformers or by measuring the resistance of the electrical load using the optical fiber) and compares the assessed electrical resistance to a theoretical resistance. Controller 714 may adjust the voltage output of transformer 704 in response to differences between the assessed resistance and the theoretical resistance. In some embodiments, the theoretical resistance is an ideal resistance for operation of the electrical load. In some embodiments, the theoretical resistance varies over time due to other changes in the electrical load (for example, temperature of the electrical load).

[1076] In some embodiments, controller 714 is programmable to cycle tap changer 692 between two or more taps 690 to achieve intermediate voltage outputs (for example, a voltage output between two tap voltage outputs). Controller 714 may adjust the time tap changer 692 is on each of the taps cycled between to obtain an average voltage at or near the desired intermediate voltage output. For example, controller 714 may keep tap changer 692 at two taps approximately 50% of the time each to maintain an average voltage approximately midway between the voltages at the two taps.

[1077] In some embodiments, controller 714 is programmable to limit the numbers of voltage changes (movement of tap changer 692 between taps 690 or cycles of tap changes) over a period of time. For example, controller 714 may only allow 1 tap change every 30 minutes or 2 tap changes per hour. Limiting the number of tap changes over the period of time reduces the stress on the electrical load (for example, a heater) from changes in voltage to the load. Reducing the stresses applied to the electrical load may increase the lifetime of the electrical load. Limiting the number of tap changes may also increase the lifetime of the tap changer apparatus. In some embodiments, the number of tap changes over the period of time is adjustable using the controller. For example, a user may be allowed to adjust the cycle limit for tap changes on transformer 704.

[1078] In some embodiments, controller 714 is programmable to power the electrical load in a start up sequence. For example, subsurface heaters may require a certain start up protocol (such as high current during early times of heating and lower current as the temperature of the heater reaches a set point). Ramping up power to the heaters in a desired procedure may reduce mechanical stresses on the heaters from materials expanding at different rates. In some embodiments, controller 714 ramps up power to the electrical load with controlled increases in voltage steps over time. In some embodiments, controller 714 ramps up power to the electrical load with controlled increases in watts per hour. Controller 714 may be programmed to automatically start up the electrical load according to a user input start up procedure or a pre-programmed start up procedure.

[1079] In some embodiments, controller 714 is programmable to turn off power to the electrical load in a shut down sequence. For example, subsurface heaters may require a certain shut down protocol to inhibit the heaters from cooling to quickly. Controller 714 may be programmed to automatically shut down the electrical load according to a user input shut down procedure or a pre-programmed shut down procedure.

[1080] In some embodiments, controller 714 is programmable to power the electrical load in a moisture removal sequence. For example, subsurface heaters or motors may require start up at second voltages to remove moisture from the system before application of higher voltages. In some embodiments, controller 714 inhibits increases in voltage until required electrical load resistance values are met. Limiting increases in voltage may inhibit transformer 704 from applying voltages that cause shorting due to moisture in the system. Controller 714 may be programmed to automatically start up the electrical load according to a user input moisture removal sequence or a pre-programmed moisture removal procedure.

[1081] In some embodiments, controller 714 is programmable to reduce power to the electrical load based on changes in the voltage input to primary winding 686. For example, the power to the electrical load may be reduced during brownouts or other power supply shortages. Reducing the power to the electrical load may compensate for the reduced power supply.

[1082] In some embodiments, controller 714 is programmable to protect the electrical load from being overloaded. Controller 714 may be programmed to automatically and immediately reduce the voltage output if the current to the electrical load increases above a selected value. The voltage output may be stepped down as fast as possible while sensing the current. Sensing of the current occurs on a faster time scale than the step downs in voltage so the voltage may be stepped down as fast as possible until the current drops below a selected level. In some embodiments, tap changes (voltage steps) may be inhibited above higher current levels. At the higher current levels, secondary fusing may be used to limit the current. Reducing the tap setting in response to the higher current levels may allow for continued operation of the transformer even after partial failure or quenching of electrical loads such as heaters.

[1083] In some embodiments, controller 714 records or tracks data from the operation of the electrical load and/or transformer 704. For example, controller 714 may record changes in the resistance or other properties of the electrical load or transformer 704. In some embodiments, controller 714 records faults in operation of transformer 704 (for example, missed step changes).

[1084] In certain embodiments, controller 714 includes communication modules. The communication modules may be programmed to provide status, data, and/or diagnostics for any device or system coupled to the controller such as the electrical load or transformer 704. The communication modules may communicate using RS485 serial communication, Ethernet, fiber, wireless, and/or other communication technologies known in the art. The communication modules may be used to transmit information remotely to another site so that controller 714 and transformer 704 are operated in a self-contained or automatic manner but are able to report to another location (for example, a central monitoring location). The central monitoring location may monitor several controllers and transformers (for example, controllers and transformers located in a hydrocarbon processing field). In some embodiments, users or equipment at the central monitoring location are able to remotely operate one or more of the controllers using the communications modules.

[1085] FIG. 148 depicts a representation of an embodiment of transformer 704 and controller 714. In certain embodiments, transformer 704 is enclosed in enclosure 716. Enclosure 716 may be a cylindrical can. Enclosure 716 may be any other suitable enclosure known in the art (for example, a substation style rectangular enclosure). Controller 714 may be mounted to the outside of enclosure 716. Bushings 706, 708, 710, and 712 may be open air, high voltage bushings located on the outside of enclosure 716 for coupling transformer 704 to the power supply and the electrical load.

[1086] In certain embodiments, enclosure 716 is mounted on a pole or otherwise supported off the ground. In some embodiments, one or more enclosures 716 are mounted on an elevated platform supported by a pole or elevated mounting support. Mounting enclosure 716 on a pole or mounting support increases air circulation around and in the enclosure and transformer 704. Increasing air circulation decreases operating temperatures and increases efficiency of the transformer. In certain embodiments, components of transformer 704 are coupled to the top of enclosure 716 so that the components are removed as a single unit from the enclosure by removing the top of the enclosure.

[1087] In certain embodiments, three transformers 704 are used to operate three, or multiples of three, electrical loads in a three-phase configuration. The three transformers may be monitored to assess if the tap positions in each transformer are in sync (at the same tap position). In some embodiments, one controller 714 is used to control the three transformers. The controller may monitor the transformers to ensure that the transformers are in sync.

[1088] 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.

[1089] 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.

[1090] 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.

[1091] 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.

[1092] 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.

[1093] FIGS. 149-152 depict side view representations of embodiments for producing mobilized fluids from tar sands formations. In FIGS. 149-152, heaters 352 have substantially horizontal heating sections in hydrocarbon layer 510 (as shown, the heaters have heating sections that go into and out of the page). Hydrocarbon layer 510 may be below overburden 520. FIG. 149 depicts a side view representation of an embodiment for producing mobilized fluids from a tar sands formation with a relatively thin hydrocarbon layer. FIG. 150 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. 149. FIG. 151 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. 150. FIG. 152 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.

[1094] In FIG. 149, heaters 352 are placed in an alternating triangular pattern in hydrocarbon layer 510. In FIGS. 150, 151, and 152, heaters 352 are placed in an alternating triangular pattern in hydrocarbon layer 510 that repeats vertically to encompass a majority or all of the hydrocarbon layer. In FIG. 152, the alternating triangular pattern of heaters 352 in hydrocarbon layer 510 repeats uninterrupted across shale break 718. In FIGS. 149-152, heaters 352 may be equidistantly spaced from each other. In the embodiments depicted in FIGS. 149-152, the number of vertical rows of heaters 352 depends on factors such as, but not limited to, the desired spacing between the heaters, the thickness of hydrocarbon layer 510, and/or the number and location of shale breaks 718. In some embodiments, heaters 352 are arranged in other patterns. For example, heaters 352 may be arranged in patterns such as, but not limited to, hexagonal patterns, square patterns, or rectangular patterns.

[1095] In the embodiments depicted in FIGS. 149-152, heaters 352 provide heat that mobilizes hydrocarbons (reduces the viscosity of the hydrocarbons) in hydrocarbon layer 510. In certain embodiments, heaters 352 provide heat that reduces the viscosity of the hydrocarbons in hydrocarbon layer 510 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 352 and/or the heat output of the heaters may be designed and/or controlled to reduce the viscosity of the hydrocarbons in hydrocarbon layer 510 to desirable values. Heat provided by heaters 352 may be controlled so that little or no pyrolyzation occurs in hydrocarbon layer 510. 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 352 so that heat from the heaters superimposes over the production wells. The superimposition of heat from heaters 352 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 510 tend to flow towards the bottommost heaters 352, 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 510.

[1096] In certain embodiments, hydrocarbon layer 510 has sufficient permeability to allow mobilized fluids to drain to production wells 206A and/or production wells 206B. For example, hydrocarbon layer 510 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 510 has a relatively large vertical permeability to horizontal permeability ratio (K_v/K_h). For example, hydrocarbon layer 510 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.

[1097] In certain embodiments, fluids are produced through production wells 206A located near heaters 352 in the lower portion of hydrocarbon layer 510. In some embodiments, fluids are produced through production wells 206B located below and approximately midway between heaters 352 in the lower portion of hydrocarbon layer 510. At least a portion of production wells 206A and/or production wells 206B may be oriented substantially horizontal in hydrocarbon layer 510 (as shown in FIGS. 149-152, 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 352 or the bottommost heaters.

[1098] In some embodiments, production wells 206A are positioned substantially vertically below the bottommost heaters in hydrocarbon layer 510. Production wells 206A may be located below heaters 352 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. 149-152). Locating production wells 206A substantially vertically below the bottommost heaters may allow for efficient collection of mobilized fluids from hydrocarbon layer 510.

[1099] In certain embodiments, the bottommost heaters are located between about 2 m and about 10 m from the bottom of hydrocarbon layer 510, 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 352 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. 149-152). 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 ⅓ 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 510, 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.

[1100] In some embodiments, at least some production wells 206A are located substantially vertically below heaters 352 near shale break 718, as depicted in FIG. 152. Production wells 206A may be located between heaters 352 and shale break 718 to produce fluids that flow and collect above the shale break. Shale break 718 may be an impermeable barrier in hydrocarbon layer 510. In some embodiments, shale break 718 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 352 and shale break 718 may produce fluids from the upper portion of hydrocarbon layer 510 (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. 152. 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.

[1101] In some embodiments, shale break 718 breaks down (is desiccated or decomposes) as the shale break is heated by heaters 352 on either side of the shale break. As shale break 718 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 718, 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 510 and be produced there.

[1102] In certain embodiments, the bottommost heaters above shale break 718 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 718, 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 718, 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.

[1103] In some embodiments, heat is provided in production wells 206A and/or production wells 206B, depicted in FIGS. 149-152. 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 352 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).

[1104] 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.

[1105] 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.

[1106] 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.

[1107] 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).

[1108] 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.

[1109] 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.

[1110] 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.

[1111] 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.

[1112] 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.

[1113] 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.

[1114] 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.

[1115] 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.

[1116] 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. 149-152. 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.

[1117] 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 352 depicted in the embodiments depicted in FIGS. 149-152 may be modified to create the gas cap at or near overburden 520 of hydrocarbon layer 510. 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.

[1118] 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.

[1119] 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.

[1120] 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.

[1121] 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.

[1122] 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.

[1123] 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.

[1124] 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.

[1125] 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.

[1126] 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.

[1127] 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).

[1128] 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.

[1129] 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.

[1130] 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.

[1131] 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.

[1132] 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.

[1133] 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 controlled 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).

[1134] 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°, or 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.

[1135] 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).

[1136] 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.

[1137] 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.

[1138] FIG. 153 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 720 and production wells 206 are substantially vertical wells. Heaters 352 are long substantially horizontal heaters positioned so that the heaters pass in the vicinity of injection wells 720. Heaters 352 intersect the vertical well patterns slightly displaced from the vertical wells.

[1139] The vertical location of heaters 352 with respect to injection wells 720 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 352 may be located near the bottom of the hydrocarbon layer 510, as shown in FIG. 154. 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. 149-152). The vertical spacing between the horizontal heaters in such formations may correspond to the distance between the heaters and the injection wells. Heaters 352 are located in the vicinity of injection wells 720 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 352 and injection wells 720 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.

[1140] 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.