In situ thermal processing of an oil shale formation using a pattern of heat sources

[0001] This application claims priority to Provisional Patent Application No. 60/286,062 entitled “IN SITU THERMAL PROCESSING OF OIL SHALE” filed on Apr. 24, 2001 and to Provisional Patent Application No. 60/337,249 entitled “1IN SITU THERMAL PROCESSING OF AN OIL SHALE FORMATION” filed on Oct. 24, 2001. This application is a continuation of U.S. patent application Ser. No. 10/131,286 entitled “IN SITU THERMAL PROCESSING OF AN OIL SHALE FORMATION USING A PATTERN OF HEAT SOURCES” filed Apr. 24, 2002, now U.S. Pat. No. 6,991,032, the disclosure of which is hereby incorporated by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates generally to methods and systems for production of hydrocarbons, hydrogen, and/or other products from various oil shale formations. Certain embodiments relate to in situ conversion of hydrocarbons to produce hydrocarbons, hydrogen, and/or novel product streams from underground oil shale formations.

[0004] 2. Description of Related Art

[0005] Hydrocarbons obtained from subterranean (e.g., sedimentary) formations are often used as energy resources, as feedstocks, and as consumer products. Concerns over depletion of available hydrocarbon resources and 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 within 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 within 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.

[0006] 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.

[0007] 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 within 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.

[0008] 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 within a viscous oil within 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.

[0009] 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 within 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.

[0010] 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.

[0011] 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.

[0012] Combustion of a fuel may be used to heat a formation. Combusting a fuel to heat a formation may be more economical than using electricity to heat a formation. Several different types of heaters may use fuel combustion as a heat source that heats a formation. The combustion may take place in the formation, in a well, and/or near the surface. Combustion in the formation may be a fireflood. An oxidizer may be pumped into the formation. The oxidizer may be ignited to advance a fire front towards a production well. Oxidizer pumped into the formation may flow through the formation along fracture lines in the formation. Ignition of the oxidizer may not result in the fire front flowing uniformly through the formation.

[0013] A flameless combustor may be used to combust a fuel within a well. U.S. Pat. No. 5,255,742 to Mikus, U.S. Pat. No. 5,404,952 to Vinegar et al., U.S. Pat. No. 5,862,858 to Wellington et al., and U.S. Pat. No. 5,899,269 to Wellington et al., which are incorporated by reference as if fully set forth herein, describe flameless combustors. Flameless combustion may be accomplished by preheating a fuel and combustion air to a temperature above an auto-ignition temperature of the mixture. The fuel and combustion air may be mixed in a heating zone to combust. In the heating zone of the flameless combustor, a catalytic surface may be provided to lower the auto-ignition temperature of the fuel and air mixture.

[0014] Heat may be supplied to a formation from a surface heater. The surface heater may produce combustion gases that are circulated through wellbores to heat the formation. Alternately, a surface burner may be used to heat a heat transfer fluid that is passed through a wellbore to heat the formation. Examples of fired heaters, or surface burners that may be used to heat a subterranean formation, are illustrated in U.S. Pat. No. 6,056,057 to Vinegar et al. and U.S. Pat. No. 6,079,499 to Mikus et al., which are both incorporated by reference as if fully set forth herein.

[0015] Synthesis gas may be produced in reactors or in situ within a subterranean formation. Synthesis gas may be produced within a reactor by partially oxidizing methane with oxygen. In situ production of synthesis gas may be economically desirable to avoid the expense of building, operating, and maintaining a surface synthesis gas production facility. U.S. Pat. No. 4,250,230 to Terry, which is incorporated by reference as if fully set forth herein, describes a system for in situ gasification of coal. A subterranean coal seam is burned from a first well towards a production well. Methane, hydrocarbons, H₂, CO, and other fluids may be removed from the formation through the production well. The H₂ and CO may be separated from the remaining fluid. The H₂ and CO may be sent to fuel cells to generate electricity.

[0016] U.S. Pat. No. 4,057,293 to Garrett, which is incorporated by reference as if fully set forth herein, discloses a process for producing synthesis gas. A portion of a rubble pile is burned to heat the rubble pile to a temperature that generates liquid and gaseous hydrocarbons by pyrolysis. After pyrolysis, the rubble is further heated, and steam or steam and air are introduced to the rubble pile to generate synthesis gas.

[0017] U.S. Pat. No. 5,554,453 to Steinfeld et al., which is incorporated by reference as if fully set forth herein, describes an ex situ coal gasifier that supplies fuel gas to a fuel cell. The fuel cell produces electricity. A catalytic burner is used to burn exhaust gas from the fuel cell with an oxidant gas to generate heat in the gasifier.

[0018] Carbon dioxide may be produced from combustion of fuel and from many chemical processes. Carbon dioxide may be used for various purposes, such as, but not limited to, a feed stream for a dry ice production facility, supercritical fluid in a low temperature supercritical fluid process, a flooding agent for coal bed demethanation, and a flooding agent for enhanced oil recovery. Although some carbon dioxide is productively used, many tons of carbon dioxide are vented to the atmosphere.

[0019] Retorting processes for oil shale may be generally divided into two major types: aboveground (surface) and underground (in situ). Aboveground retorting of oil shale typically involves mining and construction of metal vessels capable of withstanding high temperatures. The quality of oil produced from such retorting may typically be poor, thereby requiring costly upgrading. Aboveground retorting may also adversely affect environmental and water resources due to mining, transporting, processing, and/or disposing of the retorted material. Many U.S. patents have been issued relating to aboveground retorting of oil shale. Currently available aboveground retorting processes include, for example, direct, indirect, and/or combination heating methods.

[0020] In situ retorting typically involves retorting oil shale without removing the oil shale from the ground by mining. “Modified” in situ processes typically require some mining to develop underground retort chambers. An example of a “modified” in situ process includes a method developed by Occidental Petroleum that involves mining approximately 20% of the oil shale in a formation, explosively rubblizing the remainder of the oil shale to fill up the mined out area, and combusting the oil shale by gravity stable combustion in which combustion is initiated from the top of the retort. Other examples of “modified” in situ processes include the “Rubble In Situ Extraction” (“RISE”) method developed by the Lawrence Livermore Laboratory (“LLL”) and radio-frequency methods developed by IIT Research Institute (“IITRI”) and LLL, which involve tunneling and mining drifts to install an array of radio-frequency antennas in an oil shale formation.

[0021] Obtaining permeability within an oil shale formation (e.g., 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, including: hydraulic fracturing such as methods investigated by Dow Chemical and Laramie Energy Research Center; electrical fracturing (e.g., by methods investigated by Laramie Energy Research Center); acid leaching of limestone cavities (e.g., by methods investigated by Dow Chemical); steam injection into permeable nahcolite zones to dissolve the nahcolite (e.g., by methods investigated by Shell Oil and Equity Oil); fracturing with chemical explosives (e.g., by methods investigated by Talley Energy Systems); fracturing with nuclear explosives (e.g., by methods investigated by Project Bronco); and combinations of these methods. Many of such methods, however, have relatively high operating costs and lack sufficient injection capacity.

[0022] An example of an in situ retorting process is illustrated in U.S. Pat. No. 3,241,611 to Dougan, assigned to Equity Oil Company, which is incorporated by reference as if fully set forth herein. For example, Dougan discloses a method involving the use of natural gas for conveying kerogen-decomposing heat to the formation. The heated natural gas may be used as a solvent for thermally decomposed kerogen. The heated natural gas exercises a solvent-stripping action with respect to the oil shale by penetrating pores that exist in the shale. The natural gas carrier fluid, accompanied by decomposition product vapors and gases, passes upwardly through extraction wells into product recovery lines, and into and through condensers interposed in such lines, where the decomposition vapors condense, leaving the natural gas carrier fluid to flow through a heater and into an injection well drilled into the deposit of oil shale.

[0023] U.S. Pat. No. 5,297,626 Vinegar et al. and U.S. Pat. No. 5,392,854 to Vinegar et al., which are incorporated by reference as if fully set forth herein, describe a process wherein an oil containing subterranean formation is heated. The following patents are incorporated herein by reference: U.S. Pat. No. 6,152,987 to Ma et al.; U.S. Pat. No. 5,525,322 to Willms; U.S. Pat. No. 5,861,137 to Edlund; and U.S. Pat. No. 5,229,102 to Minet et al.

[0024] 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 oil shale formations. At present, however, there are still many oil shale 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 oil shale formations.

[0025] In an embodiment, hydrocarbons within an oil shale formation may be converted in situ within the formation to yield a mixture of relatively high quality hydrocarbon products, hydrogen, and/or other products. One or more heat sources may be used to heat a portion of the oil shale formation to temperatures that allow pyrolysis of the hydrocarbons. Hydrocarbons, hydrogen, and other formation fluids may be removed from the formation through one or more production wells. In some embodiments, formation fluids may be removed in a vapor phase. In other embodiments, formation fluids may be removed in liquid and vapor phases or in a liquid phase. Temperature and pressure in at least a portion of the formation may be controlled during pyrolysis to yield improved products from the formation.

[0026] In an embodiment, one or more heat sources may be installed into a formation to heat the formation. Heat sources may be installed by drilling openings (well bores) into the formation. In some embodiments, openings may be formed in the formation using a drill with a steerable motor and an accelerometer. Alternatively, an opening may be formed into the formation by geosteered drilling. Alternately, an opening may be formed into the formation by sonic drilling.

[0027] One or more heat sources may be disposed within the opening such that the heat sources transfer heat to the formation. For example, a heat source may be placed in an open wellbore in the formation. Heat may conductively and radiatively transfer from the heat source to the formation. Alternatively, a heat source may be placed within a heater well that may be packed with gravel, sand, and/or cement. The cement may be a refractory cement.

[0028] In some embodiments, one or more heat sources may be placed in a pattern within the formation. For example, in one embodiment, an in situ conversion process for hydrocarbons may include heating at least a portion of an oil shale formation with an array of heat sources disposed within the formation. In some embodiments, the array of heat sources can be positioned substantially equidistant from a production well. Certain patterns (e.g., triangular arrays, hexagonal arrays, or other array patterns) may be more desirable for specific applications. In addition, the array of heat sources may be disposed such that a distance between each heat source may be less than about 70 feet (21 m). In addition, the in situ conversion process for hydrocarbons may include heating at least a portion of the formation with heat sources disposed substantially parallel to a boundary of the hydrocarbons. Regardless of the arrangement of or distance between the heat sources, in certain embodiments, a ratio of heat sources to production wells disposed within a formation may be greater than about 3, 5, 8, 10, 20, or more.

[0029] Certain embodiments may also include allowing heat to transfer from one or more of the heat sources to a selected section of the heated portion. In an embodiment, the selected section may be disposed between one or more heat sources. For example, the in situ conversion process may also include allowing heat to transfer from one or more heat sources to a selected section of the formation such that heat from one or more of the heat sources pyrolyzes at least some hydrocarbons within the selected section. The in situ conversion process may include heating at least a portion of an oil shale formation above a pyrolyzation temperature of hydrocarbons in the formation. For example, a pyrolyzation temperature may include a temperature of at least about 270° C. Heat may be allowed to transfer from one or more of the heat sources to the selected section substantially by conduction.

[0030] One or more heat sources may be located within the formation such that superposition of heat produced from one or more heat sources may occur. Superposition of heat may increase a temperature of the selected section to a temperature sufficient for pyrolysis of at least some of the hydrocarbons within the selected section. Superposition of heat may vary depending on, for example, a spacing between heat sources. The spacing between heat sources may be selected to optimize heating of the section selected for treatment. Therefore, hydrocarbons may be pyrolyzed within a larger area of the portion. Spacing between heat sources may be selected to increase the effectiveness of the heat sources, thereby increasing the economic viability of a selected in situ conversion process for hydrocarbons. Superposition of heat tends to increase the uniformity of heat distribution in the section of the formation selected for treatment.

[0031] Various systems and methods may be used to provide heat sources. In an embodiment, a natural distributed combustor system and method may heat at least a portion of an oil shale formation. The system and method may first include heating a first portion of the formation to a temperature sufficient to support oxidation of at least some of the hydrocarbons therein. One or more conduits may be disposed within one or more openings. One or more of the conduits may provide an oxidizing fluid from an oxidizing fluid source into an opening in the formation. The oxidizing fluid may oxidize at least a portion of the hydrocarbons at a reaction zone within the formation. Oxidation may generate heat at the reaction zone. The generated heat may transfer from the reaction zone to a pyrolysis zone in the formation. The heat may transfer by conduction, radiation, and/or convection. A heated portion of the formation may include the reaction zone and the pyrolysis zone. The heated portion may also be located adjacent to the opening. One or more of the conduits may remove one or more oxidation products from the reaction zone and/or the opening in the formation. Alternatively, additional conduits may remove one or more oxidation products from the reaction zone and/or formation.

[0032] In certain embodiments, the flow of oxidizing fluid may be controlled along at least a portion of the length of the reaction zone. In some embodiments, hydrogen may be allowed to transfer into the reaction zone.

[0033] In an embodiment, a system and a method may include an opening in the formation extending from a first location on the surface of the earth to a second location on the surface of the earth. For example, the opening may be substantially U-shaped. Heat sources may be placed within the opening to provide heat to at least a portion of the formation.

[0034] A conduit may be positioned in the opening extending from the first location to the second location. In an embodiment, a heat source may be positioned proximate and/or in the conduit to provide heat to the conduit. Transfer of the heat through the conduit may provide heat to a selected section of the formation. In some embodiments, an additional heater may be placed in an additional conduit to provide heat to the selected section of the formation through the additional conduit.

[0035] In some embodiments, an annulus is formed between a wall of the opening and a wall of the conduit placed within the opening extending from the first location to the second location. A heat source may be place proximate and/or in the annulus to provide heat to a portion the opening. The provided heat may transfer through the annulus to a selected section of the formation.

[0036] In an embodiment, a system and method for heating an oil shale formation may include one or more insulated conductors disposed in one or more openings in the formation. The openings may be uncased. Alternatively, the openings may include a casing. As such, the insulated conductors may provide conductive, radiant, or convective heat to at least a portion of the formation. In addition, the system and method may allow heat to transfer from the insulated conductor to a section of the formation. In some embodiments, the insulated conductor may include a copper-nickel alloy. In some embodiments, the insulated conductor may be electrically coupled to two additional insulated conductors in a 3-phase Y configuration.

[0037] An embodiment of a system and method for heating an oil shale formation may include a conductor placed within a conduit (e.g., a conductor-in-conduit heat source). The conduit may be disposed within the opening. An electric current may be applied to the conductor to provide heat to a portion of the formation. The system may allow heat to transfer from the conductor to a section of the formation during use. In some embodiments, an oxidizing fluid source may be placed proximate an opening in the formation extending from the first location on the earth's surface to the second location on the earth's surface. The oxidizing fluid source may provide oxidizing fluid to a conduit in the opening. The oxidizing fluid may transfer from the conduit to a reaction zone in the formation. In an embodiment, an electrical current may be provided to the conduit to heat a portion of the conduit. The heat may transfer to the reaction zone in the oil shale formation. Oxidizing fluid may then be provided to the conduit. The oxidizing fluid may oxidize hydrocarbons in the reaction zone, thereby generating heat. The generated heat may transfer to a pyrolysis zone and the transferred heat may pyrolyze hydrocarbons within the pyrolysis zone.

[0038] In some embodiments, an insulation layer may be coupled to a portion of the conductor. The insulation layer may electrically insulate at least a portion of the conductor from the conduit during use.

[0039] In an embodiment, a conductor-in-conduit heat source having a desired length may be assembled. A conductor may be placed within the conduit to form the conductor-in-conduit heat source. Two or more conductor-in-conduit heat sources may be coupled together to form a heat source having the desired length. The conductors of the conductor-in-conduit heat sources may be electrically coupled together. In addition, the conduits may be electrically coupled together. A desired length of the conductor-in-conduit may be placed in an opening in the oil shale formation. In some embodiments, individual sections of the conductor-in-conduit heat source may be coupled using shielded active gas welding.

[0040] In some embodiments, a centralizer may be used to inhibit movement of the conductor within the conduit. A centralizer may be placed on the conductor as a heat source is made. In certain embodiments, a protrusion may be placed on the conductor to maintain the location of a centralizer.

[0041] In certain embodiments, a heat source of a desired length may be assembled proximate the oil shale formation. The assembled heat source may then be coiled. The heat source may be placed in the oil shale formation by uncoiling the heat source into the opening in the oil shale formation.

[0042] In certain embodiments, portions of the conductors may include an electrically conductive material. Use of the electrically conductive material on a portion (e.g., in the overburden portion) of the conductor may lower an electrical resistance of the conductor.

[0043] A conductor placed in a conduit may be treated to increase the emissivity of the conductor, in some embodiments. The emissivity of the conductor may be increased by roughening at least a portion of the surface of the conductor. In certain embodiments, the conductor may be treated to increase the emissivity prior to being placed within the conduit. In some embodiments, the conduit may be treated to increase the emissivity of the conduit.

[0044] In an embodiment, a system and method may include one or more elongated members disposed in an opening in the formation. Each of the elongated members may provide heat to at least a portion of the formation. One or more conduits may be disposed in the opening. One or more of the conduits may provide an oxidizing fluid from an oxidizing fluid source into the opening. In certain embodiments, the oxidizing fluid may inhibit carbon deposition on or proximate the elongated member.

[0045] In certain embodiments, an expansion mechanism may be coupled to a heat source. The expansion mechanism may allow the heat source to move during use. For example, the expansion mechanism may allow for the expansion of the heat source during use.

[0046] In one embodiment, an in situ method and system for heating an oil shale formation may include providing oxidizing fluid to a first oxidizer placed in an opening in the formation. Fuel may be provided to the first oxidizer and at least some fuel may be oxidized in the first oxidizer. Oxidizing fluid may be provided to a second oxidizer placed in the opening in the formation. Fuel may be provided to the second oxidizer and at least some fuel may be oxidized in the second oxidizer. Heat from oxidation of fuel may be allowed to transfer to a portion of the formation.

[0047] An opening in an oil shale formation may include a first elongated portion, a second elongated portion, and a third elongated portion. Certain embodiments of a method and system for heating an oil shale formation may include providing heat from a first heater placed in the second elongated portion. The second elongated portion may diverge from the first elongated portion in a first direction. The third elongated portion may diverge from the first elongated portion in a second direction. The first direction may be substantially different than the second direction. Heat may be provided from a second heater placed in the third elongated portion of the opening in the formation. Heat from the first heater and the second heater may be allowed to transfer to a portion of the formation.

[0048] An embodiment of a method and system for heating an oil shale formation may include providing oxidizing fluid to a first oxidizer placed in an opening in the formation. Fuel may be provided to the first oxidizer and at least some fuel may be oxidized in the first oxidizer. The method may further include allowing heat from oxidation of fuel to transfer to a portion of the formation and allowing heat to transfer from a heater placed in the opening to a portion of the formation.

[0049] In an embodiment, a system and method for heating an oil shale formation may include oxidizing a fuel fluid in a heater. The method may further include providing at least a portion of the oxidized fuel fluid into a conduit disposed in an opening in the formation. In addition, additional heat may be transferred from an electric heater disposed in the opening to the section of the formation. Heat may be allowed to transfer uniformly along a length of the opening.

[0050] Energy input costs may be reduced in some embodiments of systems and methods described above. For example, an energy input cost may be reduced by heating a portion of an oil shale formation by oxidation in combination with heating the portion of the formation by an electric heater. The electric heater may be turned down and/or off when the oxidation reaction begins to provide sufficient heat to the formation. Electrical energy costs associated with heating at least a portion of a formation with an electric heater may be reduced. Thus, a more economical process may be provided for heating an oil shale formation in comparison to heating by a conventional method. In addition, the oxidation reaction may be propagated slowly through a greater portion of the formation such that fewer heat sources may be required to heat such a greater portion in comparison to heating by a conventional method.

[0051] Certain embodiments as described herein may provide a lower cost system and method for heating an oil shale formation. For example, certain embodiments may more uniformly transfer heat along a length of a heater. Such a length of a heater may be greater than about 300 m or possibly greater than about 600 m. In addition, in certain embodiments, heat may be provided to the formation more efficiently by radiation. Furthermore, certain embodiments of systems may have a substantially longer lifetime than presently available systems.

[0052] In an embodiment, an in situ conversion system and method for hydrocarbons may include maintaining a portion of the formation in a substantially unheated condition. The portion may provide structural strength to the formation and/or confinement/isolation to certain regions of the formation. A processed oil shale formation may have alternating heated and substantially unheated portions arranged in a pattern that may, in some embodiments, resemble a checkerboard pattern, or a pattern of alternating areas (e.g., strips) of heated and unheated portions.

[0053] In an embodiment, a heat source may advantageously heat only along a selected portion or selected portions of a length of the heater. For example, a formation may include several hydrocarbon containing layers. One or more of the hydrocarbon containing layers may be separated by layers containing little or no hydrocarbons. A heat source may include several discrete high heating zones that may be separated by low heating zones. The high heating zones may be disposed proximate hydrocarbon containing layers such that the layers may be heated. The low heating zones may be disposed proximate layers containing little or no hydrocarbons such that the layers may not be substantially heated. For example, an electric heater may include one or more low resistance heater sections and one or more high resistance heater sections. Low resistance heater sections of the electric heater may be disposed in and/or proximate layers containing little or no hydrocarbons. In addition, high resistance heater sections of the electric heater may be disposed proximate hydrocarbon containing layers. In an additional example, a fueled heater (e.g., surface burner) may include insulated sections. Insulated sections of the fueled heater may be placed proximate or adjacent to layers containing little or no hydrocarbons. Alternately, a heater with distributed air and/or fuel may be configured such that little or no fuel may be combusted proximate or adjacent to layers containing little or no hydrocarbons. Such a fueled heater may include flameless combustors and natural distributed combustors.

[0054] In certain embodiments, the permeability of an oil shale formation may vary within the formation. For example, a first section may have a lower permeability than a second section. In an embodiment, heat may be provided to the formation to pyrolyze hydrocarbons within the lower permeability first section. Pyrolysis products may be produced from the higher permeability second section in a mixture of hydrocarbons.

[0055] In an embodiment, a heating rate of the formation may be slowly raised through the pyrolysis temperature range. For example, an in situ conversion process for hydrocarbons may include heating at least a portion of an oil shale formation to raise an average temperature of the portion above about 270° C. by a rate less than a selected amount (e.g., about 10° C., 5° C., 3° C., 1° C., 0.5° C., or 0.1° C.) per day. In a further embodiment, the portion may be heated such that an average temperature of the selected section may be less than about 375° C. or, in some embodiments, less than about 400° C.

[0056] In an embodiment, a temperature of the portion may be monitored through a test well disposed in a formation. For example, the test well may be positioned in a formation between a first heat source and a second heat source. Certain systems and methods may include controlling the heat from the first heat source and/or the second heat source to raise the monitored temperature at the test well at a rate of less than about a selected amount per day. In addition or alternatively, a temperature of the portion may be monitored at a production well. An in situ conversion process for hydrocarbons may include controlling the heat from the first heat source and/or the second heat source to raise the monitored temperature at the production well at a rate of less than a selected amount per day.

[0057] An embodiment of an in situ method of measuring a temperature within a wellbore may include providing a pressure wave from a pressure wave source into the wellbore. The wellbore may include a plurality of discontinuities along a length of the wellbore. The method further includes measuring a reflection signal of the pressure wave and using the reflection signal to assess at least one temperature between at least two discontinuities.

[0058] Certain embodiments may include heating a selected volume of an oil shale formation. Heat may be provided to the selected volume by providing power to one or more heat sources. Power may be defined as heating energy per day provided to the selected volume. A power (Pwr) required to generate a heating rate (h, in units of, for example, ° C./day) in a selected volume (V) of an oil shale formation may be determined by EQN. 1:
Pwr=h*V*C_v*ρ_B.  (1)

[0059] In this equation, an average heat capacity of the formation (C_v) and an average bulk density of the formation (ρ_B) may be estimated or determined using one or more samples taken from the oil shale formation.

[0060] Certain embodiments may include raising and maintaining a pressure in an oil shale formation. Pressure may be, for example, controlled within a range of about 2 bars absolute to about 20 bars absolute. For example, the process may include controlling a pressure within a majority of a selected section of a heated portion of the formation. The controlled pressure may be above about 2 bars absolute during pyrolysis. In an alternate embodiment, an in situ conversion process for hydrocarbons may include raising and maintaining the pressure in the formation within a range of about 20 bars absolute to about 36 bars absolute.

[0061] In an embodiment, compositions and properties of formation fluids produced by an in situ conversion process for hydrocarbons may vary depending on, for example, conditions within an oil shale formation.

[0062] Certain embodiments may include controlling the heat provided to at least a portion of the formation such that production of less desirable products in the portion may be inhibited. Controlling the heat provided to at least a portion of the formation may also increase the uniformity of permeability within the formation. For example, controlling the heating of the formation to inhibit production of less desirable products may, in some embodiments, include controlling the heating rate to less than a selected amount (e.g., 10° C., 5° C., 3° C., 1° C., 0.5° C., 0.1° C.) per day.

[0063] Controlling pressure, heat and/or heating rates of a selected section in a formation may increase production of selected formation fluids. For example, the amount and/or rate of heating may be controlled to produce formation fluids having an American Petroleum Institute (“API”) gravity greater than about 25°. Heat and/or pressure may be controlled to inhibit production of olefins in the produced fluids.

[0064] Controlling formation conditions to control the pressure of hydrogen in the produced fluid may result in improved qualities of the produced fluids. In some embodiments, it may be desirable to control formation conditions so that the partial pressure of hydrogen in a produced fluid is greater than about 0.5 bars absolute, as measured at a production well.

[0065] In one embodiment, a method of treating an oil shale formation in situ may include adding hydrogen to the selected section after a temperature of the selected section is at least about 270° C. Other embodiments may include controlling a temperature of the formation by selectively adding hydrogen to the formation.

[0066] In certain embodiments, an oil shale formation may be treated in situ with a heat transfer fluid such as steam. In an embodiment, a method of formation may include injecting a heat transfer fluid into a formation. Heat from the heat transfer fluid may transfer to a selected section of the formation. The heat from the heat transfer fluid may pyrolyze a substantial portion of the hydrocarbons within the selected section of the formation. The produced gas mixture may include hydrocarbons with an average API gravity greater than about 25°.

[0067] Furthermore, treating an oil shale formation with a heat transfer fluid may also mobilize hydrocarbons in the formation. In an embodiment, a method of treating a formation may include injecting a heat transfer fluid into a formation, allowing the heat from the heat transfer fluid to transfer to a selected first section of the formation, and mobilizing and pyrolyzing at least some of the hydrocarbons within the selected first section of the formation. At least some of the mobilized hydrocarbons may flow from the selected first section of the formation to a selected second section of the formation. The heat may pyrolyze at least some of the hydrocarbons within the selected second section of the formation. A gas mixture may be produced from the formation.

[0068] Another embodiment of treating a formation with a heat transfer fluid may include a moving heat transfer fluid front. A method may include injecting a heat transfer fluid into a formation and allowing the heat transfer fluid to migrate through the formation. A size of a selected section may increase as a heat transfer fluid front migrates through an untreated portion of the formation. The selected section is a portion of the formation treated by the heat transfer fluid. Heat from the heat transfer fluid may transfer heat to the selected section. The heat may pyrolyze at least some of the hydrocarbons within the selected section of the formation. The heat may also mobilize at least some of the hydrocarbons at the heat transfer fluid front. The mobilized hydrocarbons may flow substantially parallel to the heat transfer fluid front. The heat may pyrolyze at least a portion of the hydrocarbons in the mobilized fluid and a gas mixture may be produced from the formation.

[0069] Simulations may be utilized to increase an understanding of in situ processes. Simulations may model heating of the formation from heat sources and the transfer of heat to a selected section of the formation. Simulations may require the input of model parameters, properties of the formation, operating conditions, process characteristics, and/or desired parameters to determine operating conditions. Simulations may assess various aspects of an in situ process. For example, various aspects may include, but not be limited to, deformation characteristics, heating rates, temperatures within the formation, pressures, time to first produced fluids, and/or compositions of produced fluids.

[0070] Systems utilized in conducting simulations may include a central processing unit (CPU), a data memory, and a system memory. The system memory and the data memory may be coupled to the CPU. Computer programs executable to implement simulations may be stored on the system memory. Carrier mediums may include program instructions that are computer-executable to simulate the in situ processes.

[0071] In one embodiment, a computer-implemented method and system of treating an oil shale formation may include providing to a computational system at least one set of operating conditions of an in situ system being used to apply heat to a formation. The in situ system may include at least one heat source. The method may further include providing to the computational system at least one desired parameter for the in situ system. The computational system may be used to determine at least one additional operating condition of the formation to achieve the desired parameter.

[0072] In an embodiment, operating conditions may be determined by measuring at least one property of the formation. At least one measured property may be input into a computer executable program. At least one property of formation fluids selected to be produced from the formation may also be input into the computer executable program. The program may be operable to determine a set of operating conditions from at least the one or more measured properties. The program may also determine the set of operating conditions from at least one property of the selected formation fluids. The determined set of operating conditions may increase production of selected formation fluids from the formation.

[0073] In some embodiments, a property of the formation and an operating condition used in the in situ process may be provided to a computer system to model the in situ process to determine a process characteristic.

[0074] In an embodiment, a heat input rate for an in situ process from two or more heat sources may be simulated on a computer system. A desired parameter of the in situ process may be provided to the simulation. The heat input rate from the heat sources may be controlled to achieve the desired parameter.

[0075] Alternatively, a heat input property may be provided to a computer system to assess heat injection rate data using a simulation. In addition, a property of the formation may be provided to the computer system. The property and the heat injection rate data may be utilized by a second simulation to determine a process characteristic for the in situ process as a function of time.

[0076] Values for the model parameters may be adjusted using process characteristics from a series of simulations. The model parameters may be adjusted such that the simulated process characteristics correspond to process characteristics in situ. After the model parameters have been modified to correspond to the in situ process, a process characteristic or a set of process characteristics based on the modified model parameters may be determined. In certain embodiments, multiple simulations may be run such that the simulated process characteristics correspond to the process characteristics in situ.

[0077] In some embodiments, operating conditions may be supplied to a simulation to assess a process characteristic. Additionally, a desired value of a process characteristic for the in situ process may be provided to the simulation to assess an operating condition that yields the desired value.

[0078] In certain embodiments, databases in memory on a computer may be used to store relationships between model parameters, properties of the formation, operating conditions, process characteristics, desired parameters, etc. These databases may be accessed by the simulations to obtain inputs. For example, after desired values of process characteristics are provided to simulations, an operating condition may be assessed to achieve the desired values using these databases.

[0079] In some embodiments, computer systems may utilize inputs in a simulation to assess information about the in situ process. In some embodiments, the assessed information may be used to operate the in situ process. Alternatively, the assessed information and a desired parameter may be provided to a second simulation to obtain information. This obtained information may be used to operate the in situ process.

[0080] In an embodiment, a method of modeling may include simulating one or more stages of the in situ process. Operating conditions from the one or more stages may be provided to a simulation to assess a process characteristic of the one or more stages.

[0081] In an embodiment, operating conditions may be assessed by measuring at least one property of the formation. At least the measured properties may be input into a computer executable program. At least one property of formation fluids selected to be produced from the formation may also be input into the computer executable program. The program may be operable to assess a set of operating conditions from at least the one or more measured properties. The program may also determine the set of operating conditions from at least one property of the selected formation fluids. The assessed set of operating conditions may increase production of selected formation fluids from the formation.

[0082] In one embodiment, a method for controlling an in situ system of treating an oil shale formation may include monitoring at least one acoustic event within the formation using at least one acoustic detector placed within a wellbore in the formation. At least one acoustic event may be recorded with an acoustic monitoring system. The method may also include analyzing the at least one acoustic event to determine at least one property of the formation. The in situ system may be controlled based on the analysis of the at least one acoustic event.

[0083] An embodiment of a method of determining a heating rate for treating an oil shale formation in situ may include conducting an experiment at a relatively constant heating rate. The results of the experiment may be used to determine a heating rate for treating the formation in situ. The determined heating rate may be used to determine a well spacing in the formation.

[0084] In an embodiment, a method of predicting characteristics of a formation fluid may include determining an isothermal heating temperature that corresponds to a selected heating rate for the formation. The determined isothermal temperature may be used in an experiment to determine at least one product characteristic of the formation fluid produced from the formation for the selected heating rate. Certain embodiments may include altering a composition of formation fluids produced from an oil shale formation by altering a location of a production well with respect to a heater well. For example, a production well may be located with respect to a heater well such that a non-condensable gas fraction of produced hydrocarbon fluids may be larger than a condensable gas fraction of the produced hydrocarbon fluids.

[0085] Condensable hydrocarbons produced from the formation will typically include paraffins, cycloalkanes, mono-aromatics, and di-aromatics as major components. Such condensable hydrocarbons may also include other components such as tri-aromatics, etc.

[0086] In certain embodiments, a majority of the hydrocarbons in produced fluid may have a carbon number of less than approximately 25. Alternatively, less than about 15 weight % of the hydrocarbons in the fluid may have a carbon number greater than approximately 25. In other embodiments, fluid produced may have a weight ratio of hydrocarbons having carbon numbers from 2 through 4, to methane, of greater than approximately 1 (e.g., for oil shale). The non-condensable hydrocarbons may include, but are not limited to, hydrocarbons having carbon numbers less than 5.

[0087] In certain embodiments, the API gravity of the hydrocarbons in produced fluid may be approximately 25° or above (e.g., 30°, 40°, 50°, etc.). In certain embodiments, the hydrogen to carbon atomic ratio in produced fluid may be at least approximately 1.7 (e.g., 1.8, 1.9, etc.).

[0088] In certain embodiments, fluid produced from a formation may include oxygenated hydrocarbons. In an example, the condensable hydrocarbons may include an amount of oxygenated hydrocarbons greater than about 5 weight % of the condensable hydrocarbons.

[0089] Condensable hydrocarbons of a produced fluid may also include olefins. For example, the olefin content of the condensable hydrocarbons may be from about 0.1 weight % to about 15 weight %. Alternatively, the olefin content of the condensable hydrocarbons may be from about 0.1 weight % to about 2.5 weight % or, in some embodiments, less than about 5 weight %.

[0090] Non-condensable hydrocarbons of a produced fluid may also include olefins. For example, the olefin content of the non-condensable hydrocarbons may be gauged using the ethene/ethane molar ratio. In certain embodiments, the ethene/ethane molar ratio may range from about 0.001 to about 0.15.

[0091] Fluid produced from the formation may include aromatic compounds. For example, the condensable hydrocarbons may include an amount of aromatic compounds greater than about 20 weight % or about 25 weight % of the condensable hydrocarbons. The condensable hydrocarbons may also include relatively low amounts of compounds with more than two rings in them (e.g., tri-aromatics or above). For example, the condensable hydrocarbons may include less than about 1 weight %, 2 weight %, or about 5 weight % of tri-aromatics or above in the condensable hydrocarbons.

[0092] In particular, in certain embodiments, asphaltenes (i.e., large multi-ring aromatics that are substantially insoluble in hydrocarbons) make up less than about 0.1 weight % of the condensable hydrocarbons. For example, the condensable hydrocarbons may include an asphaltene component of from about 0.0 weight % to about 0.1 weight % or, in some embodiments, less than about 0.3 weight %.

[0093] Condensable hydrocarbons of a produced fluid may also include relatively large amounts of cycloalkanes. For example, the condensable hydrocarbons may include a cycloalkane component of up to 30 weight % (e.g., from about 5 weight % to about 30 weight %) of the condensable hydrocarbons.

[0094] In certain embodiments, the condensable hydrocarbons of the fluid produced from a formation may include compounds containing nitrogen. For example, less than about 1 weight % (when calculated on an elemental basis) of the condensable hydrocarbons is nitrogen (e.g., typically the nitrogen is in nitrogen containing compounds such as pyridines, amines, amides, etc.).

[0095] In certain embodiments, the condensable hydrocarbons of the fluid produced from a formation may include compounds containing oxygen. For example, in certain embodiments (e.g., for oil shale), less than about 1 weight % (when calculated on an elemental basis) of the condensable hydrocarbons is oxygen (e.g., typically the oxygen is in oxygen containing compounds such as phenols, substituted phenols, ketones, etc.). In some instances, certain compounds containing oxygen (e.g., phenols) may be valuable and, as such, may be economically separated from the produced fluid.

[0096] In certain embodiments, the condensable hydrocarbons of the fluid produced from a formation may include compounds containing sulfur. For example, less than about 1 weight % (when calculated on an elemental basis) of the condensable hydrocarbons is sulfur (e.g., typically the sulfur is in sulfur containing compounds such as thiophenes, mercaptans, etc.).

[0097] Furthermore, the fluid produced from the formation may include ammonia (typically the ammonia condenses with the water, if any, produced from the formation). For example, the fluid produced from the formation may in certain embodiments include about 0.05 weight % or more of ammonia. Certain formations may produce larger amounts of ammonia (e.g., up to about 10 weight % of the total fluid produced may be ammonia).

[0098] Furthermore, a produced fluid from the formation may also include molecular hydrogen (H₂), water, carbon dioxide, hydrogen sulfide, etc. For example, the fluid may include a H₂ content between about 10 volume % and about 80 volume % of the non-condensable hydrocarbons.

[0099] Certain embodiments may include heating to yield at least about 15 weight % of a total organic carbon content of at least some of the oil shale formation into formation fluids.

[0100] In an embodiment, an in situ conversion process for treating an oil shale formation may include providing heat to a section of the formation to yield greater than about 60 weight % of the potential hydrocarbon products and hydrogen, as measured by the Fischer Assay.

[0101] In certain embodiments, heating of the selected section of the formation may be controlled to pyrolyze at least about 20 weight % (or in some embodiments about 25 weight %) of the hydrocarbons within the selected section of the formation.

[0102] Formation fluids produced from a section of the formation may contain one or more components that may be separated from the formation fluids. In addition, conditions within the formation may be controlled to increase production of a desired component.

[0103] In certain embodiments, a method of converting pyrolysis fluids into olefins may include converting formation fluids into olefins. An embodiment may include separating olefins from fluids produced from a formation.

[0104] In an embodiment, a method of enhancing phenol production from an in situ oil shale formation may include controlling at least one condition within at least a portion of the formation to enhance production of phenols in formation fluid. In other embodiments, production of phenols from an oil shale formation may be controlled by converting at least a portion of formation fluid into phenols. Furthermore, phenols may be separated from fluids produced from an in situ oil shale formation.

[0105] An embodiment of a method of enhancing BTEX compounds (i.e., benzene, toluene, ethylbenzene, and xylene compounds) produced in situ in an oil shale formation may include controlling at least one condition within a portion of the formation to enhance production of BTEX compounds in formation fluid. In another embodiment, a method may include separating at least a portion of the BTEX compounds from the formation fluid. In addition, the BTEX compounds may be separated from the formation fluids after the formation fluids are produced. In other embodiments, at least a portion of the produced formation fluids may be converted into BTEX compounds.

[0106] In one embodiment, a method of enhancing naphthalene production from an in situ oil shale formation may include controlling at least one condition within at least a portion of the formation to enhance production of naphthalene in formation fluid. In another embodiment, naphthalene may be separated from produced formation fluids.

[0107] Certain embodiments of a method of enhancing anthracene production from an in situ oil shale formation may include controlling at least one condition within at least a portion of the formation to enhance production of anthracene in formation fluid. In an embodiment, anthracene may be separated from produced formation fluids.

[0108] In one embodiment, a method of separating ammonia from fluids produced from an in situ oil shale formation may include separating at least a portion of the ammonia from the produced fluid. Furthermore, an embodiment of a method of generating ammonia from fluids produced from a formation may include hydrotreating at least a portion of the produced fluids to generate ammonia.

[0109] In an embodiment, a method of enhancing pyridines production from an in situ oil shale formation may include controlling at least one condition within at least a portion of the formation to enhance production of pyridines in formation fluid. Additionally, pyridines may be separated from produced formation fluids.

[0110] In certain embodiments, a method of selecting an oil shale formation to be treated in situ such that production of pyridines is enhanced may include examining pyridines concentrations in a plurality of samples from oil shale formations. The method may further include selecting a formation for treatment at least partially based on the pyridines concentrations. Consequently, the production of pyridines to be produced from the formation may be enhanced.

[0111] In an embodiment, a method of enhancing pyrroles production from an in situ oil shale formation may include controlling at least one condition within at least a portion of the formation to enhance production of pyrroles in formation fluid. In addition, pyrroles may be separated from produced formation fluids.

[0112] In certain embodiments, an oil shale formation to be treated in situ may be selected such that production of pyrroles is enhanced. The method may include examining pyrroles concentrations in a plurality of samples from oil shale formations. The formation may be selected for treatment at least partially based on the pyrroles concentrations, thereby enhancing the production of pyrroles to be produced from such formation.

[0113] In one embodiment, thiophenes production from an in situ oil shale formation may be enhanced by controlling at least one condition within at least a portion of the formation to enhance production of thiophenes in formation fluid. Additionally, the thiophenes may be separated from produced formation fluids.

[0114] An embodiment of a method of selecting an oil shale formation to be treated in situ such that production of thiophenes is enhanced may include examining thiophenes concentrations in a plurality of samples from oil shale formations. The method may further include selecting a formation for treatment at least partially based on the thiophenes concentrations, thereby enhancing the production of thiophenes from such formations.

[0115] Certain embodiments may include providing a reducing agent to at least a portion of the formation. A reducing agent provided to a portion of the formation during heating may increase production of selected formation fluids. A reducing agent may include, but is not limited to, molecular hydrogen. For example, pyrolyzing at least some hydrocarbons in an oil shale formation may include forming hydrocarbon fragments. Such hydrocarbon fragments may react with each other and other compounds present in the formation. Reaction of these hydrocarbon fragments may increase production of olefin and aromatic compounds from the formation. Therefore, a reducing agent provided to the formation may react with hydrocarbon fragments to form selected products and/or inhibit the production of non-selected products.

[0116] In an embodiment, a hydrogenation reaction between a reducing agent provided to an oil shale formation and at least some of the hydrocarbons within the formation may generate heat. The generated heat may be allowed to transfer such that at least a portion of the formation may be heated. A reducing agent such as molecular hydrogen may also be autogenously generated within a portion of an oil shale formation during an in situ conversion process for hydrocarbons. The autogenously generated molecular hydrogen may hydrogenate formation fluids within the formation. Allowing formation waters to contact hot carbon in the spent formation may generate molecular hydrogen. Cracking an injected hydrocarbon fluid may also generate molecular hydrogen.

[0117] Certain embodiments may also include providing a fluid produced in a first portion of an oil shale formation to a second portion of the formation. A fluid produced in a first portion of an oil shale formation may be used to produce a reducing environment in a second portion of the formation. For example, molecular hydrogen generated in a first portion of a formation may be provided to a second portion of the formation. Alternatively, at least a portion of formation fluids produced from a first portion of the formation may be provided to a second portion of the formation to provide a reducing environment within the second portion.

[0118] In an embodiment, a method for hydrotreating a compound in a heated formation in situ may include controlling the H₂ partial pressure in a selected section of the formation, such that sufficient H₂ may be present in the selected section of the formation for hydrotreating. The method may further include providing a compound for hydrotreating to at least the selected section of the formation and producing a mixture from the formation that includes at least some of the hydrotreated compound.

[0119] Certain embodiments may include controlling heat provided to at least a portion of the formation such that a thermal conductivity of the portion may be increased to greater than about 0.5 W/(m ° C.) or, in some embodiments, greater than about 0.6 W/(m ° C.).

[0120] In certain embodiments, a mass of at least a portion of the formation may be reduced due, for example, to the production of formation fluids from the formation. As such, a permeability and porosity of at least a portion of the formation may increase. In addition, removing water during the heating may also increase the permeability and porosity of at least a portion of the formation.

[0121] Certain embodiments may include increasing a permeability of at least a portion of an oil shale formation to greater than about 0.01, 0.1, 1, 10, 20, or 50 darcy. In addition, certain embodiments may include substantially uniformly increasing a permeability of at least a portion of an oil shale formation. Some embodiments may include increasing a porosity of at least a portion of an oil shale formation substantially uniformly.

[0122] Hydrocarbon fluids produced from the formation may vary depending on conditions within the formation. For example, a heating rate of a selected pyrolyzation section may be controlled to increase the production of selected products. In addition, pressure within the formation may be controlled to vary the composition of the produced fluids.

[0123] In an embodiment, heat is provided from a first set of heat sources to a first section of an oil shale formation to pyrolyze a portion of the hydrocarbons in the first section. Heat may also be provided from a second set of heat sources to a second section of the formation. The heat may reduce the viscosity of hydrocarbons in the second section so that a portion of the hydrocarbons in the second section are able to move. A portion of the hydrocarbons from the second section may be induced to flow into the first section. A mixture of hydrocarbons may be produced from the formation. The produced mixture may include at least some pyrolyzed hydrocarbons.

[0124] In an embodiment, heat is provided from heat sources to a portion of an oil shale formation. The heat may transfer from the heat sources to a selected section of the formation to decrease a viscosity of hydrocarbons within the selected section. A gas may be provided to the selected section of the formation. The gas may displace hydrocarbons from the selected section towards a production well or production wells. A mixture of hydrocarbons may be produced from the selected section through the production well or production wells.

[0125] In some embodiments, energy supplied to a heat source or to a section of a heat source may be selectively limited to control temperature and to inhibit coke formation at or near the heat source. In some embodiments, a mixture of hydrocarbons may be produced through portions of a heat source that are operated to inhibit coke formation.

[0126] In certain embodiments, a quality of a produced mixture may be controlled by varying a location for producing the mixture. The location of production may be varied by varying the depth in the formation from which fluid is produced relative to an overburden or underburden. The location of production may also be varied by varying which production wells are used to produce fluid. In some embodiments, the production wells used to remove fluid may be chosen based on a distance of the production wells from activated heat sources.

[0127] In some embodiments, heat may be provided to a selected section of an oil shale formation to pyrolyze some hydrocarbons in a lower portion of the formation. A mixture of hydrocarbons may be produced from an upper portion of the formation. The mixture of hydrocarbons may include at least some pyrolyzed hydrocarbons from the lower portion of the formation.

[0128] In certain embodiments, a production rate of fluid from the formation may be controlled to adjust an average time that hydrocarbons are in, or flowing into, a pyrolysis zone or exposed to pyrolysis temperatures. Controlling the production rate may allow for production of a large quantity of hydrocarbons of a desired quality from the formation.

[0129] A heated formation may also be used to produce synthesis gas. Synthesis gas may be produced from the formation prior to or subsequent to producing a formation fluid from the formation. For example, synthesis gas generation may be commenced before and/or after formation fluid production decreases to an uneconomical level. Heat provided to pyrolyze hydrocarbons within the formation may also be used to generate synthesis gas. For example, if a portion of the formation is at a temperature from approximately 270° C. to approximately 375° C. (or 400° C. in some embodiments) after pyrolyzation, then less additional heat is generally required to heat such portion to a temperature sufficient to support synthesis gas generation.

[0130] In certain embodiments, synthesis gas is produced after production of pyrolysis fluids. For example, after pyrolysis of a portion of a formation, synthesis gas may be produced from carbon and/or hydrocarbons remaining within the formation. Pyrolysis of the portion may produce a relatively high, substantially uniform permeability throughout the portion. Such a relatively high, substantially uniform permeability may allow generation of synthesis gas from a significant portion of the formation at relatively low pressures. The portion may also have a large surface area and/or surface area/volume. The large surface area may allow synthesis gas producing reactions to be substantially at equilibrium conditions during synthesis gas generation. The relatively high, substantially uniform permeability may result in a relatively high recovery efficiency of synthesis gas, as compared to synthesis gas generation in an oil shale formation that has not been so treated.

[0131] Pyrolysis of at least some hydrocarbons may in some embodiments convert about 15 weight % or more of the carbon initially available. Synthesis gas generation may convert approximately up to an additional 80 weight % or more of carbon initially available within the portion. In situ production of synthesis gas from an oil shale formation may allow conversion of larger amounts of carbon initially available within the portion. The amount of conversion achieved may, in some embodiments, be limited by subsidence concerns.

[0132] Certain embodiments may include providing heat from one or more heat sources to heat the formation to a temperature sufficient to allow synthesis gas generation (e.g., in a range of approximately 400° C. to approximately 1200° C. or higher). At a lower end of the temperature range, generated synthesis gas may have a high hydrogen (H₂) to carbon monoxide (CO) ratio. At an upper end of the temperature range, generated synthesis gas may include mostly H₂ and CO in lower ratios (e.g., approximately a 1:1 ratio).

[0133] Heat sources for synthesis gas production may include any of the heat sources as described in any of the embodiments set forth herein. Alternatively, heating may include transferring heat from a heat transfer fluid (e.g., steam or combustion products from a burner) flowing within a plurality of wellbores within the formation.

[0134] A synthesis gas generating fluid (e.g., liquid water, steam, carbon dioxide, air, oxygen, hydrocarbons, and mixtures thereof) may be provided to the formation. For example, the synthesis gas generating fluid mixture may include steam and oxygen. In an embodiment, a synthesis gas generating fluid may include aqueous fluid produced by pyrolysis of at least some hydrocarbons within one or more other portions of the formation. Providing the synthesis gas generating fluid may alternatively include raising a water table of the formation to allow water to flow into it. Synthesis gas generating fluid may also be provided through at least one injection wellbore. The synthesis gas generating fluid will generally react with carbon in the formation to form H₂, water, methane, CO₂, and/or CO. A portion of the carbon dioxide may react with carbon in the formation to generate carbon monoxide. Hydrocarbons such as ethane may be added to a synthesis gas generating fluid. When introduced into the formation, the hydrocarbons may crack to form hydrogen and/or methane. The presence of methane in produced synthesis gas may increase the heating value of the produced synthesis gas.

[0135] Synthesis gas generation is, in some embodiments, an endothermic process. Additional heat may be added to the formation during synthesis gas generation to maintain a high temperature within the formation. The heat may be added from heater wells and/or from oxidizing carbon and/or hydrocarbons within the formation.

[0136] In an embodiment, an oxidant may be added to a synthesis gas generating fluid. The oxidant may include, but is not limited to, air, oxygen enriched air, oxygen, hydrogen peroxide, other oxidizing fluids, or combinations thereof. The oxidant may react with carbon within the formation to exothermically generate heat. Reaction of an oxidant with carbon in the formation may result in production of CO₂ and/or CO. Introduction of an oxidant to react with carbon in the formation may economically allow raising the formation temperature high enough to result in generation of significant quantities of H₂ and CO from hydrocarbons within the formation. Synthesis gas generation may be via a batch process or a continuous process.

[0137] Synthesis gas may be produced from the formation through one or more producer wells that include one or more heat sources. Such heat sources may operate to promote production of the synthesis gas with a desired composition.

[0138] Certain embodiments may include monitoring a composition of the produced synthesis gas and then controlling heating and/or controlling input of the synthesis gas generating fluid to maintain the composition of the produced synthesis gas within a desired range. For example, in some embodiments (e.g., such as when the synthesis gas will be used as a feedstock for a Fischer-Tropsch process), a desired composition of the produced synthesis gas may have a ratio of hydrogen to carbon monoxide of about 1.8:1 to 2.2:1 (e.g., about 2:1 or about 2.1:1). In some embodiments (such as when the synthesis gas will be used as a feedstock to make methanol), such ratio may be about 3:1 (e.g., about 2.8:1 to 3.2:1).

[0139] Certain embodiments may include blending a first synthesis gas with a second synthesis gas to produce synthesis gas of a desired composition. The first and the second synthesis gases may be produced from different portions of the formation.

[0140] Synthesis gases may be converted to heavier condensable hydrocarbons. For example, a Fischer-Tropsch hydrocarbon synthesis process may convert synthesis gas to branched and unbranched paraffins. Paraffins produced from the Fischer-Tropsch process may be used to produce other products such as diesel, jet fuel, and naphtha products. The produced synthesis gas may also be used in a catalytic methanation process to produce methane. Alternatively, the produced synthesis gas may be used for production of methanol, gasoline and diesel fuel, ammonia, and middle distillates. Produced synthesis gas may be used to heat the formation as a combustion fuel. Hydrogen in produced synthesis gas may be used to upgrade oil.

[0141] Synthesis gas may also be used for other purposes. Synthesis gas may be combusted as fuel. Synthesis gas may also be used for synthesizing a wide range of organic and/or inorganic compounds, such as hydrocarbons and ammonia. Synthesis gas may be used to generate electricity by combusting it as a fuel, by reducing the pressure of the synthesis gas in turbines, and/or using the temperature of the synthesis gas to make steam (and then run turbines). Synthesis gas may also be used in an energy generation unit such as a molten carbonate fuel cell, a solid oxide fuel cell, or other type of fuel cell.

[0142] Certain embodiments may include separating a fuel cell feed stream from fluids produced from pyrolysis of at least some of the hydrocarbons within a formation. The fuel cell feed stream may include H₂, hydrocarbons, and/or carbon monoxide. In addition, certain embodiments may include directing the fuel cell feed stream to a fuel cell to produce electricity. The electricity generated from the synthesis gas or the pyrolyzation fluids in the fuel cell may power electric heaters, which may heat at least a portion of the formation. Certain embodiments may include separating carbon dioxide from a fluid exiting the fuel cell. Carbon dioxide produced from a fuel cell or a formation may be used for a variety of purposes.

[0143] In certain embodiments, synthesis gas produced from a heated formation may be transferred to an additional area of the formation and stored within the additional area of the formation for a length of time. The conditions of the additional area of the formation may inhibit reaction of the synthesis gas. The synthesis gas may be produced from the additional area of the formation at a later time.

[0144] In some embodiments, treating a formation may include injecting fluids into the formation. The method may include providing heat to the formation, allowing the heat to transfer to a selected section of the formation, injecting a fluid into the selected section, and producing another fluid from the formation. Additional heat may be provided to at least a portion of the formation, and the additional heat may be allowed to transfer from at least the portion to the selected section of the formation. At least some hydrocarbons may be pyrolyzed within the selected section and a mixture may be produced from the formation. Another embodiment may include leaving a section of the formation proximate the selected section substantially unleached. The unleached section may inhibit the flow of water into the selected section.

[0145] In an embodiment, heat may be provided to the formation. The heat may be allowed to transfer to a selected section of the formation such that dissociation of carbonate minerals is inhibited. At least some hydrocarbons may be pyrolyzed within the selected section and a mixture produced from the formation. The method may further include reducing a temperature of the selected section and injecting a fluid into the selected section. Another fluid may be produced from the formation. Alternatively, subsequent to providing heat and allowing heat to transfer, a method may include injecting a fluid into the selected section and producing another fluid from the formation. Similarly, a method may include injecting a fluid into the selected section and pyrolyzing at least some hydrocarbons within the selected section of the formation after providing heat and allowing heat to transfer to the selected section.

[0146] In an embodiment that includes injecting fluids, a method of treating a formation may include providing heat from one or more heat sources and allowing the heat to transfer to a selected section of the formation such that a temperature of the selected section is less than about a temperature at which nahcolite dissociates. A fluid may be injected into the selected section and another fluid may be produced from the formation. The method may further include providing additional heat to the formation, allowing the additional heat to transfer to the selected section of the formation, and pyrolyzing at least some hydrocarbons within the selected section. A mixture may then be produced from the formation.

[0147] Certain embodiments that include injecting fluids may also include controlling the heating of the formation. A method may include providing heat to the formation, controlling the heat such that a selected section is at a first temperature, injecting a fluid into the selected section, and producing another fluid from the formation. The method may further include controlling the heat such that the selected section is at a second temperature that is greater than the first temperature. Heat may be allowed to transfer from the selected section, and at least some hydrocarbons may be pyrolyzed within the selected section of the formation. A mixture may be produced from the formation.

[0148] A further embodiment that includes injecting fluids may include providing heat to a formation, allowing the heat to transfer to a selected section of the formation, injecting a first fluid into the selected section, and producing a second fluid from the formation. The method may further include providing additional heat, allowing the additional heat to transfer to the selected section of the formation, pyrolyzing at least some hydrocarbons within the selected section of the formation, and producing a mixture from the formation. In addition, a temperature of the selected section may be reduced and a third fluid may be injected into the selected section. A fourth fluid may be produced from the formation.

[0149] In some embodiments, migration of fluids into and/or out of a treatment area may be inhibited. Inhibition of migration of fluids may occur before, during, and/or after an in situ treatment process. For example, migration of fluids may be inhibited while heat is provided from one or more heat sources to at least a portion of the treatment area. The heat may be allowed to transfer to at least a portion of the treatment area. Fluids may be produced from the treatment area.

[0150] Barriers may be used to inhibit migration of fluids into and/or out of a treatment area in a formation. Barriers may include, but are not limited to naturally occurring portions (e.g., overburden and/or underburden), frozen barrier zones, low temperature barrier zones, grout walls, sulfur wells, dewatering wells, and/or injection wells. Barriers may define the treatment area. Alternatively, barriers may be provided to a portion of the treatment area.

[0151] In an embodiment, a method of treating an oil shale formation in situ may include providing a refrigerant to a plurality of barrier wells to form a low temperature barrier zone. The method may further include establishing a low temperature barrier zone. In some embodiments, the temperature within the low temperature barrier zone may be lowered to inhibit the flow of water into or out of at least a portion of a treatment area in the formation.

[0152] Certain embodiments of treating an oil shale formation in situ may include providing a refrigerant to a plurality of barrier wells to form a frozen barrier zone. The frozen barrier zone may inhibit migration of fluids into and/or out of the treatment area. In certain embodiments, a portion of the treatment area is below a water table of the formation. In addition, the method may include controlling pressure to maintain a fluid pressure within the treatment area above a hydrostatic pressure of the formation and producing a mixture of fluids from the formation.

[0153] Barriers may be provided to a portion of the formation prior to, during, and after providing heat from one or more heat sources to the treatment area. For example, a barrier may be provided to a portion of the formation that has previously undergone a conversion process.

[0154] Fluid may be introduced to a portion of the formation that has previously undergone an in situ conversion process. The fluid may be produced from the formation in a mixture, which may contain additional fluids present in the formation. In some embodiments, the produced mixture may be provided to an energy producing unit.

[0155] In some embodiments, one or more conditions in a selected section may be controlled during an in situ conversion process to inhibit formation of carbon dioxide. Conditions may be controlled to produce fluids having a carbon dioxide emission level that is less than a selected carbon dioxide level. For example, heat provided to the formation may be controlled to inhibit generation of carbon dioxide, while increasing production of molecular hydrogen.

[0156] In a similar manner, a method for producing methane from an oil shale formation in situ while minimizing production of CO₂ may include controlling the heat from the one or more heat sources to enhance production of methane in the produced mixture and generating heat via at least one or more of the heat sources in a manner that minimizes CO₂ production. The methane may further include controlling a temperature proximate the production wellbore at or above a decomposition temperature of ethane.

[0157] In certain embodiments, a method for producing products from a heated formation may include controlling a condition within a selected section of the formation to produce a mixture having a carbon dioxide emission level below a selected baseline carbon dioxide emission level. In some embodiments, the mixture may be blended with a fluid to generate a product having a carbon dioxide emission level below the baseline.

[0158] In an embodiment, a method for producing methane from a heated formation in situ may include providing heat from one or more heat sources to at least one portion of the formation and allowing the heat to transfer to a selected section of the formation. The method may further include providing hydrocarbon compounds to at least the selected section of the formation and producing a mixture including methane from the hydrocarbons in the formation.

[0159] One embodiment of a method for producing hydrocarbons in a heated formation may include forming a temperature gradient in at least a portion of a selected section of the heated formation and providing a hydrocarbon mixture to at least the selected section of the formation. A mixture may then be produced from a production well.

[0160] In certain embodiments, a method for upgrading hydrocarbons in a heated formation may include providing hydrocarbons to a selected section of the heated formation and allowing the hydrocarbons to crack in the heated formation. The cracked hydrocarbons may be a higher grade than the provided hydrocarbons. The upgraded hydrocarbons may be produced from the formation.

[0161] Cooling a portion of the formation after an in situ conversion process may provide certain benefits, such as increasing the strength of the rock in the formation (thereby mitigating subsidence), increasing absorptive capacity of the formation, etc.

[0162] In an embodiment, a portion of a formation that has been pyrolyzed and/or subjected to synthesis gas generation may be allowed to cool or may be cooled to form a cooled, spent portion within the formation. For example, a heated portion of a formation may be allowed to cool by transference of heat to an adjacent portion of the formation. The transference of heat may occur naturally or may be forced by the introduction of heat transfer fluids through the heated portion and into a cooler portion of the formation.

[0163] In alternate embodiments, recovering thermal energy from a post treatment oil shale formation may include injecting a heat recovery fluid into a portion of the formation. Heat from the formation may transfer to the heat recovery fluid. The heat recovery fluid may be produced from the formation. For example, introducing water to a portion of the formation may cool the portion. Water introduced into the portion may be removed from the formation as steam. The removed steam or hot water may be injected into a hot portion of the formation to create synthesis gas

[0164] In an embodiment, hydrocarbons may be recovered from a post treatment oil shale formation by injecting a heat recovery fluid into a portion of the formation. Heat may vaporize at least some of the heat recovery fluid and at least some hydrocarbons in the formation. A portion of the vaporized recovery fluid and the vaporized hydrocarbons may be produced from the formation.

[0165] In certain embodiments, fluids in the formation may be removed from a post treatment oil shale formation by injecting a heat recovery fluid into a portion of the formation. Heat may transfer to the heat recovery fluid and a portion of the fluid may be produced from the formation. The heat recovery fluid produced from the formation may include at least some of the fluids in the formation.

[0166] In one embodiment, a method of recovering excess heat from a heated formation may include providing a product stream to the heated formation, such that heat transfers from the heated formation to the product stream. The method may further include producing the product stream from the heated formation and directing the product stream to a processing unit. The heat of the product stream may then be transferred to the processing unit. In an alternate method for recovering excess heat from a heated formation, the heated product stream may be directed to another formation, such that heat transfers from the product stream to the other formation.

[0167] In one embodiment, a method of utilizing heat of a heated formation may include placing a conduit in the formation, such that conduit input may be located separately from conduit output. The conduit may be heated by the heated formation to produce a region of reaction in at least a portion of the conduit. The method may further include directing a material through the conduit to the region of reaction. The material may undergo change in the region of reaction. A product may be produced from the conduit.

[0168] An embodiment of a method of utilizing heat of a heated formation may include providing heat from one or more heat sources to at least one portion of the formation and allowing the heat to transfer to a region of reaction in the formation. Material may be directed to the region of reaction and allowed to react in the region of reaction. A mixture may then be produced from the formation.

[0169] In an embodiment, a portion of an oil shale formation may be used to store and/or sequester materials (e.g., formation fluids, carbon dioxide). The conditions within the portion of the formation may inhibit reactions of the materials. Materials may be stored in the portion for a length of time. In addition, materials may be produced from the portion at a later time. Materials stored within the portion may have been previously produced from the portion of the formation, and/or another portion of the formation.

[0170] After an in situ conversion process has been completed in a portion of the formation, fluid may be sequestered within the formation. In some embodiments, to store a significant amount of fluid within the formation, a temperature of the formation will often need to be less than about 100° C. Water may be introduced into at least a portion of the formation to generate steam and reduce a temperature of the formation. The steam may be removed from the formation. The steam may be utilized for various purposes, including, but not limited to, heating another portion of the formation, generating synthesis gas in an adjacent portion of the formation, generating electricity, and/or as a steam flood in a oil reservoir. After the formation has cooled, fluid (e.g., carbon dioxide) may be pressurized and sequestered in the formation. Sequestering fluid within the formation may result in a significant reduction or elimination of fluid that is released to the environment due to operation of the in situ conversion process.

[0171] In alternate embodiments, carbon dioxide may be injected under pressure into the portion of the formation. The injected carbon dioxide may adsorb onto hydrocarbons in the formation and/or reside in void spaces such as pores in the formation. The carbon dioxide may be generated during pyrolysis, synthesis gas generation, and/or extraction of useful energy. In some embodiments, carbon dioxide may be stored in relatively deep oil shale formations and used to desorb methane.

[0172] In one embodiment, a method for sequestering carbon dioxide in a heated formation may include precipitating carbonate compounds from carbon dioxide provided to a portion of the formation. In some embodiments, the portion may have previously undergone an in situ conversion process. Carbon dioxide and a fluid may be provided to the portion of the formation. The fluid may combine with carbon dioxide in the portion to precipitate carbonate compounds.

[0173] In an alternate embodiment, methane may be recovered from an oil shale formation by providing heat to the formation. The heat may desorb a substantial portion of the methane within the selected section of the formation. At least a portion of the methane may be produced from the formation.

[0174] In an embodiment, a method for purifying water in a spent formation may include providing water to the formation and filtering the provided water in the formation. The filtered water may then be produced from the formation.

[0175] In an embodiment, treating an oil shale formation in situ may include injecting a recovery fluid into the formation. Heat may be provided from one or more heat sources to the formation. The heat may transfer from one or more of the heat sources to a selected section of the formation and vaporize a substantial portion of recovery fluid in at least a portion of the selected section. The heat from the heat sources and the vaporized recovery fluid may pyrolyze at least some hydrocarbons within the selected section. A gas mixture may be produced from the formation. The produced gas mixture may include hydrocarbons with an average API gravity greater than about 25°.

[0176] In certain embodiments, a method of shutting-in an in situ treatment process in an oil shale formation may include terminating heating from one or more heat sources providing heat to a portion of the formation. A pressure may be monitored and controlled in at least a portion of the formation. The pressure may be maintained approximately below a fracturing or breakthrough pressure of the formation.

[0177] One embodiment of a method of shutting-in an in situ treatment process in an oil shale formation may include terminating heating from one or more heat sources providing heat to a portion of the formation. Hydrocarbon vapor may be produced from the formation. At least a portion of the produced hydrocarbon vapor may be injected into a portion of a storage formation. The hydrocarbon vapor may be injected into a relatively high temperature formation. A substantial portion of injected hydrocarbons may be converted to coke and H₂ in the relatively high temperature formation. Alternatively, the hydrocarbon vapor may be stored in a depleted formation.

[0468] 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.

[0469] The following description generally relates to systems and methods for treating an oil shale formation. Such formations may be treated to yield relatively high quality hydrocarbon products, hydrogen, and other products.

[0470] “Hydrocarbons” are organic material with molecular structures containing carbon and hydrogen. 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 within or adjacent to mineral matrices within 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 (e.g., hydrogen (“H₂”), nitrogen (“N₂”), carbon monoxide, carbon dioxide, hydrogen sulfide, water, and ammonia).

[0471] A “formation” includes one or more hydrocarbon containing layers, one or more non-hydrocarbon layers, an overburden, and/or an underburden. An “overburden” and/or an “underburden” includes one or more different types of impermeable materials. For example, overburden and/or underburden may include rock, shale, mudstone, or wet/tight carbonate (i.e., an impermeable carbonate without hydrocarbons). In some embodiments of in situ conversion processes, an overburden and/or an underburden may include a hydrocarbon containing layer or hydrocarbon containing layers that are relatively impermeable and are not subjected to temperatures during in situ conversion processing that results in significant characteristic changes of the hydrocarbon containing layers of the overburden and/or underburden. For example, an underburden may contain shale or mudstone. In some cases, the overburden and/or underburden may be somewhat permeable.

[0472] “Kerogen” is a solid, insoluble hydrocarbon that has been converted by natural degradation (e.g., by diagenesis) and that principally contains carbon, hydrogen, nitrogen, oxygen, and sulfur. Oil shale contains kerogens. “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.

[0473] The terms “formation fluids” and “produced fluids” refer to fluids removed from an oil shale formation and may include pyrolyzation fluid, synthesis gas, mobilized hydrocarbon, and water (steam). The term “mobilized fluid” refers to fluids within the formation that are able to flow because of thermal treatment of the formation. Formation fluids may include hydrocarbon fluids as well as non-hydrocarbon fluids.

[0474] “Carbon number” refers to a number of carbon atoms within a molecule. A hydrocarbon fluid may include various hydrocarbons having varying numbers of carbon atoms. 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.

[0475] 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 within a conduit, as described in embodiments herein. A heat source may also include heat sources that generate heat by burning a fuel external to or within a formation, such as surface burners, downhole gas burners, flameless distributed combustors, and natural distributed combustors, as described in embodiments herein. In addition, it is envisioned that 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 media 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 (e.g., chemical reactions, solar energy, wind energy, biomass, or other sources of renewable energy). A chemical reaction may include an exothermic reaction (e.g., an oxidation reaction). A heat source may also include a heater that may provide heat to a zone proximate and/or surrounding a heating location such as a heater well.

[0476] A “heater” is any system for generating heat in a well or a near wellbore region. Heaters may be, but are not limited to, electric heaters, burners, combustors (e.g., natural distributed combustors) that react with material in or produced from a formation, and/or combinations thereof. A “unit of heat sources” refers to a number of heat sources that form a template that is repeated to create a pattern of heat sources within a formation.

[0477] 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 other cross-sectional shapes (e.g., circles, ovals, squares, rectangles, triangles, slits, or other regular or irregular shapes). As used herein, the terms “well” and “opening,” when referring to an opening in the formation may be used interchangeably with the term “wellbore.”

[0478] “Natural distributed combustor” refers to a heater that uses an oxidant to oxidize at least a portion of the carbon in the formation to generate heat, and wherein the oxidation takes place in a vicinity proximate a wellbore. Most of the combustion products produced in the natural distributed combustor are removed through the wellbore.

[0479] “Orifices” refer to openings (e.g., 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.

[0480] “Reaction zone” refers to a volume of an oil shale formation that is subjected to a chemical reaction such as an oxidation reaction.

[0481] “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. The term “self-controls” refers to controlling an output of a heater without external control of any type.

[0482] “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.

[0483] “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 that is reacted or reacting to form a pyrolyzation fluid.

[0484] “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.

[0485] “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.

[0486] “Fingering” refers to injected fluids bypassing portions of a formation because of variations in transport characteristics of the formation (e.g., permeability or porosity).

[0487] “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.

[0488] “Fluid pressure” is a pressure generated by a fluid within a formation. “Lithostatic pressure” (sometimes referred to as “lithostatic stress”) is a pressure within a formation equal to a weight per unit area of an overlying rock mass. “Hydrostatic pressure” is a pressure within a formation exerted by a column of water.

[0489] “Condensable hydrocarbons” are hydrocarbons that condense at 25° C. at 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.

[0490] “Olefins” are molecules that include unsaturated hydrocarbons having one or more non-aromatic carbon-to-carbon double bonds.

[0491] “Urea” describes a compound represented by the molecular formula of NH₂—CO—NH₂. Urea may be used as a fertilizer.

[0492] “Synthesis gas” is a mixture including hydrogen and carbon monoxide used for synthesizing a wide range of compounds. 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.

[0493] “Reforming” is a reaction of hydrocarbons (such as methane or naphtha) with steam to produce CO and H₂ as major products. Generally, it is conducted in the presence of a catalyst, although it can be performed thermally without the presence of a catalyst.

[0494] “Sequestration” refers to storing a gas that is a by-product of a process rather than venting the gas to the atmosphere.

[0495] “Dipping” refers to a formation that slopes downward or inclines from a plane parallel to the earth's surface, assuming the plane is flat (i.e., a “horizontal” plane). A “dip” is an angle that a stratum or similar feature makes with a horizontal plane. A “steeply dipping” oil shale formation refers to an oil shale formation lying at an angle of at least 20° from a horizontal plane. “Down dip” refers to downward along a direction parallel to a dip in a formation. “Up dip” refers to upward along a direction parallel to a dip of a formation. “Strike” refers to the course or bearing of hydrocarbon material that is normal to the direction of dip.

[0496] “Subsidence” is a downward movement of a portion of a formation relative to an initial elevation of the surface.

[0497] “Thickness” of a layer refers to the thickness of a cross section of a layer, wherein the cross section is normal to a face of the layer.

[0498] “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.

[0499] A “surface unit” is an ex situ treatment unit.

[0500] “Middle distillates” refers to hydrocarbon mixtures with a boiling point range that corresponds substantially with that of kerosene and gas oil fractions obtained in a conventional atmospheric distillation of crude oil material. The middle distillate boiling point range may include temperatures between about 150° C. and about 360° C., with a fraction boiling point between about 200° C. and about 360° C. Middle distillates may be referred to as gas oil.

[0501] A “boiling point cut” is a hydrocarbon liquid fraction that may be separated from hydrocarbon liquids when the hydrocarbon liquids are heated to a boiling point range of the fraction.

[0502] “Selected mobilized section” refers to a section of a formation that is at an average temperature within a mobilization temperature range. “Selected pyrolyzation section” refers to a section of a formation that is at an average temperature within a pyrolyzation temperature range.

[0503] “Enriched air” refers to air having a larger mole fraction of oxygen than air in the atmosphere. Enrichment of air is typically done to increase its combustion-supporting ability.

[0504] “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 also include aromatics or other complex ring hydrocarbons.

[0505] “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°.

[0506] “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.

[0507] “Off peak” times refers to times of operation when utility energy is less commonly used and, therefore, less expensive.

[0508] “Thermal fracture” refers to fractures created in a formation caused by expansion or contraction of a formation and/or fluids within the formation, which is in turn caused by increasing/decreasing the temperature of the formation and/or fluids within the formation, and/or by increasing/decreasing a pressure of fluids within the formation due to heating.

[0509] “Vertical hydraulic fracture” refers to a fracture at least partially propagated along a vertical plane in a formation, wherein the fracture is created through injection of fluids into a formation.

[0510] Hydrocarbons in formations may be treated in various ways to produce many different products. In certain embodiments, such formations may be treated in stages. FIG. 1 illustrates several stages of heating an oil shale formation. FIG. 1 also depicts an example of yield (barrels of oil equivalent per ton) (y axis) of formation fluids from an oil shale formation versus temperature (° C.) (x axis) of the formation.

[0511] Desorption of methane and vaporization of water occurs during stage 1 heating. Heating of the formation through stage 1 may be performed as quickly as possible. For example, when an oil shale formation is initially heated, hydrocarbons in the formation may desorb adsorbed methane. The desorbed methane may be produced from the formation. If the oil shale formation is heated further, water within the oil shale formation may be vaporized. Water may occupy, in some oil shale formations, between about 10% to about 50% of the pore volume in the formation. In other formations, water may occupy larger or smaller portions of the pore volume. Water typically is vaporized in a formation between about 160° C. and about 285° C. for pressures of about 6 bars absolute to 70 bars absolute. In some embodiments, the vaporized water may produce wettability changes in the formation and/or increase formation pressure. The wettability changes and/or increased pressure may affect pyrolysis reactions or other reactions in the formation. In certain embodiments, the vaporized water may be produced from the formation. In other embodiments, the vaporized water may be used for steam extraction and/or distillation in the formation or outside the formation. Removing the water from and increasing the pore volume in the formation may increase the storage space for hydrocarbons within the pore volume.

[0512] After stage 1 heating, the formation may be heated further, such that a temperature within the formation reaches (at least) an initial pyrolyzation temperature (e.g., a temperature at the lower end of the temperature range shown as stage 2). Hydrocarbons within the formation may be pyrolyzed throughout stage 2. A pyrolysis temperature range may vary depending on types of hydrocarbons within the formation. A pyrolysis temperature range may include temperatures between about 250° C. and about 900° C. A pyrolysis temperature range for producing desired products may extend through only a portion of the total pyrolysis temperature range. In some embodiments, a pyrolysis temperature range for producing desired products may include temperatures between about 250° C. to about 400° C. If a temperature of hydrocarbons in a formation is slowly raised through a temperature range from about 250° C. to about 400° C., production of pyrolysis products may be substantially complete when the temperature approaches 400° C. Heating the oil shale formation with a plurality of heat sources may establish thermal gradients around the heat sources that slowly raise the temperature of hydrocarbons in the formation through a pyrolysis temperature range.

[0513] In some in situ conversion embodiments, a temperature of the hydrocarbons to be subjected to pyrolysis may not be slowly increased throughout a temperature range from about 250° C. to about 400° C. The hydrocarbons in the formation may be heated to a desired temperature (e.g., about 325° C.). Other temperatures may be selected as the desired temperature. Superposition of heat from heat sources may allow 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 the desired temperature. The hydrocarbons may be maintained substantially at the desired temperature until pyrolysis declines such that production of desired formation fluids from the formation becomes uneconomical.

[0514] Formation fluids including pyrolyzation fluids may be produced from the formation. The pyrolyzation fluids may include, but are not limited to, hydrocarbons, hydrogen, carbon dioxide, carbon monoxide, hydrogen sulfide, ammonia, nitrogen, water, and mixtures thereof. As the temperature of the formation increases, the amount of condensable hydrocarbons in the produced formation fluid tends to decrease. At high temperatures, the formation may produce mostly methane and/or hydrogen. If an oil shale formation is heated throughout an entire pyrolysis range, the formation may produce only small amounts of hydrogen towards an upper limit of the pyrolysis range. After all of the available hydrogen is depleted, a minimal amount of fluid production from the formation will typically occur.

[0515] After pyrolysis of hydrocarbons, a large amount of carbon and some hydrogen may still be present in the formation. A significant portion of remaining carbon in the formation can be produced from the formation in the form of synthesis gas. Synthesis gas generation may take place during stage 3 heating depicted in FIG. 1. Stage 3 may include heating an oil shale formation to a temperature sufficient to allow synthesis gas generation. For example, synthesis gas may be produced within a temperature range from about 400° C. to about 1200° C. The temperature of the formation when the synthesis gas generating fluid is introduced to the formation may determine the composition of synthesis gas produced within the formation. If a synthesis gas generating fluid is introduced into a formation at a temperature sufficient to allow synthesis gas generation, synthesis gas may be generated within the formation. The generated synthesis gas may be removed from the formation through a production well or production wells. A large volume of synthesis gas may be produced during generation of synthesis gas.

[0516] Total energy content of fluids produced from an oil shale formation may stay relatively constant throughout pyrolysis and synthesis gas generation. During pyrolysis at relatively low formation temperatures, a significant portion of the produced fluid may be condensable hydrocarbons that have a high energy content. At higher pyrolysis temperatures, however, less of the formation fluid may include condensable hydrocarbons. More non-condensable formation fluids may be produced from the formation. Energy content per unit volume of the produced fluid may decline slightly during generation of predominantly non-condensable formation fluids. During synthesis gas generation, energy content per unit volume of produced synthesis gas declines significantly compared to energy content of pyrolyzation fluid. The volume of the produced synthesis gas, however, will in many instances increase substantially, thereby compensating for the decreased energy content.

[0517] FIG. 2 depicts a van Krevelen diagram. The van Krevelen diagram is a plot of atomic hydrogen to carbon ratio (y axis) versus atomic oxygen to carbon ratio (x axis) for various types of kerogen. The van Krevelen diagram shows the maturation sequence for various types of kerogen that typically occurs over geologic time due to temperature, pressure, and biochemical degradation. The maturation sequence may be accelerated by heating in situ at a controlled rate and/or a controlled pressure.

[0518] A van Krevelen diagram may be useful for selecting a resource for practicing various embodiments. Treating a formation containing kerogen in region 5 may produce carbon dioxide, non-condensable hydrocarbons, hydrogen, and water, along with a relatively small amount of condensable hydrocarbons. Treating a formation containing kerogen in region 7 may produce condensable and non-condensable hydrocarbons, carbon dioxide, hydrogen, and water. Treating a formation containing kerogen in region 9 will in many instances produce methane and hydrogen. A formation containing kerogen in region 7 may be selected for treatment because treating region 7 kerogen may produce large quantities of valuable hydrocarbons, and low quantities of undesirable products such as carbon dioxide and water. A region 7 kerogen may produce large quantities of valuable hydrocarbons and low quantities of undesirable products because the region 7 kerogen has already undergone dehydration and/or decarboxylation over geological time. In addition, region 7 kerogen can be further treated to make other useful products (e.g., methane, hydrogen, and/or synthesis gas) as the kerogen transforms to region 9 kerogen.

[0519] If a formation containing kerogen in region 5 or region 7 is selected for in situ conversion, in situ thermal treatment may accelerate maturation of the kerogen along paths represented by arrows in FIG. 2. For example, region 5 kerogen may transform to region 7 kerogen and possibly then to region 9 kerogen. Region 7 kerogen may transform to region 9 kerogen. In situ conversion may expedite maturation of kerogen and allow production of valuable products from the kerogen.

[0520] If region 5 kerogen is treated, a substantial amount of carbon dioxide may be produced due to decarboxylation of hydrocarbons in the formation. In addition to carbon dioxide, region 5 kerogen may produce some hydrocarbons (e.g., methane). Treating region 5 kerogen may produce substantial amounts of water due to dehydration of kerogen in the formation. Production of water from kerogen may leave hydrocarbons remaining in the formation enriched in carbon. Oxygen content of the hydrocarbons may decrease faster than hydrogen content of the hydrocarbons during production of such water and carbon dioxide from the formation. Therefore, production of such water and carbon dioxide from region 5 kerogen may result in a larger decrease in the atomic oxygen to carbon ratio than a decrease in the atomic hydrogen to carbon ratio (see region 5 arrows in FIG. 2 which depict more horizontal than vertical movement).

[0521] If region 7 kerogen is treated, some of the hydrocarbons in the formation may be pyrolyzed to produce condensable and non-condensable hydrocarbons. For example, treating region 7 kerogen may result in production of oil from hydrocarbons, as well as some carbon dioxide and water. In situ conversion of region 7 kerogen may produce significantly less carbon dioxide and water than is produced during in situ conversion of region 5 kerogen. Therefore, the atomic hydrogen to carbon ratio of the kerogen may decrease rapidly as the kerogen in region 7 is treated. The atomic oxygen to carbon ratio of the region 7 kerogen may decrease much slower than the atomic hydrogen to carbon ratio of the region 7 kerogen.

[0522] Kerogen in region 9 may be treated to generate methane and hydrogen. For example, if such kerogen was previously treated (e.g., it was previously region 7 kerogen), then after pyrolysis longer hydrocarbon chains of the hydrocarbons may have cracked and been produced from the formation. Carbon and hydrogen, however, may still be present in the formation.

[0523] If kerogen in region 9 were heated to a synthesis gas generating temperature and a synthesis gas generating fluid (e.g., steam) were added to the region 9 kerogen, then at least a portion of remaining hydrocarbons in the formation may be produced from the formation in the form of synthesis gas. For region 9 kerogen, the atomic hydrogen to carbon ratio and the atomic oxygen to carbon ratio in the hydrocarbons may significantly decrease as the temperature rises. Hydrocarbons in the formation may be transformed into relatively pure carbon in region 9. Heating region 9 kerogen to still higher temperatures will tend to transform such kerogen into graphite 11.

[0524] An oil shale formation may have a number of properties that depend on a composition of the hydrocarbons within the formation. Such properties may affect the composition and amount of products that are produced from an oil shale formation during in situ conversion. Properties of an oil shale formation may be used to determine if and/or how an oil shale formation is to be subjected to in situ conversion.

[0525] Kerogen is composed of organic matter that has been transformed due to a maturation process. The maturation process for kerogen may include two stages: a biochemical stage and a geochemical stage. The biochemical stage typically involves degradation of organic material by aerobic and/or anaerobic organisms. The geochemical stage typically involves conversion of organic matter due to temperature changes and significant pressures. During maturation, oil and gas may be produced as the organic matter of the kerogen is transformed.

[0526] The van Krevelen diagram shown in FIG. 2 classifies various natural deposits of kerogen. For example, kerogen may be classified into four distinct groups: type I, type II, type III, and type IV, which are illustrated by the four branches of the van Krevelen diagram. The van Krevelen diagram shows the maturation sequence for kerogen that typically occurs over geological time due to temperature and pressure. Classification of kerogen type may depend upon precursor materials of the kerogen. The precursor materials transform over time into macerals. Macerals are microscopic structures that have different structures and properties depending on the precursor materials from which they are derived. Oil shale may be described as a kerogen type I or type II, and may primarily contain macerals from the liptinite group. Liptinites are derived from plants, specifically the lipid rich and resinous parts. The concentration of hydrogen within liptinite may be as high as 9 weight %. In addition, liptinite has a relatively high hydrogen to carbon ratio and a relatively low atomic oxygen to carbon ratio.

[0527] A type I kerogen may be classified as an alginite, since type I kerogen developed primarily from algal bodies. Type I kerogen may result from deposits made in lacustrine environments. Type II kerogen may develop from organic matter that was deposited in marine environments.

[0528] Type III kerogen may generally include vitrinite macerals. Vitrinite is derived from cell walls and/or woody tissues (e.g., stems, branches, leaves, and roots of plants). Type III kerogen may be present in most humic coals. Type III kerogen may develop from organic matter that was deposited in swamps. Type IV kerogen includes the inertinite maceral group. The inertinite maceral group is composed of plant material such as leaves, bark, and stems that have undergone oxidation during the early peat stages of burial diagenesis. Inertinite maceral is chemically similar to vitrinite, but has a high carbon and low hydrogen content.

[0529] The dashed lines in FIG. 2 correspond to vitrinite reflectance. Vitrinite reflectance is a measure of maturation. As kerogen undergoes maturation, the composition of the kerogen usually changes due to expulsion of volatile matter (e.g., carbon dioxide, methane, and oil) from the kerogen. Rank classifications of kerogen indicate the level to which kerogen has matured. For example, as kerogen undergoes maturation, the rank of kerogen increases. As rank increases, the volatile matter within, and producible from, the kerogen tends to decrease. In addition, the moisture content of kerogen generally decreases as the rank increases. At higher ranks, the moisture content may reach a relatively constant value. Higher rank kerogens that have undergone significant maturation tend to have a higher carbon content and a lower volatile matter content than lower rank kerogens such as lignite.

[0530] Oil shale formations may be selected for in situ conversion based on properties of at least a portion of the formation. For example, a formation may be selected based on richness, thickness, and/or depth (i.e., thickness of overburden) of the formation. In addition, the types of fluids producible from the formation may be a factor in the selection of a formation for in situ conversion. In certain embodiments, the quality of the fluids to be produced may be assessed in advance of treatment. Assessment of the products that may be produced from a formation may generate significant cost savings since only formations that will produce desired products need to be subjected to in situ conversion. Properties that may be used to assess hydrocarbons in a formation include, but are not limited to, an amount of hydrocarbon liquids that may be produced from the hydrocarbons, a likely API gravity of the produced hydrocarbon liquids, an amount of hydrocarbon gas producible from the formation, and/or an amount of carbon dioxide and water that in situ conversion will generate.

[0531] Another property that may be used to assess the quality of fluids produced from certain kerogen containing formations is vitrinite reflectance. Such formations include, but are not limited to, oil shale formations. Oil shale formations that include kerogen may be assessed/selected for treatment based on a vitrinite reflectance of the kerogen. Vitrinite reflectance is often related to a hydrogen to carbon atomic ratio of a kerogen and an oxygen to carbon atomic ratio of the kerogen, as shown by the dashed lines in FIG. 2. A van Krevelen diagram may be useful in selecting a resource for an in situ conversion process.

[0532] Vitrinite reflectance of a kerogen in an oil shale formation may indicate which fluids are producible from a formation upon heating. For example, a vitrinite reflectance of approximately 0.5% to approximately 1.5% may indicate that the kerogen will produce a large quantity of condensable fluids. In addition, a vitrinite reflectance of approximately 1.5% to 3.0% may indicate a kerogen in region 9 as described above. If an oil shale formation having such kerogen is heated, a significant amount (e.g., a majority) of the fluid produced by such heating may include methane and hydrogen. The formation may be used to generate synthesis gas if the temperature is raised sufficiently high and a synthesis gas generating fluid is introduced into the formation.

[0533] A kerogen containing formation to be subjected to in situ conversion may be chosen based on a vitrinite reflectance. The vitrinite reflectance of the kerogen may indicate that the formation will produce high quality fluids when subjected to in situ conversion. In some in situ conversion embodiments, a portion of the kerogen containing formation to be subjected to in situ conversion may have a vitrinite reflectance in a range between about 0.2% and about 3.0%. In some in situ conversion embodiments, a portion of the kerogen containing formation may have a vitrinite reflectance from about 0.5% to about 2.0%. In some in situ conversion embodiments, a portion of the kerogen containing formation may have a vitrinite reflectance from about 0.5% to about 1.0%.

[0534] In some in situ conversion embodiments, an oil shale formation may be selected for treatment based on a hydrogen content within the hydrocarbons in the formation. For example, a method of treating an oil shale formation may include selecting a portion of the oil shale formation for treatment having hydrocarbons with a hydrogen content greater than about 3 weight %, 3.5 weight %, or 4 weight % when measured on a dry, ash-free basis. In addition, a selected section of an oil shale formation may include hydrocarbons with an atomic hydrogen to carbon ratio that falls within a range from about 0.5 to about 2, and in many instances from about 0.70 to about 1.65.

[0535] Hydrogen content of an oil shale formation may significantly influence a composition of hydrocarbon fluids producible from the formation. Pyrolysis of hydrocarbons within heated portions of the formation may generate hydrocarbon fluids that include a double bond or a radical. Hydrogen within the formation may reduce the double bond to a single bond. Reaction of generated hydrocarbon fluids with each other and/or with additional components in the formation may be inhibited. For example, reduction of a double bond of the generated hydrocarbon fluids to a single bond may reduce polymerization of the generated hydrocarbons. Such polymerization may reduce the amount of fluids produced and may reduce the quality of fluid produced from the formation.

[0536] Hydrogen within the formation may neutralize radicals in the generated hydrocarbon fluids. Hydrogen present in the formation may inhibit reaction of hydrocarbon fragments by transforming the hydrocarbon fragments into relatively short chain hydrocarbon fluids. The hydrocarbon fluids may enter a vapor phase. Vapor phase hydrocarbons may move relatively easily through the formation to production wells. Increase in the hydrocarbon fluids in the vapor phase may significantly reduce a potential for producing less desirable products within the selected section of the formation.

[0537] A lack of bound and free hydrogen in the formation may negatively affect the amount and quality of fluids that can be produced from the formation. If too little hydrogen is naturally present, then hydrogen or other reducing fluids may be added to the formation.

[0538] When heating a portion of an oil shale formation, oxygen within the portion may form carbon dioxide. A formation may be chosen and/or conditions in a formation may be adjusted to inhibit production of carbon dioxide and other oxides. In an embodiment, production of carbon dioxide may be reduced by selecting and treating a portion of an oil shale formation having a vitrinite reflectance of greater than about 0.5%.

[0539] An amount of carbon dioxide that can be produced from a kerogen containing formation may be dependent on an oxygen content initially present in the formation and/or an atomic oxygen to carbon ratio of the kerogen. In some in situ conversion embodiments, formations to be subjected to in situ conversion may include kerogen with an atomic oxygen weight percentage of less than about 20 weight %, 15 weight %, and/or 10 weight %. In some in situ conversion embodiments, formations to be subjected to in situ conversion may include kerogen with an atomic oxygen to carbon ratio of less than about 0.15. In some in situ conversion embodiments, a formation selected for treatment may have an atomic oxygen to carbon ratio of about 0.03 to about 0.12.

[0540] Heating an oil shale formation may include providing a large amount of energy to heat sources located within the formation. Oil shale formations may also contain some water. A significant portion of energy initially provided to a formation may be used to heat water within the formation. An initial rate of temperature increase may be reduced by the presence of water in the formation. Excessive amounts of heat and/or time may be required to heat a formation having a high moisture content to a temperature sufficient to pyrolyze hydrocarbons in the formation. In certain embodiments, water may be inhibited from flowing into a formation subjected to in situ conversion. A formation to be subjected to in situ conversion may have a low initial moisture content. The formation may have an initial moisture content that is less than about 15 weight %. Some formations that are to be subjected to in situ conversion may have an initial moisture content of less than about 10 weight %. Other formations that are to be processed using an in situ conversion process may have initial moisture contents that are greater than about 15 weight %. Formations with initial moisture contents above about 15 weight % may incur significant energy costs to remove the water that is initially present in the formation during heating to pyrolysis temperatures.

[0541] An oil shale formation may be selected for treatment based on additional factors such as, but not limited to, thickness of hydrocarbon containing layers within the formation, assessed liquid production content, location of the formation, and depth of hydrocarbon containing layers. An oil shale formation may include multiple layers. Such layers may include hydrocarbon containing layers, as well as layers that are hydrocarbon free or have relatively low amounts of hydrocarbons. Conditions during formation may determine the thickness of hydrocarbon and non-hydrocarbon layers in an oil shale formation. An oil shale formation to be subjected to in situ conversion will typically include at least one hydrocarbon containing layer having a thickness sufficient for economical production of formation fluids. Richness of a hydrocarbon containing layer may be a factor used to determine if a formation will be treated by in situ conversion. A thin and rich hydrocarbon layer may be able to produce significantly more valuable hydrocarbons than a much thicker, less rich hydrocarbon layer. Producing hydrocarbons from a formation that is both thick and rich is desirable.

[0542] Each hydrocarbon containing layer of a formation may have a potential formation fluid yield or richness. The richness of a hydrocarbon layer may vary in a hydrocarbon layer and between different hydrocarbon layers in a formation. Richness may depend on many factors including the conditions under which the hydrocarbon containing layer was formed, an amount of hydrocarbons in the layer, and/or a composition of hydrocarbons in the layer. Richness of a hydrocarbon layer may be estimated in various ways. For example, richness may be measured by a Fischer Assay. The Fischer Assay is a standard method which involves heating a sample of a hydrocarbon containing layer to approximately 500° C. in one hour, collecting products produced from the heated sample, and quantifying the amount of products produced. A sample of a hydrocarbon containing layer may be obtained from an oil shale formation by a method such as coring or any other sample retrieval method.

[0543] An in situ conversion process may be used to treat formations with hydrocarbon layers that have thicknesses greater than about 10 m. Thick formations may allow for placement of heat sources so that superposition of heat from the heat sources efficiently heats the formation to a desired temperature. Formations having hydrocarbon layers that are less than 10 m thick may also be treated using an in situ conversion process. In some in situ conversion embodiments of thin hydrocarbon layer formations, heat sources may be inserted in or adjacent to the hydrocarbon layer along a length of the hydrocarbon layer (e.g., with horizontal or directional drilling). Heat losses to layers above and below the thin hydrocarbon layer or thin hydrocarbon layers may be offset by an amount and/or quality of fluid produced from the formation.

[0544] FIG. 3 shows a schematic view of an embodiment of a portion of an in situ conversion system for treating an oil shale formation. Heat sources 100 may be placed within at least a portion of the oil shale formation. Heat sources 100 may include, for example, electric heaters such as insulated conductors, conductor-in-conduit heaters, surface burners, flameless distributed combustors, and/or natural distributed combustors. Heat sources 100 may also include other types of heaters. Heat sources 100 may provide heat to at least a portion of an oil shale formation. Energy may be supplied to the heat sources 100 through supply lines 102. The supply lines may be structurally different depending on the type of heat source or heat sources being used to heat the formation. Supply lines for heat sources may transmit electricity for electric heaters, may transport fuel for combustors, or may transport heat exchange fluid that is circulated within the formation.

[0545] Production wells 104 may be used to remove formation fluid from the formation. Formation fluid produced from production wells 104 may be transported through collection piping 106 to treatment facilities 108. Formation fluids may also be produced from heat sources 100. For example, fluid may be produced from heat sources 100 to control pressure within the formation adjacent to the heat sources. Fluid produced from heat sources 100 may be transported through tubing or piping to collection piping 106 or the produced fluid may be transported through tubing or piping directly to treatment facilities 108. Treatment facilities 108 may include separation units, reaction units, upgrading units, fuel cells, turbines, storage vessels, and other systems and units for processing produced formation fluids.

[0546] An in situ conversion system for treating hydrocarbons may include dewatering wells 110 (wells shown with reference number 110 may, in some embodiments, be capture, barrier, and/or isolation wells). Dewatering wells 110 or vacuum wells may remove liquid water and/or inhibit liquid water from entering a portion of an oil shale formation to be heated, or to a formation being heated. A plurality of water wells may surround all or a portion of a formation to be heated. In the embodiment depicted in FIG. 3, dewatering wells 110 are shown extending only along one side of heat sources 100, but dewatering wells typically encircle all heat sources 100 used, or to be used, to heat the formation.

[0547] Dewatering wells 110 may be placed in one or more rings surrounding selected portions of the formation. New dewatering wells may need to be installed as an area being treated by the in situ conversion process expands. An outermost row of dewatering wells may inhibit a significant amount of water from flowing into the portion of formation that is heated or to be heated. Water produced from the outermost row of dewatering wells should be substantially clean, and may require little or no treatment before being released. An innermost row of dewatering wells may inhibit water that bypasses the outermost row from flowing into the portion of formation that is heated or to be heated. The innermost row of dewatering wells may also inhibit outward migration of vapor from a heated portion of the formation into surrounding portions of the formation. Water produced by the innermost row of dewatering wells may include some hydrocarbons. The water may need to be treated before being released. Alternately, water with hydrocarbons may be stored and used to produce synthesis gas from a portion of the formation during a synthesis gas phase of the in situ conversion process. The dewatering wells may reduce heat loss to surrounding portions of the formation, may increase production of vapors from the heated portion, and/or may inhibit contamination of a water table proximate the heated portion of the formation.

[0548] In some embodiments, pressure differences between successive rows of dewatering wells may be minimized (e.g., maintained relatively low or near zero) to create a “no or low flow” boundary between rows.

[0549] In some in situ conversion process embodiments, a fluid may be injected in the innermost row of wells. The injected fluid may maintain a sufficient pressure around a pyrolysis zone to inhibit migration of fluid from the pyrolysis zone through the formation. The fluid may act as an isolation barrier between the outermost wells and the pyrolysis fluids. The fluid may improve the efficiency of the dewatering wells.

[0550] In certain embodiments, wells initially used for one purpose may be later used for one or more other purposes, thereby lowering project costs and/or decreasing the time required to perform certain tasks. For instance, production wells (and in some circumstances heater wells) may initially be used as dewatering wells (e.g., before heating is begun and/or when heating is initially started). In addition, in some circumstances dewatering wells can later be used as production wells (and in some circumstances heater wells). As such, the dewatering wells may be placed and/or designed so that such wells can be later used as production wells and/or heater wells. The heater wells may be placed and/or designed so that such wells can be later used as production wells and/or dewatering wells. The production wells may be placed and/or designed so that such wells can be later used as dewatering wells and/or heater wells. Similarly, injection wells may be wells that initially were used for other purposes (e.g., heating, production, dewatering, monitoring, etc.), and injection wells may later be used for other purposes. Similarly, monitoring wells may be wells that initially were used for other purposes (e.g., heating, production, dewatering, injection, etc.), and monitoring wells may later be used for other purposes.

[0551] Hydrocarbons to be subjected to in situ conversion may be located under a large area. The in situ conversion system may be used to treat small portions of the formation, and other sections of the formation may be treated as time progresses. In an embodiment of a system for treating a formation (e.g., an oil shale formation), a field layout for 24 years of development may be divided into 24 individual plots that represent individual drilling years. Each plot may include 120 “tiles” (repeating matrix patterns) wherein each plot is made of 6 rows by 20 columns of tiles. Each tile may include 1 production well and 12 or 18 heater wells. The heater wells may be placed in an equilateral triangle pattern with a well spacing of about 12 m. Production wells may be located in centers of equilateral triangles of heater wells, or the production wells may be located approximately at a midpoint between two adjacent heater wells.

[0552] In certain embodiments, heat sources will be placed within a heater well formed within an oil shale formation. The heater well may include an opening through an overburden of the formation. The heater may extend into or through at least one hydrocarbon containing section (or hydrocarbon containing layer) of the formation. As shown in FIG. 4, an embodiment of heater well 224 may include an opening in hydrocarbon layer 222 that has a helical or spiral shape. A spiral heater well may increase contact with the formation as opposed to a vertically positioned heater. A spiral heater well may provide expansion room that inhibits buckling or other modes of failure when the heater well is heated or cooled. In some embodiments, heater wells may include substantially straight sections through overburden 220. Use of a straight section of heater well through the overburden may decrease heat loss to the overburden and reduce the cost of the heater well.

[0553] As shown in FIG. 5, a heat source embodiment may be placed into heater well 224. Heater well 224 may be substantially “U” shaped. The legs of the “U” may be wider or more narrow depending on the particular heater well and formation characteristics. First portion 226 and third portion 228 of heater well 224 may be arranged substantially perpendicular to an upper surface of hydrocarbon layer 222 in some embodiments. In addition, the first and the third portion of the heater well may extend substantially vertically through overburden 220. Second portion 230 of heater well 224 may be substantially parallel to the upper surface of the hydrocarbon layer.

[0554] Multiple heat sources (e.g., 2, 3, 4, 5, 10 heat sources or more) may extend from a heater well in some situations. As shown in FIG. 6, heat sources 232, 234, and 236 extend through overburden 220 into hydrocarbon layer 222 from heater well 224. Multiple wells extending from a single wellbore may be used when surface considerations (e.g., aesthetics, surface land use concerns, and/or unfavorable soil conditions near the surface) make it desirable to concentrate well platforms in a small area. For example, in areas where the soil is frozen and/or marshy, it may be more cost-effective to have a minimal number of well platforms located at selected sites.

[0555] In certain embodiments, a first portion of a heater well may extend from the ground surface, through an overburden, and into an oil shale formation. A second portion of the heater well may include one or more heater wells in the oil shale formation. The one or more heater wells may be disposed within the oil shale formation at various angles. In some embodiments, at least one of the heater wells may be disposed substantially parallel to a boundary of the oil shale formation. In alternate embodiments, at least one of the heater wells may be substantially perpendicular to the oil shale formation. In addition, one of the one or more heater wells may be positioned at an angle between perpendicular and parallel to a layer in the formation.

[0556] FIG. 7 illustrates a schematic of view of multilateral or side tracked lateral heaters branched from a single well in an oil shale formation. In relatively thin and deep layers found in an oil shale formation, it may be advantageous to place more than one heater substantially horizontally within the relatively thin layer of hydrocarbons. For example, an oil shale layer may have a richness greater than about 0.06 L/kg and a relatively low initial thermal conductivity. Heat provided to a thin layer with a low thermal conductivity from a horizontal wellbore may be more effectively trapped within the thin layer and reduce heat losses from the layer. Substantially vertical opening 6108 may be placed in hydrocarbon layer 6100. Substantially vertical opening 6108 may be an elongated portion of an opening formed in hydrocarbon layer 6100. Hydrocarbon layer 6100 may be below overburden 540.

[0557] One or more substantially horizontal openings 6102 may also be placed in hydrocarbon layer 6100. Horizontal openings 6102 may, in some embodiments, contain perforated liners. The horizontal openings 6102 may be coupled to vertical opening 6108. Horizontal openings 6102 may be elongated portions that diverge from the elongated portion of vertical opening 6108. Horizontal openings 6102 may be formed in hydrocarbon layer 6100 after vertical opening 6108 has been formed. In certain embodiments, openings 6102 may be angled upwards to facilitate flow of formation fluids towards the production conduit.

[0558] Each horizontal opening 6102 may lie above or below an adjacent horizontal opening. In an embodiment, six horizontal openings 6102 may be formed in hydrocarbon layer 6100. Three horizontal openings 6102 may face 180°, or in a substantially opposite direction, from three additional horizontal openings 6102. Two horizontal openings facing substantially opposite directions may lie in a substantially identical vertical plane within the formation. Any number of horizontal openings 6102 may be coupled to a single vertical opening 6108, depending on, but not limited to, a thickness of hydrocarbon layer 6100, a type of formation, a desired heating rate in the formation, and a desired production rate.

[0559] Production conduit 6106 may be placed substantially vertically within vertical opening 6108. Production conduit 6106 may be substantially centered within vertical opening 6108. Pump 6107 may be coupled to production conduit 6106. Such a pump may be used, in some embodiments, to pump formation fluids from the bottom of the well. Pump 6107 may be a rod pump, progressing cavity pump (PCP), centrifugal pump, jet pump, gas lift pump, submersible pump, rotary pump, etc.

[0560] One or more heaters 6104 may be placed within each horizontal opening 6102. Heaters 6104 may be placed in hydrocarbon layer 6100 through vertical opening 6108 and into horizontal opening 6102.

[0561] In some embodiments, heater 6104 may be used to generate heat along a length of the heater within vertical opening 6108 and horizontal opening 6102. In other embodiments, heater 6104 may be used to generate heat only within horizontal opening 6102. In certain embodiments, heat generated by heater 6104 may be varied along its length and/or varied between vertical opening 6108 and horizontal opening 6102. For example, less heat may be generated by heater 6104 in vertical opening 6108 and more heat may be generated by the heater in horizontal opening 6102. It may be advantageous to have at least some heating within vertical opening 6108. This may maintain fluids produced from the formation in a vapor phase in production conduit 6106 and/or may upgrade the produced fluids within the production well. Having production conduit 6106 and heaters 6104 installed into a formation through a single opening in the formation may reduce costs associated with forming openings in the formation and installing production equipment and heaters within the formation.

[0562] FIG. 8 depicts a schematic view from an elevated position of the embodiment of FIG. 7. One or more vertical openings 6108 may be formed in hydrocarbon layer 6100. Each of vertical openings 6108 may lie along a single plane in hydrocarbon layer 6100. Horizontal openings 6102 may extend in a plane substantially perpendicular to the plane of vertical openings 6108. Additional horizontal openings 6102 may lie in a plane below the horizontal openings as shown in the schematic depiction of FIG. 7. A number of vertical openings 6108 and/or a spacing between vertical openings 6108 may be determined by, for example, a desired heating rate or a desired production rate. In some embodiments, spacing between vertical openings may be about 4 m to about 30 m. Longer or shorter spacings may be used to meet specific formation needs. A length of a horizontal opening 6102 may be up to about 1600 m. However, a length of horizontal openings 6102 may vary depending on, for example, a maximum installation cost, an area of hydrocarbon layer 6100, or a maximum producible heater length.

[0563] In an in situ conversion process embodiment, a formation having one or more thin hydrocarbon layers may be treated. The hydrocarbon layer may be, but is not limited to, a rich, thin oil shale. In some in situ conversion process embodiments, such formations may be treated with heat sources that are positioned substantially horizontal within and/or adjacent to the thin hydrocarbon layer or thin hydrocarbon layers. A relatively thin hydrocarbon layer may be at a substantial depth below a ground surface. For example, a formation may have an overburden of up to about 650 m in depth. The cost of drilling a large number of substantially vertical wells within a formation to a significant depth may be expensive. It may be advantageous to place heaters horizontally within these formations to heat large portions of the formation for lengths up to about 1600 m. Using horizontal heaters may reduce the number of vertical wells that are needed to place a sufficient number of heaters within the formation.

[0564] FIG. 9 illustrates an embodiment of hydrocarbon containing layer 200 that may be at a near-horizontal angle with respect to an upper surface of ground 204. An angle of hydrocarbon containing layer 200, however, may vary. For example, hydrocarbon containing layer 200 may dip or be steeply dipping. Economically viable production of a steeply dipping hydrocarbon containing layer may not be possible using presently available mining methods.

[0565] A dipping or relatively steeply dipping hydrocarbon containing layer may be subjected to an in situ conversion process. For example, a set of production wells may be disposed near a highest portion of a dipping hydrocarbon layer of an oil shale formation. Hydrocarbon portions adjacent to and below the production wells may be heated to pyrolysis temperatures. Pyrolysis fluid may be produced from the production wells. As production from the top portion declines, deeper portions of the formation may be heated to pyrolysis temperatures. Vapors may be produced from the hydrocarbon containing layer by transporting vapor through the previously pyrolyzed hydrocarbons. High permeability resulting from pyrolysis and production of fluid from the upper portion of the formation may allow for vapor phase transport with minimal pressure loss. Vapor phase transport of fluids produced in the formation may eliminate a need to have deep production wells in addition to the set of production wells. A number of production wells required to process the formation may be reduced. Reducing the number of production wells required for production may increase economic viability of an in situ conversion process.

[0566] In steeply dipping formations, directional drilling may be used to form an opening in the formation for a heater well or production well. Directional drilling may include drilling an opening in which the route/course of the opening may be planned before drilling. Such an opening may usually be drilled with rotary equipment. In directional drilling, a route/course of an opening may be controlled by deflection wedges, etc.

[0567] A wellbore may be formed using a drill equipped with a steerable motor and an accelerometer. The steerable motor and accelerometer may allow the wellbore to follow a layer in the oil shale formation. A steerable motor may maintain a substantially constant distance between heater well 202 and a boundary of hydrocarbon containing layer 200 throughout drilling of the opening.

[0568] In some in situ conversion embodiments, geosteered drilling may be used to drill a wellbore in an oil shale formation. Geosteered drilling may include determining or estimating a distance from an edge of hydrocarbon containing layer 200 to the wellbore with a sensor. The sensor may monitor variations in characteristics or signals in the formation. The characteristic or signal variance may allow for determination of a desired drill path. The sensor may monitor resistance, acoustic signals, magnetic signals, gamma rays, and/or other signals within the formation. A drilling apparatus for geosteered drilling may include a steerable motor. The steerable motor may be controlled to maintain a predetermined distance from an edge of a hydrocarbon containing layer based on data collected by the sensor.

[0569] In some in situ conversion embodiments, wellbores may be formed in a formation using other techniques. Wellbores may be formed by impaction techniques and/or by sonic drilling techniques. The method used to form wellbores may be determined based on a number of factors. The factors may include, but are not limited to, accessibility of the site, depth of the wellbore, properties of the overburden, and properties of the hydrocarbon containing layer or layers.

[0570] FIG. 10 illustrates an embodiment of a plurality of heater wells 210 formed in hydrocarbon layer 212. Hydrocarbon layer 212 may be a steeply dipping layer. One or more of heater wells 210 may be formed in the formation such that two or more of the heater wells are substantially parallel to each other, and/or such that at least one heater well is substantially parallel to a boundary of hydrocarbon layer 212. For example, one or more of heater wells 210 may be formed in hydrocarbon layer 212 by a magnetic steering method. An example of a magnetic steering method is illustrated in U.S. Pat. No. 5,676,212 to Kuckes, which is incorporated by reference as if fully set forth herein. Magnetic steering may include drilling heater well 210 parallel to an adjacent heater well. The adjacent well may have been previously drilled. In addition, magnetic steering may include directing the drilling by sensing and/or determining a magnetic field produced in an adjacent heater well. For example, the magnetic field may be produced in the adjacent heater well by flowing a current through an insulated current-carrying wireline disposed in the adjacent heater well.

[0571] Magnetic steering may include directing the drilling by sensing and/or determining a magnetic field produced in an adjacent well. For example, the magnetic field may be produced in the adjacent well by flowing a current through an insulated current-carrying wireline disposed in the adjacent well. In some embodiments, magnetostatic steering may be used to form openings adjacent to a first opening. U.S. Pat. No. 5,541,517, issued to Hartmann et al., which is incorporated by reference as if fully set forth herein, describes a method for drilling a wellbore relative to a second wellbore that has magnetized casing portions.

[0572] When drilling a wellbore (opening), a magnet or magnets may be inserted into a first opening to provide a magnetic field used to guide a drilling mechanism that forms an adjacent opening or adjacent openings. The magnetic field may be detected by a 3-axis fluxgate magnetometer in the opening being drilled. A control system may use information detected by the magnetometer to determine and implement operation parameters needed to form an opening that is a selected distance away (e.g., parallel) from the first opening (within desired tolerances). Some types of wells may require or need close tolerances. For example, freeze wells may need to be positioned parallel to each other with small or no variance in parallel alignment to allow for formation of a continuous frozen barrier around a treatment area. Also, vertical and/or horizontally positioned heater wells and/or production wells may need to be positioned parallel to each other with small or no variance in parallel alignment to allow for substantially uniform heating and/or production from a treatment area in a formation.

[0573] FIG. 11 depicts a schematic representation of an embodiment of a magnetostatic drilling operation to form an opening that is a selected distance away from (e.g., substantially parallel to) a drilled opening. Opening 514 may be formed in formation 6100. Opening 514 may be a cased opening or an open hole opening. Magnetic string 9678 may be inserted into opening 514. Magnetic string 9678 may be unwound from a reel into opening 514. In an embodiment, magnetic string includes several segments 9680 of magnets within casing 6152.

[0574] In some embodiments, casing 6152 may be a conduit made of a material that is not significantly influenced by a magnetic field (e.g., non-magnetic alloy such as non-magnetic stainless steel (e.g., 304, 310, 316 stainless steel), reinforced polymer pipe, or brass tubing). The casing may be a conduit of a conductor-in-conduit heater, or it may be perforated liner or casing. If the casing is not significantly influenced by a magnetic field, then the magnetic flux will not be shielded. In other embodiments, the casing may be made of a material that is influenced by a magnetic field (e.g., carbon steel). The use of a material that is influenced by a magnetic field may weaken the strength of the magnetic field to be detected by drilling apparatus 9684 in adjacent opening 9685.

[0575] Magnets may be inserted in conduits 9681 in segments 9680. Conduits 9681 may be threaded or seamless coiled tubing (e.g., tubing having an inside diameter of about 5 cm). Members 9682 (e.g., pins) may be placed between segments 9680 to inhibit movement of segments 9680 relative to conduit 9681. Magnets from adjoining segments of conduit may be close to each other or touch each other as, for example, threaded sections of conduit are tightened together. A segment may be made of several north-south aligned magnets. Alignment of the magnets allows each segment to effectively be a long magnet. In an embodiment, a segment may include one magnet. Magnets may be Alnico magnets or other types of magnets having significant magnetic strength. Two adjacent segments may be oriented to have opposite polarities so that the segments repel each other.

[0576] The magnetic string may include 2 or more magnetic segments. Segments may range in length from about 1.5 m to about 15 m. Magnetic segments may be formed from several magnets. Magnets used to form segments may have diameters greater than about 1 cm (about 4.5 cm). The magnets may be oriented so that the magnets are attracted to each other. For example, a segment may be made of 40 magnets each having a length of about 0.15 m.

[0577] FIG. 12 depicts a schematic of a portion of magnetic string. Segments 9680 may be positioned such that adjacent segments 9680 have opposing polarities. In some embodiments, force may be applied to minimize distance 9692 between segments 9680. Additional segments may be added to increase a length of magnetic string 9678. Magnetic strings may be coiled after assembling. Installation of the magnetic string may include uncoiling the magnetic string.

[0578] For example, first segment 9697 may be positioned north-south in the conduit and second segment 9698 may be positioned south-north such that the south poles of segments 9697, 9698 are proximate each other. Third segment 9696 may be positioned in the conduit in a south-north orientation such that the north poles of segments 9697, 9696 are proximate each other. Magnet strings may include multiple south-south and north-north interfaces. As shown in FIG. 12, this configuration may induce a series of magnetic fields 9694.

[0579] Alternating the polarity of the segments within a magnetic string may provide several magnetic field differentials that allow for reduction in the amount of deviation that is a selected distance between the openings. Increasing a length of the segments within the magnetic string may increase the radial distance at which the magnetometer may detect a magnetic field. In some embodiments, the length of segments within the magnetic string may be varied. For example, more magnets may be used in the segment proximate the earth's surface than in segments positioned in the formation.

[0580] In an embodiment, when the separation distance between two wellbores increases, then the segment length of the magnetic strings may also be increased, and vice versa. With shorter segment lengths, while the overall strength of the magnetic field is decreased, variations in the magnetic field occur more frequently, thus providing more guidance to the drilling operation. For example, segments having a length of about 6 m may induce a magnetic field sufficient to allow drilling of adjacent openings at distances of less than about 16 m. This configuration may allow a desired tolerance between the adjacent openings to be achieved.

[0581] In alternate embodiments, the strength of the magnets used may affect a strength of the magnetic field induced. For example, when using magnets having a lower strength than those in the example above, a segment length of about 6 m may induce a magnetic field sufficient to drill adjacent openings at distances of less than about 6 m. In some embodiments, a segment length of about 6 m may induce a magnetic field sufficient to drill adjacent openings at distances of less than about 10 m.

[0582] A length of the magnetic string may be based on an economic balance between cost of the string and the cost of having to reposition the string during drilling. A string length may range from about 30 m to about 500 m. In an embodiment, a magnetic string may have a length of about 150 m. Thus, in some embodiments, the magnetic string may need to be repositioned if the openings being drilled are longer than the length of the string.

[0583] When multiple wellbores are to be drilled, it is possible to initially drill a center wellbore, and then use magnetic strings in that center wellbore to guide the drilling of the other wellbores substantially surrounding the center wellbore. In this manner cumulative errors may be limited since, for example, movement of the magnetic string may be minimized. In addition, only the center well in this embodiment will include a more expensive nonmagnetic liner.

[0584] In some embodiments, heated portion 310 may extend radially from heat source 300, as shown in FIG. 13. For example, a width of heated portion 310, in a direction extending radially from heat source 300, may be about 0 m to about 10 m. A width of heated portion 310 may vary, however, depending upon, for example, heat provided by heat source 300 and the characteristics of the formation. Heat provided by heat source 300 will typically transfer through the heated portion to create a temperature gradient within the heated portion. For example, a temperature proximate the heater well will generally be higher than a temperature proximate an outer lateral boundary of the heated portion. A temperature gradient within the heated portion may vary within the heated portion depending on various factors (e.g., thermal conductivity of the formation, density, and porosity).

[0585] As heat transfers through heated portion 310 of the oil shale formation, a temperature within at least a section of the heated portion may be within a pyrolysis temperature range. As the heat transfers away from the heat source, a front at which pyrolysis occurs will in many instances travel outward from the heat source. For example, heat from the heat source may be allowed to transfer into a selected section of the heated portion such that heat from the heat source pyrolyzes at least some of the hydrocarbons within the selected section. Pyrolysis may occur within selected section 315 of the heated portion, and pyrolyzation fluids will be generated in the selected section.

[0586] Selected section 315 may have a width radially extending from the inner lateral boundary of the selected section. For a single heat source as depicted in FIG. 13, width of the selected section may be dependent on a number of factors. The factors may include, but are not limited to, time that heat source 300 is supplying energy to the formation, thermal conductivity properties of the formation, extent of pyrolyzation of hydrocarbons in the formation. A width of selected section 315 may expand for a significant time after initialization of heat source 300. A width of selected section 315 may initially be zero and may expand to 10 m or more after initialization of heat source 300.

[0587] An inner boundary of selected section 315 may be radially spaced from the heat source. The inner boundary may define a volume of spent hydrocarbons 317. Spent hydrocarbons 317 may include a volume of hydrocarbon material that is transformed to coke due to the proximity and heat of heat source 300. Coking may occur by pyrolysis reactions that occur due to a rapid increase in temperature in a short time period. Applying heat to a formation at a controlled rate may allow for avoidance of significant coking, however, some coking may occur in the vicinity of heat sources. Spent hydrocarbons 317 may also include a volume of material that has been subjected to pyrolysis and the removal of pyrolysis fluids. The volume of material that has been subjected to pyrolysis and the removal of pyrolysis fluids may produce insignificant amounts or no additional pyrolysis fluids with increases in temperature. The inner lateral boundary may advance radially outwards as time progresses during operation of an in situ conversion process.

[0588] In some embodiments, a plurality of heated portions may exist within a unit of heat sources. A unit of heat sources refers to a minimal number of heat sources that form a template that is repeated to create a pattern of heat sources within the formation. The heat sources may be located within the formation such that superposition (overlapping) of heat produced from the heat sources occurs. For example, as illustrated in FIG. 14, transfer of heat from two or more heat sources 330 results in superposition of heat to region 332 between the heat sources 330. Superposition of heat may occur between two, three, four, five, six, or more heat sources. Region 332 is an area in which temperature is influenced by various heat sources. Superposition of heat may provide the ability to efficiently raise the temperature of large volumes of a formation to pyrolysis temperatures. The size of region 332 may be significantly affected by the spacing between heat sources.

[0589] Superposition of heat may increase a temperature in at least a portion of the formation to a temperature sufficient for pyrolysis of hydrocarbons within the portion. Superposition of heat to region 332 may increase the quantity of hydrocarbons in a formation that are subjected to pyrolysis. Selected sections of a formation that are subjected to pyrolysis may include regions 334 brought into a pyrolysis temperature range by heat transfer from substantially only one heat source. Selected sections of a formation that are subjected to pyrolysis may also include regions 332 brought into a pyrolysis temperature range by superposition of heat from multiple heat sources.

[0590] A pattern of heat sources will often include many units of heat sources. There will typically be many heated portions, as well as many selected sections within the pattern of heat sources. Superposition of heat within a pattern of heat sources may decrease the time necessary to reach pyrolysis temperatures within the multitude of heated portions. Superposition of heat may allow for a relatively large spacing between adjacent heat sources. In some embodiments, a large spacing may provide for a relatively slow heating rate of hydrocarbon material. The slow heating rate may allow for pyrolysis of hydrocarbon material with minimal coking or no coking within the formation away from areas in the vicinity of the heat sources. Heating from heat sources allows the selected section to reach pyrolysis temperatures so that all hydrocarbons within the selected section may be subject to pyrolysis reactions. In some in situ conversion embodiments, a majority of pyrolysis fluids are produced when the selected section is within a range from about 0 m to about 25 m from a heat source.

[0591] In an in situ conversion process embodiment, a heating rate may be controlled to minimize costs associated with heating a selected section. The costs may include, for example, input energy costs and equipment costs. In certain embodiments, a cost associated with heating a selected section may be minimized by reducing a heating rate when the cost associated with heating is relatively high and increasing the heating rate when the cost associated with heating is relatively low. For example, a heating rate of about 330 watts/m may be used when the associated cost is relatively high, and a heating rate of about 1640 watts/m may be used when the associated cost is relatively low. The cost associated with heating may be relatively high at peak times of energy use, such as during the daytime. For example, energy use may be high in warm climates during the daytime in the summer due to energy use for air conditioning. Low times of energy use may be, for example, at night or during weekends, when energy demand tends to be lower. In an embodiment, the heating rate may be varied from a higher heating rate during low energy usage times, such as during the night, to a lower heating rate during high energy usage times, such as during the day.

[0592] As shown in FIG. 3, in addition to heat sources 100, one or more production wells 104 will typically be placed within the portion of the oil shale formation. Formation fluids may be produced through production well 104. In some embodiments, production well 104 may include a heat source. The heat source may heat the portions of the formation at or near the production well and allow for vapor phase removal of formation fluids. The need for high temperature pumping of liquids from the production well may be reduced or eliminated. Avoiding or limiting high temperature pumping of liquids may significantly decrease production costs. 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, and/or (3) increase formation permeability at or proximate the production well. In some in situ conversion process embodiments, an amount of heat supplied to production wells is significantly less than an amount of heat applied to heat sources that heat the formation.

[0593] Because permeability and/or porosity increases in the heated formation, produced vapors may flow considerable distances through the formation with relatively little pressure differential. Increases in permeability may result from a reduction of mass of the heated portion due to vaporization of water, removal of hydrocarbons, and/or creation of fractures. Fluids may flow more easily through the heated portion. In some embodiments, production wells may be provided in upper portions of hydrocarbon layers. As shown in FIG. 9, production wells 206 may extend into an oil shale formation near the top of heated portion 208. Extending production wells significantly into the depth of the heated hydrocarbon layer may be unnecessary.

[0594] Fluid generated within an oil shale formation may move a considerable distance through the oil shale formation as a vapor. The considerable distance may be over 1000 m depending on various factors (e.g., permeability of the formation, properties of the fluid, temperature of the formation, and pressure gradient allowing movement of the fluid). Due to increased permeability in formations subjected to in situ conversion and formation fluid removal, production wells may only need to be provided in every other unit of heat sources or every third, fourth, fifth, or sixth units of heat sources.

[0595] Embodiments of a production well may include valves that alter, maintain, and/or control a pressure of at least a portion of the formation. Production wells may be cased wells. Production wells may have production screens or perforated casings adjacent to production zones. In addition, the production wells may be surrounded by sand, gravel or other packing materials adjacent to production zones. Production wells 104 may be coupled to treatment facilities 108, as shown in FIG. 3.

[0596] During an in situ process, production wells may be operated such that the production wells are at a lower pressure than other portions of the formation. In some embodiments, a vacuum may be drawn at the production wells. Maintaining the production wells at lower pressures may inhibit fluids in the formation from migrating outside of the in situ treatment area.

[0597] FIG. 15 illustrates an embodiment of production well 6109 placed in hydrocarbon layer 6100. Production well 6109 may be used to produce formation fluids from hydrocarbon layer 6100. Hydrocarbon layer 6100 may be treated using an in situ conversion process. Production conduit 6106 may be placed within production well 6109. In an embodiment, production conduit 6106 is a hollow sucker rod placed in production well 6109. Production well 6109 may have a casing, or lining, placed along the length of the production well. The casing may have openings, or perforations, to allow formation fluids to enter production well 6109. Formation fluids may include vapors and/or liquids. Production conduit 6106 and production well 6109 may include non-corrosive materials such as steel.

[0598] In certain embodiments, production conduit 6106 may include heat source 6105. Heat source 6105 may be a heater placed inside or outside production conduit 6106 or formed as part of the production conduit. Heat source 6105 may be a heater such as an insulated conductor heater, a conductor-in-conduit heater, or a skin-effect heater. A skin-effect heater is an electric heater that uses eddy current heating to induce resistive losses in production conduit 6106 to heat the production conduit. An example of a skin-effect heater is obtainable from Dagang Oil Products (China).

[0599] Heating of production conduit 6106 may inhibit condensation and/or refluxing in the production conduit or within production well 6109. In certain embodiments, heating of production conduit 6106 may inhibit plugging of pump 6107 by liquids (e.g., heavy hydrocarbons). For example, heat source 6105 may heat production conduit 6106 to about 35° C. to maintain the mobility of liquids in the production conduit to inhibit plugging of pump 6107 or the production conduit. In certain embodiments (e.g., for formations greater than about 100 m in depth), heat source 6105 may heat production conduit 6106 and/or production well 6109 to temperatures of about 200° C. to about 250° C. to maintain produced fluids substantially in a vapor phase by inhibiting condensation and/or reflux of fluids in the production well.

[0600] Pump 6107 may be coupled to production conduit 6106. Pump 6107 may be used to pump formation fluids from hydrocarbon layer 6100 into production conduit 6106. Pump 6107 may be any pump used to pump fluids, such as a rod pump, PCP, jet pump, gas lift pump, centrifugal pump, rotary pump, or submersible pump. Pump 6107 may be used to pump fluids through production conduit 6106 to a surface of the formation above overburden 540.

[0601] In certain embodiments, pump 6107 can be used to pump formation fluids that may be liquids. Liquids may be produced from hydrocarbon layer 6100 prior to production well 6109 being heated to a temperature sufficient to vaporize liquids within the production well. In some embodiments, liquids produced from the formation tend to include water. Removing liquids from the formation before heating the formation, or during early times of heating before pyrolysis occurs, tends to reduce the amount of heat input that is needed to produce hydrocarbons from the formation.

[0602] In an embodiment, formation fluids that are liquids may be produced through production conduit 6106 using pump 6107. Formation fluids that are vapors may be simultaneously produced through an annulus of production well 6109 outside of production conduit 6106.

[0603] Insulation may be placed on a wall of production well 6109 in a section of the production well within overburden 540. The insulation may be cement or any other suitable low heat transfer material. Insulating the overburden section of production well 6109 may inhibit transfer of heat from fluids being produced from the formation into the overburden.

[0604] In an in situ conversion process embodiment, a mixture may be produced from an oil shale formation. The mixture may be produced through a heater well disposed in the formation. Producing the mixture through the heater well may increase a production rate of the mixture as compared to a production rate of a mixture produced through a non-heater well. A non-heater well may include a production well. In some embodiments, a production well may be heated to increase a production rate.

[0605] A heated production well may inhibit condensation of higher carbon numbers (C₅ or above) in the production well. A heated production well may inhibit problems associated with producing a hot, multi-phase fluid from a formation.

[0606] A heated production well may have an improved production rate as compared to a non-heated production well. Heat applied to the formation adjacent to the production well 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. A heater in a lower portion of a production well may be turned off when superposition of heat from heat sources heats the formation sufficiently to counteract benefits provided by heating from within the production well. In some embodiments, a heater in an upper portion of a production well may remain on after a heater in a lower portion of the well is deactivated. The heater in the upper portion of the well may inhibit condensation and reflux of formation fluid.

[0607] In some embodiments, heated production wells may improve product quality by causing production through a hot zone in the formation adjacent to the heated production well. A final phase of thermal cracking may exist in the hot zone adjacent to the production well. Producing through a hot zone adjacent to a heated production well may allow for an increased olefin content in non-condensable hydrocarbons and/or condensable hydrocarbons in the formation fluids. The hot zone may produce formation fluids with a greater percentage of non-condensable hydrocarbons due to thermal cracking in the hot zone. The extent of thermal cracking may depend on a temperature of the hot zone and/or on a residence time in the hot zone. A heater can be deliberately run hotter to promote the further in situ upgrading of hydrocarbons.

[0608] In an embodiment, heating in or proximate a production well may be controlled such that a desired mixture is produced through the production well. The desired mixture may have a selected yield of non-condensable hydrocarbons. For example, the selected yield of non-condensable hydrocarbons may be about 75 weight % non-condensable hydrocarbons or, in some embodiments, about 50 weight % to about 100 weight %. In other embodiments, the desired mixture may have a selected yield of condensable hydrocarbons. The selected yield of condensable hydrocarbons may be about 75 weight % condensable hydrocarbons or, in some embodiments, about 50 weight % to about 95 weight %.

[0609] A temperature and a pressure may be controlled within the formation to inhibit the production of carbon dioxide and increase production of carbon monoxide and molecular hydrogen during synthesis gas production. In an embodiment, the mixture is produced through a production well (or heater well), which may be heated to inhibit the production of carbon dioxide. In some embodiments, a mixture produced from a first portion of the formation may be recycled into a second portion of the formation to inhibit the production of carbon dioxide. The mixture produced from the first portion may be at a lower temperature than the mixture produced from the second portion of the formation.

[0610] A desired volume ratio of molecular hydrogen to carbon monoxide in synthesis gas may be produced from the formation. The desired volume ratio may be about 2.0:1. In an embodiment, the volume ratio may be maintained between about 1.8:1 and 2.2:1 for synthesis gas.

[0611] FIG. 16 illustrates a pattern of heat sources 400 and production wells 402 that may be used to treat an oil shale formation. Heat sources 400 may be arranged in a unit of heat sources such as triangular pattern 401. Heat sources 400, however, may be arranged in a variety of patterns including, but not limited to, squares, hexagons, and other polygons. The pattern may include a regular polygon to promote uniform heating of the formation in which the heat sources are placed. The pattern may also be a line drive pattern. A line drive pattern generally includes a first linear array of heater wells, a second linear array of heater wells, and a production well or a linear array of production wells between the first and second linear array of heater wells.

[0612] A distance from a node of a polygon to a centroid of the polygon is smallest for a 3-sided polygon and increases with increasing number of sides of the polygon. The distance from a node to the centroid for an equilateral triangle is (length/2)/(square root(3)/2) or 0.5774 times the length. For a square, the distance from a node to the centroid is (length/2)/(square root(2)/2) or 0.7071 times the length. For a hexagon, the distance from a node to the centroid is (length/2)/(½) or the length. The difference in distance between a heat source and a midpoint to a second heat source (length/2) and the distance from a heat source to the centroid for an equilateral pattern (0.5774 times the length) is significantly less for the equilateral triangle pattern than for any higher order polygon pattern. The small difference means that superposition of heat may develop more rapidly and that the formation may rise to a more uniform temperature between heat sources using an equilateral triangle pattern rather than a higher order polygon pattern.

[0613] Triangular patterns tend to provide more uniform heating to a portion of the formation in comparison to other patterns such as squares and/or hexagons. Triangular patterns tend to provide faster heating to a predetermined temperature in comparison to other patterns such as squares or hexagons. The use of triangular patterns may result in smaller volumes of a formation being overheated. A plurality of units of heat sources such as triangular pattern 401 may be arranged substantially adjacent to each other to form a repetitive pattern of units over an area of the formation. For example, triangular patterns 401 may be arranged substantially adjacent to each other in a repetitive pattern of units by inverting an orientation of adjacent triangles 401. Other patterns of heat sources 400 may also be arranged such that smaller patterns may be disposed adjacent to each other to form larger patterns.

[0614] Production wells may be disposed in the formation in a repetitive pattern of units. In certain embodiments, production well 402 may be disposed proximate a center of every third triangle 401 arranged in the pattern. Production well 402, however, may be disposed in every triangle 401 or within just a few triangles. In some embodiments, a production well may be placed within every 13, 20, or 30 heater well triangles. For example, a ratio of heat sources in the repetitive pattern of units to production wells in the repetitive pattern of units may be more than approximately 5 (e.g., more than 6, 7, 8, or 9). In some well pattern embodiments, three or more production wells may be located within an area defined by a repetitive pattern of units. For example, as shown in FIG. 16, production wells 410 may be located within an area defined by repetitive pattern of units 412. Production wells 410 may be located in the formation in a unit of production wells. The location of production wells 402, 410 within a pattern of heat sources 400 may be determined by, for example, a desired heating rate of the oil shale formation, a heating rate of the heat sources, the type of heat sources used, the type of oil shale formation (and its thickness), the composition of the oil shale formation, permeability of the formation, the desired composition to be produced from the formation, and/or a desired production rate.

[0615] One or more injection wells may be disposed within a repetitive pattern of units. For example, as shown in FIG. 16, injection wells 414 may be located within an area defined by repetitive pattern of units 416. Injection wells 414 may also be located in the formation in a unit of injection wells. For example, the unit of injection wells may be a triangular pattern. Injection wells 414, however, may be disposed in any other pattern. In certain embodiments, one or more production wells and one or more injection wells may be disposed in a repetitive pattern of units. For example, as shown in FIG. 16, production wells 418 and injection wells 420 may be located within an area defined by repetitive pattern of units 422. Production wells 418 may be located in the formation in a unit of production wells, which may be arranged in a first triangular pattern. In addition, injection wells 420 may be located within the formation in a unit of production wells, which are arranged in a second triangular pattern. The first triangular pattern may be different than the second triangular pattern. For example, areas defined by the first and second triangular patterns may be different.

[0616] One or more monitoring wells may be disposed within a repetitive pattern of units. Monitoring wells may include one or more devices that measure temperature, pressure, and/or fluid properties. In some embodiments, logging tools may be placed in monitoring well wellbores to measure properties within a formation. The logging tools may be moved to other monitoring well wellbores as needed. The monitoring well wellbores may be cased or uncased wellbores. As shown in FIG. 16, monitoring wells 424 may be located within an area defined by repetitive pattern of units 426. Monitoring wells 424 may be located in the formation in a unit of monitoring wells, which may be arranged in a triangular pattern. Monitoring wells 424, however, may be disposed in any of the other patterns within repetitive pattern of units 426.

[0617] It is to be understood that a geometrical pattern of heat sources 400 and production wells 402 is described herein by example. A pattern of heat sources and production wells will in many instances vary depending on, for example, the type of oil shale formation to be treated. For example, for relatively thin layers, heater wells may be aligned along one or more layers along strike or along dip. For relatively thick layers, heat sources may be at an angle to one or more layers (e.g., orthogonally or diagonally).

[0618] A triangular pattern of heat sources may treat a hydrocarbon layer having a thickness of about 10 m or more. For a thin hydrocarbon layer (e.g., about 10 m thick or less) a line and/or staggered line pattern of heat sources may treat the hydrocarbon layer.

[0619] For certain thin layers, heating wells may be placed close to an edge of the layer (e.g., in a staggered line instead of a line placed in the center of the layer) to increase the amount of hydrocarbons produced per unit of energy input. A portion of input heating energy may heat non-hydrocarbon portions of the formation, but the staggered pattern may allow superposition of heat to heat a majority of the hydrocarbon layers to pyrolysis temperatures. If the thin formation is heated by placing one or more heater wells in the layer along a center of the thickness, a significant portion of the hydrocarbon layers may not be heated to pyrolysis temperatures. In some embodiments, placing heater wells closer to an edge of the layer may increase the volume of layer undergoing pyrolysis per unit of energy input.

[0620] Exact placement of heater wells, production wells, etc. will depend on variables specific to the formation (e.g., thickness of the layer or composition of the layer), project economics, etc. In certain embodiments, heater wells may be substantially horizontal while production wells may be vertical, or vice versa. In some embodiments, wells may be aligned along dip or strike or oriented at an angle between dip and strike.

[0621] The spacing between heat sources may vary depending on a number of factors. The factors may include, but are not limited to, the type of an oil shale formation, the selected heating rate, and/or the selected average temperature to be obtained within the heated portion. In some well pattern embodiments, the spacing between heat sources may be within a range of about 5 m to about 25 m. In some well pattern embodiments, spacing between heat sources may be within a range of about 8 m to about 15 m.

[0622] The spacing between heat sources may influence the composition of fluids produced from an oil shale formation. In an embodiment, a computer-implemented simulation may be used to determine optimum heat source spacings within an oil shale formation. At least one property of a portion of oil shale formation can usually be measured. The measured property may include, but is not limited to, vitrinite reflectance, hydrogen content, atomic hydrogen to carbon ratio, oxygen content, atomic oxygen to carbon ratio, water content, thickness of the oil shale formation, and/or the amount of stratification of the oil shale formation into separate layers of rock and hydrocarbons.

[0623] In certain embodiments, a computer-implemented simulation may include providing at least one measured property to a computer system. One or more sets of heat source spacings in the formation may also be provided to the computer system. For example, a spacing between heat sources may be less than about 30 m. Alternatively, a spacing between heat sources may be less than about 15 m. The simulation may include determining properties of fluids produced from the portion as a function of time for each set of heat source spacings. The produced fluids may include formation fluids such as pyrolyzation fluids or synthesis gas. The determined properties may include, but are not limited to, API gravity, carbon number distribution, olefin content, hydrogen content, carbon monoxide content, and/or carbon dioxide content. The determined set of properties of the produced fluid may be compared to a set of selected properties of a produced fluid. Sets of properties that match the set of selected properties may be determined. Furthermore, heat source spacings may be matched to heat source spacings associated with desired properties.

[0624] As shown in FIG. 16, unit cell 404 will often include a number of heat sources 400 disposed within a formation around each production well 402. An area of unit cell 404 may be determined by midlines 406 that may be equidistant and perpendicular to a line connecting two production wells 402. Vertices 408 of the unit cell may be at the intersection of two midlines 406 between production wells 402. Heat sources 400 may be disposed in any arrangement within the area of unit cell 404. For example, heat sources 400 may be located within the formation such that a distance between each heat source varies by less than approximately 10%, 20%, or 30%. In addition, heat sources 400 may be disposed such that an approximately equal space exists between each of the heat sources. Other arrangements of heat sources 400 within unit cell 404 may be used. A ratio of heat sources 400 to production wells 402 may be determined by counting the number of heat sources 400 and production wells 402 within unit cell 404 or over the total field.

[0625] FIG. 17 illustrates an embodiment of unit cell 404. Unit cell 404 includes heat sources 400 and production well 402. Unit cell 404 may have six full heat sources 400a and six partial heat sources 400b. Full heat sources 400a may be closer to production well 402 than partial heat sources 400b. In addition, an entirety of each of full heat sources 400a may be located within unit cell 404. Partial heat sources 400b may be partially disposed within unit cell 404. Only a portion of heat source 400b disposed within unit cell 404 may provide heat to a portion of an oil shale formation disposed within unit cell 404. A remaining portion of heat source 400b disposed outside of unit cell 404 may provide heat to a remaining portion of the oil shale formation outside of unit cell 404. To determine a number of heat sources within unit cell 404, partial heat source 400b may be counted as one-half of full heat source 400a. In other unit cell embodiments, fractions other than ½ (e.g., ⅓) may more accurately describe the amount of heat applied to a portion from a partial heat source based on geometrical considerations.

[0626] The total number of heat sources 400 in unit cell 404 may include six full heat sources 400a that are each counted as one heat source, and six partial heat sources 400b that are each counted as one-half of a heat source. Therefore, a ratio of heat sources 400 to production wells 402 in unit cell 404 may be determined as 9:1. A ratio of heat sources to production wells may be varied, however, depending on, for example, the desired heating rate of the oil shale formation, the heating rate of the heat sources, the type of heat source, the type of oil shale formation, the composition of oil shale formation, the desired composition of the produced fluid, and/or the desired production rate. Providing more heat source wells per unit area will allow faster heating of the selected portion and thus hasten the onset of production. However, adding more heat sources will generally cost more money in installation and equipment. An appropriate ratio of heat sources to production wells may include ratios greater than about 5:1. In some embodiments, an appropriate ratio of heat sources to production wells may be about 10:1, 20:1, 50:1, or greater. If larger ratios are used, then project costs tend to decrease since less wells and equipment are needed.

[0627] A selected section is generally the volume of formation that is within a perimeter defined by the location of the outermost heat sources (assuming that the formation is viewed from above). For example, if four heat sources were located in a single square pattern with an area of about 100 m² (with each source located at a corner of the square), and if the formation had an average thickness of approximately 5 m across this area, then the selected section would be a volume of about 500 m³ (i.e., the area multiplied by the average formation thickness across the area). In many commercial applications, many heat sources (e.g., hundreds or thousands) may be adjacent to each other to heat a selected section, and therefore only the outermost heat sources (i.e., edge heat sources) would define the perimeter of the selected section.

[0628] FIG. 18 illustrates a typical computational system 6250 that is suitable for implementing various embodiments of the system and method for in situ processing of a formation. Each computational system 6250 typically includes components such as one or more central processing units (CPU) 6252 with associated memory mediums, represented by floppy disks or compact discs (CDs) 6260. The memory mediums may store program instructions for computer programs, wherein the program instructions are executable by CPU 6252. Computational system 6250 may further include one or more display devices such as monitor 6254, one or more alphanumeric input devices such as keyboard 6256, and one or more directional input devices such as mouse 6258. Computational system 6250 is operable to execute the computer programs to implement (e.g., control, design, simulate, and/or operate) in situ processing of formation systems and methods.

[0629] Computational system 6250 preferably includes one or more memory mediums on which computer programs according to various embodiments may be stored. The term “memory medium” may include an installation medium, e.g., CD-ROM or floppy disks 6260, a computational system memory such as DRAM, SRAM, EDO DRAM, SDRAM, DDR SDRAM, Rambus RAM, etc., or a non-volatile memory such as a magnetic media (e.g., a hard drive) or optical storage. The memory medium may include other types of memory as well, or combinations thereof. In addition, the memory medium may be located in a first computer that is used to execute the programs. Alternatively, the memory medium may be located in a second computer, or other computers, connected to the first computer (e.g., over a network). In the latter case, the second computer provides the program instructions to the first computer for execution. Also, computational system 6250 may take various forms, including a personal computer, mainframe computational system, workstation, network appliance, Internet appliance, personal digital assistant (PDA), television system, or other device. In general, the term “computational system” can be broadly defined to encompass any device, or system of devices, having a processor that executes instructions from a memory medium.

[0630] The memory medium preferably stores a software program or programs for event-triggered transaction processing. The software program(s) may be implemented in any of various ways, including procedure-based techniques, component-based techniques, and/or object-oriented techniques, among others. For example, the software program may be implemented using ActiveX controls, C++ objects, JavaBeans, Microsoft Foundation Classes (MFC), or other technologies or methodologies, as desired. A CPU, such as host CPU 6252, executing code and data from the memory medium, includes a system/process for creating and executing the software program or programs according to the methods and/or block diagrams described below.

[0631] In one embodiment, the computer programs executable by computational system 6250 may be implemented in an object-oriented programming language. In an object-oriented programming language, data and related methods can be grouped together or encapsulated to form an entity known as an object. All objects in an object-oriented programming system belong to a class, which can be thought of as a category of like objects that describes the characteristics of those objects. Each object is created as an instance of the class by a program. The objects may therefore be said to have been instantiated from the class. The class sets out variables and methods for objects that belong to that class. The definition of the class does not itself create any objects. The class may define initial values for its variables, and it normally defines the methods associated with the class (e.g., includes the program code which is executed when a method is invoked). The class may thereby provide all of the program code that will be used by objects in the class, hence maximizing re-use of code that is shared by objects in the class.

[0632] Turning now to FIG. 19, a block diagram of one embodiment of computational system 6270 including processor 6293 coupled to a variety of system components through bus bridge 6292 is shown. Other embodiments are possible and contemplated. In the depicted system, main memory 6296 is coupled to bus bridge 6292 through memory bus 6294, and graphics controller 6288 is coupled to bus bridge 6292 through AGP bus 6290. Finally, a plurality of PCI devices 6282 and 6284 are coupled to bus bridge 6292 through PCI bus 6276. Secondary bus bridge 6274 may further be provided to accommodate an electrical interface to one or more EISA or ISA devices 6280 through EISA/ISA bus 6278. Processor 6293 is coupled to bus bridge 6292 through CPU bus 6295 and to optional L2 cache 6297.

[0633] Bus bridge 6292 provides an interface between processor 6293, main memory 6296, graphics controller 6288, and devices attached to PCI bus 6276. When an operation is received from one of the devices connected to bus bridge 6292, bus bridge 6292 identifies the target of the operation (e.g., a particular device or, in the case of PCI bus 6276, that the target is on PCI bus 6276). Bus bridge 6292 routes the operation to the targeted device. Bus bridge 6292 generally translates an operation from the protocol used by the source device or bus to the protocol used by the target device or bus.

[0634] In addition to providing an interface to an ISA/EISA bus for PCI bus 6276, secondary bus bridge 6274 may further incorporate additional functionality, as desired. An input/output controller (not shown), either external from or integrated with secondary bus bridge 6274, may also be included within computational system 6270 to provide operational support for keyboard and mouse 6272 and for various serial and parallel ports, as desired. An external cache unit (not shown) may further be coupled to CPU bus 6295 between processor 6293 and bus bridge 6292 in other embodiments. Alternatively, the external cache may be coupled to bus bridge 6292 and cache control logic for the external cache may be integrated into bus bridge 6292. L2 cache 6297 is further shown in a backside configuration to processor 6293. It is noted that L2 cache 6297 may be separate from processor 6293, integrated into a cartridge (e.g., slot 1 or slot A) with processor 6293, or even integrated onto a semiconductor substrate with processor 6293.

[0635] Main memory 6296 is a memory in which application programs are stored and from which processor 6293 primarily executes. A suitable main memory 6296 comprises DRAM (Dynamic Random Access Memory). For example, a plurality of banks of SDRAM (Synchronous DRAM), DDR (Double Data Rate) SDRAM, or Rambus DRAM (RDRAM) may be suitable.

[0636] PCI devices 6282 and 6284 are illustrative of a variety of peripheral devices such as, for example, network interface cards, video accelerators, audio cards, hard or floppy disk drives or drive controllers, SCSI (Small Computer Systems Interface) adapters, and telephony cards. Similarly, ISA device 6280 is illustrative of various types of peripheral devices, such as a modem, a sound card, and a variety of data acquisition cards such as GPIB or field bus interface cards.

[0637] Graphics controller 6288 is provided to control the rendering of text and images on display 6286. Graphics controller 6288 may embody a typical graphics accelerator generally known in the art to render three-dimensional data structures that can be effectively shifted into and from main memory 6296. Graphics controller 6288 may therefore be a master of AGP bus 6290 in that it can request and receive access to a target interface within bus bridge 6292 to thereby obtain access to main memory 6296. A dedicated graphics bus accommodates rapid retrieval of data from main memory 6296. For certain operations, graphics controller 6288 may generate PCI protocol transactions on AGP bus 6290. The AGP interface of bus bridge 6292 may thus include functionality to support both AGP protocol transactions as well as PCI protocol target and initiator transactions. Display 6286 is any electronic display upon which an image or text can be presented. A suitable display 6286 includes a cathode ray tube (“CRT”), a liquid crystal display (“LCD”), etc.

[0638] It is noted that, while the AGP, PCI, and ISA or EISA buses have been used as examples in the above description, any bus architectures may be substituted as desired. It is further noted that computational system 6270 may be a multiprocessing computational system including additional processors (e.g., processor 6291 shown as an optional component of computational system 6270). Processor 6291 may be similar to processor 6293. More particularly, processor 6291 may be an identical copy of processor 6293. Processor 6291 may be connected to bus bridge 6292 via an independent bus (as shown in FIG. 19) or may share CPU bus 6295 with processor 6293. Furthermore, processor 6291 may be coupled to an optional L2 cache 6298 similar to L2 cache 6297.

[0639] FIG. 20 illustrates a flow chart of a computer-implemented method for treating an oil shale formation based on a characteristic of the formation. At least one characteristic 6370 may be input into computational system 6250. Computational system 6250 may process at least one characteristic 6370 using a software executable to determine a set of operating conditions 6372 for treating the formation with in situ process 6310. The software executable may process equations relating to formation characteristics and/or the relationships between formation characteristics. At least one characteristic 6370 may include, but is not limited to, an overburden thickness, depth of the formation, vitrinite reflectance, type of formation, permeability, density, porosity, moisture content, and other organic maturity indicators, oil saturation, water saturation, volatile matter content, kerogen composition, oil chemistry, ash content, net-to-gross ratio, carbon content, hydrogen content, oxygen content, sulfur content, nitrogen content, mineralology, soluble compound content, elemental composition, hydrogeology, water zones, gas zones, barren zones, mechanical properties, or top seal character. Computational system 6250 may be used to control in situ process 6310 using determined set of operating conditions 6372.

[0640] FIG. 21 illustrates a schematic of an embodiment used to control an in situ conversion process (ICP) in formation 6600. Barrier well 6602, monitor well 6604, production well 6606, and heater well 6608 may be placed in formation 6600. Barrier well 6602 may be used to control water conditions within formation 6600. Monitoring well 6604 may be used to monitor subsurface conditions in the formation, such as, but not limited to, pressure, temperature, product quality, or fracture progression. Production well 6606 may be used to produce formation fluids (e.g., oil, gas, and water) from the formation. Heater well 6608 may be used to provide heat to the formation. Formation conditions such as, but not limited to, pressure, temperature, fracture progression (monitored, for instance, by acoustical sensor data), and fluid quality (e.g., product quality or water quality) may be monitored through one or more of wells 6602, 6604, 6606, and 6608.

[0641] Surface data such as pump status (e.g., pump on or off), fluid flow rate, surface pressure/temperature, and heater power may be monitored by instruments placed at each well or certain wells. Similarly, subsurface data such as pressure, temperature, fluid quality, and acoustical sensor data may be monitored by instruments placed at each well or certain wells. Surface data 6610 from barrier well 6602 may include pump status, flow rate, and surface pressure/temperature. Surface data 6612 from production well 6606 may include pump status, flow rate, and surface pressure/temperature. Subsurface data 6614 from barrier well 6602 may include pressure, temperature, water quality, and acoustical sensor data. Subsurface data 6616 from monitoring well 6604 may include pressure, temperature, product quality, and acoustical sensor data. Subsurface data 6618 from production well 6606 may include pressure, temperature, product quality, and acoustical sensor data. Subsurface data 6620 from heater well 6608 may include pressure, temperature, and acoustical sensor data.

[0642] Surface data 6610 and 6612 and subsurface data 6614, 6616, 6618, and 6620 may be monitored as analog data 6621 from one or more measuring instruments. Analog data 6621 may be converted to digital data 6623 in analog-to-digital converter 6622. Digital data 6623 may be provided to computational system 6250. Alternatively, one or more measuring instruments may provide digital data to computational system 6250. Computational system 6250 may include a distributed central processing unit (CPU). Computational system 6250 may process digital data 6623 to interpret analog data 6621. Output from computational system 6250 may be provided to remote display 6624, data storage 6626, display 6628, or to a surface facility 6630. Surface facility 6630 may include, for example, a hydrotreating plant, a liquid processing plant, or a gas processing plant. Computational system 6250 may provide digital output 6632 to digital-to-analog converter 6634. Digital-to-analog converter 6634 may convert digital output 6632 to analog output 6636.

[0643] Analog output 6636 may include instructions to control one or more conditions of formation 6600. Analog output 6636 may include instructions to control the ICP within formation 6600. Analog output 6636 may include instructions to adjust one or more parameters of the ICP. The one or more parameters may include, but are not limited to, pressure, temperature, product composition, and product quality. Analog output 6636 may include instructions for control of pump status 6640 or flow rate 6642 at barrier well 6602. Analog output 6636 may include instructions for control of pump status 6644 or flow rate 6646 at production well 6606. Analog output 6636 may also include instructions for control of heater power 6648 at heater well 6608. Analog output 6636 may include instructions to vary one or more conditions such as pump status, flow rate, or heater power. Analog output 6636 may also include instructions to turn on and/or off pumps, heaters, or monitoring instruments located at each well.

[0644] Remote input data 6638 may also be provided to computational system 6250 to control conditions within formation 6600. Remote input data 6638 may include data used to adjust conditions of formation 6600. Remote input data 6638 may include data such as, but not limited to, electricity cost, gas or oil prices, pipeline tariffs, data from simulations, plant emissions, or refinery availability. Remote input data 6638 may be used by computational system 6250 to adjust digital output 6632 to a desired value. In some embodiments, surface facility data 6650 may be provided to computational system 6250.

[0645] An in situ conversion process (ICP) may be monitored using a feedback control process. Conditions within a formation may be monitored and used within the feedback control process. A formation being treated using an in situ conversion process may undergo changes in mechanical properties due to the conversion of solids and viscous liquids to vapors, fracture propagation (e.g., to overburden, underburden, water tables, etc.), increases in permeability or porosity and decreases in density, moisture evaporation, and/or thermal instability of matrix minerals (leading to dehydration and decarbonation reactions and shifts in stable mineral assemblages).

[0646] Remote monitoring techniques that will sense these changes in reservoir properties may include, but are not limited to, 4D (4 dimension) time lapse seismic monitoring, 3D/3C (3 dimension/3 component) seismic passive acoustic monitoring of fracturing, time lapse 3D seismic passive acoustic monitoring of fracturing, electrical resistivity, thermal mapping, surface or downhole tilt meters, surveying permanent surface monuments, chemical sniffing or laser sensors for surface gas abundance, and gravimetrics. More direct subsurface-based monitoring techniques may include high temperature downhole instrumentation (such as thermocouples and other temperature sensing mechanisms, stress sensors, or instrumentation in the producer well to detect gas flows on a finely incremental basis).

[0647] In certain embodiments, a “base” seismic monitoring may be conducted, and then subsequent seismic results can be compared to determine changes.

[0648] Simulation methods on a computer system may be used to model an in situ process for treating a formation. Simulations may determine and/or predict operating conditions (e.g., pressure, temperature, etc.), products that may be produced from the formation at given operating conditions, and/or product characteristics (e.g., API gravity, aromatic to paraffin ratio, etc.) for the process. In certain embodiments, a computer simulation may be used to model fluid mechanics (including mass transfer and heat transfer) and kinetics within the formation to determine characteristics of products produced during heating of the formation. A formation may be modeled using commercially available simulation programs such as STARS, THERM, FLUENT, or CFX. In addition, combinations of simulation programs may be used to more accurately determine or predict characteristics of the in situ process. Results of the simulations may be used to determine operating conditions within the formation prior to actual treatment of the formation. Results of the simulations may also be used to adjust operating conditions during treatment of the formation based on a change in a property of the formation and/or a change in a desired property of a product produced from the formation.

[0649] FIG. 22 illustrates a flow chart of an embodiment of method 9470 for modeling an in situ process for treating an oil shale formation using a computer system. Method 9470 may include providing at least one property 9472 of the formation to the computer system. Properties of the formation may include, but are not limited to, porosity, permeability, saturation, thermal conductivity, volumetric heat capacity, compressibility, composition, and number and types of phases in the formation. Properties may also include chemical components, chemical reactions, and kinetic parameters. At least one operating condition 9474 of the process may also be provided to the computer system. For instance, operating conditions may include, but are not limited to, pressure, temperature, heating rate, heat input rate, process time, weight percentage of gases, production characteristics (e.g., flow rates, locations, compositions), and peripheral water recovery or injection. In addition, operating conditions may include characteristics of the well pattern such as producer well location, producer well orientation, ratio of producer wells to heater wells, heater well spacing, type of heater well pattern, heater well orientation, and distance between an overburden and horizontal heater wells.

[0650] Furthermore, a method may include assessing at least one process characteristic 9478 of the in situ process using simulation method 9476 on the computer system. At least one process characteristic may be assessed as a function of time from at least one property of the formation and at least one operating condition. Process characteristics may include properties of a produced fluid such as API gravity, olefin content, carbon number distribution, ethene to ethane ratio, atomic carbon to hydrogen ratio, and ratio of non-condensable hydrocarbons to condensable hydrocarbons (gas/oil ratio). Process characteristics may also include a pressure and temperature in the formation, total mass recovery from the formation, and/or production rate of fluid produced from the formation.

[0651] In some embodiments, a simulation method may include a numerical simulation method used/performed on the computer system. The numerical simulation method may employ finite difference methods to solve fluid mechanics, heat transfer, and chemical reaction equations as a function of time. A finite difference method may use a body-fitted grid system with unstructured grids to model a formation. An unstructured grid employs a wide variety of shapes to model a formation geometry, in contrast to a structured grid. A body-fitted finite difference simulation method may calculate fluid flow and heat transfer in a formation. Heat transfer mechanisms may include conduction, convection, and radiation. The body-fitted finite difference simulation method may also be used to treat chemical reactions in the formation. Simulations with a finite difference simulation method may employ closed value thermal conduction equations to calculate heat transfer and temperature distributions in the formation. A finite difference simulation method may determine values for heat injection rate data.

[0652] In an embodiment, a body-fitted finite difference simulation method may be well suited for simulating systems that include sharp interfaces in physical properties or conditions. In general, a body-fitted finite difference simulation method may be more accurate, in certain circumstances, than space-fitted methods due to the use of finer, unstructured grids in body-fitted methods. For instance, it may be advantageous to use a body-fitted finite difference simulation method to calculate heat transfer in a heater well and in the region near or close to a heater well. The temperature profile in and near a heater well may be relatively sharp. A region near a heater well may be referred to as a “near wellbore region.” The size or radius of a near wellbore region may depend on the type of formation. A general criteria for determining or estimating the radius of a “near wellbore region” may be a distance at which heat transfer by the mechanism of convection contributes significantly to overall heat transfer. Heat transfer in the near wellbore region is typically limited to contributions from conductive and/or radiative heat transfer. Convective heat transfer tends to contribute significantly to overall heat transfer at locations where fluids flow within the formation (i.e., convective heat transfer is significant where the flow of mass contributes to heat transfer).

[0653] In general, the radius of a near wellbore region in a formation decreases with both increasing convection and increasing variation of thermal properties with temperature in the formation.

[0654] An oil shale formation may have a relatively large near wellbore region due to the relatively small contribution of convection for heat transfer and a small variation in thermal properties with temperature. For example, an oil shale formation may have a near wellbore region with a radius between about 5 m and about 7 m. In other embodiments, the radius may be between about 7 m and about 10 m.

[0655] In a simulation of a heater well and near wellbore region, a body-fitted finite difference simulation method may calculate the heat input rate that corresponds to a given temperature in a heater well. The method may also calculate the temperature distributions both inside the wellbore and at the near wellbore region.

[0656] CFX supplied by AEA Technologies in the United Kingdom is an example of a commercially available body-fitted finite difference simulation method. FLUENT is another commercially available body-fitted finite difference simulation method from FLUENT, Inc. located in Lebanon, N.H. FLUENT may simulate models of a formation that include porous media and heater wells. The porous media models may include one or more materials and/or phases with variable fractions. The materials may have user-specified temperature dependent thermal properties and densities. The user may also specify the initial spatial distribution of the materials in a model. In one modeling scheme of a porous media, a combustion reaction may only involve a reaction between carbon and oxygen. In a model of hydrocarbon combustion, the volume fraction and porosity of the formation tend to decrease. In addition, a gas phase may be modeled by one or more species in FLUENT, for example, nitrogen, oxygen, and carbon dioxide.

[0657] In an embodiment, the simulation method may include a numerical simulation method on a computer system that uses a space-fitted finite difference method with structured grids. The space-fitted finite difference simulation method may be a reservoir simulation method. A reservoir simulation method may calculate fluid mechanics, mass balances, heat transfer, and/or kinetics in the formation. A reservoir simulation method may be particularly useful for modeling multiphase porous media in which convection (e.g., the flow of hot fluids) is a relatively important mechanism of heat transfer.

[0658] STARS is an example of a reservoir simulation method provided by Computer Modeling Group, Ltd. of Alberta, Canada. STARS is designed for simulating steam flood, steam cycling, steam-with-additives, dry and wet combustion, along with many types of chemical additive processes, using a wide range of grid and porosity models in both field and laboratory scales. STARS includes options such as thermal applications, steam injection, fireflood, horizontal wells, dual porosity/permeability, directional permeability, and flexible grids. STARS allows for complex temperature dependent models of thermal and physical properties. STARS may also simulate pressure dependent chemical reactions. STARS may simulate a formation using a combination of structured space-fitted grids and unstructured body-fitted grids. Additionally, THERM is an example of a reservoir simulation method provided by Scientific Software Intercomp.

[0659] In certain embodiments, a simulation method may use properties of a formation. In general, the properties of a formation for a model of an in situ process depend on the type of formation. In a model of an oil shale formation, for example, a porosity value may be used to model an amount of kerogen and hydrated mineral matter in the formation. The kerogen and hydrated mineral matter used in a model may be determined or approximated by the amount of kerogen and hydrated mineral matter necessary to generate the oil, gas and water produced in laboratory experiments. The remainder of the volume of the oil shale may be modeled as inert mineral matter, which may be assumed to remain intact at all simulated temperatures. During a simulation, hydrated mineral matter decomposes to produce water and minerals. In addition, kerogen pyrolyzes during the simulation to produce hydrocarbons and other compounds resulting in a rise in fluid porosity. In some embodiments, the change in porosity during a simulation may be determined by monitoring the amount of solids that are treated/transformed, and fluids that are generated.

[0660] Some embodiments of a simulation method may require an initial permeability of a formation and a relationship for the dependence of permeability on conditions of the formation. An initial permeability of a formation may be determined from experimental measurements of a sample (e.g., a core sample) of a formation. In some embodiments, a ratio of vertical permeability to horizontal permeability may be adjusted to take into consideration cleating in the formation.

[0661] In some embodiments, the porosity of a formation may be used to model the change in permeability of the formation during a simulation. For example, the permeability of oil shale often increases with temperature due to the loss of solid matter from the decomposition of mineral matter and the pyrolysis of kerogen. In one embodiment, the dependence of porosity on permeability may be described by an analytical relationship. For example, the effect of pyrolysis on permeability, K, may be governed by a Carman-Kozeny type formula shown in EQN. 2:
K(φ_f)=K₀(φ_f/φ_f,0)^CKpower[(1−φ_f,0)/(1−φ_f)]²  (2)
where φ_f is the current fluid porosity, φ_f,0 is the initial fluid porosity, K₀ is the permeability at initial and CKpower is a user-defined exponent. The value of CKpower may be fitted by matching or approximating the pressure gradient in an experiment in a formation. The porosity-permeability relationship 9350 is plotted in FIG. 23 for a value of the initial porosity of 0.935 millidarcy and CKpower=0.95.

[0662] In certain embodiments, the thermal conductivity of a model of a formation may be expressed in terms of the thermal conductivities of constituent materials. For example, the thermal conductivity may be expressed in terms of solid phase components and fluid phase components. The solid phase in oil shale formations may be composed of inert mineral matter and organic solid matter. One or more fluid phases in the formations may include, for example, a water phase, an oil phase, and a gas phase. In some embodiments, the dependence of the thermal conductivity on constituent materials in an oil shale formation may be modeled according to EQN. 3:
k_th=φ_f×(k_th,w×S_w+k_th,o×S_o+k_th,g×S_g) +(1−φ)×k_th,r+(φ−φ_f)×k_th,s
where φ is the porosity of the formation, φ_f is the instantaneous fluid porosity, k_th,i is the thermal conductivity phase i=(w,o,g)=(water,oil,gas), S_i is the saturation of phase i=(w,o,g)=(water,oil,gas), k_th,r is the thermal conductivity of rock (inert mineral matter), and k_th,s is the thermal conductivity of solid-phase components. The thermal conductivity, from EQN. 3, may be a function of temperature due to the temperature dependence of the solid phase components. The thermal conductivity also changes with temperature due to the change in composition of the fluid phase and porosity.

[0663] In some embodiments, a model may take into account the effect of different geological strata on properties of the formation. A property of a formation may be calculated for a given mineralogical composition.

[0664] In an embodiment, the volumetric heat capacity, ρ_bC_p, may also be modeled as a direct function of temperature. However, the volumetric heat capacity also depends on the composition of the formation material through the density, which is affected by temperature.

[0665] In one embodiment, properties of the formation may include one or more phases with one or more chemical components. For example, fluid phases may include water, oil, and gas. Solid phases may include mineral matter and organic matter. Each of the fluid phases in an in situ process may include a variety of chemical components such as hydrocarbons, H₂, CO₂, etc. The chemical components may be products of one or more chemical reactions, such as pyrolysis reactions, that occur in the formation. Some embodiments of a model of an in situ process may include modeling individual chemical components known to be present in a formation. However, inclusion of chemical components in a model of an in situ process may be limited by available experimental composition and kinetic data for the components. In addition, a simulation method may also place numerical and solution time limitations on the number of components that may be modeled.

[0666] In some embodiments, one or more chemical components may be modeled as a single component called a pseudo-component. In certain embodiments, the oil phase may be modeled by two volatile pseudo-components, a light oil and a heavy oil. The oil and at least some of the gas phase components are generated by pyrolysis of organic matter in the formation. The light oil and the heavy oil may be modeled as having an API gravity that is consistent with laboratory or experimental field data. For example, the light oil may have an API gravity of between about 20° and about 70°. The heavy oil may have an API gravity less than about 20°.

[0667] In some embodiments, hydrocarbon gases in a formation of one or more carbon numbers may be modeled as a single pseudo-component. In other embodiments, non-hydrocarbon gases and hydrocarbon gases may be modeled as a single component. For example, hydrocarbon gases between a carbon number of one to a carbon number of five and nitrogen and hydrogen sulfide may be modeled as a single component. In some embodiments, the multiple components modeled as a single component have relatively similar molecular weights. A molecular weight of the hydrocarbon gas pseudo-component may be set such that the pseudo-component is similar to a hydrocarbon gas generated in a laboratory pyrolysis experiment at a specified pressure.

[0668] In some embodiments of an in situ process, the composition of the generated hydrocarbon gas may vary with pressure. As pressure increases, the ratio of a higher molecular weight component to a lower molecular component tends to increase. For example, as pressure increases, the ratio of hydrocarbon gases with carbon numbers between about three and about five to hydrocarbon gases with one and two carbon numbers tends to increase. Consequently, the molecular weight of the pseudo-component that models a mixture of component gases may vary with pressure.

[0669] TABLE 1 lists components in a model of an in situ process in an oil shale formation according to an embodiment.

[0670]

TABLE 1
CHEMICAL COMPONENTS IN A MODEL OF AN OIL SHALE
FORMATION.
	Component	Phase	MW
	H2O	Aqueous	18.016
	heavy oil	Oil	317.96
	light oil	Oil	154.11
	HC gas	Gas	26.895
	H2	Gas	2.016
	CO2	Gas	44.01
	CO	Gas	28.01
	Hydramin	Solid	15.153
	Kerogen	Solid	15.153
	Prechar	Solid	12.72

[0671] The pseudo-component, HCgas, generated from pyrolysis in an oil shale formation, as shown in TABLE 1, may have critical properties very close to those of ethane. The HCgas pseudo-components may model hydrocarbons between a carbon number of about one and a carbon number of about five. The molecular weight of the pseudo-component in TABLE 1 generally reflects the composition of the hydrocarbon gas that was generated in a laboratory experiment at a pressure of about 6.9 bars absolute.

[0672] In some embodiments, the solid phase in a formation may be modeled with one or more components. The components in a kerogen formation may include kerogen and a hydrated mineral phase (hydramin), as shown in TABLE 1. The hydrated mineral component may be included to model water and carbon dioxide generated in an oil shale formation at temperatures below a pyrolysis temperature of kerogen. The hydrated minerals, for example, may include illite and nahcolite.

[0673] Kerogen may be the source of most or all of the hydrocarbon fluids generated by the pyrolysis. Kerogen may also be the source of some of the water and carbon dioxide that is generated at temperatures below a pyrolysis temperature.

[0674] In an embodiment, the solid phase model may also include one or more intermediate components that are artifacts of the reactions that model the pyrolysis. An oil shale formation may include at least one intermediate component, prechar, as shown in TABLE 1. The prechar solid-phase components may model carbon residue in a formation that may contain H₂ and low molecular weight hydrocarbons. In one embodiment, the number of intermediate components may be increased to improve the match or agreement between simulation results and experimental results.

[0675] In one embodiment, a model of an in situ process may include one or more chemical reactions. A number of chemical reactions are known to occur in an in situ process for an oil shale formation. The chemical reactions may belong to one of several categories of reactions. The categories may include, but not be limited to, generation of pre-pyrolysis water and carbon dioxide, generation of hydrocarbons, coking and cracking of hydrocarbons, formation of synthesis gas, and combustion and oxidation of coke.

[0676] In one embodiment, the rate of change of the concentration of species X due to a chemical reaction, for example:
X→products  (I)
may be expressed in terms of a rate law:
d[X]/dt=−k[X]ⁿ  (II)

[0677] Species X in the chemical reaction undergoes chemical transformation to the products. [X] is the concentration of species X, t is the time, k is the reaction rate constant, and n is the order of the reaction. The reaction rate constant, k, may be defined by the Arrhenius equation:
k=A exp[−E_a/RT]  (III)
where A is the frequency factor, E_a is the activation energy, R is the universal gas constant, and T is the temperature. Kinetic parameters, such as k, A, E_a, and n, may be determined from experimental measurements. A simulation method may include one or more rate laws for assessing the change in concentration of species in an in situ process as a function of time. Experimentally determined kinetic parameters for one or more chemical reactions may be used as input to the simulation method.

[0678] In some embodiments, the number and categories of reactions in a model of an in situ process may depend on the availability of experimental kinetic data and/or numerical limitations of a simulation method. Generally, chemical reactions and kinetic parameters for a model may be chosen such that simulation results match or approximate quantitative and qualitative experimental trends.

[0679] In some embodiments, reactions that model the generation of pre-pyrolysis water and carbon dioxide account for the bound water, carbon dioxide, and carbon monoxide generated in a temperature range below a pyrolysis temperature. For example, pre-pyrolysis water may be generated from hydrated mineral matter. In one embodiment, the temperature range may be between about 100° C. and about 270° C. In other embodiments, the temperature range may be between about 80° C. and about 300° C. Reactions in the temperature range below a pyrolysis temperature may account for between about 45% and about 60% of the total water generated and up to about 30% of the total carbon dioxide observed in laboratory experiments of pyrolysis.

[0680] In an embodiment, the pressure dependence of the chemical reactions may be modeled. To account for the pressure dependence, a single reaction with variable stoichiometric coefficients may be used to model the generation of pre-pyrolysis fluids. Alternatively, the pressure dependence may be modeled with two or more reactions with pressure dependent kinetic parameters such as frequency factors.

[0681] For example, experimental results indicate that the reaction that generates pre-pyrolysis fluids from oil shale is a function of pressure. The amount of water generated generally decreases with pressure while the amount of carbon dioxide generated generally increases with pressure. In an embodiment, the generation of pre-pyrolysis fluids may be modeled with two reactions to account for the pressure dependence. One reaction may be dominant at high pressures while the other may be prevalent at lower pressures. For example, a molar stoichiometry of two reactions according to one embodiment may be written as follows:
1 mol hydramin→0.5884 mol H₂O+0.0962 mol CO₂+0.0114 mol CO  (4)
1 mol hydramin→0.8234 mol H₂O+0.0 mol CO₂+0.0114 mol CO  (5)

[0682] Experimentally determined kinetic parameters for Reactions (4) and (5) are shown in TABLE 2. TABLE 2 shows that pressure dependence of Reactions (4) and (5) is taken into account by the frequency factor. The frequency factor increases with increasing pressure for Reaction (4), which results in an increase in the rate of product formation with pressure. The rate of product formation increases due to the increase in the rate constant. In addition, the frequency factor decreases with increasing pressure for Reaction (5), which results in a decrease in the rate of product formation with increasing pressure. Therefore, the values of the frequency factor in TABLE 2 indicate that Reaction (4) dominates at high pressures while Reaction (5) dominates at low pressures. In addition, the molar balances for Reactions (4) and (5) indicate that Reaction (4) generates less water and more carbon dioxide than Reaction (5).

[0683] In one embodiment, a reaction enthalpy may be used by a simulation method such as STARS to assess the thermodynamic properties of a formation. In TABLES 2–5, the reaction enthalpy is a negative number if a chemical reaction is endothermic and positive if a chemical reaction is exothermic.

[0684]

TABLE 2
KINETIC PARAMETERS OF PRE-PYROLYSIS FLUID
GENERATION REACTIONS IN AN OIL SHALE FORMATION.
	Pressure	Frequency	Activation		Reaction
	(bars	Factor	Energy		Enthalpy
Reaction	absolute)	[(day)−1]	(kJ/kgmole)	Order	(kJ/kgmole)
4	1.0342	1.197 × 109	125,600	1	0
	4.482	7.938 × 1010
	7.929	2.170 × 1011
	11.376	4.353 × 1011
	14.824	7.545 × 1011
	18.271	1.197 × 1012
5	1.0342	1.197 × 1012	125,600	1	0
	4.482	5.176 × 1011
	7.929	2.037 × 1011
	11.376	6.941 × 1010
	14.824	1.810 × 1010
	18.271	1.197 × 109

[0685] In other embodiments, the generation of hydrocarbons in a pyrolysis temperature range in a formation may be modeled with one or more reactions. One or more reactions may model the amount of hydrocarbon fluids and carbon residue that are generated in a pyrolysis temperature range. Hydrocarbons generated may include light oil, heavy oil, and non-condensable gases. Pyrolysis reactions may also generate water, H₂, and CO₂.

[0686] Experimental results indicate that the composition of products generated in a pyrolysis temperature range may depend on operating conditions such as pressure. For example, the production rate of hydrocarbons generally decreases with pressure. In addition, the amount of produced hydrogen gas generally decreases substantially with pressure, the amount of carbon residue generally increases with pressure, and the amount of condensable hydrocarbons generally decreases with pressure. Furthermore, the amount of non-condensable hydrocarbons generally increases with pressure such that the sum of condensable hydrocarbons and non-condensable hydrocarbons generally remains approximately constant with a change in pressure. In addition, the API gravity of the generated hydrocarbons increases with pressure.

[0687] In an embodiment, the generation of hydrocarbons in a pyrolysis temperature range in an oil shale formation may be modeled with two reactions. One of the reactions may be dominant at high pressures, the other prevailing at low pressures. For example, the molar stoichiometry of the two reactions according to one embodiment may be as follows:
1 mol kerogen→0.02691 mol H₂O+0.009588 mol heavy oil+0.01780 mol light oil+0.04475 mol HCgas+0.01049 mol H₂+0.00541 mol CO₂+0.5827 mol prechar  (6)
1 mol kerogen→0.02691 mol H₂O+0.009588 mol heavy oil+0.01780 mol light oil+0.04475 mol HCgas+0.07930 mol H₂+0.00541 mol CO₂+0.5718 mol prechar  (7)

[0688] Experimentally determined kinetic parameters are shown in TABLE 3. Reactions (6) and (7) model the pressure dependence of hydrogen and carbon residue on pressure. However, the reactions do not take into account the pressure dependence of hydrocarbon production. In one embodiment, the pressure dependence of the production of hydrocarbons may be taken into account by a set of cracking/coking reactions. Alternatively, pressure dependence of hydrocarbon production may be modeled by hydrocarbon generation reactions without cracking/coking reactions.

[0689]

TABLE 3
KINETIC PARAMETERS OF PRE-PYROLYSIS GENERATION
REACTIONS IN AN OIL SHALE FORMATION.
	Pressure	Frequency	Activation		Reaction
	(bars	Factor	Energy		Enthalpy
Reaction	absolute)	[(day)−1]	(kJ/kgmole)	Order	(kJ/kgmole)
6	1.0342	1.000 × 109	161600	1	0
	4.482	2.620 × 1012
	7.929	2.610 × 1012
	11.376	1.975 × 1012
	14.824	1.620 × 1012
	18.271	1.317 × 1012
7	1.0342	4.935 × 1012	161600	1	0
	4.482	1.195 × 1012
	7.929	2.940 × 1011
	11.376	7.250 × 1010
	14.824	1.840 × 1010
	18.271	1.100 × 1010

[0690] In one embodiment, one or more reactions may model the cracking and coking in a formation. Cracking reactions involve the reaction of condensable hydrocarbons (e.g., light oil and heavy oil) to form lighter compounds (e.g., light oil and non-condensable gases) and carbon residue. The coking reactions model the polymerization and condensation of hydrocarbon molecules. Coking reactions lead to formation of char, lower molecular weight hydrocarbons, and hydrogen. Gaseous hydrocarbons may undergo coking reactions to form carbon residue and H₂. Coking and cracking may account for the deposition of coke in the vicinity of heater wells where the temperature may be substantially greater than a pyrolysis temperature. For example, the molar stoichiometry of the cracking and coking reactions in an oil shale formation according to one embodiment may be as follows:
1 mol heavy oil (gas phase)→1.8530 mol light oil+0.045 mol HCgas+2.4515 mol prechar  (8)
1 mol light oil (gas phase)→5.730 mol HCgas  (9)
1 mol heavy oil (liquid phase)→0.2063 mol light oil+2.365 mol HCgas+17.497 mol prechar  (10)
1 mol light oil (liquid phase)→0.5730 mol HCgas+10.904 mol prechar  (11)
1 mol HCgas→2.8 mol H_2+1.6706 mol char  (12)
Kinetic parameters for Reactions 8 to 12 are listed in TABLE 4. The kinetic parameters of the cracking reactions were chosen to match or approximate the oil and gas production observed in laboratory experiments. The kinetic parameter of the coking reaction was derived from experimental data on pyrolysis reactions.

[0691]

TABLE 4
KINETIC PARAMETERS OF CRACKING AND COKING
REACTIONS IN AN OIL SHALE FORMATION.
	Pressure	Frequency	Activation		Reaction
	(bars	Factor	Energy		Enthalpy
Reaction	absolute)	[(day)−1]	(kJ/kgmole)	Order	(kJ/kgmole)
8	1.0342	6.250 × 1016	206034	1	0
	4.482	7.950 × 1016
	7.929
	11.376
	14.824
	18.271
9	1.0342	9.850 × 1016	219328	1	0
	4.482
	7.929
	11.376
	14.824
	18.271	5.850 × 1016
10	—	2.647 × 1020	206034	1	0
11	—	3.820 × 1020	219328	1	0
12	—	7.660 × 1020	311432	1	0

[0692] In addition, reactions may model the generation of water at a temperature below or within a pyrolysis temperature range and the generation of hydrocarbons at a temperature in a pyrolysis temperature range in a coal formation. For example, according to one embodiment, the reactions may include:
1 mol coal→0.01894 mol H₂O+0.0004.91 mol HCgas+0.000047 mol H₂+0.0006.8 mol CO₂+0.99883 mol coalbtm  (13)
1 mol coalbtm→0.02553 mol H₂O+0.00136 mol heavy oil+0.003174 mol light oil+0.01618 mol HCgas+0.0032 mol H₂+0.005599 mol CO₂+0.0008.26 mol CO+0.91306 mol prechar  (14)
1 mol prechar→0.02764 mol H₂O+0.05764 mol HCgas+0.02823 mol H₂+0.0154 mol CO₂+0.006.465 mol CO+0.90598 mol char  (15)

[0693] Reaction (13) models the generation of water in a temperature range below a pyrolysis temperature. Reaction (14) models the generation of hydrocarbons, such as oil and gas, generated in a pyrolysis temperature range. Reaction (15) models gas generated at temperatures between about 370° C. and about 600° C.

[0694] In certain embodiments, the generation of synthesis gas in a formation may be modeled by one or more reactions. For example, the molar stoichiometry of four synthesis gas reactions may be according to one embodiment:
1 mol 0.9442 char+1.0 mol CO₂→2.0 mol CO  (16)
1.0 mol CO→0.5 mol CO₂+0.4721 mol char  (17)
0.94426 mol char+1.0 mol H₂O→1.0 mol H₂+1.0 mol CO  (18)
1.0 mol H₂+1.0 mol CO→0.94426 mol char+1.0 mol H₂O  (19)

[0695] The kinetic parameters of the four reactions are tabulated in TABLE 5. Kinetic parameters for Reactions 16–19 were based on literature data that were adjusted to fit the results of a cube laboratory experiment. Pressure dependence of reactions in the formation is not taken in to account in TABLE 5. In one embodiment, pressure dependence of the reactions in the formation may be modeled, for example, with pressure dependent frequency factors.

[0696]

TABLE 5
KINETIC PARAMETERS FOR SYNTHESIS GAS REACTIONS
IN A FORMATION.
	Frequency
	Factor	Activation Energy		Reaction Enthalpy
Reaction	(day × bar)−1	(kJ/kgmole)	Order	(kJ/kgmole)
16	2.47 × 1011	169970	1	−173033
17	201.6	148.6	1	86516
18	6.44 × 1014	237015	1	−135138
19	2.73 × 107	103191	1	135138

[0697] In one embodiment, a combustion and oxidation reaction of coke to carbon dioxide may be modeled in a formation. For example, the molar stoichiometry of a reaction according to one embodiment may be:
0.9442 mol char+1.0 mol O₂→1.0 mol CO₂  (20)

[0698] Experimentally derived kinetic parameters include a frequency factor of 1.0×10⁴ (day)⁻¹, an activation energy of 58,614 kJ/kgmole, an order of 1, and a reaction enthalpy of 427,977 kJ/kgmole.

[0699] In an embodiment, a method of modeling an in situ process of treating an oil shale formation using a computer system may include simulating a heat input rate to the formation from two or more heat sources. FIG. 24 illustrates method 9360 for simulating heat transfer in a formation. Simulation method 9361 may simulate heat input rate 9368 from two or more heat sources in the formation. For example, the simulation method may be a body-fitted finite difference simulation method. The heat may be allowed to transfer from the heat sources to a selected section of the formation. In an embodiment, the superposition of heat from the two or more heat sources may pyrolyze at least some hydrocarbons within the selected section of the formation. In one embodiment, two or more heat sources may be simulated with a model of heat sources with symmetry boundary conditions.

[0700] In some embodiments, the method may further include providing at least one desired parameter 9366 of the in situ process to the computer system. For example, the desired parameter may be a desired temperature in the formation. In particular, the desired parameter may be a maximum temperature at specific locations in the formation. In addition, the desired parameter may be a desired heating rate or a desired product composition. Desired parameters may also include other parameters such as a desired pressure, process time, production rate, time to obtain a given production rate, and product composition. Process characteristics 9362 determined by simulation method 9361 may be compared 9364 to at least one desired parameter 9366. The method may further include controlling 9363 the heat input rate from the heat sources (or some other process parameter) to achieve at least one desired parameter. Consequently, the heat input rate from the two or more heat sources during a simulation may be time dependent.

[0701] In an embodiment, heat injection into a formation may be initiated by imposing a constant flux per unit area at the interface between a heater and the formation. When a point in the formation, such as the interface, reaches a specified maximum temperature, the heat flux may be varied to maintain the maximum temperature. The specified maximum temperature may correspond to the maximum temperature allowed for a heater well casing (e.g., a maximum operating temperature for the metallurgy in the heater well). In one embodiment, the maximum temperature may be between about 600° C. and about 700° C. In other embodiments, the maximum temperature may be between about 700° C. and about 800° C. In some embodiments, the maximum temperature may be greater than about 800° C.

[0702] FIG. 25 illustrates a model for simulating a heat transfer rate in a formation. Model 9370 represents an aerial view of 1/12^th of a seven spot heater pattern in a formation. The pattern is composed of body-fitted grid elements 9371. The model includes horizontal heater 9372 and producer 9374. A pattern of heaters in a formation is modeled by imposing symmetry boundary conditions. The elements near the heaters and in the region near the heaters are substantially smaller than other portions of the formation to more effectively model a steep temperature profile.

[0703] In one embodiment, an in situ process may be modeled with more than one simulation methods. FIG. 26 illustrates a flow chart of an embodiment of method 8630 for modeling an in situ process for treating an oil shale formation using a computer system. At least one heat input property 8632 may be provided to the computer system. The computer system may include first simulation method 8634. At least one heat input property 8632 may include a heat transfer property of the formation. For example, the heat transfer property of the formation may include heat capacities or thermal conductivities of one or more components in the formation. In certain embodiments, at least one heat input property 8632 includes an initial heat input property of the formation. Initial heat input properties may also include, but are not limited to, volumetric heat capacity, thermal conductivity, porosity, permeability, saturation, compressibility, composition, and the number and types of phases. Properties may also include chemical components, chemical reactions, and kinetic parameters.

[0704] In certain embodiments, first simulation method 8634 may simulate heating of the formation. For example, the first simulation method may simulate heating the wellbore and the near wellbore region. Simulation of heating of the formation may assess (i.e., estimate, calculate, or determine) heat injection rate data 8636 for the formation. In one embodiment, heat injection rate data may be assessed to achieve at least one desired parameter of the formation, such as a desired temperature or composition of fluids produced from the formation. First simulation method 8634 may use at least one heat input property 8632 to assess heat injection rate data 8636 for the formation. First simulation method 8634 may be a numerical simulation method. The numerical simulation may be a body-fitted finite difference simulation method. In certain embodiments, first simulation method 8634 may use at least one heat input property 8632, which is an initial heat input property. First simulation method 8634 may use the initial heat input property to assess heat input properties at later times during treatment (e.g., heating) of the formation.

[0705] Heat injection rate data 8636 may be used as input into second simulation method 8640. In some embodiments, heat injection rate data 8636 may be modified or altered for input into second simulation method 8640. For example, heat injection rate data 8636 may be modified as a boundary condition for second simulation method 8640. At least one property 8638 of the formation may also be input for use by second simulation method 8640. Heat injection rate data 8636 may include a temperature profile in the formation at any time during heating of the formation. Heat injection rate data 8636 may also include heat flux data for the formation. Heat injection rate data 8636 may also include properties of the formation.

[0706] Second simulation method 8640 may be a numerical simulation and/or a reservoir simulation method. In certain embodiments, second simulation method 8640 may be a space-fitted finite difference simulation (e.g., STARS). Second simulation method 8640 may include simulations of fluid mechanics, mass balances, and/or kinetics within the formation. The method may further include providing at least one property 8638 of the formation to the computer system. At least one property 8638 may include chemical components, reactions, and kinetic parameters for the reactions that occur within the formation. At least one property 8638 may also include other properties of the formation such as, but not limited to, permeability, porosities, and/or a location and orientation of heat sources, injection wells, or production wells.

[0707] Second simulation method 8640 may assess at least one process characteristic 8642 as a function of time based on heat injection rate data 8636 and at least one property 8638. In some embodiments, second simulation method 8640 may assess an approximate solution for at least one process characteristic 8642. The approximate solution may be a calculated estimation of at least one process characteristic 8642 based on the heat injection rate data and at least one property. The approximate solution may be assessed using a numerical method in second simulation method 8640. At least one process characteristic 8642 may include one or more parameters produced by treating an oil shale formation in situ. For example, at least one process characteristic 8642 may include, but is not limited to, a production rate of one or more produced fluids, an API gravity of a produced fluid, a weight percentage of a produced component, a total mass recovery from the formation, and operating conditions in the formation such as pressure or temperature.

[0708] In some embodiments, first simulation method 8634 and second simulation method 8640 may be used to predict process characteristics using parameters based on laboratory data. For example, experimentally based parameters may include chemical components, chemical reactions, kinetic parameters, and one or more formation properties. The simulations may further be used to assess operating conditions that can be used to produce desired properties in fluids produced from the formation. In additional embodiments, the simulations may be used to predict changes in process characteristics based on changes in operating conditions and/or formation properties.

[0709] In certain embodiments, one or more of the heat input properties may be initial values of the heat input properties. Similarly, one or more of the properties of the formation may be initial values of the properties. The heat input properties and the reservoir properties may change during a simulation of the formation using the first and second simulation methods. For example, the chemical composition, porosity, permeability, volumetric heat capacity, thermal conductivity, and/or saturation may change with time. Consequently, the heat input rate assessed by the first simulation method may not be adequate input for the second simulation method to achieve a desired parameter of the process. In some embodiments, the method may further include assessing modified heat injection rate data at a specified time of the second simulation. At least one heat input property 8641 of the formation assessed at the specified time of the second simulation method may be used as input by first simulation method 8634 to calculate the modified heat input data. Alternatively, the heat input rate may be controlled to achieve a desired parameter during a simulation of the formation using the second simulation method.

[0710] In some embodiments, one or more model parameters for input into a simulation method may be based on laboratory or field test data of an in situ process for treating an oil shale formation. FIG. 27 illustrates a flow chart of an embodiment of method 9390 for calibrating model parameters to match or approximate laboratory or field data for an in situ process. The method may include providing one or more model parameters 9392 for the in situ process. The model parameters may include properties of the formation. In addition, the model parameters may also include relationships for the dependence of properties on the changes in conditions, such as temperature and pressure, in the formation. For example, model parameters may include a relationship for the dependence of porosity on pressure in the formation. Model parameters may also include an expression for the dependence of permeability on porosity. Model parameters may include an expression for the dependence of thermal conductivity on composition of the formation. In addition, model parameters may include chemical components, the number and types of reactions in the formation, and kinetic parameters. Kinetic parameters may include the order of a reaction, activation energy, reaction enthalpy, and frequency factor.

[0711] In some embodiments, the method may include assessing one or more simulated process characteristics 9396 based on the one or more model parameters. Simulated process characteristics 9396 may be assessed using simulation method 9394. Simulation method 9394 may be a body-fitted finite difference simulation method. Alternatively, simulation method 9394 may be a reservoir simulation method.

[0712] In an embodiment, simulated process characteristics 9396 may be compared 9398 to real process characteristics 9400. Real process characteristics may be process characteristics obtained from laboratory or field tests of an in situ process. Comparing process characteristics may include comparing the simulated process characteristics with the real process characteristics as a function of time. Differences between a simulated process characteristic and a real process characteristic may be associated with one or more model parameters. For example, a higher ratio of gas to oil of produced fluids from a real in situ process may be due to a lack of pressure dependence of kinetic parameters. The method may further include modifying 9399 the one or more model parameters such that at least one simulated process characteristic matches or approximates at least one real process characteristic. One or more model parameters may be modified to account for a difference between a simulated process characteristic and a real process characteristic. For example, an additional chemical reaction may be added to account for pressure dependence or a discrepancy of an amount of a particular component in produced fluids.

[0713] Some embodiments may include assessing one or more modified simulated process characteristics from simulation method 9394 based on modified model parameters 9397. Modified model parameters may include one or both of model parameters 9392 that have been modified and that have not been modified. In an embodiment, the simulation method may use modified model parameters 9397 to assess at least one operating condition of the in situ process to achieve at least one desired parameter.

[0714] Method 9390 may be used to calibrate model parameters for generation reactions of pre-pyrolysis fluids and generation of hydrocarbons from pyrolysis. For example, field test results may show a larger amount of H₂ produced from the formation than the simulation results. The discrepancy may be due to the generation of synthesis gas in the formation in the field test. Synthesis gas may be generated from water in the formation, particularly near heater wells. The temperatures near heater wells may approach a synthesis gas generating temperature range even when the majority of the formation is below synthesis gas generating temperatures. Therefore, the model parameters for the simulation method may be modified to include some synthesis gas reactions.

[0715] In addition, model parameters may be calibrated to account for the pressure dependence of the production of low molecular weight hydrocarbons in a formation. The pressure dependence may arise in both laboratory and field scale experiments. As pressure increases, fluids tend to remain in a laboratory vessel or a formation for longer periods of time. The fluids tend to undergo increased cracking and/or coking with increased residence time in the laboratory vessel or the formation. As a result, larger amounts of lower molecular weight hydrocarbons may be generated. Increased cracking of fluids may be more pronounced in a field scale experiment (as compared to a laboratory experiment, or as compared to calculated cracking) due to longer residence times since fluids may be required to pass through significant distances (e.g., tens of meters) of formation before being produced from a formation.

[0716] Simulations may be used to calibrate kinetic parameters that account for the pressure dependence. For example, pressure dependence may be accounted for by introducing cracking and coking reactions into a simulation. The reactions may include pressure dependent kinetic parameters to account for the pressure dependence. Kinetic parameters may be chosen to match or approximate hydrocarbon production reactions parameters from experiments.

[0717] In certain embodiments, a simulation method based on a set of model parameters may be used to design an in situ process. A field test of an in situ process based on the design may be used to calibrate the model parameters. FIG. 28 illustrates a flow chart of an embodiment of method 9405 for calibrating model parameters. Method 9405 may include assessing at least one operating condition 9414 of the in situ process using simulation method 9410 based on one or more model parameters. Operating conditions may include pressure, temperature, heating rate, heat input rate, process time, weight percentage of gases, peripheral water recovery or injection. Operating conditions may also include characteristics of the well pattern such as producer well location, producer well orientation, ratio of producer wells to heater wells, heater well spacing, type of heater well pattern, heater well orientation, and distance between an overburden and horizontal heater wells. In one embodiment, at least one operating condition may be assessed such that the in situ process achieves at least one desired parameter.

[0718] In some embodiments, at least one operating condition 9414 may be used in real in situ process 9418. In an embodiment, the real in situ process may be a field test, or a field operation, operating with at least one operating condition. The real in situ process may have one or more real process characteristics 9420. Simulation method 9410 may assess one or more simulated process characteristics 9412. In an embodiment, simulated process characteristics 9412 may be compared 9416 to real process characteristics 9420. The one or more model parameters may be modified such that at least one simulated process characteristic 9412 from a simulation of the in situ process matches or approximates at least one real process characteristic 9420 from the in situ process. The in situ process may then be based on at least one operating condition. The method may further include assessing one or more modified simulated process characteristics based on the modified model parameters 9417. In some embodiments, simulation method 9410 may be used to control the in situ process such that the in situ process has at least one desired parameter.

[0719] In one embodiment, a first simulation method may be more effective than a second simulation method in assessing process characteristics under a first set of conditions. Alternatively, the second simulation method may be more effective in assessing process characteristics under a second set of conditions. A first simulation method may include a body-fitted finite difference simulation method. A first set of conditions may include, for example, a relatively sharp interface in an in situ process. In an embodiment, a first simulation method may use a finer grid than a second simulation method. Thus, the first simulation method may be more effective in modeling a sharp interface. A sharp interface refers to a relatively large change in one or more process characteristics in a relatively small region in the formation. A sharp interface may include a relatively steep temperature gradient that may exist in a near wellbore region of a heater well. A relatively steep gradient in pressure and composition, due to pyrolysis, may also exist in the near wellbore region. A sharp interface may also be present at a combustion or reaction front as it propagates through a formation. A steep gradient in temperature, pressure, and composition may be present at a reaction front.

[0720] In certain embodiments, a second simulation method may include a space-fitted finite difference simulation method such as a reservoir simulation method. A second set of conditions may include conditions in which heat transfer by convection is significant. In addition, a second set of conditions may also include condensation of fluids in a formation.

[0721] In some embodiments, model parameters for the second simulation method may be calibrated such that the second simulation method effectively assesses process characteristics under both the first set and the second set of conditions. FIG. 29 illustrates a flow chart of an embodiment of method 9430 for calibrating model parameters for a second simulation method using a first simulation method. Method 9430 may include providing one or more model parameters 9431 to a computer system. One or more first process characteristics 9434 based on one or more model parameters 9431 may be assessed using first simulation method 9432 in memory on the computer system. First simulation method 9432 may be a body-fitted finite difference simulation method. The model parameters may include relationships for the dependence of properties such as porosity, permeability, thermal conductivity, and heat capacity on the changes in conditions (e.g., temperature and pressure) in the formation. In addition, model parameters may include chemical components, the number and types of reactions in the formation, and kinetic parameters. Kinetic parameters may include the order of a reaction, activation energy, reaction enthalpy, and frequency factor. Process characteristics may include, but are not limited to, a temperature profile, pressure, composition of produced fluids, and a velocity of a reaction or combustion front.

[0722] In certain embodiments, one or more second process characteristics 9440 based on one or more model parameters 9431 may be assessed using second simulation method 9438. Second simulation method 9438 may be a space-fitted finite difference simulation method, such as a reservoir simulation method. One or more first process characteristics 9434 may be compared 9436 to one or more second process characteristics 9440. The method may further include modifying one or more model parameters 9431 such that at least one first process characteristic 9434 matches or approximates at least one second process characteristic 9440. For example, the order or the activation energy of the one or more chemical reactions may be modified to account for differences between the first and second process characteristics. In addition, a single reaction may be expressed as two or more reactions. In some embodiments, one or more third process characteristics based on the one or more modified model parameters 9442 may be assessed using the second simulation method.

[0723] In one embodiment, simulations of an in situ process for treating an oil shale formation may be used to design and/or control a real in situ process. Design and/or control of an in situ process may include assessing at least one operating condition that achieves a desired parameter of the in situ process. FIG. 30 illustrates a flow chart of an embodiment of method 9450 for the design and/or control of an in situ process. The method may include providing to the computer system one or more values of at least one operating condition 9452 of the in situ process for use as input to simulation method 9454. The simulation method may be a space-fitted finite difference simulation method such as a reservoir simulation method or it may be a body-fitted simulation method such as FLUENT. At least one operating condition may include, but is not limited to, pressure, temperature, heating rate, heat input rate, process time, weight percentage of gases, peripheral water recovery or injection, production rate, and time to reach a given production rate. In addition, operating conditions may include characteristics of the well pattern such as producer well location, producer well orientation, ratio of producer wells to heater wells, heater well spacing, type of heater well pattern, heater well orientation, and distance between an overburden and horizontal heater wells.

[0724] In one embodiment, the method may include assessing one or more values of at least one process characteristic 9456 corresponding to one or more values of at least one operating condition 9452 from one or more simulations using simulation method 9454. In certain embodiments, a value of at least one process characteristic may include the process characteristic as a function of time. A desired value of at least one process characteristic 9460 for the in situ process may also be provided to the computer system. An embodiment of the method may further include assessing 9458 desired value of at least one operating condition 9462 to achieve desired value of at least one process characteristic 9460. Desired value of at least one operating condition 9462 may be assessed from the values of at least one process characteristic 9456 and values of at least one operating condition 9452. For example, desired value 9462 may be obtained by interpolation of values 9456 and values 9452. In some embodiments, a value of at least one process characteristic may be assessed from the desired value of at least one operating condition 9462 using simulation method 9454. In some embodiments, an operating condition to achieve a desired parameter may be assessed by comparing a process characteristic as a function of time for different operating conditions. In an embodiment, the method may include operating the in situ system using the desired value of at least one additional operating condition.

[0725] In an alternate embodiment, a desired value of at least one operating condition to achieve the desired value of at least one process characteristic may be assessed by using a relationship between at least one process characteristic and at least one operating condition of the in situ process. The relationship may be assessed from a simulation method. The relationship may be stored on a database accessible by the computer system. The relationship may include one or more values of at least one process characteristic and corresponding values of at least one operating condition. Alternatively, the relationship may be an analytical function.

[0726] In an embodiment, a desired process characteristic may be a selected composition of fluids produced from a formation. A selected composition may correspond to a ratio of non-condensable hydrocarbons to condensable hydrocarbons. In certain embodiments, increasing the pressure in the formation may increase the ratio of non-condensable hydrocarbons to condensable hydrocarbons of produced fluids. The pressure in the formation may be controlled by increasing the pressure at a production well in an in situ process. In an alternate embodiment, another operating condition may be controlled simultaneously (e.g., the heat input rate).

[0727] In an embodiment, the pressure corresponding to the selected composition may be assessed from two or more simulations at two or more pressures. In one embodiment, at least one of the pressures of the simulations may be estimated from EQN. 21:

[0728] $\begin{array}{cc}p={\exp }^{\left[\frac{A}{T}+B\right]}& \left(21\right)\end{array}$
where p is measured in psia (pounds per square inch absolute), T is measured in Kelvin, and A and B are parameters dependent on the value of the desired process characteristic for a given type of formation. Values of A and B may be assessed from experimental data for a process characteristic in a given formation and may be used as input to EQN. 21. The pressure corresponding to the desired value of the process characteristic may then be estimated for use as input into a simulation.

[0729] The two or more simulations may provide a relationship between pressure and the composition of produced fluids. The pressure corresponding to the desired composition may be interpolated from the relationship. A simulation at the interpolated pressure may be performed to assess a composition and one or more additional process characteristics. The accuracy of the interpolated pressure may be assessed by comparing the selected composition with the composition from the simulation. The pressure at the production well may be set to the interpolated pressure to obtain produced fluids with the selected composition.

[0730] In certain embodiments, the pressure of a formation may be readily controlled at certain stages of an in situ process. At some stages of the in situ process, however, pressure control may be relatively difficult. For example, during a relatively short period of time after heating has begun, the permeability of the formation may be relatively low. At such early stages, the heat transfer front at which pyrolysis occurs may be at a relatively large distance from a producer well (i.e., the point at which pressure may be controlled). Therefore, there may be a significant pressure drop between the producer well and the heat transfer front. Consequently, adjusting the pressure at a producer well may have a relatively small influence on the pressure at which pyrolysis occurs at early stages of the in situ process. At later stages of the in situ process when permeability has developed relatively uniformly throughout the formation, the pressure of the producer well corresponds to the pressure in the formation. Therefore, the pressure at the producer well may be used to control the pressure at which pyrolysis occurs.

[0731] In some embodiments, a similar procedure may be followed to assess heater well pattern and producer well pattern characteristics that correspond to a desired process characteristic. For example, a relationship between the spacing of the heater wells and composition of produced fluids may be obtained from two or more simulations with different heater well spacings.

[0732] In one embodiment, a simulation method on a computer system may be used in a method for modeling one or more stages of a process for treating an oil shale formation in situ. The simulation method may be, for example, a reservoir simulation method. The simulation method may simulate heating of the formation, fluid flow, mass transfer, heat transfer, and chemical reactions in one or more of the stages of the process. In some embodiments, the simulation method may also simulate removal of contaminants from the formation, recovery of heat from the formation, and injection of fluids into the formation.

[0733] Method 9588 of modeling the one or more stages of a treatment process is depicted in a flow chart in FIG. 31. The one or more stages may include heating stage 9574, pyrolyzation stage 9576, synthesis gas generation stage 9579, remediation stage 9582, and/or shut-in stage 9585. The method may include providing at least one property 9572 of the formation to the computer system. In addition, operating conditions 9573, 9577, 9580, 9583, and/or 9586 for one or more of the stages of the in situ process may be provided to the computer system. Operating conditions may include, but not be limited to, pressure, temperature, heating rates, etc. In addition, operating conditions of a remediation stage may include a flow rate of ground water and injected water into the formation, size of treatment area, and type of drive fluid.

[0734] In certain embodiments, the method may include assessing process characteristics 9575, 9578, 9581, 9584, and/or 9587 of the one or more stages using the simulation method. Process characteristics may include properties of a produced fluid such as API gravity and gas/oil ratio. Process characteristics may also include a pressure and temperature in the formation, total mass recovery from the formation, and production rate of fluid produced from the formation. In addition, a process characteristic of the remediation stage may include the type and concentration of contaminants remaining in the formation.

[0735] In one embodiment, a simulation method may be used to assess operating conditions of at least one of the stages of an in situ process that results in desired process characteristics. FIG. 32 illustrates a flow chart of an embodiment of method 9770 for designing and controlling heating stage 9771, pyrolyzation stage 9772, synthesis gas generating stage 9773, remediation stage 9774, and/or shut-in stage 9775 of an in situ process with a simulation method on a computer system. The method may include providing sets of operating conditions 9776, 9777, 9778, 9779, and/or 9780 for at least one of the stages of the in situ process. In addition, desired process characteristics 9781, 9782, 9783, 9784, and/or 9785 for at least one of the stages of the in situ process may also be provided. The method may further include assessing at least one additional operating condition 9786, 9787, 9788, 9789, and/or 9790 for at least one of the stages that achieves the desired process characteristics of one or more stages.

[0736] In an embodiment, in situ treatment of an oil shale formation may substantially change physical and mechanical properties of the formation. The physical and mechanical properties may be affected by chemical properties of a formation, operating conditions, and process characteristics.

[0737] Changes in physical and mechanical properties due to treatment of a formation may result in deformation of the formation. Deformation characteristics may include, but are not limited to, subsidence, compaction, heave, and shear deformation. Subsidence is a vertical decrease in the surface of a formation over a treated portion of a formation. Heave is a vertical increase at the surface above a treated portion of a formation. Surface displacement may result from several concurrent subsurface effects, such as the thermal expansion of layers of the formation, the compaction of the richest and weakest layers, and the constraining force exerted by cooler rock that surrounds the treated portion of the formation. In general, in the initial stages of heating a formation, the surface above the treated portion may show a heave due to thermal expansion of incompletely pyrolyzed formation material in the treated portion of the formation. As a significant portion of formation becomes pyrolyzed, the formation is weakened and pore pressure in the treated portion declines. The pore pressure is the pressure of the liquid and gas that exists in the pores of a formation. The pore pressure may be influenced by the thermal expansion of the organic matter in the formation and the withdrawal of fluids from the formation. The decrease in the pore pressure tends to increase the effective stress in the treated portion. Since the pore pressure affects the effective stress on the treated portion of a formation, pore pressure influences the extent of subsurface compaction in the formation. Compaction, another deformation characteristic, is a vertical decrease of a subsurface portion above or in the treated portion of the formation. In addition, shear deformation of layers both above and in the treated portion of the formation may also occur. In some embodiments, deformation may adversely affect the in situ treatment process. For example, deformation may seriously damage surface facilities and wellbores.

[0738] In certain embodiments, an in situ treatment process may be designed and controlled such that the adverse influence of deformation is minimized or substantially eliminated. Computer simulation methods may be useful for design and control of an in situ process since simulation methods may predict deformation characteristics. For example, simulation methods may predict subsidence, compaction, heave, and shear deformation in a formation from a model of an in situ process. The models may include physical, mechanical, and chemical properties of a formation. Simulation methods may be used to study the influence of properties of a formation, operating conditions, and process characteristics on deformation characteristics of the formation.

[0739] FIG. 33 illustrates model 9791 of a formation that may be used in simulations of deformation characteristics according to one embodiment. The formation model is a vertical cross section that may include treated portion 9792 with thickness 9793 and width or radius 9794. Treated portion 9792 may include several layers or regions that vary in mineral composition and richness of organic matter. For example, in a model of an oil shale formation, treated portion 9792 may include layers of lean kerogenous chalk rich kerogenous chalk, and silicified kerogenous chalk. In one embodiment, treated portion 9792 may be a dipping layer that is at an angle to the surface of the formation. The model may also include untreated portions such as overburden 9795 and base rock 9796. Overburden 9795 may have thickness 9797. Overburden 9795 may also include one or more portions, for example, portion 9798 and portion 9799 that differ in composition. For example, portion 9799 may have a composition similar to treated portion 9792 prior to treatment. Portion 9798 may be composed of organic material, soil, rock, etc. Base rock 9796 may include barren rock with at least some organic material.

[0740] In some embodiments, an in situ process may be designed such that it includes an untreated portion or strip between treated portions of the formation. FIG. 34 illustrates a schematic of a strip development according to one embodiment. The formation includes treated portion 9523 and treated portion 9525 with thicknesses 9531 and widths 9533 (thicknesses 9531 and widths 9533 may vary between portion 9523 and portion 9525). Untreated portion 9527 with width 9529 separates treated portion 9523 from treated portion 9525. In some embodiments, width 9529 is substantially less than widths 9533 since only smaller sections need to remain untreated to provide structural support. In some embodiments, the use of an untreated portion may decrease the amount of subsidence, heave, compaction, or shear deformation at and above the treated portions of the formation.

[0741] In an embodiment, an in situ treatment process may be represented by a three-dimensional model. FIG. 35 depicts a schematic illustration of a treated portion that may be modeled with a simulation. The treated portion includes a well pattern with heat sources 9524 and producers 9526. Dashed lines 9528 correspond to three planes of symmetry that may divide the pattern into six equivalent sections. Solid lines between heat sources 9524 merely depict the pattern of heat sources (i.e., the solid lines do not represent actual equipment between the heat sources). In some embodiments, a geomechanical model of the pattern may include one of the six symmetry segments.

[0742] FIG. 36 depicts a horizontal cross section of a model of a formation for use by a simulation method according to one embodiment. The model includes grid elements 9530. Treated portion 9532 is located in the lower left corner of the model. Grid elements in the treated portion may be sufficiently small to take into account the large variations in conditions in the treated portion. In addition, distance 9537 and distance 9539 may be sufficiently large such that the deformation furthest from the treated portion is substantially negligible. Alternatively, a model may be approximated by a shape, such as a cylinder. The diameter and height of the cylinder may correspond to the size and height of the treated portion.

[0743] In certain embodiments, heat sources may be modeled by line sources that inject heat at a fixed rate. The heat sources may generate a reasonably accurate temperature distribution in the vicinity of the heat sources. Alternatively, a time-dependent temperature distribution may be imposed as an average boundary condition.

[0744] FIG. 37 illustrates a flow chart of an embodiment of method 9543 for modeling deformation due to treatment of an oil shale formation in situ. The method may include providing at least one property 9534 of the formation to a computer system. The formation may include a treated portion and an untreated portion. Properties may include mechanical, chemical, thermal, and physical properties of the portions of the formation. For example, the mechanical properties may include compressive strength, confining pressure, creep parameters, elastic modulus, Poisson's ratio, cohesion stress, friction angle, and cap eccentricity. Thermal and physical properties may include a coefficient of thermal expansion, volumetric heat capacity, and thermal conductivity. Properties may also include the porosity, permeability, saturation, compressibility, and density of the formation. Chemical properties may include, for example, the richness and/or organic content of the portions of the formation.

[0745] In addition, at least one operating condition 9535 may be provided to the computer system. For instance, operating conditions may include, but are not limited to, pressure, temperature, process time, rate of pressure increase, heating rate, and characteristics of the well pattern. In addition, an operating condition may include the overburden thickness and thickness and width or radius of the treated portion of the formation. An operating condition may also include untreated portions between treated portions of the formation, along with the horizontal distance between treated portions of a formation.

[0746] In certain embodiments, the properties may include initial properties of the formation. Furthermore, the model may include relationships for the dependence of the mechanical, thermal, and physical properties on conditions such as temperature, pressure, and richness in the treated portions of the formation. For example, the compressive strength in the treated portion of the formation may be a function of richness, temperature, and pressure. The volumetric heat capacity may depend on the richness and the coefficient of thermal expansion may be a function of the temperature and richness. Additionally, the permeability, porosity, and density may be dependent upon the richness of the formation.

[0747] In some embodiments, physical and mechanical properties for a model of a formation may be assessed from samples extracted from a geological formation targeted for treatment. Properties of the samples may be measured at various temperatures and pressures. For example, mechanical properties may be measured using uniaxial, triaxial, and creep experiments. In addition, chemical properties (e.g., richness) of the samples may also be measured. Richness of the samples may be measured by the Fischer Assay method. The dependence of properties on temperature, pressure, and richness may then be assessed from the measurements. In certain embodiments, the properties may be mapped on to a model using known sample locations. For instance, FIG. 38 depicts a profile of richness versus depth in a model of an oil shale formation. The treated portion is represented by region 9545. Similarly, the overburden and base rock are represented by region 9547 and region 9549, respectively. In FIG. 38, richness is measured in m³ of kerogen per metric ton of oil shale.

[0748] In certain embodiments, assessing deformation using a simulation method may require a material or constitutive model. A constitutive model relates the stress in the formation to the strain or displacement. Mechanical properties may be entered into a suitable constitutive model to calculate the deformation of the formation. In one embodiment, the Drucker-Prager-with-cap material model may be used to model the time-independent deformation of the formation.

[0749] In an embodiment, the time-dependent creep or secondary creep strain of the formation may also be modeled. For example, the time-dependent creep in a formation may be modeled with a power law in EQN. 22:
ε=C×(σ₁−σ₃)^D×t  (22)
where ε is the secondary creep strain, C is a creep multiplier, σ₁ is the axial stress, σ₃ is the confining pressure, D is a stress exponent, and t is the time. The values of C and D may be obtained from fitting experimental data. In one embodiment, the creep rate may be expressed by EQN. 23:
dε/dt=A×(σ₁/σ_u)^D  (23)
where A is a multiplier obtained from fitting experimental data and σ_u is the ultimate strength in uniaxial compression.

[0750] The method shown in FIG. 37 may further include assessing 9536 at least one process characteristic 9538 of the treated portion of the formation. At least one process characteristic 9538 may include a pore pressure distribution, a heat input rate, or a time dependent temperature distribution in the treated portion of the formation.

[0751] At least one process characteristic may be assessed by a simulation method. For example, a heat input rate may be estimated using a body-fitted finite difference simulation package such as FLUENT. Similarly, the pore pressure distribution may be assessed from a space-fitted or body-fitted simulation method such as STARS. In other embodiments, the pore pressure may be assessed by a finite element simulation method such as ABAQUS. The finite element simulation method may employ line sinks of fluid to simulate the performance of production wells.

[0752] Alternatively, process characteristics such as temperature distribution and pore pressure distribution may be approximated by other means. For example, the temperature distribution may be imposed as an average boundary condition in the calculation of deformation characteristics. The temperature distribution may be estimated from results of detailed calculations of a heating rate of a formation. For example, a treated portion may be heated to a pyrolyzation temperature for a specified period of time by heat sources and the temperature distribution assessed during heating of the treated portion. In an embodiment, the heat sources may be uniformly distributed and inject a constant amount of heat. The temperature distribution inside most of the treated portion may be substantially uniform during the specified period of time. Some heat may be allowed to diffuse from the treated portion into the overburden, base rock, and lateral rock. The treated portion may be maintained at a selected temperature for a selected period of time after the specified period of time by injecting heat from the heat sources as needed.

[0753] Similarly, the pore pressure distribution may also be imposed as an average boundary condition. The initial pore pressure distribution may be assumed to be lithostatic. The pore pressure distribution may then be gradually reduced to a selected pressure during the remainder of the simulation of the deformation characteristics.

[0754] In some embodiments, as shown in FIG. 37, the method may include assessing at least one deformation characteristic 9542 of the formation using simulation method 9540 on the computer system as a function of time. At least one deformation characteristic may be assessed from at least one property 9534, at least one process characteristic 9538, and at least one operating condition 9535. In certain embodiments, process characteristic 9538 may be assessed by a simulation or process characteristic 9538 may be measured. Deformation characteristics may include, but are not limited to, subsidence, compaction, heave, and shear deformation in the formation.

[0755] Simulation method 9540 may be a finite element simulation method for calculating elastic, plastic, and time dependent behavior of materials. For example, ABAQUS is a commercially available finite element simulation method from Hibbitt, Karlsson & Sorensen, Inc. located in Pawtucket, R.I. ABAQUS is capable of describing the elastic, plastic, and time dependent (creep) behavior of a broad class of materials such as mineral matter, soils, and metals. In general, ABAQUS may treat materials whose properties may be specified by user-defined constitutive laws. ABAQUS may also calculate heat transfer and treat the effect of pore pressure variations on rock deformation.

[0756] Computer simulations may be used to assess operating conditions of an in situ process in a formation that may result in desired deformation characteristics. FIG. 39 illustrates a flow chart of an embodiment of method 9544 for designing and controlling an in situ process using a computer system. The method may include providing to the computer system at least one set of operating conditions 9546 for the in situ process. For instance, operating conditions may include pressure, temperature, process time, rate of pressure increase, heating rate, characteristics of the well pattern, the overburden thickness, thickness and width of the treated portion of the formation and/or untreated portions between treated portions of the formation, and the horizontal distance between treated portions of a formation.

[0757] In addition, at least one desired deformation characteristic 9548 for the in situ process may be provided to the computer system. The desired deformation characteristic may be a selected subsidence, selected heave, selected compaction, or selected shear deformation. In some embodiments, at least one additional operating condition 9551 may be assessed using simulation method 9550 that achieves at least one desired deformation characteristic 9548. A desired deformation characteristic may be a value that does not adversely affect the operation of an in situ process. For example, a minimum overburden necessary to achieve a desired maximum value of subsidence may be assessed. In an embodiment, at least one additional operating condition 9551 may be used to operate in situ process 9552.

[0758] In one embodiment, operating conditions to obtain desired deformation characteristics may be assessed from simulations of an in situ process based on multiple operating conditions. FIG. 40 illustrates a flow chart of an embodiment of method 9554 for assessing operating conditions to obtain desired deformation characteristics. The method may include providing one or more values of at least one operating condition 9556 to a computer system for use as input to simulation method 9558. The simulation method may be a finite element simulation method for calculating elastic, plastic, and creep behavior.

[0759] In some embodiments, the method may further include assessing one or more values of deformation characteristics 9560 using simulation method 9558 based on the one or more values of at least one operating condition 9556. In one embodiment, a value of at least one deformation characteristic may include the deformation characteristic as a function of time. A desired value of at least one deformation characteristic 9564 for the in situ process may also be provided to the computer system. An embodiment of the method may include assessing 9562 desired value of at least one operating condition 9566 to achieve desired value of at least one deformation characteristic 9564.

[0760] Desired value of at least one operating condition 9566 may be assessed from the values of at least one deformation characteristic 9560 and the values of at least one operating condition 9556. For example, desired value 9566 may be obtained by interpolation of values 9560 and values 9556. In some embodiments, a value of at least one deformation characteristic may be assessed 9565 from the desired value of at least one operating condition 9566 using simulation method 9558. In some embodiments, an operating condition to achieve a desired deformation characteristic may be assessed by comparing a deformation characteristic as a function of time for different operating conditions.

[0761] In an alternate embodiment, a desired value of at least one operating condition to achieve the desired value of at least one deformation characteristic may be assessed using a relationship between at least one deformation characteristic and at least one operating condition of the in situ process. The relationship may be assessed using a simulation method. Such relationship may be stored on a database accessible by the computer system. The relationship may include one or more values of at least one deformation characteristic and corresponding values of at least one operating condition. Alternatively, the relationship may be an analytical function.

[0762] Simulations have been used to investigate the effect of various operating conditions on the deformation characteristics of an oil shale formation. In one set of simulations, the formation was modeled as either a cylinder or a rectangular slab. In the case of a cylinder, the model of the formation is described by a thickness of the treated portion, a radius, and a thickness of the overburden. The rectangular slab is described by a width rather than a radius and by a thickness of the treated section and overburden. FIG. 41 illustrates the influence of operating pressure on subsidence in a cylindrical model of a formation from a finite element simulation. The thickness of the treated portion is 189 m, the radius of the treated portion is 305 m, and the overburden thickness is 201 m. FIG. 41 shows the vertical surface displacement in meters over a period of years. Curve 9568 corresponds to an operating pressure of 27.6 bars absolute and curve 9569 to an operating pressure of 6.9 bars absolute. It is to be understood that the surface displacements set forth in FIG. 41 are only illustrative (actual surface displacements will generally differ from those shown in FIG. 41). FIG. 41 demonstrates, however, that increasing the operating pressure may substantially reduce subsidence.

[0763] FIGS. 42 and 43 illustrate the influence of the use of an untreated portion between two treated portions. FIG. 42 is the subsidence in a rectangular slab model with a treated portion thickness of 189 m, treated portion width of 649 m, and overburden thickness of 201 m. FIG. 43 represents the subsidence in a rectangular slab model with two treated portions separated by an untreated portion, as pictured in FIG. 34. The thickness of the treated portion and the overburden are the same as the model corresponding to FIG. 42. The width of each treated portion is one half of the width of the treated portion of the model in FIG. 42. Therefore, the total width of the treated portions is the same for each model. The operating pressure in each case is 6.9 bars absolute. As with FIG. 41, the surface displacements in FIGS. 42 and 43 are only illustrative. A comparison of FIGS. 42 and 43, however, shows that the use of an untreated portion reduces the subsidence by about 25%. In addition, the initial heave is also reduced.

[0764] In another set of simulations, the calculation of the shear deformation in a treated oil shale formation was demonstrated. The model included a symmetry element of a pattern of heat sources and producer wells. Boundary conditions imposed in the model were such that the vertical planes bounding the formation were symmetry planes. FIG. 44 represents the shear deformation of the formation at the location of selected heat sources as a function of depth. Curve 9570 and curve 9571 represent the shear deformation as a function of depth at 10 months and 12 months, respectively. The curves, which correspond to the predicted shape of the heat injection wells, show that shear deformation increases with depth in the formation.

[0765] In certain embodiments, a computer system may be used to operate an in situ process for treating an oil shale formation. The in situ process may include providing heat from one or more heat sources to at least one portion of the formation. In addition, the in situ process may also include allowing the heat to transfer from the one or more heat sources to a selected section of the formation. FIG. 45 illustrates method 9480 for operating an in situ process using a computer system. The method may include operating in situ process 9482 using one or more operating parameters. Operating parameters may include properties of the formation, such as heat capacity, density, permeability, thermal conductivity, porosity, and/or chemical reaction data. In addition, operating parameters may include operating conditions. Operating conditions may include, but are not limited to, thickness and area of heated portion of the formation, pressure, temperature, heating rate, heat input rate, process time, production rate, time to obtain a given production rate, weight percentage of gases, and/or peripheral water recovery or injection. Operating conditions may also include characteristics of the well pattern such as producer well location, producer well orientation, ratio of producer wells to heater wells, heater well spacing, type of heater well pattern, heater well orientation, and/or distance between an overburden and horizontal heater wells. Operating parameters may also include mechanical properties of the formation. Operating parameters may include deformation characteristics, such as fracture, strain, subsidence, heave, compaction, and/or shear deformation.

[0766] In certain embodiments, at least one operating parameter 9484 of in situ process 9482 may be provided to computer system 9486. Computer system 9486 may be at or near in situ process 9482. Alternatively, computer system 9486 may be at a location remote from in situ process 9482. The computer system may include a first simulation method for simulating a model of in situ process 9482. In one embodiment, the first simulation method may include method 9470 illustrated in FIG. 22, method 9360 illustrated in FIG. 24, method 8630 illustrated in FIG. 26, method 9390 illustrated in FIG. 27, method 9405 illustrated in FIG. 28, method 9430 illustrated in FIG. 29, and/or method 9450 illustrated in FIG. 30. The first simulation method may include a body-fitted finite difference simulation method such as FLUENT or space-fitted finite difference simulation method such as STARS. The first simulation method may perform a reservoir simulation. A reservoir simulation method may be used to determine operating parameters including, but not limited to, pressure, temperature, heating rate, heat input rate, process time, production rate, time to obtain a given production rate, weight percentage of gases, and peripheral water recovery or injection.

[0767] In an embodiment, the first simulation method may also calculate deformation in a formation. A simulation method for calculating deformation characteristics may include a finite element simulation method such as ABAQUS. The first simulation method may calculate fracture progression, strain, subsidence, heave, compaction, and shear deformation. A simulation method used for calculating deformation characteristics may include method 9543 illustrated in FIG. 37 and/or method 9554 illustrated in FIG. 40.

[0768] The method may further include using at least one parameter 9484 with a first simulation method and the computer system to provide assessed information 9488 about in situ process 9482. Operating parameters from the simulation may be compared to operating parameters of in situ process 9482. Assessed information from a simulation may include a simulated relationship between one or more operating parameters with at least one parameter 9484. For example, the assessed information may include a relationship between operating parameters such as pressure, temperature, heating input rate, or heating rate and operating parameters relating to product quality.

[0769] In some embodiments, assessed information may include inconsistencies between operating parameters from simulation and operating parameters from in situ process 9482. For example, the temperature, pressure, product quality, or production rate from the first simulation method may differ from in situ process 9482. The source of the inconsistencies may be assessed from the operating parameters provided by simulation. The source of the inconsistencies may include differences between certain properties used in a simulated model of in situ process 9482 and in situ process 9482. Certain properties may include, but are not limited to, thermal conductivity, heat capacity, density, permeability, or chemical reaction data. Certain properties may also include mechanical properties such as compressive strength, confining pressure, creep parameters, elastic modulus, Poisson's ratio, cohesion stress, friction angle, and cap eccentricity.

[0770] In one embodiment, assessed information may include adjustments in one or more operating parameters of in situ process 9482. The adjustments may compensate for inconsistencies between simulated operating parameters and operating parameters from in situ process 9482. Adjustments may be assessed from a simulated relationship between at least one parameter 9484 and one or more operating parameters.

[0771] For example, an in situ process may have a particular hydrocarbon fluid production rate, e.g., 1 m³/day, after a particular period of time (e.g., 90 days). A theoretical temperature at an observation well (e.g., 100° C.) may be calculated using given properties of the formation. However, a measured temperature at an observation well (e.g., 80° C.) may be lower than the theoretical temperature. A simulation on a computer system may be performed using the measured temperature. The simulation may provide operating parameters of the in situ process that correspond to the measured temperature. The operating parameters from simulation may be used to assess a relationship between, for example, temperature or heat input rate and the production rate of the in situ process. The relationship may indicate that the heat capacity or thermal conductivity of the formation used in the simulation is inconsistent with the formation.

[0772] In some embodiments, the method may further include using assessed information 9488 to operate in situ process 9482. As used herein, “operate” refers to controlling or changing operating conditions of an in situ process. For example, the assessed information may indicate that the thermal conductivity of the formation in the above example is lower than the thermal conductivity used in the simulation. Therefore, the heat input rate to in situ process 9482 may be increased to operate at the theoretical temperature.

[0773] In other embodiments, the method may include obtaining 9492 information 9494 from a second simulation method and the computer system using assessed information 9488 and desired parameter 9490. In one embodiment, the first simulation method may be the same as the second simulation method. In another embodiment, the first and second simulation methods may be different. Simulations may provide a relationship between at least one operating parameter and at least one other parameter. Additionally, obtained information 9494 may be used to operate in situ process 9482.

[0774] Obtained information 9494 may include at least one operating parameter for use in the in situ process that achieves the desired parameter. In one embodiment, simulation method 9450 illustrated in FIG. 30 may be used to obtain at least one operating parameter that achieves the desired parameter. For example, a desired hydrocarbon fluid production rate for an in situ process may be 6 m³/day. One or more simulations may be used to determine the operating parameters necessary to achieve a hydrocarbon fluid production rate of 6 m³/day. In some embodiments, model parameters used by simulation method 9450 may be calibrated to account for differences observed between simulations and in situ process 9482. In one embodiment, simulation method 9390 illustrated in FIG. 27 may be used to calibrate model parameters. In another embodiment, simulation method 9554 illustrated in FIG. 40 may be used to obtain at least one operating parameter that achieves a desired deformation characteristic.

[0775] FIG. 46 illustrates a schematic of an embodiment for controlling in situ process 9701 in a formation using a computer simulation method. In situ process 9701 may include sensor 9702 for monitoring operating parameters. Sensor 9702 may be located in a barrier well, a monitoring well, a production well, or a heater well. Sensor 9702 may monitor operating parameters such as subsurface and surface conditions in the formation. Subsurface conditions may include pressure, temperature, product quality, and deformation characteristics, such as fracture progression. Sensor 9702 may also monitor surface data such as pump status (i.e., on or off), fluid flow rate, surface pressure/temperature, and heater power. The surface data may be monitored with instruments placed at a well.

[0776] In addition, at least one operating parameter 9704 measured by sensor 9702 may be provided to local computer system 9708. Alternatively, operating parameter 9704 may be provided to remote computer system 9706. Computer system 9706 may be, for example, a personal desktop computer system, a laptop, or personal digital assistant such as a palm pilot. FIG. 47 illustrates several ways that information may be transmitted from in situ process 9701 to remote computer system 9706. Information may be transmitted by means of internet 9718, hardwire telephone lines 9720, and wireless communications 9722. Wireless communications 9722 may include transmission via satellite 9724.

[0777] In some embodiments, as shown in FIG. 46, operating parameter 9704 may be provided to computer system 9708 or 9706 automatically during the treatment of a formation. Computer systems 9706 and 9708 may include a simulation method for simulating a model of the in situ treatment process 9701. The simulation method may be used to obtain information 9710 about the in situ process.

[0778] In an embodiment, a simulation of in situ process 9701 may be performed manually at a desired time. Alternatively, a simulation may be performed automatically when a desired condition is met. For instance, a simulation may be performed when one or more operating parameters reach, or fail to reach, a particular value at a particular time. For example, a simulation may be performed when the production rate fails to reach a particular value at a particular time.

[0779] In some embodiments, information 9710 relating to in situ process 9701 may be provided automatically by computer system 9706 or 9708 for use in controlling in situ process 9701. Information 9710 may include instructions relating to control of in situ process 9701. Information 9710 may be transmitted from computer system 9706 via internet, hardwire, wireless, or satellite transmission. Information 9710 may be provided to computer system 9712. Computer system 9712 may also be at a location remote from the in situ process. Computer system 9712 may process information 9710 for use in controlling in situ process 9701. For example, computer system 9712 may use information 9710 to determine adjustments in one or more operating parameters. Computer system 9712 may then automatically adjust 9716 one or more operating parameters of in situ process 9701. Alternatively, one or more operating parameters of in situ process 9701 may be displayed and then, optionally, adjusted manually 9714.

[0780] FIG. 48 illustrates a schematic of an embodiment for controlling in situ process 9701 in a formation using information 9710. Information 9710 may be obtained using a simulation method and a computer system. Information 9710 may be provided to computer system 9712. Information 9710 may include information that relates to adjusting one or more operating parameters. Output 9713 from computer system 9712 may be provided to display 9719, data storage 9721, or surface facility 9723. Output 9713 may also be used to automatically control conditions in the formation by adjusting one or more operating parameters. Output 9713 may include instructions to adjust pump status and flow rate at a barrier well 9726, adjust pump status and flow rate at a production well 9728, and/or adjust the heater power at a heater well 9730. Output 9713 may also include instructions to heating pattern 9732 of in situ process 9701. For example, an instruction may be to add one or more heater wells at particular locations. In addition, output 9713 may include instructions to shut-in the formation 9734.

[0781] Alternatively, output 9713 may be viewed by operators of the in situ process on display 9719. The operators may then use output 9713 to manually adjust one or more operating parameters.

[0782] FIG. 49 illustrates a schematic of an embodiment for controlling in situ process 9701 in a formation using a simulation method and a computer system. At least one operating parameter 9704 from in situ process 9701 may be provided to computer system 9736. Computer system 9736 may include a simulation method for simulating a model of in situ process 9701. Computer system 9736 may use the simulation method to obtain information 9738 about in situ process 9701. Information 9738 may be provided to data storage 9740, display 9742, and analysis 9743. In an embodiment, information 9738 may be automatically provided to in situ process 9701. Information 9738 may then be used to operate in situ process 9701.

[0783] Analysis 9743 may include review of information 9738 and/or use of information 9738 to operate in situ process 9701. Analysis 9743 may include obtaining additional information 9750 using one or more simulations 9746 of in situ process 9701. One or more simulations may be used to obtain additional or modified model parameters of in situ process 9701. The additional or modified model parameters may be used to further assess in situ process 9701. Simulation method 9390 illustrated in FIG. 27 may be used to determine additional or modified model parameters. Method 9390 may use at least one operating parameter 9704 and information 9738 to calibrate model parameters. For example, at least one operating parameter 9704 may be compared to at least one simulated operating parameter. Model parameters may be modified such that at least one simulated operating parameter matches or approximates at least one operating parameter 9704.

[0784] In an embodiment, analysis 9743 may include obtaining 9744 additional information 9748 about properties of in situ process 9701. Properties may include, for example, thermal conductivity, heat capacity, porosity, or permeability of one or more portions of the formation. Properties may also include chemical reaction data such as chemical reactions, chemical components, and chemical reaction parameters. Properties may be obtained from the literature or from field or laboratory experiments. For example, properties of core samples of the treated formation may be measured in a laboratory. Additional information 9748 may be used to operate in situ process 9701. Alternatively, additional information 9748 may be used in one or more simulations 9746 to obtain additional information 9750. For example, additional information 9750 may include one or more operating parameters that may be used to operate in situ process 9701. In one embodiment, method 9450 illustrated in FIG. 30 may be used to determine operating parameters to achieve a desired parameter. The operating parameters may then be used to operate in situ process 9701.

[0785] An in situ process for treating a formation may include treating a selected section of the formation with a minimum average overburden thickness. The minimum average overburden thickness may depend on a type of hydrocarbon resource and geological formation surrounding the hydrocarbon resource. An overburden may, in some embodiments, be substantially impermeable so that fluids produced in the selected section are inhibited from passing to the ground surface through the overburden. A minimum overburden thickness may be determined as the minimum overburden needed to inhibit the escape of fluids produced in the formation and to inhibit breakthrough to the surface due to increased pressure within the formation during in the situ conversion process. Determining this minimum overburden thickness may be dependent on, for example, composition of the overburden, maximum pressure to be reached in the formation during the in situ conversion process, permeability of the overburden, composition of fluids produced in the formation, and/or temperatures in the formation or overburden. A ratio of overburden thickness to hydrocarbon resource thickness may be used during selection of resources to produce using an in situ thermal conversion process.

[0786] Selected factors may be used to determine a minimum overburden thickness. These selected factors may include overall thickness of the overburden, lithology and/or rock properties of the overburden, earth stresses, expected extent of subsidence and/or reservoir compaction, a pressure of a process to be used in the formation, and extent and connectivity of natural fracture systems surrounding the formation.

[0787] For oil shale, a minimum overburden thickness may be about 100 m or between about 25 m and 300 m. A minimum overburden to resource thickness may be between about 0.25:1 and 100:1.

[0788] FIG. 50 illustrates a flow chart of a computer-implemented method for determining a selected overburden thickness. Selected section properties 6366 may be input into computational system 6250. Properties of the selected section may include type of formation, density, permeability, porosity, earth stresses, etc. Selected section properties 6366 may be used by a software executable to determine minimum overburden thickness 6368 for the selected section. The software executable may be, for example, ABAQUS. The software executable may incorporate selected factors. Computational system 6250 may also run a simulation to determine minimum overburden thickness 6368. The minimum overburden thickness may be determined so that fractures that allow formation fluid to pass to the ground surface will not form within the overburden during an in situ process. A formation may be selected for treatment by computational system 6250 based on properties of the formation and/or properties of the overburden as determined herein. Overburden properties 6364 may also be input into computational system 6250. Properties of the overburden may include a type of material in the overburden, density of the overburden, permeability of the overburden, earth stresses, etc. Computational system 6250 may also be used to determine operating conditions and/or control operating conditions for an in situ process of treating a formation.

[0789] Heating of the formation may be monitored during an in situ conversion process. Monitoring heating of a selected section may include continuously monitoring acoustical data associated with the selected section. Acoustical data may include seismic data or any acoustical data that may be measured, for example, using geophones, hydrophones, or other acoustical sensors. In an embodiment, a continuous acoustical monitoring system can be used to monitor (e.g., intermittently or constantly) the formation. The formation can be monitored (e.g., using geophones at 2 kilohertz, recording measurements every ⅛ of a millisecond) for undesirable formation conditions. In an embodiment, a continuous acoustical monitoring system may be obtained from Oyo Instruments (Houston, Tex.).

[0790] Acoustical data may be acquired by recording information using underground acoustical sensors located within and/or proximate a treated formation area. Acoustical data may be used to determine a type and/or location of fractures developing within the selected section. Acoustical data may be input into a computational system to determine the type and/or location of fractures. Also, heating profiles of the formation or selected section may be determined by the computational system using the acoustical data. The computational system may run a software executable to process the acoustical data. The computational system may be used to determine a set of operating conditions for treating the formation in situ. The computational system may also be used to control the set of operating conditions for treating the formation in situ based on the acoustical data. Other properties, such as a temperature of the formation, may also be input into the computational system.

[0791] An in situ conversion process may be controlled by using some of the production wells as injection wells for injection of steam and/or other process modifying fluids (e.g., hydrogen, which may affect a product composition through in situ hydrogenation).

[0792] In certain embodiments, it may be possible to use well technologies that may operate at high temperatures. These technologies may include both sensors and control mechanisms. The heat injection profiles and hydrocarbon vapor production may be adjusted on a more discrete basis. It may be possible to adjust heat profiles and production on a bed-by-bed basis or in meter-by-meter increments. This may allow the ICP to compensate, for example, for different thermal properties and/or organic contents in an interbedded lithology. Thus, cold and hot spots may be inhibited from forming, the formation may not be overpressurized, and/or the integrity of the formation may not be highly stressed, which could cause deformations and/or damage to wellbore integrity.

[0793] FIGS. 51 and 52 illustrate schematic diagrams of a plan view and a cross-sectional representation, respectively, of a zone being treated using an in situ conversion process (ICP). The ICP may cause microseismic failures, or fractures, within the treatment zone from which a seismic wave may be emitted. Treatment zone 6400 may be heated using heat provided from heater 6410 placed in heater well 6402. Pressure in treatment zone 6400 may be controlled by producing some formation fluid through heater wells 6402 and/or production wells. Heat from heater 6410 may cause failure 6406 in a portion of the formation proximate treatment zone 6400. Failure 6406 may be a localized rock failure within a rock volume of the formation. Failure 6406 may be an instantaneous failure.

[0794] Failure 6406 tends to produce seismic disturbance 6408. Seismic disturbance 6408 may be an elastic or microseismic disturbance that propagates as a body wave in the formation surrounding the failure. Magnitude and direction of seismic disturbance as measured by sensors may indicate a type of macro-scale failure that occurs within the formation and/or treatment zone 6400. For example, seismic disturbance 6408 may be evaluated to indicate a location, orientation, and/or extent of one or more macro-scale failures that occurred in the formation due to heat treatment of the treatment zone 6400.

[0795] Seismic disturbance 6408 from one or more failures 6406 may be detected with one or more sensors 6412. Sensor 6412 may be a geophone, hydrophone, accelerometer, and/or other seismic sensing device. Sensors 6412 may be placed in monitoring well 6404 or monitoring wells. Monitoring wells 6404 may be placed in the formation proximate heater well 6402 and treatment zone 6400. In certain embodiments, three monitoring wells 6404 are placed in the formation such that a location of failure 6406 may be triangulated using sensors 6412 in each monitoring well.

[0796] In an in situ conversion process embodiment, sensors 6412 may measure a signal of seismic disturbance 6408. The signal may include a wave or set of waves emitted from failure 6406. The signals may be used to determine an approximate location of failure 6406. An approximate time at which failure 6406 occurred, causing seismic disturbance 6408, may also be determined from the signal. This approximate location and approximate time of failure 6406 may be used to determine if failure 6406 can propagate into an undesired zone of the formation. The undesired zone may include a water aquifer, a zone of the formation undesired for treatment, overburden 540 of the formation, and/or underburden 6416 of the formation. An aquifer may also lie above overburden 540 or below underburden 6416. Overburden 540 and/or underburden 6416 may include one or more rock layers that can be fractured and allow formation fluid to undesirably escape from the in situ conversion process. Sensors 6412 may be used to monitor a progression of failure 6406 (i.e., an increase in extent of the failure) over a period of time.

[0797] In certain embodiments, a location of failure 6406 may be more precisely determined using a vertical distribution of sensors 6412 along each monitoring well 6404. The vertical distribution of sensors 6412 may also include at least one sensor above overburden 540 and/or below underburden 6416. The sensors above overburden 540 and/or below underburden 6416 may be used to monitor penetration (or an absence of penetration) of a failure through the overburden or underburden.

[0798] If failure 6406 propagates into an undesired zone of the formation, a parameter for treatment of treatment zone 6400 controlled through heater well 6402 may be altered to inhibit propagation of the failure. The parameter of treatment may include a pressure in treatment zone 6400, a volume (or flow rate) of fluids injected into the treatment zone or removed from the treatment zone, or a heat input rate from heater 6410 into the treatment zone.

[0799] FIG. 53 illustrates a flow chart of an embodiment of a method used to monitor treatment of a formation. Treatment plan 6420 may be provided for a treatment zone (e.g., treatment zone 6400 in FIGS. 51 and 52). Parameters 6422 for treatment plan 6420 may include, but are not limited to, pressure in the treatment zone, heating rate of the treatment zone, and average temperature in the treatment zone. Treatment parameters 6422 may be controlled to treat through heat sources, production wells, and/or injection wells. A failure or failures may occur during treatment of the treatment zone for a given set of parameters. Seismic disturbances that indicate a failure may be detected by sensors placed in one or more monitoring wells in monitoring step 6424. The seismic disturbances may be used to determine a location, a time, and/or extent of the one or more failures in determination step 6426. Determination step 6426 may include imaging the seismic disturbances to determine a spatial location of a failure or failures and/or a time at which the failure or failures occurred. The location, time, and/or extent of the failure or failures may be processed to determine if treatment parameters 6422 can be altered to inhibit the propagation of a failure or failures into an undesired zone of the formation in interpretation step 6428.

[0800] In an in situ conversion process embodiment, a recording system may be used to continuously monitor signals from sensors placed in a formation. The recording system may continuously record the signals from sensors. The recording system may save the signals as data. The data may be permanently saved by the recording system. The recording system may simultaneously monitor signals from sensors. The signals may be monitored at a selected sampling rate (e.g., about once every 0.25 milliseconds). In some embodiments, two recording systems may be used to continuously monitor signals from sensors. A recording system may be used to record each signal from the sensors at the selected sampling rate for a desired time period. A controller may be used when the recording system is used to monitor a signal. The controller may be a computational system or computer. In an embodiment using two or more recording systems, the controller may direct which recording system is used for a selected time period. The controller may include a global positioning satellite (GPS) clock. The GPS clock may be used to provide a specific time for a recording system to begin monitoring signals (e.g., a trigger time) and a time period for the monitoring of signals. The controller may provide the specific time for the recording system to begin monitoring signals to a trigger box. The trigger box may be used to supply a trigger pulse to a recording system to begin monitoring signals.

[0801] A storage device may be used to record signals monitored by a recording system. The storage device may include a tape drive (e.g., a high-speed, high-capacity tape drive) or any device capable of recording relatively large amounts of data at very short time intervals. In an embodiment using two recording systems, the storage device may receive data from the first recording system while the second recording system is monitoring signals from one or more sensors, or vice versa. This enables continuous data coverage so that all or substantially all microseismic events that occur will be detected. In some embodiments, heat progress through the formation may be monitored by measuring microseismic events caused by heating of various portions of the formation.

[0802] In some embodiments, monitoring heating of a selected section of the formation may include electromagnetic monitoring of the selected section. Electromagnetic monitoring may include measuring a resistivity between at least two electrodes within the selected section. Data from electromagnetic monitoring may be input into a computational system and processed as described above.

[0803] A relationship between a change in characteristics of formation fluids with temperature in an in situ conversion process may be developed. The relationship may relate the change in characteristics with temperature to a heating rate and temperature for the formation. The relationship may be used to select a temperature which can be used in an isothermal experiment to determine a quantity and quality of a product produced by ICP in a formation without having to use one or more slow heating rate experiments. The isothermal experiment may be conducted in a laboratory or similar test facility. The isothermal experiment may be conducted much more quickly than experiments that slowly increase temperatures. An appropriate selection of a temperature for an isothermal experiment may be significant for prediction of characteristics of formation fluids. The experiment may include conducting an experiment on a sample of a formation. The experiment may include producing hydrocarbons from the sample.

[0804] For example, first order kinetics may be generally assumed for a reaction producing a product. Assuming first order kinetics and a linear heating rate, the change in concentration (a characteristic of a formation fluid being the concentration of a component) with temperature may be defined by the equation:
dC/dT=−(k₀/m)×e^(−E/RT)C;  (24)
in which C is the concentration of a component, T is temperature in Kelvin, k₀ is the frequency factor of the reaction, m is the heating rate, E is the activation energy, and R is the gas constant.

[0805] EQN. 24 may be solved for a concentration at a selected temperature based on an initial concentration at a first temperature. The result is the equation:

[0806] $\begin{array}{cc}C=C_0\times e^{\frac{k_0{\mathrm{RT}}^2{e}^{-E/\mathrm{RT}}}{\mathrm{mE}}};& \left(25\right)\end{array}$
in which C is the concentration of a component at temperature T and C₀ is an initial concentration of the component.

[0807] Substituting EQN. 25 into EQN. 24 yields the expression:

[0808] $\begin{array}{cc}\frac{dC}{dT}=-\frac{k_0{C}_0}{m}\times e^{\left(-\frac{E}{\mathrm{RT}}-\frac{k_0{\mathrm{RT}}^2}{\mathrm{mE}}\times e^{-\frac{E}{\mathrm{RT}}}\right)};& \left(26\right)\end{array}$
which relates the change in concentration C with temperature T for first-order kinetics and a linear heating rate.

[0809] Typically, in application of an ICP to an oil shale formation, the heating rate may not be linear due to temperature limitations in heat sources and/or in heater wells. For example, heating may be reduced at higher temperatures so that a temperature in a heater well is maintained below a desired temperature (e.g., about 650° C.). This may provide a non-linear heating rate that is relatively slower than a linear heating rate. The non-linear heating rate may be expressed as:
T=m×tⁿ;  (27)
in which t is time and n is an exponential decay term for the heating rate, and in which n is typically less than 1 (e.g., about 0.75).

[0810] Using EQN. 27 in a first-order kinetics equation gives the expression:

[0811] $\begin{array}{cc}C=C_0\times e^{\left(-\frac{k_0{\mathrm{RT}}^{\frac{n+1}{n}}}{m^{1/n}n}\times e^{\frac{-E}{\mathrm{RT}}}\right)};& \left(28\right)\end{array}$
which is a generalization of EQN. 25 for a non-linear heating rate.

[0812] An isothermal experiment may be conducted at a selected temperature to determine a quality and a quantity of a product produced using an ICP in a formation. The selected temperature may be a temperature at which half the initial concentration, C₀, has been converted into product (i.e., C/C₀=½). EQN. 28 may be solved for this value, giving the expression:

[0813] $\begin{array}{cc}\ln \left(\frac{k_{0}R}{m^{1/n}n}\right)-\ln \left(\ln 2\right)=\frac{E}{{\mathrm{RT}}_{1/2}}-\frac{n+1}{n}\times \ln T_{1/2};& \left(29\right)\end{array}$
in which T_1/2 is the selected temperature which corresponds to converting half of the initial concentration into product. Alternatively, an equation such as EQN. 26 may be used with a heating rate that approximates a heating rate expected in a temperature range where in situ conversion of hydrocarbons is expected. EQN. 29 may be used to determine a selected temperature based on a heating rate that may be expected for ICP in at least a portion of a formation. The heating rate may be selected based on parameters such as, but not limited to, heater well spacing, heater well installation economics (e.g., drilling costs, heater costs, etc.), and maximum heater output. At least one property of the formation may also be used to determine the heating rate. At least one property may include, but is not limited to, a type of formation, formation heat capacity, formation depth, permeability, thermal conductivity, and total organic content. The selected temperature may be used in an isothermal experiment to determine product quality and/or quantity. The product quality and/or quantity may also be determined at a selected pressure in the isothermal experiment. The selected pressure may be a pressure used for an ICP. The selected pressure may be adjusted to produce a desired product quality and/or quantity in the isothermal experiment. The adjusted selected pressure may be used in an ICP to produce the desired product quality and/or quantity from the formation.

[0814] In some embodiments, EQN. 29 may be used to determine a heating rate (m or mⁿ) used in an ICP based on results from an isothermal experiment at a selected temperature (T_1/2). For example, isothermal experiments may be performed at a variety of temperatures. The selected temperature may be chosen as a temperature at which a product of desired quality and/or quantity is produced. The selected temperature may be used in EQN. 29 to determine the desired heating rate during ICP to produce a product of the desired quality and/or quantity.

[0815] Alternatively, if a heating rate is estimated, at least in a first instance, by optimizing costs and incomes such as heater well costs and the time required to produce hydrocarbons, then constants for an equation such as EQN. 29 may be determined by data from an experiment when the temperature is raised at a constant rate. With the constants of EQN. 29 estimated and heating rates estimated, a temperature for isothermal experiments may be calculated. Isothermal experiments may be performed much more quickly than experiments at anticipated heating rates (i.e., relatively slow heating rates). Thus, the effect of variables (such as pressure) and the effect of applying additional gases (such as, for example, steam and hydrogen) may be determined by relatively fast experiments.

[0816] In an embodiment, an oil shale formation may be heated with a natural distributed combustor system located in the formation. The generated heat may be allowed to transfer to a selected section of the formation. A natural distributed combustor may oxidize hydrocarbons in a formation in the vicinity of a wellbore to provide heat to a selected section of the formation.

[0817] A temperature sufficient to support oxidation may be at least about 200° C. or 250° C. The temperature sufficient to support oxidation will tend to vary depending on many factors (e.g., a composition of the hydrocarbons in the oil shale formation, water content of the formation, and/or type and amount of oxidant). Some water may be removed from the formation prior to heating. For example, the water may be pumped from the formation by dewatering wells. The heated portion of the formation may be near or substantially adjacent to an opening in the oil shale formation. The opening in the formation may be a heater well formed in the formation. The heated portion of the oil shale formation may extend radially from the opening to a width of about 0.3 m to about 1.2 m. The width, however, may also be less than about 0.9 m. A width of the heated portion may vary with time. In certain embodiments, the variance depends on factors including a width of formation necessary to generate sufficient heat during oxidation of carbon to maintain the oxidation reaction without providing heat from an additional heat source.

[0818] After the portion of the formation reaches a temperature sufficient to support oxidation, an oxidizing fluid may be provided into the opening to oxidize at least a portion of the hydrocarbons at a reaction zone or a heat source zone within the formation. Oxidation of the hydrocarbons will generate heat at the reaction zone. The generated heat will in most embodiments transfer from the reaction zone to a pyrolysis zone in the formation. In certain embodiments, the generated heat transfers at a rate between about 650 watts per meter and 1650 watts per meter as measured along a depth of the reaction zone. Upon oxidation of at least some of the hydrocarbons in the formation, energy supplied to the heater for initially heating the formation to the temperature sufficient to support oxidation may be reduced or turned off. Energy input costs may be significantly reduced using natural distributed combustors, thereby providing a significantly more efficient system for heating the formation.

[0819] In an embodiment, a conduit may be disposed in the opening to provide oxidizing fluid into the opening. The conduit may have flow orifices or other flow control mechanisms (i.e., slits, venturi meters, valves, etc.) to allow the oxidizing fluid to enter the opening. The term “orifices” includes openings having a wide variety of cross-sectional shapes including, but not limited to, circles, ovals, squares, rectangles, triangles, slits, or other regular or irregular shapes. The flow orifices may be critical flow orifices in some embodiments. The flow orifices may provide a substantially constant flow of oxidizing fluid into the opening, regardless of the pressure in the opening.

[0820] In some embodiments, the number of flow orifices may be limited by the diameter of the orifices and a desired spacing between orifices for a length of the conduit. For example, as the diameter of the orifices decreases, the number of flow orifices may increase, and vice versa. In addition, as the desired spacing increases, the number of flow orifices may decrease, and vice versa. The diameter of the orifices may be determined by a pressure in the conduit and/or a desired flow rate through the orifices. For example, for a flow rate of about 1.7 standard cubic meters per minute and a pressure of about 7 bars absolute, an orifice diameter may be about 1.3 mm with a spacing between orifices of about 2 m. Smaller diameter orifices may plug more readily than larger diameter orifices. Orifices may plug for a variety of reasons. The reasons may include, but are not limited to, contaminants in the fluid flowing in the conduit and/or solid deposition within or proximate the orifices.

[0821] In some embodiments, the number and diameter of the orifices are chosen such that a more even or nearly uniform heating profile will be obtained along a depth of the opening in the formation. A depth of a heated formation that is intended to have an approximately uniform heating profile may be greater than about 300 m, or even greater than about 600 m. Such a depth may vary, however, depending on, for example, a type of formation to be heated and/or a desired production rate.

[0822] In some embodiments, flow orifices may be disposed in a helical pattern around the conduit within the opening. The flow orifices may be spaced by about 0.3 m to about 3 m between orifices in the helical pattern. In some embodiments, the spacing may be about 1 m to about 2 m or, for example, about 1.5 m.

[0823] The flow of oxidizing fluid into the opening may be controlled such that a rate of oxidation at the reaction zone is controlled. Transfer of heat between incoming oxidant and outgoing oxidation products may heat the oxidizing fluid. The transfer of heat may also maintain the conduit below a maximum operating temperature of the conduit.

[0824] FIG. 54 illustrates an embodiment of a natural distributed combustor that may heat an oil shale formation. Conduit 512 may be placed into opening 514 in hydrocarbon layer 516. Conduit 512 may have inner conduit 513. Oxidizing fluid source 508 may provide oxidizing fluid 517 into inner conduit 513. Inner conduit 513 may have critical flow orifices 515 along its length. Critical flow orifices 515 may be disposed in a helical pattern (or any other pattern) along a length of inner conduit 513 in opening 514. For example, critical flow orifices 515 may be arranged in a helical pattern with a distance of about 1 m to about 2.5 m between adjacent orifices. Inner conduit 513 may be sealed at the bottom. Oxidizing fluid 517 may be provided into opening 514 through critical flow orifices 515 of inner conduit 513.

[0825] Critical flow orifices 515 may be designed such that substantially the same flow rate of oxidizing fluid 517 may be provided through each critical flow orifice. Critical flow orifices 515 may also provide substantially uniform flow of oxidizing fluid 517 along a length of conduit 512. Such flow may provide substantially uniform heating of hydrocarbon layer 516 along the length of conduit 512.

[0826] Packing material 542 may enclose conduit 512 in overburden 540 of the formation. Packing material 542 may inhibit flow of fluids from opening 514 to surface 550. Packing material 542 may include any material that inhibits flow of fluids to surface 550 such as cement or consolidated sand or gravel. A conduit or opening through the packing may provide a path for oxidation products to reach the surface.

[0827] Oxidation products 519 typically enter conduit 512 from opening 514. Oxidation products 519 may include carbon dioxide, oxides of nitrogen, oxides of sulfur, carbon monoxide, and/or other products resulting from a reaction of oxygen with hydrocarbons and/or carbon. Oxidation products 519 may be removed through conduit 512 to surface 550. Oxidation products 519 may flow along a face of reaction zone 524 in opening 514 until proximate an upper end of opening 514 where oxidation products 519 may flow into conduit 512. Oxidation products 519 may also be removed through one or more conduits disposed in opening 514 and/or in hydrocarbon layer 516. For example, oxidation products 519 may be removed through a second conduit disposed in opening 514. Removing oxidation products 519 through a conduit may inhibit oxidation products 519 from flowing to a production well disposed in the formation. Critical flow orifices 515 may also inhibit oxidation products 519 from entering inner conduit 513.

[0828] A flow rate of oxidation products 519 may be balanced with a flow rate of oxidizing fluid 517 such that a substantially constant pressure is maintained within opening 514. For a 100 m length of heated section, a flow rate of oxidizing fluid may be between about 0.5 standard cubic meters per minute to about 5 standard cubic meters per minute, or about 1.0 standard cubic meter per minute to about 4.0 standard cubic meters per minute, or, for example, about 1.7 standard cubic meters per minute. A flow rate of oxidizing fluid into the formation may be incrementally increased during use to accommodate expansion of the reaction zone. A pressure in the opening may be, for example, about 8 bars absolute. Oxidizing fluid 517 may oxidize at least a portion of the hydrocarbons in heated portion 518 of hydrocarbon layer 516 at reaction zone 524. Heated portion 518 may have been initially heated to a temperature sufficient to support oxidation by an electric heater, as shown in FIG. 55. In some embodiments, an electric heater may be placed inside or strapped to the outside of inner conduit 513.

[0829] In certain embodiments, controlling the pressure within opening 514 may inhibit oxidation products and/or oxidation fluids from flowing into the pyrolysis zone of the formation. In some instances, pressure within opening 514 may be controlled to be slightly greater than a pressure in the formation to allow fluid within the opening to pass into the formation but to inhibit formation of a pressure gradient that allows the transport of the fluid a significant distance into the formation.

[0830] Although the heat from the oxidation is transferred to the formation, oxidation products 519 (and excess oxidation fluid such as air) may be inhibited from flowing through the formation and/or to a production well within the formation. Instead, oxidation products 519 and/or excess oxidation fluid may be removed from the formation. In some embodiments, the oxidation products and/or excess oxidation fluid are removed through conduit 512. Removing oxidation products and/or excess oxidation fluid may allow heat from oxidation reactions to transfer to the pyrolysis zone without significant amounts of oxidation products and/or excess oxidation fluid entering the pyrolysis zone.

[0831] In certain embodiments, some pyrolysis product near reaction zone 524 may be oxidized in reaction zone 524 in addition to the carbon. Oxidation of the pyrolysis product in reaction zone 524 may provide additional heating of hydrocarbon layer 516. When oxidation of pyrolysis product occurs, oxidation products from the oxidation of pyrolysis product may be removed near the reaction zone (e.g., through a conduit such as conduit 512). Removing the oxidation products of a pyrolysis product may inhibit contamination of other pyrolysis products in the formation with oxidation products.

[0832] Conduit 512 may, in some embodiments, remove oxidation products 519 from opening 514 in hydrocarbon layer 516. Oxidizing fluid 517 in inner conduit 513 may be heated by heat exchange with conduit 512. A portion of heat transfer between conduit 512 and inner conduit 513 may occur in overburden section 540. Oxidation products 519 may be cooled by transferring heat to oxidizing fluid 517. Heating the incoming oxidizing fluid 517 tends to improve the efficiency of heating the formation.

[0833] Oxidizing fluid 517 may transport through reaction zone 524, or heat source zone, by gas phase diffusion and/or convection. Diffusion of oxidizing fluid 517 through reaction zone 524 may be more efficient at the relatively high temperatures of oxidation. Diffusion of oxidizing fluid 517 may inhibit development of localized overheating and fingering in the formation. Diffusion of oxidizing fluid 517 through hydrocarbon layer 516 is generally a mass transfer process. In the absence of an external force, a rate of diffusion for oxidizing fluid 517 may depend upon concentration, pressure, and/or temperature of oxidizing fluid 517 within hydrocarbon layer 516. The rate of diffusion may also depend upon the diffusion coefficient of oxidizing fluid 517 through hydrocarbon layer 516. The diffusion coefficient may be determined by measurement or calculation based on the kinetic theory of gases. In general, random motion of oxidizing fluid 517 may transfer the oxidizing fluid through hydrocarbon layer 516 from a region of high concentration to a region of low concentration.

[0834] With time, reaction zone 524 may slowly extend radially to greater diameters from opening 514 as hydrocarbons are oxidized. Reaction zone 524 may, in many embodiments, maintain a relatively constant width. For an oil shale formation, reaction zone 524 may extend radially about 2 m in the first year and at a lower rate in subsequent years due to an increase in volume of reaction zone 524 as the reaction zone extends radially. Such a lower rate may be about 1 m per year to about 1.5 m per year. Reaction zone 524 may extend at slower rates for hydrocarbon rich formations and at faster rates for formations with more inorganic material since more hydrocarbons per volume are available for combustion in the hydrocarbon rich formations.

[0835] A flow rate of oxidizing fluid 517 into opening 514 may be increased as a diameter of reaction zone 524 increases to maintain the rate of oxidation per unit volume at a substantially steady state. Thus, a temperature within reaction zone 524 may be maintained substantially constant in some embodiments. The temperature within reaction zone 524 may be between about 650° C. to about 900° C. or, for example, about 760° C. The temperature may be maintained below a temperature that results in production of oxides of nitrogen (NO_x). Oxides of nitrogen are often produced at temperatures above about 1200° C.

[0836] The temperature within reaction zone 524 may be varied to achieve a desired heating rate of selected section 526. The temperature within reaction zone 524 may be increased or decreased by increasing or decreasing a flow rate of oxidizing fluid 517 into opening 514. A temperature of conduit 512, inner conduit 513, and/or any metallurgical materials within opening 514 may be controlled to not exceed a maximum operating temperature of the material. Maintaining the temperature below the maximum operating temperature of a material may inhibit excessive deformation and/or corrosion of the material.

[0837] An increase in the diameter of reaction zone 524 may allow for relatively rapid heating of hydrocarbon layer 516. As the diameter of reaction zone 524 increases, an amount of heat generated per time in reaction zone 524 may also increase. Increasing an amount of heat generated per time in the reaction zone will in many instances increase a heating rate of hydrocarbon layer 516 over a period of time, even without increasing the temperature in the reaction zone or the temperature at conduit 513. Thus, increased heating may be achieved over time without installing additional heat sources and without increasing temperatures adjacent to wellbores. In some embodiments, the heating rates may be increased while allowing the temperatures to decrease (allowing temperatures to decrease may often lengthen the life of the equipment used).

[0838] By utilizing the carbon in the formation as a fuel, the natural distributed combustor may save significantly on energy costs. Thus, an economical process may be provided for heating formations that would otherwise be economically unsuitable for heating by other types of heat sources. Using natural distributed combustors may allow fewer heaters to be inserted into a formation for heating a desired volume of the formation as compared to heating the formation using other types of heat sources. Heating a formation using natural distributed combustors may allow for reduced equipment costs as compared to heating the formation using other types of heat sources.

[0839] Heat generated at reaction zone 524 may transfer by thermal conduction to selected section 526 of hydrocarbon layer 516. In addition, generated heat may transfer from a reaction zone to the selected section to a lesser extent by convective heat transfer. Selected section 526, sometimes referred as the “pyrolysis zone,” may be substantially adjacent to reaction zone 524. Removing oxidation products (and excess oxidation fluid such as air) may allow the pyrolysis zone to receive heat from the reaction zone without being exposed to oxidation products, or oxidants, that are in the reaction zone. Oxidation products and/or oxidation fluids may cause the formation of undesirable products if they are present in the pyrolysis zone. Removing oxidation products and/or oxidation fluids may allow a reducing environment to be maintained in the pyrolysis zone.

[0840] In an in situ conversion process embodiment, natural distributed combustors may be used to heat a formation. FIG. 54 depicts an embodiment of a natural distributed combustor. A flow of oxidizing fluid 517 may be controlled along a length of opening 514 or reaction zone 524. Opening 514 may be referred to as an “elongated opening,” such that reaction zone 524 and opening 514 may have a common boundary along a determined length of the opening. The flow of oxidizing fluid may be controlled using one or more orifices 515 (the orifices may be critical flow orifices). The flow of oxidizing fluid may be controlled by a diameter of orifices 515, a number of orifices 515, and/or by a pressure within inner conduit 513 (a pressure behind orifices 515). Controlling the flow of oxidizing fluid may control a temperature at a face of reaction zone 524 in opening 514. For example, an increased flow of oxidizing fluid 517 will tend to increase a temperature at the face of reaction zone 524. Increasing the flow of oxidizing fluid into the opening tends to increase a rate of oxidation of hydrocarbons in the reaction zone. Since the oxidation of hydrocarbons is an exothermic reaction, increasing the rate of oxidation tends to increase the temperature in the reaction zone.

[0841] In certain natural distributed combustor embodiments, the flow of oxidizing fluid 517 may be varied along the length of inner conduit 513 (e.g., using critical flow orifices 515) such that the temperature at the face of reaction zone 524 is variable. The temperature at the face of reaction zone 524, or within opening 514, may be varied to control a rate of heat transfer within reaction zone 524 and/or a heating rate within selected section 526. Increasing the temperature at the face of reaction zone 524 may increase the heating rate within selected section 526. A property of oxidation products 519 may be monitored (e.g., oxygen content, nitrogen content, temperature, etc.). The property of oxidation products 519 may be monitored and used to control input properties (e.g., oxidizing fluid input) into the natural distributed combustor.

[0842] A rate of diffusion of oxidizing fluid 517 through reaction zone 524 may vary with a temperature of and adjacent to the reaction zone. In general, the higher the temperature, the faster a gas will diffuse because of the increased energy in the gas. A temperature within the opening may be assessed (e.g., measured by a thermocouple) and related to a temperature of the reaction zone. The temperature within the opening may be controlled by controlling the flow of oxidizing fluid into the opening from inner conduit 513. For example, increasing a flow of oxidizing fluid into the opening may increase the temperature within the opening. Decreasing the flow of oxidizing fluid into the opening may decrease the temperature within the opening. In an embodiment, a flow of oxidizing fluid may be increased until a selected temperature below the metallurgical temperature limits of the equipment being used is reached. For example, the flow of oxidizing fluid can be increased until a working temperature limit of a metal used in a conduit placed in the opening is reached. The temperature of the metal may be directly measured using a thermocouple or other temperature measurement device.

[0843] In a natural distributed combustor embodiment, production of carbon dioxide within reaction zone 524 may be inhibited. An increase in a concentration of hydrogen in the reaction zone may inhibit production of carbon dioxide within the reaction zone. The concentration of hydrogen may be increased by transferring hydrogen into the reaction zone. In an embodiment, hydrogen may be transferred into the reaction zone from selected section 526. Hydrogen may be produced during the pyrolysis of hydrocarbons in the selected section. Hydrogen may transfer by diffusion and/or convection into the reaction zone from the selected section. In addition, additional hydrogen may be provided into opening 514 or another opening in the formation through a conduit placed in the opening. The additional hydrogen may transfer into the reaction zone from opening 514.

[0844] In some natural distributed combustor embodiments, heat may be supplied to the formation from a second heat source in the wellbore of the natural distributed combustor. For example, an electric heater (e.g., an insulated conductor heater or a conductor-in-conduit heater) used to preheat a portion of the formation may also be used to provide heat to the formation along with heat from the natural distributed combustor. In addition, an additional electric heater may be placed in an opening in the formation to provide additional heat to the formation. The electric heater may be used to provide heat to the formation so that heat provided from the combination of the electric heater and the natural distributed combustor is maintained at a constant heat input rate. Heat input into the formation from the electric heater may be varied as heat input from the natural distributed combustor varies, or vice versa. Providing heat from more than one type of heat source may allow for substantially uniform heating of the formation.

[0845] In certain in situ conversion process embodiments, up to 10%, 25%, or 50% of the total heat input into the formation may be provided from electric heaters. A percentage of heat input into the formation from electric heaters may be varied depending on, for example, electricity cost, natural distributed combustor heat input, etc. Heat from electric heaters can be used to compensate for low heat output from natural distributed combustors to maintain a substantially constant heating rate in the formation. If electrical costs rise, more heat may be generated from natural distributed combustors to reduce the amount of heat supplied by electric heaters. In some embodiments, heat from electric heaters may vary due to the source of electricity (e.g., solar or wind power). In such embodiments, more or less heat may be provided by natural distributed combustors to compensate for changes in electrical heat input.

[0846] In a heat source embodiment, an electric heater may be used to inhibit a natural distributed combustor from “burning out.” A natural distributed combustor may “burn out” if a portion of the formation cools below a temperature sufficient to support combustion. Additional heat from the electric heater may be needed to provide heat to the portion and/or another portion of the formation to heat a portion to a temperature sufficient to support oxidation of hydrocarbons and maintain the natural distributed combustor heating process.

[0847] In some natural distributed combustor embodiments, electric heaters may be used to provide more heat to a formation proximate an upper portion and/or a lower portion of the formation. Using the additional heat from the electric heaters may compensate for heat losses in the upper and/or lower portions of the formation. Providing additional heat with the electric heaters proximate the upper and/or lower portions may produce more uniform heating of the formation. In some embodiments, electric heaters may be used for similar purposes (e.g., provide heat at upper and/or lower portions, provide supplemental heat, provide heat to maintain a minimum combustion temperature, etc.) in combination with other types of fueled heaters, such as flameless distributed combustors or downhole combustors.

[0848] In some in situ conversion process embodiments, exhaust fluids from a fueled heater (e.g., a natural distributed combustor or downhole combustor) may be used in an air compressor located at a surface of the formation proximate an opening used for the fueled heater. The exhaust fluids may be used to drive the air compressor and reduce a cost associated with compressing air for use in the fueled heater. Electricity may also be generated using the exhaust fluids in a turbine or similar device. In some embodiments, fluids (e.g., oxidizing fluid and/or fuel) used for one or more fueled heaters may be provided using a compressor or a series of compressors. A compressor may provide oxidizing fluid and/or fuel for one heater or more than one heater. In addition, oxidizing fluid and/or fuel may be provided from a centralized facility for use in a single heater or more than one heater.

[0849] Pyrolysis of hydrocarbons, or other heat-controlled processes, may take place in heated selected section 526. Selected section 526 may be at a temperature between about 270° C. and about 400° C. for pyrolysis. The temperature of selected section 526 may be increased by heat transfer from reaction zone 524.

[0850] A temperature within opening 514 may be monitored with a thermocouple disposed in opening 514. Alternatively, a thermocouple may be coupled to conduit 512 and/or disposed on a face of reaction zone 524. Power input or oxidant introduced into the formation may be controlled based upon the monitored temperature to maintain the temperature in a selected range. The selected range may vary or be varied depending on location of the thermocouple, a desired heating rate of hydrocarbon layer 516, and other factors. If a temperature within opening 514 falls below a minimum temperature of the selected temperature range, the flow rate of oxidizing fluid 517 may be increased to increase combustion and thereby increase the temperature within opening 514.

[0851] In certain embodiments, one or more natural distributed combustors may be placed along strike of a hydrocarbon layer and/or horizontally. Placing natural distributed combustors along strike or horizontally may reduce pressure differentials along the heated length of the heat source. Reduced pressure differentials may make the temperature generated along a length of the heater more uniform and easier to control.

[0852] In some embodiments, presence of air or oxygen (O₂) in oxidation products 519 may be monitored. Alternatively, an amount of nitrogen, carbon monoxide, carbon dioxide, oxides of nitrogen, oxides of sulfur, etc. may be monitored in oxidation products 519. Monitoring the composition and/or quantity of exhaust products (e.g., oxidation products 519) may be useful for heat balances, for process diagnostics, process control, etc.

[0853] FIG. 56 illustrates a cross-sectional representation of an embodiment of a natural distributed combustor having a second conduit 6200 disposed in opening 514 in hydrocarbon layer 516. Second conduit 6200 may be used to remove oxidation products from opening 514. Second conduit 6200 may have orifices 515 disposed along its length. In certain embodiments, oxidation products are removed from an upper region of opening 514 through orifices 515 disposed on second conduit 6200. Orifices 515 may be disposed along the length of conduit 6200 such that more oxidation products are removed from the upper region of opening 514.

[0854] In certain natural distributed combustor embodiments, orifices 515 on second conduit 6200 may face away from orifices 515 on conduit 513. The orientation may inhibit oxidizing fluid provided through conduit 513 from passing directly into second conduit 6200.

[0855] In some embodiments, conduit 6200 may have a higher density of orifices 515 (and/or relatively larger diameter orifices 515) towards the upper region of opening 514. The preferential removal of oxidation products from the upper region of opening 514 may produce a substantially uniform concentration of oxidizing fluid along the length of opening 514. Oxidation products produced from reaction zone 524 tend to be more concentrated proximate the upper region of opening 514. The large concentration of oxidation products 519 in the upper region of opening 514 tends to dilute a concentration of oxidizing fluid 517 in the upper region. Removing a significant portion of the more concentrated oxidation products from the upper region of opening 514 may produce a more uniform concentration of oxidizing fluid 517 throughout opening 514. Having a more uniform concentration of oxidizing fluid throughout the opening may produce a more uniform driving force for oxidizing fluid to flow into reaction zone 524. The more uniform driving force may produce a more uniform oxidation rate within reaction zone 524, and thus produce a more uniform heating rate in selected section 526 and/or a more uniform temperature within opening 514.

[0856] In a natural distributed combustor embodiment, the concentration of air and/or oxygen in the reaction zone may be controlled. A more even distribution of oxygen (or oxygen concentration) in the reaction zone may be desirable. The rate of reaction may be controlled as a function of the rate in which oxygen diffuses in the reaction zone. The rate of oxygen diffusion correlates to the oxygen concentration. Thus, controlling the oxygen concentration in the reaction zone (e.g., by controlling oxidizing fluid flow rates, the removal of oxidation products along some or all of the length of the reaction zone, and/or the distribution of the oxidizing fluid along some or all of the length of the reaction zone) may control oxygen diffusion in the reaction zone and thereby control the reaction rates in the reaction zone.

[0857] In the embodiment shown in FIG. 57, conductor 580 is placed in opening 514. Conductor 580 may extend from first end 6170 of opening 514 to second end 6172 of opening 514. In certain embodiments, conductor 580 may be placed in opening 514 within hydrocarbon layer 516. One or more low resistance sections 584 may be coupled to conductor 580 and used in overburden 540. In some embodiments, conductor 580 and/or low resistance sections 584 may extend above the surface of the formation.

[0858] In some heat source embodiments, an electric current may be applied to conductor 580 to increase a temperature of the conductor. Heat may transfer from conductor 580 to heated portion 518 of hydrocarbon layer 516. Heat may transfer from conductor 580 to heated portion 518 substantially by radiation. Some heat may also transfer by convection or conduction. Current may be provided to the conductor until a temperature within heated portion 518 is sufficient to support the oxidation of hydrocarbons within the heated portion. As shown in FIG. 57, oxidizing fluid may be provided into conductor 580 from oxidizing fluid source 508 at one or both ends 6170, 6172 of opening 514. A flow of the oxidizing fluid from conductor 580 into opening 514 may be controlled by orifices 515. The orifices may be critical flow orifices. The flow of oxidizing fluid from orifices 515 may be controlled by a diameter of the orifices, a number of orifices, and/or by a pressure within conductor 580 (i.e., a pressure behind the orifices).

[0859] Reaction of oxidizing fluids with hydrocarbons in reaction zone 524 may generate heat. The rate of heat generated in reaction zone 524 may be controlled by a flow rate of the oxidizing fluid into the formation, the rate of diffusion of oxidizing fluid through the reaction zone, and/or a removal rate of oxidation products from the formation. In an embodiment, oxidation products from the reaction of oxidizing fluid with hydrocarbons in the formation are removed through one or both ends of opening 514. In some embodiments, a conduit may be placed in opening 514 to remove oxidation products. All or portions of the oxidation products may be recycled and/or reused in other oxidation type heaters (e.g., natural distributed combustors, surface burners, downhole combustors, etc.). Heat generated in reaction zone 524 may transfer to a surrounding portion (e.g., selected section) of the formation. The transfer of heat between reaction zone 524 and a selected section may be substantially by conduction. In certain embodiments, the transferred heat may increase a temperature of the selected section above a minimum mobilization temperature of the hydrocarbons and/or a minimum pyrolysis temperature of the hydrocarbons.

[0860] In some heat source embodiments, a conduit may be placed in the opening. The opening may extend through the formation contacting a surface of the earth at a first location and a second location. Oxidizing fluid may be provided to the conduit from the oxidizing fluid source at the first location and/or the second location after a portion of the formation that has been heated to a temperature sufficient to support oxidation of hydrocarbons by the oxidizing fluid.

[0861] FIG. 58 illustrates an embodiment of a section of overburden with a natural distributed combustor as described in FIG. 54. Overburden casing 541 may be disposed in overburden 540 of hydrocarbon layer 516. Overburden casing 541 may be surrounded by materials (e.g., an insulating material such as cement) that inhibit heating of overburden 540. Overburden casing 541 may be made of a metal material such as, but not limited to, carbon steel or 304 stainless steel.

[0862] Overburden casing 541 may be placed in reinforcing material 544 in overburden 540. Reinforcing material 544 may be, but is not limited to, cement, gravel, sand, and/or concrete. Packing material 542 may be disposed between overburden casing 541 and opening 514 in the formation. Packing material 542 may be any substantially non-porous material (e.g., cement, concrete, grout, etc.). Packing material 542 may inhibit flow of fluid outside of conduit 512 and between opening 514 and surface 550. Inner conduit 513 may introduce fluid into opening 514 in hydrocarbon layer 516. Conduit 512 may remove combustion product (or excess oxidation fluid) from opening 514 in hydrocarbon layer 516. Diameter of conduit 512 may be determined by an amount of the combustion product produced by oxidation in the natural distributed combustor. For example, a larger diameter may be required for a greater amount of exhaust product produced by the natural distributed combustor heater.

[0863] In some heat source embodiments, a portion of the formation adjacent to a wellbore may be heated to a temperature and at a heating rate that converts hydrocarbons to coke or char adjacent to the wellbore by a first heat source. Coke and/or char may be formed at temperatures above about 400° C. In the presence of an oxidizing fluid, the coke or char will oxidize. The wellbore may be used as a natural distributed combustor subsequent to the formation of coke and/or char. Heat may be generated from the oxidation of coke or char.

[0864] FIG. 59 illustrates an embodiment of a natural distributed combustor heater. Insulated conductor 562 may be coupled to conduit 532 and placed in opening 514 in hydrocarbon layer 516. Insulated conductor 562 may be disposed internal to conduit 532 (thereby allowing retrieval of insulated conductor 562), or, alternately, coupled to an external surface of conduit 532. Insulating material for the conductor may include, but is not limited to, mineral coating and/or ceramic coating. Conduit 532 may have critical flow orifices 515 disposed along its length within opening 514. Electrical current may be applied to insulated conductor 562 to generate radiant heat in opening 514. Conduit 532 may serve as a return for current. Insulated conductor 562 may heat portion 518 of hydrocarbon layer 516 to a temperature sufficient to support oxidation of hydrocarbons.

[0865] Oxidizing fluid source 508 may provide oxidizing fluid into conduit 532. Oxidizing fluid may be provided into opening 514 through critical flow orifices 515 in conduit 532. Oxidizing fluid may oxidize at least a portion of the hydrocarbon layer in reaction zone 524. A portion of heat generated at reaction zone 524 may transfer to selected section 526 by convection, radiation, and/or conduction. Oxidation products may be removed through a separate conduit placed in opening 514 or through opening 543 in overburden casing 541.

[0866] FIG. 60 illustrates an embodiment of a natural distributed combustor heater with an added fuel conduit. Fuel conduit 536 may be placed in opening 514. Fuel conduit may be placed adjacent to conduit 533 in certain embodiments. Fuel conduit 536 may have critical flow orifices 535 along a portion of the length within opening 514. Conduit 533 may have critical flow orifices 515 along a portion of the length within opening 514. The critical flow orifices 535, 515 may be positioned so that a fuel fluid provided through fuel conduit 536 and an oxidizing fluid provided through conduit 533 do not react to heat the fuel conduit and the conduit. Heat from reaction of the fuel fluid with oxidizing fluid may heat fuel conduit 536 and/or conduit 533 to a temperature sufficient to begin melting metallurgical materials in fuel conduit 536 and/or conduit 533 if the reaction takes place proximate fuel conduit 536 and/or conduit 533. Critical flow orifices 535 on fuel conduit 536 and critical flow orifices 515 on conduit 533 may be positioned so that the fuel fluid and the oxidizing fluid do not react proximate the conduits. For example, conduits 536 and 533 may be positioned such that orifices that spiral around the conduits are oriented in opposite directions.

[0867] Reaction of the fuel fluid and the oxidizing fluid may produce heat. In some embodiments, the fuel fluid may be methane, ethane, hydrogen, or synthesis gas that is generated by in situ conversion in another part of the formation. The produced heat may heat portion 518 to a temperature sufficient to support oxidation of hydrocarbons. Upon heating of portion 518 to a temperature sufficient to support oxidation, a flow of fuel fluid into opening 514 may be turned down or may be turned off. In some embodiments, the supply of fuel may be continued throughout the heating of the formation.

[0868] The oxidizing fluid may oxidize at least a portion of the hydrocarbons at reaction zone 524. Generated heat may transfer heat to selected section 526 by radiation, convection, and/or conduction. An oxidation product may be removed through a separate conduit placed in opening 514 or through opening 543 in overburden casing 541.

[0869] FIG. 55 illustrates an embodiment of a system that may heat an oil shale formation. Electric heater 510 may be disposed within opening 514 in hydrocarbon layer 516. Opening 514 may be formed through overburden 540 into hydrocarbon layer 516. Opening 514 may be at least about 5 cm in diameter. Opening 514 may, as an example, have a diameter of about 13 cm. Electric heater 510 may heat at least portion 518 of hydrocarbon layer 516 to a temperature sufficient to support oxidation (e.g., about 260° C.). Portion 518 may have a width of about 1 m. An oxidizing fluid may be provided into the opening through conduit 512 or any other appropriate fluid transfer mechanism. Conduit 512 may have critical flow orifices 515 disposed along a length of the conduit.

[0870] Conduit 512 may be a pipe or tube that provides the oxidizing fluid into opening 514 from oxidizing fluid source 508. In an embodiment, a portion of conduit 512 that may be exposed to high temperatures is a stainless steel tube and a portion of the conduit that will not be exposed to high temperatures (i.e., a portion of the tube that extends through the overburden) is carbon steel. The oxidizing fluid may include air or any other oxygen containing fluid (e.g., hydrogen peroxide, oxides of nitrogen, ozone). Mixtures of oxidizing fluids may be used. An oxidizing fluid mixture may be a fluid including fifty percent oxygen and fifty percent nitrogen. In some embodiments, the oxidizing fluid may include compounds that release oxygen when heated, such as hydrogen peroxide. The oxidizing fluid may oxidize at least a portion of the hydrocarbons in the formation.

[0871] FIG. 61 illustrates an embodiment of a system that heats an oil shale formation. Heat exchanger 520 may be disposed external to opening 514 in hydrocarbon layer 516. Opening 514 may be formed through overburden 540 into hydrocarbon layer 516. Heat exchanger 520 may provide heat from another surface process, or it may include a heater (e.g., an electric or combustion heater). Oxidizing fluid source 508 may provide an oxidizing fluid to heat exchanger 520. Heat exchanger 520 may heat an oxidizing fluid (e.g., above 200° C. or to a temperature sufficient to support oxidation of hydrocarbons). The heated oxidizing fluid may be provided into opening 514 through conduit 521. Conduit 521 may have critical flow orifices 515 disposed along a length of the conduit. The heated oxidizing fluid may heat, or at least contribute to the heating of, at least portion 518 of the formation to a temperature sufficient to support oxidation of hydrocarbons. The oxidizing fluid may oxidize at least a portion of the hydrocarbons in the formation. After temperature in the formation is sufficient to support oxidation, use of heat exchanger 520 may be reduced or phased out.

[0872] An embodiment of a natural distributed combustor may include a surface combustor (e.g., a flame-ignited heater). A fuel fluid may be oxidized in the combustor. The oxidized fuel fluid may be provided into an opening in the formation from the heater through a conduit. Oxidation products and unreacted fuel may return to the surface through another conduit. In some embodiments, one of the conduits may be placed within the other conduit. The oxidized fuel fluid may heat, or contribute to the heating of, a portion of the formation to a temperature sufficient to support oxidation of hydrocarbons. Upon reaching the temperature sufficient to support oxidation, the oxidized fuel fluid may be replaced with an oxidizing fluid. The oxidizing fluid may oxidize at least a portion of the hydrocarbons at a reaction zone within the formation.

[0873] An electric heater may heat a portion of the oil shale formation to a temperature sufficient to support oxidation of hydrocarbons. The portion may be proximate or substantially adjacent to the opening in the formation. The portion may radially extend a width of less than approximately 1 m from the opening. An oxidizing fluid may be provided to the opening for oxidation of hydrocarbons. Oxidation of the hydrocarbons may heat the oil shale formation in a process of natural distributed combustion. Electrical current applied to the electric heater may subsequently be reduced or may be turned off. Natural distributed combustion may be used in conjunction with an electric heater to provide a reduced input energy cost method to heat the oil shale formation compared to using only an electric heater.

[0874] An insulated conductor heater may be a heater element of a heat source. In an embodiment of an insulated conductor heater, the insulated conductor heater is a mineral insulated cable or rod. An insulated conductor heater may be placed in an opening in an oil shale formation. The insulated conductor heater may be placed in an uncased opening in the oil shale formation. Placing the heater in an uncased opening in the oil shale formation may allow heat transfer from the heater to the formation by radiation as well as conduction. Using an uncased opening may facilitate retrieval of the heater from the well, if necessary. Using an uncased opening may significantly reduce heat source capital cost by eliminating a need for a portion of casing able to withstand high temperature conditions. In some heat source embodiments, an insulated conductor heater may be placed within a casing in the formation; may be cemented within the formation; or may be packed in an opening with sand, gravel, or other fill material. The insulated conductor heater may be supported on a support member positioned within the opening. The support member may be a cable, rod, or a conduit (e.g., a pipe). The support member may be made of a metal, ceramic, inorganic material, or combinations thereof. Portions of a support member may be exposed to formation fluids and heat during use, so the support member may be chemically resistant and thermally resistant.

[0875] Ties, spot welds, and/or other types of connectors may be used to couple the insulated conductor heater to the support member at various locations along a length of the insulated conductor heater. The support member may be attached to a wellhead at an upper surface of the formation. In an embodiment of an insulated conductor heater, the insulated conductor heater is designed to have sufficient structural strength so that a support member is not needed. The insulated conductor heater will in many instances have some flexibility to inhibit thermal expansion damage when heated or cooled.

[0876] In certain embodiments, insulated conductor heaters may be placed in wellbores without support members and/or centralizers. An insulated conductor heater without support members and/or centralizers may have a suitable combination of temperature and corrosion resistance, creep strength, length, thickness (diameter), and metallurgy that will inhibit failure of the insulated conductor during use. For example, an insulated conductor without support members that has a working temperature limit of about 700° C. may be less than about 150 m in length and may be made of 310 stainless steel.

[0877] FIG. 62 depicts a perspective view of an end portion of an embodiment of insulated conductor heater 562. An insulated conductor heater may have any desired cross-sectional shape, such as, but not limited to round (as shown in FIG. 62), triangular, ellipsoidal, rectangular, hexagonal, or irregular shape. An insulated conductor heater may include conductor 575, electrical insulation 576, and sheath 577. Conductor 575 may resistively heat when an electrical current passes through the conductor. An alternating or direct current may be used to heat conductor 575. In an embodiment, a 60-cycle AC current is used.

[0878] In some embodiments, electrical insulation 576 may inhibit current leakage and arcing to sheath 577. Electrical insulation 576 may also thermally conduct heat generated in conductor 575 to sheath 577. Sheath 577 may radiate or conduct heat to the formation. Insulated conductor heater 562 may be 1000 m or more in length. In an embodiment of an insulated conductor heater, insulated conductor heater 562 may have a length from about 15 m to about 950 m. Longer or shorter insulated conductors may also be used to meet specific application needs. In embodiments of insulated conductor heaters, purchased insulated conductor heaters have lengths of about 100 m to 500 m (e.g., 230 m). In certain embodiments, dimensions of sheaths and/or conductors of an insulated conductor may be selected so that the insulated conductor has enough strength to be self supporting even at upper working temperature limits. Such insulated cables may be suspended from wellheads or supports positioned near an interface between an overburden and an oil shale formation without the need for support members extending into the oil shale formation along with the insulated conductors.

[0879] In an embodiment, a higher frequency current may be used to take advantage of the skin effect in certain metals. In some embodiments, a 60 cycle AC current may be used in combination with conductors made of metals that exhibit pronounced skin effects. For example, ferromagnetic metals like iron alloys and nickel may exhibit a skin effect. The skin effect confines the current to a region close to the outer surface of the conductor, thereby effectively increasing the resistance of the conductor. A high resistance may be desired to decrease the operating current, minimize ohmic losses in surface cables, and minimize the cost of surface facilities.

[0880] Insulated conductor 562 may be designed to operate at power levels of up to about 1650 watts/meter. Insulated conductor heater 562 may typically operate at a power level between about 500 watts/meter and about 1150 watts/meter when heating a formation. Insulated conductor heater 562 may be designed so that a maximum voltage level at a typical operating temperature does not cause substantial thermal and/or electrical breakdown of electrical insulation 576. The insulated conductor heater 562 may be designed so that sheath 577 does not exceed a temperature that will result in a significant reduction in corrosion resistance properties of the sheath material.

[0881] In an embodiment of insulated conductor heater 562, conductor 575 may be designed to reach temperatures within a range between about 650° C. and about 870° C. The sheath 577 may be designed to reach temperatures within a range between about 535° C. and about 760° C. Insulated conductors having other operating ranges may be formed to meet specific operational requirements. In an embodiment of insulated conductor heater 562, conductor 575 is designed to operate at about 760° C., sheath 577 is designed to operate at about 650° C., and the insulated conductor heater is designed to dissipate about 820 watts/meter.

[0882] Insulated conductor heater 562 may have one or more conductors 575. For example, a single insulated conductor heater may have three conductors within electrical insulation that are surrounded by a sheath. FIG. 62 depicts insulated conductor heater 562 having a single conductor 575. The conductor may be made of metal. The material used to form a conductor may be, but is not limited to, nichrome, nickel, and a number of alloys made from copper and nickel in increasing nickel concentrations from pure copper to Alloy 30, Alloy 60, Alloy 180, and Monel. Alloys of copper and nickel may advantageously have better electrical resistance properties than substantially pure nickel or copper.

[0883] In an embodiment, the conductor may be chosen to have a diameter and a resistivity at operating temperatures such that its resistance, as derived from Ohm's law, makes it electrically and structurally stable for the chosen power dissipation per meter, the length of the heater, and/or the maximum voltage allowed to pass through the conductor. In some embodiments, the conductor may be designed using Maxwell's equations to make use of skin effect.

[0884] The conductor may be made of different materials along a length of the insulated conductor heater. For example, a first section of the conductor may be made of a material that has a significantly lower resistance than a second section of the conductor. The first section may be placed adjacent to a formation layer that does not need to be heated to as high a temperature as a second formation layer that is adjacent to the second section. The resistivity of various sections of conductor may be adjusted by having a variable diameter and/or by having conductor sections made of different materials.

[0885] A diameter of conductor 575 may typically be between about 1.3 mm to about 10.2 mm. Smaller or larger diameters may also be used to have conductors with desired resistivity characteristics. In an embodiment of an insulated conductor heater, the conductor is made of Alloy 60 that has a diameter of about 5.8 mm.

[0886] Electrical insulator 576 of insulated conductor heater 562 may be made of a variety of materials. Pressure may be used to place electrical insulator powder between conductor 575 and sheath 577. Low flow characteristics and other properties of the powder and/or the sheaths and conductors may inhibit the powder from flowing out of the sheaths. Commonly used powders may include, but are not limited to, MgO, Al₂O₃, Zirconia, BeO, different chemical variations of Spinels, and combinations thereof. MgO may provide good thermal conductivity and electrical insulation properties. The desired electrical insulation properties include low leakage current and high dielectric strength. A low leakage current decreases the possibility of thermal breakdown and the high dielectric strength decreases the possibility of arcing across the insulator. Thermal breakdown can occur if the leakage current causes a progressive rise in the temperature of the insulator leading also to arcing across the insulator. An amount of impurities 578 in the electrical insulator powder may be tailored to provide required dielectric strength and a low level of leakage current. Impurities 578 added may be, but are not limited to, CaO, Fe₂O₃, Al₂O₃, and other metal oxides. Low porosity of the electrical insulation tends to reduce leakage current and increase dielectric strength. Low porosity may be achieved by increased packing of the MgO powder during fabrication or by filling of the pore space in the MgO powder with other granular materials, for example, Al₂O₃.

[0887] Impurities 578 added to the electrical insulator powder may have particle sizes that are smaller than the particle sizes of the powdered electrical insulator. The small particles may occupy pore space between the larger particles of the electrical insulator so that the porosity of the electrical insulator is reduced. Examples of powdered electrical insulators that may be used to form electrical insulation 576 are “H” mix manufactured by Idaho Laboratories Corporation (Idaho Falls, Id.) or Standard MgO used by Pyrotenax Cable Company (Trenton, Ontario) for high temperature applications. In addition, other powdered electrical insulators may be used.

[0888] Sheath 577 of insulated conductor heater 562 may be an outer metallic layer. Sheath 577 may be in contact with hot formation fluids. Sheath 577 may need to be made of a material having a high resistance to corrosion at elevated temperatures. Alloys that may be used in a desired operating temperature range of the sheath include, but are not limited to 304 stainless steel, 310 stainless steel, Incoloy 800, and Inconel 600. The thickness of the sheath has to be sufficient to last for three to ten years in a hot and corrosive environment. A thickness of the sheath may generally vary between about 1 mm and about 2.5 mm. For example, a 1.3 mm thick, 310 stainless steel outer layer may be used as sheath 577 to provide good chemical resistance to sulfidation corrosion in a heated zone of a formation for a period of over 3 years. Larger or smaller sheath thicknesses may be used to meet specific application requirements.

[0889] An insulated conductor heater may be tested after fabrication. The insulated conductor heater may be required to withstand 2–3 times an operating voltage at a selected operating temperature. Also, selected samples of produced insulated conductor heaters may be required to withstand 1000 VAC at 760° C. for one month.

[0890] As illustrated in FIG. 63, short flexible transition conductor 571 may be connected to lead-in conductor 572 using connection 569 made during heater installation in the field. Transition conductor 571 may be a flexible, low resistivity, stranded copper cable that is surrounded by rubber or polymer insulation. Transition conductor 571 may typically be between about 1.5 m and about 3 m, although longer or shorter transition conductors may be used to accommodate particular needs. Temperature resistant cable may be used as transition conductor 571. Transition conductor 571 may also be connected to a short length of an insulated conductor heater that is less resistive than a primary heating section of the insulated conductor heater. The less resistive portion of the insulated conductor heater may be referred to as “cold pin” 568.

[0891] Cold pin 568 may be designed to dissipate about one-tenth to about one-fifth of the power per unit length as is dissipated in a unit length of the primary heating section. Cold pins may typically be between about 1.5 m and about 15 m, although shorter or longer lengths may be used to accommodate specific application needs. In an embodiment, the conductor of a cold pin section is copper with a diameter of about 6.9 mm and a length of 9.1 m. The electrical insulation is the same type of insulation used in the primary heating section. A sheath of the cold pin may be made of Inconel 600. Chloride corrosion cracking in the cold pin region may occur, so a chloride corrosion resistant metal such as Inconel 600 may be used as the sheath.

[0892] As illustrated in FIG. 63, small, epoxy filled canister 573 may be used to create a connection between transition conductor 571 and cold pin 568. Cold pins 568 may be connected to the primary heating sections of insulated conductor 562 heaters by “splices” 567. The length of cold pin 568 may be sufficient to significantly reduce a temperature of insulated conductor heater 562. The heater section of the insulated conductor heater 562 may operate from about 530° C. to about 760° C., splice 567 may be at a temperature from about 260° C. to about 370° C., and the temperature at the lead-in cable connection to the cold pin may be from about 40° C. to about 90° C. In addition to a cold pin at a top end of the insulated conductor heater, a cold pin may also be placed at a bottom end of the insulated conductor heater. The cold pin at the bottom end may in many instances make a bottom termination easier to manufacture.

[0893] Splice material may have to withstand a temperature equal to half of a target zone operating temperature. Density of electrical insulation in the splice should in many instances be high enough to withstand the required temperature and the operating voltage.

[0894] Splice 567 may be required to withstand 1000 VAC at 480° C. Splice material may be high temperature splices made by Idaho Laboratories Corporation or by Pyrotenax Cable Company. A splice may be an internal type of splice or an external splice. An internal splice is typically made without welds on the sheath of the insulated conductor heater. The lack of weld on the sheath may avoid potential weak spots (mechanical and/or electrical) on the insulated cable heater. An external splice is a weld made to couple sheaths of two insulated conductor heaters together. An external splice may need to be leak tested prior to insertion of the insulated cable heater into a formation. Laser welds or orbital TIG (tungsten inert gas) welds may be used to form external splices. An additional strain relief assembly may be placed around an external splice to improve the splice's resistance to bending and to protect the external splice against partial or total parting.

[0895] In certain embodiments, an insulated conductor assembly, such as the assembly depicted in FIG. 64 and FIG. 63, may have to withstand a higher operating voltage than normally would be used. For example, for heaters greater than about 700 m in length, voltages greater than about 2000 V may be needed for generating heat with the insulated conductor, as compared to voltages of about 480 V that may be used with heaters having lengths of less than about 225 m. In such cases, it may be advantageous to form insulated conductor 562, cold pin 568, transition conductor 571, and lead-in conductor 572 into a single insulated conductor assembly. In some embodiments, cold pin 568 and canister 573 may not be required as shown in FIG. 63. In such an embodiment, splice 567 can be used to directly couple insulated conductor 562 to transition conductor 571.

[0896] In a heat source embodiment, insulated conductor 562, transition conductor 571, and lead-in conductor 572 each include insulated conductors of varying resistance. Resistance of the conductors may be varied, for example, by altering a type of conductor, a diameter of a conductor, and/or a length of a conductor. In an embodiment, diameters of insulated conductor 562, transition conductor 571, and lead-in conductor 572 are different. Insulated conductor 562 may have a diameter of 6 mm, transition conductor 571 may have a diameter of 7 mm, and lead-in conductor 572 may have a diameter of 8 mm. Smaller or larger diameters may be used to accommodate site conditions (e.g., heating requirements or voltage requirements). Insulated conductor 562 may have a higher resistance than either transition conductor 571 or lead-in conductor 572, such that more heat is generated in the insulated conductor. Also, transition conductor 571 may have a resistance between a resistance of insulated conductor 562 and lead-in conductor 572. Insulated conductor 562, transition conductor 571, and lead-in conductor 572 may be coupled using splice 567 and/or connection 569. Splice 567 and/or connection 569 may be required to withstand relatively large operating voltages depending on a length of insulated conductor 562 and/or lead-in conductor 572. Splice 567 and/or connection 569 may inhibit arcing and/or voltage breakdowns within the insulated conductor assembly. Using insulated conductors for each cable within an insulated conductor assembly may allow for higher operating voltages within the assembly.

[0897] An insulated conductor assembly may include heating sections, cold pins, splices, termination canisters and flexible transition conductors. The insulated conductor assembly may need to be examined and electrically tested before installation of the assembly into an opening in a formation. The assembly may need to be examined for competent welds and to make sure that there are no holes in the sheath anywhere along the whole heater (including the heated section, the cold-pins, the splices, and the termination cans). Periodic X-ray spot checking of the commercial product may need to be made. The whole cable may be immersed in water prior to electrical testing. Electrical testing of the assembly may need to show more than 2000 megaohms at 500 VAC at room temperature after water immersion. In addition, the assembly may need to be connected to 1000 VAC and show less than about 10 microamps per meter of resistive leakage current at room temperature. In addition, a check on leakage current at about 760° C. may need to show less than about 0.4 milliamps per meter.

[0898] A number of companies manufacture insulated conductor heaters. Such manufacturers include, but are not limited to, MI Cable Technologies (Calgary, Alberta), Pyrotenax Cable Company (Trenton, Ontario), Idaho Laboratories Corporation (Idaho Falls, Id.), and Watlow (St. Louis, Mo.). As an example, an insulated conductor heater may be ordered from Idaho Laboratories as cable model 355-A90-310-“H” 30′/750′/30′ with Inconel 600 sheath for the cold-pins, three-phase Y configuration and bottom jointed conductors. The specification for the heater may also include 1000 VAC, 1400° F. quality cable. The designator 355 specifies the cable OD (0.355″); A90 specifies the conductor material; 310 specifies the heated zone sheath alloy (SS 310); “H” specifies the MgO mix; and 30′/750′/30′ specifies about a 230 m heated zone with cold-pins top and bottom having about 9 m lengths. A similar part number with the same specification using high temperature Standard purity MgO cable may be ordered from Pyrotenax Cable Company.

[0899] One or more insulated conductor heaters may be placed within an opening in a formation to form a heat source or heat sources. Electrical current may be passed through each insulated conductor heater in the opening to heat the formation. Alternately, electrical current may be passed through selected insulated conductor heaters in an opening. The unused conductors may be backup heaters. Insulated conductor heaters may be electrically coupled to a power source in any convenient manner. Each end of an insulated conductor heater may be coupled to lead-in cables that pass through a wellhead. Such a configuration typically has a 180° bend (a “hairpin” bend) or turn located near a bottom of the heat source. An insulated conductor heater that includes a 180° bend or turn may not require a bottom termination, but the 180° bend or turn may be an electrical and/or structural weakness in the heater. Insulated conductor heaters may be electrically coupled together in series, in parallel, or in series and parallel combinations. In some embodiments of heat sources, electrical current may pass into the conductor of an insulated conductor heater and may be returned through the sheath of the insulated conductor heater by connecting conductor 575 to sheath 577 (shown in FIG. 62) at the bottom of the heat source.

[0900] In the embodiment of a heat source depicted in FIG. 64, three insulated conductor heaters 562 are electrically coupled in a 3-phase Y configuration to a power supply. The power supply may provide 60 cycle AC current to the electrical conductors. No bottom connection may be required for the insulated conductor heaters. Alternately, all three conductors of the three phase circuit may be connected together near the bottom of a heat source opening. The connection may be made directly at ends of heating sections of the insulated conductor heaters or at ends of cold pins coupled to the heating sections at the bottom of the insulated conductor heaters. The bottom connections may be made with insulator filled and sealed canisters or with epoxy filled canisters. The insulator may be the same composition as the insulator used as the electrical insulation.

[0901] The three insulated conductor heaters depicted in FIG. 64 may be coupled to support member 564 using centralizers 566. Alternatively, the three insulated conductor heaters may be strapped directly to the support tube using metal straps. Centralizers 566 may maintain a location or inhibit movement of insulated conductor heaters 562 on support member 564. Centralizers 566 may be made of metal, ceramic, or combinations thereof. The metal may be stainless steel or any other type of metal able to withstand a corrosive and hot environment. In some embodiments, centralizers 566 may be bowed metal strips welded to the support member at distances less than about 6 m. A ceramic used in centralizer 566 may be, but is not limited to, Al₂O₃, MgO, or other insulator. Centralizers 566 may maintain a location of insulated conductor heaters 562 on support member 564 such that movement of insulated conductor heaters is inhibited at operating temperatures of the insulated conductor heaters. Insulated conductor heaters 562 may also be somewhat flexible to withstand expansion of support member 564 during heating.

[0902] Support member 564, insulated conductor heater 562, and centralizers 566 may be placed in opening 514 in hydrocarbon layer 516. Insulated conductor heaters 562 may be coupled to bottom conductor junction 570 using cold pin transition conductor 568. Bottom conductor junction 570 may electrically couple each insulated conductor heater 562 to each other. Bottom conductor junction 570 may include materials that are electrically conducting and do not melt at temperatures found in opening 514. Cold pin transition conductor 568 may be an insulated conductor heater having lower electrical resistance than insulated conductor heater 562. As illustrated in FIG. 63, cold pin 568 may be coupled to transition conductor 571 and insulated conductor heater 562. Cold pin transition conductor 568 may provide a temperature transition between transition conductor 571 and insulated conductor heater 562.

[0903] Lead-in conductor 572 may be coupled to wellhead 590 to provide electrical power to insulated conductor heater 562. Lead-in conductor 572 may be made of a relatively low electrical resistance conductor such that relatively little heat is generated from electrical current passing through lead-in conductor 572. In some embodiments, the lead-in conductor is a rubber or polymer insulated stranded copper wire. In some embodiments, the lead-in conductor is a mineral-insulated conductor with a copper core. Lead-in conductor 572 may couple to wellhead 590 at surface 550 through a sealing flange located between overburden 540 and surface 550. The sealing flange may inhibit fluid from escaping from opening 514 to surface 550.

[0904] Packing material 542 may be placed between overburden casing 541 and opening 514. In some embodiments, reinforcing material 544 may secure overburden casing 541 to overburden 540. In an embodiment of a heat source, overburden casing is a 7.6 cm (3 inch) diameter carbon steel, schedule 40 pipe. Packing material 542 may inhibit fluid from flowing from opening 514 to surface 550. Reinforcing material 544 may include, for example, Class G or Class H Portland cement mixed with silica flour for improved high temperature performance, slag or silica flour, and/or a mixture thereof (e.g., about 1.58 grams per cubic centimeter slag/silica flour). In some heat source embodiments, reinforcing material 544 extends radially a width of from about 5 cm to about 25 cm. In some embodiments, reinforcing material 544 may extend radially a width of about 10 cm to about 15 cm. Reinforcing material 544 may inhibit heat transfer into overburden 540.

[0905] In certain embodiments, one or more conduits may be provided to supply additional components (e.g., nitrogen, carbon dioxide, reducing agents such as gas containing hydrogen, etc.) to formation openings, to bleed off fluids, and/or to control pressure. Formation pressures tend to be highest near heating sources. Providing pressure control equipment in heat sources may be beneficial. In some embodiments, adding a reducing agent proximate the heating source assists in providing a more favorable pyrolysis environment (e.g., a higher hydrogen partial pressure). Since permeability and porosity tend to increase more quickly proximate the heating source, it is often optimal to add a reducing agent proximate the heating source so that the reducing agent can more easily move into the formation.

[0906] Conduit 5000, depicted in FIG. 64, may be provided to add gas from gas source 5003, through valve 5001, and into opening 514. Opening 5004 is provided in packing material 542 to allow gas to pass into opening 514. Conduit 5000 and valve 5002 may be used at different times to bleed off pressure and/or control pressure proximate opening 514. Conduit 5010, depicted in FIG. 66, may be provided to add gas from gas source 5013, through valve 5011, and into opening 514. An opening is provided in reinforcing material 544 to allow gas to pass into opening 514. Conduit 5010 and valve 5012 may be used at different times to bleed off pressure and/or control pressure proximate opening 514. It is to be understood that any of the heating sources described herein may also be equipped with conduits to supply additional components, bleed off fluids, and/or to control pressure.

[0907] As shown in FIG. 64, support member 564 and lead-in conductor 572 may be coupled to wellhead 590 at surface 550 of the formation. Surface conductor 545 may enclose reinforcing material 544 and couple to wellhead 590. Embodiments of surface conductor 545 may have an outer diameter of about 10.16 cm to about 30.48 cm or, for example, an outer diameter of about 22 cm. Embodiments of surface conductors may extend to depths of approximately 3 m to approximately 515 m into an opening in the formation. Alternatively, the surface conductor may extend to a depth of approximately 9 m into the opening. Electrical current may be supplied from a power source to insulated conductor heater 562 to generate heat due to the electrical resistance of conductor 575 as illustrated in FIG. 62. As an example, a voltage of about 330 volts and a current of about 266 amps are supplied to insulated conductor 562 to generate a heat of about 1150 watts/meter in insulated conductor heater 562. Heat generated from the three insulated conductor heaters 562 may transfer (e.g., by radiation) within opening 514 to heat at least a portion of the hydrocarbon layer 516.

[0908] An appropriate configuration of an insulated conductor heater may be determined by optimizing a material cost of the heater based on a length of heater, a power required per meter of conductor, and a desired operating voltage. In addition, an operating current and voltage may be chosen to optimize the cost of input electrical energy in conjunction with a material cost of the insulated conductor heaters. For example, as input electrical energy increases, the cost of materials needed to withstand the higher voltage may also increase. The insulated conductor heaters may generate radiant heat of approximately 650 watts/meter of conductor to approximately 1650 watts/meter of conductor. The insulated conductor heater may operate at a temperature between approximately 530° C. and approximately 760° C. within a formation.

[0909] Heat generated by an insulated conductor heater may heat at least a portion of an oil shale formation. In some embodiments, heat may be transferred to the formation substantially by radiation of the generated heat to the formation. Some heat may be transferred by conduction or convection of heat due to gases present in the opening. The opening may be an uncased opening. An uncased opening eliminates cost associated with thermally cementing the heater to the formation, costs associated with a casing, and/or costs of packing a heater within an opening. In addition, heat transfer by radiation is typically more efficient than by conduction, so the heaters may be operated at lower temperatures in an open wellbore. Conductive heat transfer during initial operation of a heat source may be enhanced by the addition of a gas in the opening. The gas may be maintained at a pressure up to about 27 bars absolute. The gas may include, but is not limited to, carbon dioxide and/or helium. An insulated conductor heater in an open wellbore may advantageously be free to expand or contract to accommodate thermal expansion and contraction. An insulated conductor heater may advantageously be removable from an open wellbore.

[0910] In an embodiment, an insulated conductor heater may be installed or removed using a spooling assembly. More than one spooling assembly may be used to install both the insulated conductor and a support member simultaneously. 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. Alternatively, the support member may be installed using a coiled tubing unit. The heaters may be un-spooled and connected to the support as the support is inserted into the well. The electric heater and the support member may be un-spooled from the spooling assemblies. Spacers may be coupled to the support member and the heater along a length of the support member. Additional spooling assemblies may be used for additional electric heater elements.

[0911] In an in situ conversion process embodiment, a heater may be installed in a substantially horizontal wellbore. Installing a heater in a wellbore (whether vertical or horizontal) may include placing one or more heaters (e.g., three mineral insulated conductor heaters) within a conduit. FIG. 67 depicts an embodiment of a portion of three insulated conductor heaters 6232 placed within conduit 6234. Insulated conductor heaters 6232 may be spaced within conduit 6234 using spacers 6236 to locate the insulated conductor heater within the conduit.

[0912] The conduit may be reeled onto a spool. The spool may be placed on a transporting platform such as a truck bed or other platform that can be transported to a site of a wellbore. The conduit may be unreeled from the spool at the wellbore and inserted into the wellbore to install the heater within the wellbore. A welded cap may be placed at an end of the coiled conduit. The welded cap may be placed at an end of the conduit that enters the wellbore first. The conduit may allow easy installation of the heater into the wellbore. The conduit may also provide support for the heater.

[0913] In some heat source embodiments, coiled tubing installation may be used to install one or more wellbore elements placed in openings in a formation for an in situ conversion process. For example, a coiled conduit may be used to install other types of wells in a formation. The other types of wells may be, but are not limited to, monitor wells, freeze wells or portions of freeze wells, dewatering wells or portions of dewatering wells, outer casings, injection wells or portions of injection wells, production wells or portions of production wells, and heat sources or portions of heat sources. Installing one or more wellbore elements using a coiled conduit installation process may be less expensive and faster than using other installation processes.

[0914] Coiled tubing installation may reduce a number of welded and/or threaded connections in a length of casing. Welds and/or threaded connections in coiled tubing may be pre-tested for integrity (e.g., by hydraulic pressure testing). Coiled tubing is available from Quality Tubing, Inc. (Houston, Tex.), Precision Tubing (Houston, Tex.), and other manufacturers. Coiled tubing may be available in many sizes and different materials. Sizes of coiled tubing may range from about 2.5 cm (1 inch) to about 15 cm (6 inches). Coiled tubing may be available in a variety of different metals, including carbon steel. Coiled tubing may be spooled on a large diameter reel. The reel may be carried on a coiled tubing unit. Suitable coiled tubing units are available from Halliburton (Duncan, Okla.), Fleet Cementers, Inc. (Cisco, Tex.), and Coiled Tubing Solutions, Inc. (Eastland, Tex.). Coiled tubing may be unwound from the reel, passed through a straightener, and inserted into a wellbore. A wellcap may be attached (e.g., welded) to an end of the coiled tubing before inserting the coiled tubing into a well. After insertion, the coiled tubing may be cut from the coiled tubing on the reel.

[0915] In some embodiments, coiled tubing may be inserted into a previously cased opening, e.g., if a well is to be used later as a heater well, production well, or monitoring well. Alternately, coiled tubing installed within a wellbore can later be perforated (e.g., with a perforation gun) and used as a production conduit.

[0916] Embodiments of heat sources, production wells, and/or freeze wells may be installed in a formation using coiled tubing installation. Some embodiments of heat sources, production wells, and freeze wells include an element placed within an outer casing. For example, a conductor-in-conduit heater may include an outer conduit with an inner conduit placed in the outer conduit. A production well may include a heater element or heater elements placed within a casing to inhibit condensation and refluxing of vapor phase production fluids. A freeze well may include a refrigerant input line placed within a casing, or a refrigeration inlet and outlet line. Spacers may be spaced along a length of an element, or elements, positioned within a casing to inhibit the element, or elements, from contacting walls of the casing.

[0917] In some embodiments of heat sources, production wells, and freeze wells, casings may be installed using coiled tube installation. Elements may be placed within the casing after the casing is placed in the formation for heat sources or wells that include elements within the casings. In some embodiments, sections of casings may be threaded and/or welded and inserted into a wellbore using a drilling rig or workover rig. In some embodiments of heat sources, production wells, and freeze wells, elements may be placed within the casing before the casing is wound onto a reel.

[0918] Some wells may have sealed casings that inhibit fluid flow from the formation into the casing. Sealed casings also inhibit fluid flow from the casing into the formation. Some casings may be perforated, screened or have other types of openings that allow fluid to pass into the casing from the formation, or fluid from the casing to pass into the formation. In some embodiments, portions of wells are open wellbores that do not include casings.

[0919] In an embodiment, the support member may be installed using standard oil field operations and welding different sections of support. Welding may be done by using orbital welding. For example, a first section of the support member may be disposed into the well. A second section (e.g., of substantially similar length) may be coupled to the first section in the well. The second section may be coupled by welding the second section to the first section. An orbital welder disposed at the wellhead may weld the second section to the first section. This process may be repeated with subsequent sections coupled to previous sections until a support of desired length is within the well.

[0920] FIG. 65 illustrates a cross-sectional view of one embodiment of a wellhead coupled to overburden casing 541. Flange 590c may be coupled to, or may be a part of, wellhead 590. Flange 590c may be formed of carbon steel, stainless steel, or any other material. Flange 590c may be sealed with o-ring 590f, or any other sealing mechanism. Support member 564 may be coupled to flange 590c. Support member 564 may support one or more insulated conductor heaters. In an embodiment, support member 564 is sealed in flange 590c by welds 590h.

[0921] Power conductor 590a may be coupled to a lead-in cable and/or an insulated conductor heater. Power conductor 590a may provide electrical energy to the insulated conductor heater. Power conductor 590a may be sealed in sealing flange 590d. Sealing flange 590d may be sealed by compression seals or o-rings 590e. Power conductor 590a may be coupled to support member 564 with band 590i. Band 590i may include a rigid and corrosion resistant material such as stainless steel. Wellhead 590 may be sealed with weld 590h such that fluids are inhibited from escaping the formation through wellhead 590. Lift bolt 590j may lift wellhead 590 and support member 564.

[0922] Thermocouple 590g may be provided through flange 590c. Thermocouple 590g may measure a temperature on or proximate support member 564 within the heated portion of the well. Compression fittings 590k may serve to seal power cable 590a. Compression fittings 5901 may serve to seal thermocouple 590g. The compression fittings may inhibit fluids from escaping the formation. Wellhead 590 may also include a pressure control valve. The pressure control valve may control pressure within an opening in which support member 564 is disposed.

[0923] In a heat source embodiment, a control system may control electrical power supplied to an insulated conductor heater. Power supplied to the insulated conductor heater may be controlled with any appropriate type of controller. For alternating current, the controller may be, but is not limited to, a tapped transformer or a zero crossover electric heater firing SCR (silicon controlled rectifier) controller. Zero crossover electric heater firing control may be achieved by allowing full supply voltage to the insulated conductor heater to pass through the insulated conductor heater for a specific number of cycles, starting at the “crossover,” where an instantaneous voltage may be zero, continuing for a specific number of complete cycles, and discontinuing when the instantaneous voltage again crosses zero. A specific number of cycles may be blocked, allowing control of the heat output by the insulated conductor heater. For example, the control system may be arranged to block fifteen and/or twenty cycles out of each sixty cycles that are supplied by a standard 60 Hz alternating current power supply. Zero crossover firing control may be advantageously used with materials having low temperature coefficient materials. Zero crossover firing control may inhibit current spikes from occurring in an insulated conductor heater.

[0924] FIG. 66 illustrates an embodiment of a conductor-in-conduit heater that may heat an oil shale formation. Conductor 580 may be disposed in conduit 582. Conductor 580 may be a rod or conduit of electrically conductive material. Low resistance sections 584 may be present at both ends of conductor 580 to generate less heating in these sections. Low resistance section 584 may be formed by having a greater cross-sectional area of conductor 580 in that section, or the sections may be made of material having less resistance. In certain embodiments, low resistance section 584 includes a low resistance conductor coupled to conductor 580. In some heat source embodiments, conductors 580 may be 316, 304, or 310 stainless steel rods with diameters of approximately 2.8 cm. In some heat source embodiments, conductors are 316, 304, or 310 stainless steel pipes with diameters of approximately 2.5 cm. Larger or smaller diameters of rods or pipes may be used to achieve desired heating of a formation. The diameter and/or wall thickness of conductor 580 may be varied along a length of the conductor to establish different heating rates at various portions of the conductor.

[0925] Conduit 582 may be made of an electrically conductive material. For example, conduit 582 may be a 7.6 cm, schedule 40 pipe made of 316, 304, or 310 stainless steel. Conduit 582 may be disposed in opening 514 in hydrocarbon layer 516. Opening 514 has a diameter able to accommodate conduit 582. A diameter of the opening may be from about 10 cm to about 13 cm. Larger or smaller diameter openings may be used to accommodate particular conduits or designs.

[0926] Conductor 580 may be centered in conduit 582 by centralizer 581. Centralizer 581 may electrically isolate conductor 580 from conduit 582. Centralizer 581 may inhibit movement and properly locate conductor 580 within conduit 582. Centralizer 581 may be made of a ceramic material or a combination of ceramic and metallic materials. Centralizers 581 may inhibit deformation of conductor 580 in conduit 582. Centralizer 581 may be spaced at intervals between approximately 0.5 m and approximately 3 m along conductor 580. FIGS. 68, 69, and 70 depict embodiments of centralizers 581.

[0927] A second low resistance section 584 of conductor 580 may couple conductor 580 to wellhead 690, as depicted in FIG. 66. Electrical current may be applied to conductor 580 from power cable 585 through low resistance section 584 of conductor 580. Electrical current may pass from conductor 580 through sliding connector 583 to conduit 582. Conduit 582 may be electrically insulated from overburden casing 541 and from wellhead 690 to return electrical current to power cable 585. Heat may be generated in conductor 580 and conduit 582. The generated heat may radiate within conduit 582 and opening 514 to heat at least a portion of hydrocarbon layer 516. As an example, a voltage of about 330 volts and a current of about 795 amps may be supplied to conductor 580 and conduit 582 in a 229 m (750 ft) heated section to generate about 1150 watts/meter of conductor 580 and conduit 582.

[0928] Overburden casing 541 may be disposed in overburden 540. Overburden casing 541 may, in some embodiments, be surrounded by materials that inhibit heating of overburden 540. Low resistance section 584 of conductor 580 may be placed in overburden casing 541. Low resistance section 584 of conductor 580 may be made of, for example, carbon steel. Low resistance section 584 may have a diameter between about 2 cm to about 5 cm or, for example, a diameter of about 4 cm. Low resistance section 584 of conductor 580 may be centralized within overburden casing 541 using centralizers 581. Centralizers 581 may be spaced at intervals of approximately 6 m to approximately 12 m or, for example, approximately 9 m along low resistance section 584 of conductor 580. In a heat source embodiment, low resistance section 584 of conductor 580 is coupled to conductor 580 by a weld or welds. In other heat source embodiments, low resistance sections may be threaded, threaded and welded, or otherwise coupled to the conductor. Low resistance section 584 may generate little and/or no heat in overburden casing 541. Packing material 542 may be placed between overburden casing 541 and opening 514. Packing material 542 may inhibit fluid from flowing from opening 514 to surface 550.

[0929] In a heat source embodiment, overburden casing is a 7.6 cm schedule 40 carbon steel pipe. In some embodiments, the overburden casing may be cemented in the overburden. Reinforcing material 544 may be slag or silica flour or a mixture thereof (e.g., about 1.58 grams per cubic centimeter slag/silica flour). Reinforcing material 544 may extend radially a width of about 5 cm to about 25 cm. Cement 544 may also be made of material designed to inhibit flow of heat into overburden 540. In other heat source embodiments, overburden may not be cemented into the formation. Having an uncemented overburden casing may facilitate removal of conduit 582 if the need for removal should arise.

[0930] Surface conductor 545 may couple to wellhead 690. Surface conductor 545 may have a diameter of about 10 cm to about 30 cm or, in certain embodiments, a diameter of about 22 cm. Electrically insulating sealing flanges may mechanically couple low resistance section 584 of conductor 580 to wellhead 690 and to electrically couple low resistance section 584 to power cable 585. The electrically insulating sealing flanges may couple power cable 585 to wellhead 690. For example, power cable 585 may be a copper cable, wire, or other elongated member. Power cable 585 may include any material having a substantially low resistance. The power cable may be clamped to the bottom of the low resistance conductor to make electrical contact.

[0931] In an embodiment, heat may be generated in or by conduit 582. About 10% to about 30%, or, for example, about 20%, of the total heat generated by the heater may be generated in or by conduit 582. Both conductor 580 and conduit 582 may be made of stainless steel. Dimensions of conductor 580 and conduit 582 may be chosen such that the conductor will dissipate heat in a range from approximately 650 watts per meter to 1650 watts per meter. A temperature in conduit 582 may be approximately 480° C. to approximately 815° C., and a temperature in conductor 580 may be approximately 500° C. to 840° C. Substantially uniform heating of an oil shale formation may be provided along a length of conduit 582 greater than about 300 m or even greater than about 600 m.

[0932] FIG. 71 depicts a cross-sectional representation of an embodiment of a removable conductor-in-conduit heat source. Conduit 582 may be placed in opening 514 through overburden 540 such that a gap remains between the conduit and overburden casing 541. Fluids may be removed from opening 514 through the gap between conduit 582 and overburden casing 541. Fluids may be removed from the gap through conduit 5010. Conduit 582 and components of the heat source included within the conduit that are coupled to wellhead 690 may be removed from opening 514 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.

[0933] In certain embodiments, portions of a conductor-in-conduit heat source may be moved or removed to adjust a portion of the formation that is heated by the heat source. For example, in a horizontal well the conductor-in-conduit heat source may be initially almost as long as the opening in the formation. As products are produced from the formation, the conductor-in-conduit heat source may be moved so that it is placed at location further from the end of the opening in the formation. Heat may be applied to a different portion of the formation by adjusting the location of the heat source. In certain embodiments, an end of the heater may be coupled to a sealing mechanism (e.g., a packing mechanism, or a plugging mechanism) to seal off perforations in a liner or casing. The sealing mechanism may inhibit undesired fluid production from portions of the heat source wellbore from which the conductor-in-conduit heat source has been removed.

[0934] As depicted in FIG. 72, sliding connector 583 may be coupled near an end of conductor 580. Sliding connector 583 may be positioned near a bottom end of conduit 582. Sliding connector 583 may electrically couple conductor 580 to conduit 582. Sliding connector 583 may move during use to accommodate thermal expansion and/or contraction of conductor 580 and conduit 582 relative to each other. In some embodiments, sliding connector 583 may be attached to low resistance section 584 of conductor 580. The lower resistance of section 584 may allow the sliding connector to be at a temperature that does not exceed about 90° C. Maintaining sliding connector 583 at a relatively low temperature may inhibit corrosion of the sliding connector and promote good contact between the sliding connector and conduit 582.

[0935] Sliding connector 583 may include scraper 593. Scraper 593 may abut an inner surface of conduit 582 at point 595. Scraper 593 may include any metal or electrically conducting material (e.g., steel or stainless steel). Centralizer 591 may couple to conductor 580. In some embodiments, sliding connector 583 may be positioned on low resistance section 584 of conductor 580. Centralizer 591 may include any electrically conducting material (e.g., a metal or metal alloy). Spring bow 592 may couple scraper 593 to centralizer 591. Spring bow 592 may include any metal or electrically conducting material (e.g., copper-beryllium alloy). In some embodiments, centralizer 591, spring bow 592, and/or scraper 593 are welded together.

[0936] More than one sliding connector 583 may be used for redundancy and to reduce the current through each scraper 593. In addition, a thickness of conduit 582 may be increased for a length adjacent to sliding connector 583 to reduce heat generated in that portion of conduit. The length of conduit 582 with increased thickness may be, for example, approximately 6 m.

[0937] FIG. 73 illustrates an embodiment of a wellhead. Wellhead 690 may be coupled to electrical junction box 690a by flange 690n or any other suitable mechanical device. Electrical junction box 690a may control power (current and voltage) supplied to an electric heater. Power source 690t may be included in electrical junction box 690a. In a heat source embodiment, the electric heater is a conductor-in-conduit heater. Flange 690n may include stainless steel or any other suitable sealing material. Conductor 690b may electrically couple conduit 582 to power source 690t. In some embodiments, power source 690t may be located outside wellhead 690 and the power source is coupled to the wellhead with power cable 585, as shown in FIG. 66. Low resistance section 584 may be coupled to power source 690t. Compression seal 690c may seal conductor 690b at an inner surface of electrical junction box 690a.

[0938] Flange 690n may be sealed with metal o-ring 690d. Conduit 690f may couple flange 690n to flange 690m. Flange 690m may couple to an overburden casing. Flange 690m may be sealed with o-ring 690g (e.g., metal o-ring or steel o-ring). Low resistance section 584 of the conductor may couple to electrical junction box 690a. Low resistance section 584 may be passed through flange 690n. Low resistance section 584 may be sealed in flange 690n with o-ring assembly 690p. Assemblies 690p are designed to insulate low resistance section 584 from flange 690n and flange 690m. Compression seal 690c may be designed to electrically insulate conductor 690b from flange 690n and junction box 690a. Centralizer 581 may couple to low resistance section 584. Thermocouples 690i may be coupled to thermocouple flange 690q with connectors 690h and wire 690j. Thermocouples 690i may be enclosed in an electrically insulated sheath (e.g., a metal sheath). Thermocouples 690i may be sealed in thermocouple flange 690q with compression seals 690k. Thermocouples 690i may be used to monitor temperatures in the heated portion downhole. In some embodiments, fluids (e.g., vapors) may be removed through wellhead 690. For example, fluids from outside conduit 582 may be removed through flange 690r or fluids within the conduit may be removed through flange 690s.

[0939] FIG. 74 illustrates an embodiment of a conductor-in-conduit heater placed substantially horizontally within hydrocarbon layer 516. Heated section 6011 may be placed substantially horizontally within hydrocarbon layer 516. Heater casing 6014 may be placed within hydrocarbon layer 516. Heater casing 6014 may be formed of a corrosion resistant, relatively rigid material (e.g., 304 stainless steel). Heater casing 6014 may be coupled to overburden casing 541. Overburden casing 541 may include materials such as carbon steel. In an embodiment, overburden casing 541 and heater casing 6014 have a diameter of about 15 cm. Expansion mechanism 6012 may be placed at an end of heater casing 6014 to accommodate thermal expansion of the conduit during heating and/or cooling.

[0940] To install heater casing 6014 substantially horizontally within hydrocarbon layer 516, overburden casing 541 may bend from a vertical direction in overburden 540 into a horizontal direction within hydrocarbon layer 516. A curved wellbore may be formed during drilling of the wellbore in the formation. Heater casing 6014 and overburden casing 541 may be installed in the curved wellbore. A radius of curvature of the curved wellbore may be determined by properties of drilling in the overburden and the formation. For example, the radius of curvature may be about 200 m from point 6015 to point 6016.

[0941] Conduit 582 may be placed within heater casing 6014. In some embodiments, conduit 582 may be made of a corrosion resistant metal (e.g., 304 stainless steel). Conduit 582 may be heated to a high temperature. Conduit 582 may also be exposed to hot formation fluids. Conduit 582 may be treated to have a high emissivity. Conduit 582 may have upper section 6002. In some embodiments, upper section 6002 may be made of a less corrosion resistant metal than other portions of conduit 582 (e.g., carbon steel). A large portion of upper section 6002 may be positioned in overburden 540 of the formation. Upper section 6002 may not be exposed to temperatures as high as the temperatures of conduit 582. In an embodiment, conduit 582 and upper section 6002 have a diameter of about 7.6 cm.

[0942] Conductor 580 may be placed in conduit 582. A portion of the conduit placed adjacent to conductor 580 may be made of a metal that has desired electrical properties, emissivity, creep resistance and corrosion resistance at high temperatures. Conductor 580 may include, but is not limited to, 310 stainless steel, 304 stainless steel, 316 stainless steel, 347 stainless steel, and/or other steel or non-steel alloys. Conductor 580 may have a diameter of about 3 cm, however, a diameter of conductor 580 may vary depending on, but not limited to, heating requirements and power requirements. Conductor 580 may be located in conduit 582 using one or more centralizers 581. Centralizers 581 may be ceramic or a combination of metal and ceramic. Centralizers 581 may inhibit conductor 580 from contacting conduit 582. In some embodiments, centralizers 581 may be coupled to conductor 580. In other embodiments, centralizers 581 may be coupled to conduit 582. Conductor 580 may be electrically coupled to conduit 582 using sliding connector 583.

[0943] Conductor 580 may be coupled to transition conductor 6010. Transition conductor 6010 may be used as an electrical transition between lead-in conductor 6004 and conductor 580. In an embodiment, transition conductor 6010 may be carbon steel. Transition conductor 6010 may be coupled to lead-in conductor 6004 with electrical connector 6008. FIG. 75 illustrates an enlarged view of an embodiment of a junction of transition conductor 6010, electrical connector 6008, insulator 6006, and lead-in conductor 6004. Lead-in conductor 6004 may include one or more conductors (e.g., three conductors). In certain embodiments, the one or more conductors may be insulated copper conductors (e.g., rubber-insulated copper cable). In some embodiments, the one or more conductors may be insulated or un-insulated stranded copper cable. As shown in FIG. 75, insulator 6006 may be placed inside lead-in conductor 6004. Insulator 6006 may include electrically insulating materials such as fiberglass. Insulator 6006 may couple electrical connector 6008 to heater support 6000. In an embodiment, electrical current may flow from a power supply through lead-in conductor 6004, through transition conductor 6010, into conductor 580, and return through conduit 582 and upper section 6002.

[0944] Referring to FIG. 74, heater support 6000 may include a support that is used to install heated section 6011 in hydrocarbon layer 516. For example, heater support 6000 may be a sucker rod that is inserted through overburden 540 from a ground surface. The sucker rod may include one or more portions that can be coupled to each other at the surface as the rod is inserted into the formation. In some embodiments, heater support 6000 is a single piece assembled in an assembly facility. Inserting heater support 6000 into the formation may push heated section 6011 into the formation.

[0945] Overburden casing 541 may be supported within overburden 540 using reinforcing material 544. Reinforcing material may include cement (e.g., Portland cement). Surface conductor 545 may enclose reinforcing material 544 and overburden casing 541 in a portion of overburden 540 proximate the ground surface. Surface conductor 545 may include a surface casing.

[0946] FIG. 76 illustrates a schematic of an alternate embodiment of a conductor-in-conduit heater placed substantially horizontally within a formation. In an embodiment, heater support 6000 may be a low resistance conductor (e.g., low resistance section 584 as shown in FIG. 66). Heater support 6000 may include carbon steel or other electrically-conducting materials. Heater support 6000 may be electrically coupled to transition conductor 6010 and conductor 580.

[0947] In some embodiments, a heat source may be placed within an uncased wellbore in an oil shale formation. FIG. 78 illustrates a schematic of an embodiment of a conductor-in-conduit heater placed substantially horizontally within an uncased wellbore in a formation. Heated section 6011 may be placed within opening 514 in hydrocarbon layer 516. In certain embodiments, heater support 6000 may be a low resistance conductor (e.g., low resistance section 584 as shown in FIG. 66). Heater support 6000 may be electrically coupled to transition conductor 6010 and conductor 580. FIG. 77 depicts an alternate embodiment of the conductor-in-conduit heater shown in FIG. 78. In certain embodiments, perforated casing 9636 may be placed in opening 514 as shown in FIG. 77. In some embodiments, centralizers 581 may be used to support perforated casing 9636 within opening 514.

[0948] In certain heat source embodiments, a cladding section may be coupled to heater support 6000 and/or upper section 6002. FIG. 79 depicts an embodiment of cladding section 9200 coupled to heater support 6000. Cladding may also be coupled to an upper section of conduit 582. Cladding section 9200 may reduce the electrical resistance of heater support 6000 and/or the upper section of conduit 582. In an embodiment, cladding section 9200 is copper tubing coupled to the heater support and the conduit.

[0949] In other heat source embodiments, heated section 6011, as shown in FIGS. 74, 76, and 78, may be placed in a wellbore with an orientation other than substantially horizontally in hydrocarbon layer 516. For example, heated section 6011 may be placed in hydrocarbon layer 516 at an angle of about 45° or substantially vertically in the formation. In addition, elements of the heat source placed in overburden 540 (e.g., heater support 6000, overburden casing 541, upper section 6002, etc.) may have an orientation other than substantially vertical within the overburden.

[0950] In certain heat source embodiments, the heat source may be removably installed in a formation. Heater support 6000 may be used to install and/or remove the heat source, including heated section 6011, from the formation. The heat source may be removed to repair, replace, and/or use the heat source in a different wellbore. The heat source may be reused in the same formation or in a different formation. In some embodiments, a heat source or a portion of a heat source may be spooled on a coiled tubing rig and moved to another well location.

[0951] In some embodiments for heating an oil shale formation, more than one heater may be installed in a wellbore or heater well. Having more than one heater in a wellbore or heat source may provide the ability to heat a selected portion or portions of a formation at a different rate than other portions of the formation. Having more than one heater in a wellbore or heat source may provide a backup heat source in the wellbore or heat source should one or more of the heaters fail. Having more than one heater may allow a uniform temperature profile to be established along a desired portion of the wellbore. Having more than one heater may allow for rapid heating of a hydrocarbon layer or layers to a pyrolysis temperature from ambient temperature. The more than one heater may include similar types of heaters or may include different types of heaters. For example, the more than one heater may be a natural distributed combustor heater, an insulated conductor heater, a conductor-in-conduit heater, an elongated member heater, a downhole combustor (e.g., a downhole flameless combustor or a downhole combustor), etc.

[0952] In an in situ conversion process embodiment, a first heater in a wellbore may be used to selectively heat a first portion of a formation and a second heater may be used to selectively heat a second portion of the formation. The first heater and the second heater may be independently controlled. For example, heat provided by a first heater can be controlled separately from heat provided by a second heater. As another example, electrical power supplied to a first electric heater may be controlled independently of electrical power supplied to a second electric heater. The first portion and the second portion may be located at different heights or levels within a wellbore, either vertically or along a face of the wellbore. The first portion and the second portion may be separated by a third, or separate, portion of a formation. The third portion may contain hydrocarbons or may be a non-hydrocarbon containing portion of the formation. For example, the third portion may include rock or similar non-hydrocarbon containing materials. The third portion may be heated or unheated. In some embodiments, heat used to heat the first and second portions may be used to heat the third portion. Heat provided to the first and second portions may substantially uniformly heat the first, second, and third portions.

[0953] FIG. 68 illustrates a perspective view of an embodiment of centralizer 581 in conduit 582. Electrical insulator 581a may be disposed on conductor 580. Insulator 581a may be made of aluminum oxide or other electrically insulating material that has a high working temperature limit. Neck portion 581j may be a bushing which has an inside diameter that allows conductor 580 to pass through the bushing. Neck portion 581j may include electrically-insulative materials such as metal oxides and ceramics (e.g., aluminum oxide). Insulator 581a and neck portion 581j may be obtainable from manufacturers such as CoorsTek (Golden, Colo.) or Norton Ceramics (United Kingdom). In an embodiment, insulator 581a and/or neck portion 581j are made from 99% or greater purity machinable aluminum oxide. In certain embodiments, ceramic portions of a heat source may be surface glazed. Surface glazing ceramic may seal the ceramic from contamination from dirt and/or moisture. High temperature surface glazing of ceramics may be done by companies such as NGK-Locke Inc. (Baltimore, Md.) or Johannes Gebhart (Germany).

[0954] A location of insulator 581a on conductor 580 may be maintained by disc 581d. Disc 581d may be welded to conductor 580. Spring bow 581c may be coupled to insulator 581a by disc 581b. Spring bow 581c and disc 581b may be made of metals such as 310 stainless steel and/or any other thermally conducting material that may be used at relatively high temperatures. Spring bow 581c may reduce the stress on ceramic portions of the centralizer during installation or removal of the heater, and/or during use of the heater. Reducing the stress on ceramic portions of the centralizer during installation or removal may increase an operational lifetime of the heater. In some heat source embodiments, centralizer 581 may have an opening that fits over an end of conductor 580. In other embodiments, centralizer 581 may be assembled from two or more pieces around a portion of conductor 580. The pieces may be coupled to conductor 580 by fastening device 581e. Fastening device 581e may be made of any material that can be used at relatively high temperatures (e.g., steel).

[0955] FIG. 69 depicts a representation of an embodiment of centralizer 581 disposed on conductor 580. Discs 581d may maintain positions of centralizer 581 relative to conductor 580. Discs 581d may be metal discs welded to conductor 580. Discs 581d may be tack-welded to conductor 580. FIG. 70 depicts a top view representation of a centralizer embodiment. Centralizer 581 may be made of any suitable electrically insulating material able to withstand high voltage at high temperatures. Examples of such materials include, but are not limited to, aluminum oxide and/or Macor. Centralizer 581 may electrically insulate conductor 580 from conduit 582, as shown in FIGS. 69 and 70.

[0956] FIG. 80 illustrates a cross-sectional representation of an embodiment of a centralizer placed on a conductor. FIG. 81 depicts a portion of an embodiment of a conductor-in-conduit heat source with a cutout view showing a centralizer on the conductor. Centralizer 581 may be used in a conductor-in-conduit heat source. Centralizer 581 may be used to maintain a location of conductor 580 within conduit 582. Centralizer 581 may include electrically-insulating materials such as ceramics (e.g., alumina and zirconia). As shown in FIG. 80, centralizer 581 may have at least one recess 581i. Recess 581i may be, for example, an indentation or notch in centralizer 581 or a recess left by a portion removed from the centralizer. A cross-sectional shape of recess 581i may be a rectangular shape or any other geometrical shape. In certain embodiments, recess 581i has a shape that allows protrusion 581g to reside within the recess. Recess 581i may be formed such that the recess will be placed at a junction of centralizer 581 and conductor 580. In one embodiment, recess 581i is formed at a bottom of centralizer 581.

[0957] At least one protrusion 581g may be formed on conductor 580. Protrusion 581g may be welded to conductor 580. In some embodiments, protrusion 581g is a weld bead formed on conductor 580. Protrusion 581g may include electrically-conductive materials such as steel (e.g., stainless steel). In certain embodiments, protrusion 581g may include one or more protrusions formed around the circumference of conductor 580. Protrusion 581g may be used to maintain a location of centralizer 581 on conductor 580. For example, protrusion 581g may inhibit downward movement of centralizer 581 along conductor 580. In some embodiments, at least one additional recess 581i and at least one additional protrusion 581g may be placed at a top of centralizer 581 to inhibit upward movement of the centralizer along conductor 580.

[0958] In an embodiment, electrically-insulating material 581h is placed over protrusion 581g and recess 581i. Electrically-insulating material 581h may cover recess 581i such that protrusion 581g is enclosed within the recess and the electrically-insulating material. In some embodiments, electrically-insulating material 581h may partially cover recess 581i. Protrusion 581g may be enclosed so that carbon deposition (i.e., coking) on protrusion 581g during use is inhibited. Carbon may form electrically-conducting paths during use of conductor 580 and conduit 582 to heat a formation. Electrically-insulating material 581h may include materials such as, but not limited to, metal oxides and/or ceramics (e.g., alumina or zirconia). In some embodiments, electrically-insulating material 581h is a thermally conducting material. A thermal plasma spray process may be used to place electrically-insulating material 581h over protrusion 581g and recess 581i. The thermal plasma process may spray coat electrically-insulating material 581h on protrusion 581g and/or centralizer 581.

[0959] In an embodiment, centralizer 581 with recess 581i, protrusion 581g, and electrically-insulating material 581h are placed on conductor 580 within conduit 582 during installation of the conductor-in-conduit heat source in an opening in a formation. In another embodiment, centralizer 581 with recess 581i, protrusion 581g, and electrically-insulating material 581h are placed on conductor 580 within conduit 582 during assembling of the conductor-in-conduit heat source. For example, an assembling process may include forming protrusion 581g on conductor 580, placing centralizer 581 with recess 581i on conductor 580, covering the protrusion and the recess with electrically-insulating material 581h, and placing the conductor within conduit 582.

[0960] FIG. 82 depicts an alternate embodiment of centralizer 581. Neck portion 581j may be coupled to centralizer 581. In certain embodiments, neck portion 581j is an extended portion of centralizer 581. Protrusion 581g may be placed on conductor 580 to maintain a location of centralizer 581 and neck portion 581j on the conductor. Neck portion 581j may be a bushing which has an inside diameter that allows conductor 580 to pass through the bushing. Neck portion 581j may include electrically-insulative materials such as metal oxides and ceramics (e.g., aluminum oxide). For example, neck portion 581j may be a commercially available bushing from manufacturers such as Borges Technical Ceramics (Pennsburg, Pa.). In one embodiment, as shown in FIG. 82, a first neck portion 581j is coupled to an upper portion of centralizer 581 and a second neck portion 581j is coupled to a lower portion of centralizer 581.

[0961] Neck portion 581j may extend between about 1 cm and about 5 cm from centralizer 581. In an embodiment, neck portion 581j extends about 2–3 cm from centralizer 581. Neck portion 581j may extend a selected distance from centralizer 581 such that arcing (e.g., surface arcing) is inhibited. Neck portion 581j may increase a path length for arcing between conductor 580 and conduit 582. A path for arcing between conductor 580 and conduit 582 may be formed by carbon deposition on centralizer 581 and/or neck portion 581j. Increasing the path length for arcing between conductor 580 and conduit 582 may reduce the likelihood of arcing between the conductor and the conduit. Another advantage of increasing the path length for arcing between conductor 580 and conduit 582 may be an increase in a maximum operating voltage of the conductor.

[0962] In an embodiment, neck portion 581j also includes one or more grooves 581k. One or more grooves 581k may further increase the path length for arcing between conductor 580 and conduit 582. In certain embodiments, conductor 580 and conduit 582 may be oriented substantially vertically within a formation. In such an embodiment, one or more grooves 581k may also inhibit deposition of conducting particles (e.g., carbon particles or corrosion scale) along the length of neck portion 581j. Conducting particles may fall by gravity along a length of conductor 580. One or more grooves 581k may be oriented such that falling particles do not deposit into the one or more grooves. Inhibiting the deposition of conducting particles on neck portion 581j may inhibit formation of an arcing path between conductor 580 and conduit 582. In some embodiments, diameters of each of one or more grooves 581k may be varied. Varying the diameters of the grooves may further inhibit the likelihood of arcing between conductor 580 and conduit 582.

[0963] FIG. 83 depicts an embodiment of centralizer 581. Centralizer 581 may include two or more portions held together by fastening device 581e. Fastening device 581e may be a clamp, bolt, snap-lock, or screw. FIGS. 84 and 85 depict top views of embodiments of centralizer 581 placed on conductor 580. Centralizer 581 may include two portions. The two portions may be coupled together to form a centralizer in a “clam shell” configuration. The two portions may have notches and recesses that are shaped to fit together as shown in either of FIGS. 84 and 85. In some embodiments, the two portions may have notches and recesses that are tapered so that the two portions tightly couple together. The two portions may be slid together lengthwise along the notches and recesses.

[0964] In a heat source embodiment, an insulation layer may be placed between a conductor and a conduit. The insulation layer may be used to electrically insulate the conductor from the conduit. The insulation layer may also maintain a location of the conductor within the conduit. In some embodiments, the insulation layer may include a layer that remains placed on and/or in the heat source after installation. In certain embodiments, the insulation layer may be removed by heating the heat source to a selected temperature. The insulation layer may include electrically-insulating materials such as, but not limited to, metal oxides and/or ceramics. For example, the insulation layer may be Nextel™ insulation obtainable from 3M Company (St. Paul, Minn.). An insulation layer may also be used for installation of any other heat source (e.g., insulated conductor heat source, natural distributed combustor, etc.). In an embodiment, the insulation layer is fastened to the conductor. The insulation layer may be fastened to the conductor with a high temperature adhesive (e.g., a ceramic adhesive such as Cotronics 920 alumina-based adhesive available from Cotronics Corporation (Brooklyn, N.Y.)).

[0965] FIG. 86 depicts a cross-sectional representation of an embodiment of a section of a conductor-in-conduit heat source with insulation layer 9180. Insulation layer 9180 may be placed on conductor 580. Insulation layer 9180 may be spiraled around conductor 580 as shown in FIG. 86. In one embodiment, insulation layer 9180 is a single insulation layer wound around the length of conductor 580. In some embodiments, insulation layer 9180 may include one or more individual sections of insulation layers wrapped around conductor 580. Conductor 580 may be placed in conduit 582 after insulation layer 9180 has been placed on the conductor. Insulation layer 9180 may electrically insulate conductor 580 from conduit 582.

[0966] In an embodiment of a conductor-in-conduit heat source, a conduit may be pressurized with a fluid to inhibit a large pressure difference between pressure in the conduit and pressure in the formation. Balanced pressure or a small pressure difference may inhibit deformation of the conduit during use. The fluid may increase conductive heat transfer from the conductor to the conduit. The fluid may include, but is not limited to, a gas such as helium, nitrogen, air, or mixtures thereof. The fluid may inhibit arcing between the conductor and the conduit. If air and/or air mixtures are used to pressurize the conduit, the air and/or air mixtures may react with materials of the conductor and the conduit to form an oxide layer on a surface of the conductor and/or an oxide layer on an inner surface of the conduit. The oxide layer may inhibit arcing. The oxide layer may make the conductor and/or the conduit more resistant to corrosion.

[0967] Reducing the amount of heat losses to an overburden of a formation may increase an efficiency of a heat source. The efficiency of the heat source may be determined by the energy transferred into the formation through the heat source as a fraction of the energy input into the heat source. In other words, the efficiency of the heat source may be a function of energy that actually heats a desired portion of the formation divided by the electrical power (or other input power) provided to the heat source. To increase the amount of energy actually transferred to the formation, heating losses to the overburden may be reduced. Heating losses in the overburden may be reduced for electrical heat sources by the use of relatively low resistance conductors in the overburden that couple a power supply to the heat source. Alternating electrical current flowing through certain conductors (e.g., carbon steel conductors) tends to flow along the skin of the conductors. This skin depth effect may increase the resistance heating at the outer surface of the conductor (i.e., the current flows through only a small portion of the available metal) and increases heating of the overburden. Electrically conductive casings, coatings, wiring, and/or claddings may be used to reduce the electrical resistance of a conductor used in the overburden. Reducing the electrical resistance of the conductor in the overburden may reduce electricity losses to heating the conduit in the overburden portion and thereby increase the available electricity for resistive heating in portions of the conductor below the overburden.

[0968] As shown in FIG. 66, low resistance section 584 may be coupled to conductor 580. Low resistance section 584 may be placed in overburden 540. Low resistance section 584 may be, for example, a carbon steel conductor. Carbon steel may be used to provide mechanical strength for the heat source in overburden 540. In an embodiment, an electrically conductive coating may be coated on low resistance section 584 to further reduce an electrical resistance of the low resistance conductor. In some embodiments, the electrically conductive coating may be coated on low resistance section 584 during assembly of the heat source. In other embodiments, the electrically conductive coating may be coated on low resistance section 584 after installation of the heat source in opening 514.

[0969] In some embodiments, the electrically conductive coating may be sprayed on low resistance section 584. For example, the electrically conductive coating may be a sprayed on thermal plasma coating. The electrically conductive coating may include conductive materials such as, but not limited to, aluminum or copper. The electrically conductive coating may include other conductive materials that can be thermal plasma sprayed. In certain embodiments, the electrically conductive coating may be coated on low resistance section 584 such that the resistance of the low resistance conductor is reduced by a factor of greater than about 2. In some embodiments, the resistance is lowered by a factor of greater than about 4 or about 5. The electrically conductive coating may have a thickness of between 0.1 mm and 0.8 mm. In an embodiment, the electrically conductive coating may have a thickness of about 0.25 mm. The electrically conductive coating may be coated on low resistance conductors used with other types of heat sources such as, for example, insulated conductor heat sources, elongated member heat sources, etc.

[0970] In another embodiment, a cladding may be coupled to low resistance section 584 to reduce the electrical resistance in overburden 540. FIG. 87 depicts a cross-sectional view of a portion of cladding section 9200 of conductor-in-conduit heater. Cladding section 9200 may be coupled to the outer surface of low resistance section 584. Cladding sections 9200 may also be coupled to an inner surface of conduit 582. In certain embodiments, cladding sections may be coupled to inner surface of low resistance section 584 and/or outer surface of conduit 582. In some embodiments, low resistance section 584 may include one or more sections of individual low resistance sections 584 coupled together. Conduit 582 may include one or more sections of individual conduits 582 coupled together.

[0971] Individual cladding sections 9200 may be coupled to each individual low resistance section 584 and/or conduit 582, as shown in FIG. 87. A gap may remain between each cladding section 9200. The gap may be at a location of a coupling between low resistance sections 584 and/or conduits 582. For example, the gap may be at a thread or weld junction between low resistance sections 584 and/or conduits 582. The gap may be less than about 4 cm in length. In certain embodiments, the gap may be less than about 5 cm in length or less than 6 cm in length.

[0972] Cladding section 9200 may be a conduit (or tubing) of relatively electrically conductive material. Cladding section 9200 may be a conduit that tightly fits against a surface of low resistance section 584 and/or conduit 582. Cladding section 9200 may include non-ferromagnetic metals that have a relatively high electrical conductivity. For example, cladding section 9200 may include copper, aluminum, brass, bronze, or combinations thereof. Cladding section 9200 may have a thickness between about 0.2 cm and about 1 cm. In some embodiments, low resistance section 584 has an outside diameter of about 2.5 cm and conduit 582 has an inside diameter of about 7.3 cm. In an embodiment, cladding section 9200 coupled to low resistance section 584 is copper tubing with a thickness of about 0.32 cm (about ⅛ inch) and an inside diameter of about 2.5 cm. In an embodiment, cladding section 9200 coupled to conduit 582 is copper tubing with a thickness of about 0.32 cm (about ⅛ inch) and an outside diameter of about 7.3 cm. In certain embodiments, cladding section 9200 has a thickness between about 0.20 cm and about 1.2 cm.

[0973] In certain embodiments, cladding section 9200 is brazed to low resistance section 584 and/or conduit 582. In other embodiments, cladding section 9200 may be welded to low resistance section 584 and/or conduit 582. In one embodiment, cladding section 9200 is Everdur® (silicon bronze) welded to low resistance section 584 and/or conduit 582. Cladding section 9200 may be brazed or welded to low resistance section 584 and/or conduit 582 depending on the types of materials used in the cladding section, the low resistance conductor, and the conduit. For example, cladding section 9200 may include copper that is Everdur® welded to low resistance section 584, which includes carbon steel. In some embodiments, cladding section 9200 may be pre-oxidized to inhibit corrosion of the cladding section during use.

[0974] Using cladding section 9200 coupled to low resistance section 584 and/or conduit 582 may inhibit a significant temperature rise in the overburden of a formation during use of the heat source (i.e., reduce heat losses to the overburden). For example, using a copper cladding section of about 0.3 cm thickness may decrease the electrical resistance of a carbon steel low resistance conductor by a factor of about 20. The lowered resistance in the overburden section of the heat source may provide a relatively small temperature increase adjacent to the wellbore in the overburden of the formation. For example, supplying a current of about 500 A into an approximately 1.9 cm diameter low resistance conductor (schedule 40 carbon steel pipe) with a copper cladding of about 0.3 cm thickness produces a maximum temperature of about 93° C. at the low resistance conductor. This relatively low temperature in the low resistance conductor may transfer relatively little heat to the formation. For a fixed voltage at the power source, lowering the resistance of the low resistance conductor may increase the transfer of power into the heated section of the heat source (e.g., conductor 580). For example, a 600 volt power supply may be used to supply power to a heat source through about a 300 m overburden and into about a 260 m heated section. This configuration may supply about 980 watts per meter to the heated section. Using a copper cladding section of about 0.3 cm thickness with a carbon steel low resistance conductor may increase the transfer of power into the heated section by up to about 15% compared to using the carbon steel low resistance conductor only.

[0975] In some embodiments, cladding section 9200 may be coupled to conductor 580 and/or conduit 582 by a “tight fit tubing” (TFT) method. TFT is commercially available from vendors such as Kuroki (Japan) or Karasaki Steel (Japan). The TFT method includes cryogenically cooling an inner pipe or conduit, which is a tight fit to an outer pipe. The cooled inner pipe is inserted into the heated outer pipe or conduit. The assembly is then allowed to return to an ambient temperature. In some cases, the inner pipe can be hydraulically expanded to bond tightly with the outer pipe.

[0976] Another method for coupling a cladding section to a conductor or a conduit may include an explosive cladding method. In explosive cladding, an inner pipe is slid into an outer pipe. Primer cord or other type of explosive charge may be set off inside the inner pipe. The explosive blast may bond the inner pipe to the outer pipe.

[0977] Electromagnetically formed cladding may also be used for cladding section 9200. An inner pipe and an outer pipe may be placed in a water bath. Electrodes attached to the inner pipe and the outer pipe may be used to create a high potential between the inner pipe and the outer pipe. The potential may cause sudden formation of bubbles in the bath that bond the inner pipe to the outer pipe.

[0978] In another embodiment, cladding section 9200 may be arc welded to a conductor or conduit. For example, copper may be arc deposited and/or welded to a stainless steel pipe or tube.

[0979] In some embodiments, cladding section 9200 may be formed with plasma powder welding (PPW). PPW formed material may be obtained from Daido Steel Co. (Japan). In PPW, copper powder is heated to form a plasma. The hot plasma may be moved along the length of a tube (e.g., a stainless steel tube) to deposit the copper and form the copper cladding.

[0980] Cladding section 9200 may also be formed by billet co-extrusion. A large piece of cladding material may be extruded along a pipe to form a desired length of cladding along the pipe.

[0981] In certain embodiments, forge welding (e.g., shielded active gas welding) may be used to form cladding section 9200 on a conductor and/or conduit. Forge welding may be used to form a uniform weld through the cladding section and the conductor or conduit.

[0982] Another method is to start with strips of copper and carbon steel that are bonded together by tack welding or another suitable method. The composite strip is drawn through a shaping unit to form a cylindrically shaped tube. The cylindrically shaped tube is seam welded longitudinally. The resulting tube may be coiled onto a spool.

[0983] Another possible embodiment for reducing the electrical resistance of the conductor in the overburden is to form low resistance section 584 from low resistance metals (e.g., metals that are used in cladding section 9200). A polymer coating may be placed on some of these metals to inhibit corrosion of the metals (e.g., to inhibit corrosion of copper or aluminum by hydrogen sulfide).

[0984] Increasing the emissivity of a conductive heat source may increase the efficiency of heat transfer to a formation. An emissivity of a surface affects the amount of radiative heat emitted from the surface and the amount of radiative heat absorbed by the surface. In general, the higher the emissivity a surface has, the greater the radiation from the surface or the absorption of heat by the surface. Thus, increasing the emissivity of a surface increases the efficiency of heat transfer because of the increased radiation of energy from the surface into the surroundings. For example, increasing the emissivity of a conductor in a conductor-in-conduit heat source may increase the efficiency of heat transfer to the conduit, as shown by the following equation:

[0985] $\begin{array}{cc}\stackrel{.}{Q}=\frac{2\pi r_1\sigma \left(T_1^4-T_2^4\right)}{\frac{1}{{\varepsilon }_1}+\left(\frac{r_1}{r_2}\right)\left(\frac{1}{{\varepsilon }_2}-1\right)};& \left(30\right)\end{array}$
where {dot over (Q)} is the rate of heat transfer between a cylindrical conductor and a conduit, r₁ is the radius of the conductor, r₂ is the radius of the conduit, T₁ is the temperature at the conductor, T₂ is the temperature at the conduit, σ is the Stefan-Boltzmann constant (5.670×10⁻⁸ J·K⁻⁴·m⁻²·s⁻¹), ε₁ is the emissivity of the conductor, and σ₂ is the emissivity of the conduit. According to EQN. 30, increasing the emissivity of the conductor increases the heat transfer between the conductor and the conduit. Accordingly, for a constant heat transfer rate, increasing the emissivity of the conductor decreases the temperature difference between the conductor and the conduit (i.e., increases the temperature of the conduit for a given conductor temperature). Increasing the temperature of the conduit increases the amount of heat transfer to the formation.

[0986] In an embodiment, a conductor and/or conduit may be treated to increase the emissivity of the conductor and/or conduit materials. Treating the conductor and/or conduit may include roughening a surface of the conductor or conduit and/or oxidizing the conductor or conduit. In some embodiments, a conductor and/or conduit may be roughened and/or oxidized prior to assembly of a heat source. In some embodiments, a conductor and/or conduit may be roughened and/or oxidized after assembly and/or installation into a formation (e.g., an oxidizing fluid may be introduced into an annular space between the conductor and the conduit when heating a portion of the formation to pyrolysis temperatures so that the heat generated in the conductor oxidizes the conductor and the conduit). The treatment method may be used to treat inner surfaces and/or outer surfaces, or portions thereof, of conductors or conduits. In certain embodiments, the outer surface of a conductor and the inner surface of a conduit are treated to increase the emissivities of the conductor and the conduit.

[0987] In an embodiment, surfaces of a conductor, or a portion of the surface, may be roughened. The roughened surface of the conductor may be the outer surface of the conductor. The surface of the conductor may be roughened by, but is not limited to being roughened by, sandblasting or beadblasting the surface, peening the surface, emery grinding the surface, or using an electrostatic discharge method on the surface. For example, the surface of the conductor may be sand blasted with fine particles to roughen the surface. The conductor may also be treated by pre-oxidizing the surface of the conductor (i.e., heating the conductor to an oxidation temperature before use of the conductor). Pre-oxidizing the surface of the conductor may include heating the conductor to a temperature between about 850° C. and about 950° C. The conductor may be heated in an oven or furnace. The conductor may be heated in an oxidizing atmosphere (e.g., an oven with a charge of an oxidizing fluid such as air). In an embodiment, a 304H stainless steel conductor is heated in a furnace at a temperature of about 870° C. for about 2 hours. If the surface of the 304H stainless steel conductor is roughened prior to heating the conductor in the furnace, the emissivity of the 304H stainless steel conductor may be increased from about 0.5 to about 0.85. Increasing the emissivity of the conductor may reduce an operating temperature of the conductor. Operating the conductor at lower temperatures may increase an operational lifetime of the conductor. For example, operating the conductor at lower temperatures may reduce creep and/or corrosion.

[0988] In some embodiments, applying a coating to a conductor or conduit may increase the emissivity of a conductor or a conduit and increase the efficiency of heat transfer to the formation. An electrically insulating and thermally conductive coating may be placed on a conductor and/or conduit. The electrically insulating coating may inhibit arcing between the conductor and the conduit. Arcing between the conductor and the conduit may cause shorting between the conductor and the conduit. Arcing may also produce hot spots and/or cold spots on either the conductor or the conduit. In some embodiments, a coating or coatings on portions of a conduit and/or a conductor may increase emissivity, electrically insulate, and promote thermal conduction.

[0989] As shown in FIG. 66, conductor 580 and conduit 582 may be placed in opening 514 in hydrocarbon layer 516. In an embodiment, an electrically insulative, thermally conductive coating is placed on conductor 580 and conduit 582 (e.g., on an outside surface of the conductor and an inside surface of the conduit). In some embodiments, the electrically insulative, thermally conductive coating is placed on conductor 580. In other embodiments, the electrically insulative, thermally conductive coating is placed on conduit 582. The electrically insulative, thermally conductive coating may electrically insulate conductor 580 from conduit 582. The electrically insulative, thermally conductive coating may inhibit arcing between conductor 580 and conduit 582. In certain embodiments, the electrically insulative, thermally conductive coating maintains an emissivity of conductor 580 or conduit 582 (i.e., inhibits the emissivity of the conductor or conduit from decreasing). In other embodiments, the electrically insulative, thermally conductive coating increases an emissivity of conductor 580 and/or conduit 582. The electrically insulative, thermally conductive coating may include, but is not limited to, oxides of silicon, aluminum, and zirconium, or combinations thereof. For example, silicon oxide may be used to increase an emissivity of a conductor or conduit while aluminum oxide may be used to provide better electrical insulation and thermal conductivity. Thus, a combination of silicon oxide and aluminum oxide may be used to increase emissivity while providing improved electrical insulation and thermal conductivity. In an embodiment, aluminum oxide is coated on conductor 580 to electrically insulate the conductor followed by a coating of silicon oxide to increase the emissivity of the conductor.

[0990] In an embodiment, the electrically insulative, thermally conductive coating is sprayed on conductor 580 or conduit 582. The coating may be sprayed on during assembly of the conductor-in-conduit heat source. In some embodiments, the coating is sprayed on before assembling the conductor-in-conduit heat source. For example, the coating may be sprayed on conductor 580 or conduit 582 by a manufacturer of the conductor or conduit. In certain embodiments, the coating is sprayed on conductor 580 or conduit 582 before the conductor or conduit is coiled onto a spool for installation. In other embodiments, the coating is sprayed on after installation of the conductor-in-conduit heat source.

[0991] In a heat source embodiment, a perforated conduit may be placed in the opening formed in the oil shale formation proximate and external to the conduit of a conductor-in-conduit heater. The perforated conduit may remove fluids formed in an opening in the formation to reduce pressure adjacent to the heat source. A pressure may be maintained in the opening such that deformation of the first conduit is inhibited. In some embodiments, the perforated conduit may be used to introduce a fluid into the formation adjacent to the heat source. For example, in some embodiments, hydrogen gas may be injected into the formation adjacent to selected heat sources to increase a partial pressure of hydrogen during in situ conversion.

[0992] FIG. 88 illustrates an embodiment of a conductor-in-conduit heater that may heat an oil shale formation. Second conductor 586 may be disposed in conduit 582 in addition to conductor 580. Second conductor 586 may be coupled to conductor 580 using connector 587 located near a lowermost surface of conduit 582. Second conductor 586 may be a return path for the electrical current supplied to conductor 580. For example, second conductor 586 may return electrical current to wellhead 690 through low resistance second conductor 588 in overburden casing 541. Second conductor 586 and conductor 580 may be formed of elongated conductive material. Second conductor 586 and conductor 580 may be a stainless steel rod having a diameter of approximately 2.4 cm. Connector 587 may be flexible. Conduit 582 may be electrically isolated from conductor 580 and second conductor 586 using centralizers 581. The use of a second conductor may eliminate the need for a sliding connector. The absence of a sliding connector may extend the life of the heater. The absence of a sliding connector may allow for isolation of applied power from hydrocarbon layer 516.

[0993] In a heat source embodiment that utilizes second conductor 586, conductor 580 and the second conductor may be coupled by a flexible connecting cable. The bottom of the first and second conductor may have increased thicknesses to create low resistance sections. The flexible connector may be made of stranded copper covered with rubber insulation.

[0994] In a heat source embodiment, a first conductor and a second conductor may be coupled to a sliding connector within a conduit. The sliding connector may include insulating material that inhibits electrical coupling between the conductors and the conduit. The sliding connector may accommodate thermal expansion and contraction of the conductors and conduit relative to each other. The sliding connector may be coupled to low resistance sections of the conductors and/or to a low temperature portion of the conduit.

[0995] In a heat source embodiment, the conductor may be formed of sections of various metals that are welded or otherwise joined together. The cross-sectional area of the various metals may be selected to allow the resulting conductor to be long, to be creep resistant at high operating temperatures, and/or to dissipate desired amounts of heat per unit length along the entire length of the conductor. For example, a first section may be made of a creep resistant metal (such as, but not limited to, Inconel 617 or HR120), and a second section of the conductor may be made of 304 stainless steel. The creep resistant first section may help to support the second section. The cross-sectional area of the first section may be larger than the cross-sectional area of the second section. The larger cross-sectional area of the first section may allow for greater strength of the first section. Higher resistivity properties of the first section may allow the first section to dissipate the same amount of heat per unit length as the smaller cross-sectional area second section.

[0996] In some embodiments, the cross-sectional area and/or the metal used for a particular conduit section may be chosen so that a particular section provides greater (or lesser) heat dissipation per unit length than an adjacent section. More heat may be provided near an interface between a hydrocarbon layer and a non-hydrocarbon layer (e.g., the overburden and the hydrocarbon layer and/or an underburden and the hydrocarbon layer) to counteract end effects and allow for more uniform heat dissipation into the oil shale formation.

[0997] In a heat source embodiment, a conduit may have a variable wall thickness. Wall thickness may be thickest adjacent to portions of the formation that do not need to be fully heated. Portions of formation that do not need to be fully heated may include layers of formation that have low grade, little, or no hydrocarbon material.

[0998] In an embodiment of heat sources placed in a formation, a first conductor, a second conductor and a third conductor may be electrically coupled in a 3-phase Y electrical configuration. Each of the conductors may be a part of a conductor-in-conduit heater. The conductor-in-conduit heaters may be located in separate wellbores within the formation. The outer conduits may be electrically coupled together or conduits may be connected to ground. The 3-phase Y electrical configuration may provide a safer and more efficient method to heat an oil shale formation than using a single conductor. The first, second, and third conduits may be electrically isolated from the first, second, and third conductors. Each conductor-in-conduit heater in a 3-phase Y electrical configuration may be dimensioned to generate approximately 650 watts per meter of conductor to approximately 1650 watts per meter of conductor.

[0999] Heat may be generated by the conductor-in-conduit heater within an open wellbore. Generated heat may radiatively heat a portion of an oil shale formation adjacent to the conductor-in-conduit heater. To a lesser extent, gas conduction adjacent to the conductor-in-conduit heater heats the portion of the formation. Using an open wellbore completion may reduce casing and packing costs associated with filling the opening with a material to provide conductive heat transfer between the insulated conductor and the formation. In addition, heat transfer by radiation may be more efficient than heat transfer by conduction in a formation, so the heaters may be operated at lower temperatures using radiative heat transfer. Operating at a lower temperature may extend the life of the heat source and/or reduce the cost of material needed to form the heat source.

[1000] The conductor-in-conduit heater may be installed in opening 514. In an embodiment, the conductor-in-conduit heater may be installed into a well by sections. For example, a first section of the conductor-in-conduit heater may be suspended in a wellbore by a rig. The section may be about 12 m in length. A second section (e.g., of substantially similar length) may be coupled to the first section in the well. The second section may be coupled by welding the second section to the first section and/or with threads disposed on the first and second section. An orbital welder disposed at the wellhead may weld the second section to the first section. The first section may be lowered into the wellbore by the rig. This process may be repeated with subsequent sections coupled to previous sections until a heater of desired length is placed in the wellbore. In some embodiments, three sections may be welded together prior to being placed in the wellbore. The welds may be formed and tested before the rig is used to attach the three sections to a string already placed in the ground. The three sections may be lifted by a crane to the rig. Having three sections already welded together may reduce installation time of the heat source.

[1001] Assembling a heat source at a location proximate a formation (e.g., at the site of a formation) may be more economical than shipping a pre-formed heat source and/or conduits to the oil shale formation. For example, assembling the heat source at the site of the formation may reduce costs for transporting assembled heat sources over long distances. In addition, heat sources may be more easily assembled in varying lengths and/or of varying materials to meet specific formation requirements at the formation site. For example, a portion of a heat source that is to be heated may be made of a material (e.g., 304 stainless steel or other high temperature alloy) while a portion of the heat source in the overburden may be made of carbon steel. Forming the heat source at the site may allow the heat source to be specifically made for an opening in the formation so that the portion of the heat source in the overburden is carbon steel and not a more expensive, heat resistant alloy. Heat source lengths may vary due to varying formation layer depths and formation properties. For example, a formation may have a varying thickness and/or may be located underneath rolling terrain, uneven surfaces, and/or an overburden with a varying thickness. Heat sources of varying length and of varying materials may be assembled on site in lengths determined by the depth of each opening in the formation.

[1002] FIG. 89 depicts an embodiment for assembling a conductor-in-conduit heat source and installing the heat source in a formation. The conductor-in-conduit heat source may be assembled in assembly facility 8650. In some embodiments, the heat source is assembled from conduits shipped to the formation site. In other embodiments, heat sources may be made from plate stock that is formed into conduits at the assembly facility. An advantage of forming a conduit at the assembly facility may be that a surface of plate stock may be treated with a desired coating (e.g., a coating that allows the emissivity to approach one) or cladding (e.g., copper cladding) before forming the conduit so that the treated surface is an inside surface of the conduit. In some embodiments, portions of heat sources may be formed from plate stock at the assembly facility, while other portions of the heat source may be formed from conduits shipped to the formation site.

[1003] Individual conductor-in-conduit heat source 8652 may include conductor 580 and conduit 582 as shown in FIG. 90. In an embodiment, conductor 580 and conduit 582 heat sources may be made of a number of joined together sections. In an embodiment, each section is a standard 40 ft (12.2 m) section of pipe. Other section lengths may also be formed and/or utilized. In addition, sections of conductor 580 and/or conduit 582 may be treated in assembly facility 8650 before, during, or after assembly. The sections may be treated, for example, to increase an emissivity of the sections by roughening and/or oxidation of the sections.

[1004] Each conductor-in-conduit heat source 8652 may be assembled in an assembly facility. Components of conductor-in-conduit heat source 8652 may be placed on or within individual conductor-in-conduit heat source 8652 in the assembly facility. Components may include, but are not limited to, one or more centralizers, low resistance sections, sliding connectors, insulation layers, and coatings, claddings, or coupling materials.

[1005] As shown in FIG. 89, each individual conductor-in-conduit heat source 8652 may be coupled to at least one individual conductor-in-conduit heat source 8652 at coupling station 8656 to form conductor-in-conduit heat source of desired length 8654. The desired length may be, for example, a length of a conductor-in-conduit heat source specified for a selected opening in a formation. In certain embodiments, coupling individual conductor-in-conduit heat source 8652 to at least one additional individual conductor-in-conduit heat source 8652 includes welding the individual conductor-in-conduit heat source to at least one additional individual conductor-in-conduit heat source. In one embodiment, welding each individual conductor-in-conduit heat source 8652 to an additional individual conductor-in-conduit heat source is accomplished by forge welding two adjacent sections together.

[1006] In some embodiments, sections of welded together conductor-in-conduit heat source of desired length 8654 are placed on a bench, holding tray or in an opening in the ground until the entire length of the heat source is completed. Weld integrity may be tested as each weld is formed. For example, weld integrity may be tested by a non-destructive testing method such as x-ray testing, acoustic testing, and/or electromagnetic testing. After an entire length of conductor-in-conduit heat source of desired length 8654 is completed, the conductor-in-conduit heat source of desired length may be coiled onto spool 8660 in a direction of arrow 8662. Coiling conductor-in-conduit heat source of desired length 8654 may make the heat source easier to transport to an opening in a formation. For example, conductor-in-conduit heat source of desired length 8654 may be more easily transported by truck or train to an opening in the formation.

[1007] In some embodiments, a set length of welded together conductor-in-conduit may be coiled onto spool 8660 while other sections are being formed at coupling station 8656. In some embodiments, the assembly facility may be a mobile facility (e.g., placed on one or more train cars or semi-trailers) that can be moved to an opening in a formation. After forming a welded together length of conductor-in-conduit with components (e.g., centralizers, coatings, claddings, sliding connectors), the conductor-in-conduit length may be lowered into the opening in the formation.

[1008] In certain embodiments, conductor-in-conduit heat source of desired length 8654 may be tested at testing station 8658 before coiling the heat source. Testing station 8658 may be used to test a completed conductor-in-conduit heat source of desired length 8654 or sections of the conductor-in-conduit heat source of desired length. Testing station 8658 may be used to test selected properties of conductor-in-conduit heat source of desired length 8654. For example, testing station 8658 may be used to test properties such as, but not limited to, electrical conductivity, weld integrity, thermal conductivity, emissivity, and mechanical strength. In one embodiment, testing station 8658 is used to test weld integrity with an Electro-Magnetic Acoustic Transmission (EMAT) weld inspection technique.

[1009] Conductor-in-conduit heat source of desired length 8654 may be coiled onto spool 8660 for transporting from assembly facility 8650 to an opening in a formation and installation into the opening. In an embodiment, assembly facility 8650 is located at a site of the formation. For example, assembly facility 8650 may be part of a surface facility used to treat fluids from the formation or located proximate to the formation (e.g., less than about 10 km from the formation or, in some embodiments, less than about 20 km or less than about 30 km). Other types of heat sources (e.g., insulated conductor heat sources, natural distributed combustor heat sources, etc.) may also be assembled in assembly facility 8650. These other heat sources may also be spooled onto spool 8660, transported to an opening in a formation, and installed into the opening as is described for conductor-in-conduit heat source of desired length 8654.

[1010] Transportation of conductor-in-conduit heat source of desired length 8654 to an opening in a formation is represented by arrow 8664 in FIG. 89. Transporting conductor-in-conduit heat source of desired length 8654 may include transporting the heat source on a bed, trailer, a cart of a truck or train, or a coiled tubing unit. In some embodiments, more than one heat source may be placed on the bed. Each heat source may be installed in a separate opening in the formation. In one embodiment, a train system (e.g., rail system) may be set up to transport heat sources from assembly facility 8650 to each of the openings in the formation. In some instances, a lift and move track system may be used in which train tracks are lifted and moved to another location after use in one location.

[1011] After spool 8660 with conductor-in-conduit heat source of desired length 8654 has been transported to opening 514, the heat source may be uncoiled and installed into the opening in a direction of arrow 8666. Conductor-in-conduit heat source of desired length 8654 may be uncoiled from spool 8660 while the spool remains on the bed of a truck or train. In some embodiments, more than one conductor-in-conduit heat source of desired length 8654 may be installed at one time. In one embodiment, more than one heat source may be installed into one opening 514. Spool 8660 may be re-used for additional heat sources after installation of conductor-in-conduit heat source of desired length 8654. In some embodiments, spool 8660 may be used to remove conductor-in-conduit heat source of desired length 8654 from the opening. Conductor-in-conduit heat source of desired length 8654 may be re-coiled onto spool 8660 as the heat source is removed from opening 514. Subsequently, conductor-in-conduit heat source of desired length 8654 may be re-installed from spool 8660 into opening 514 or transported to an alternate opening in the formation and installed in the alternate opening.

[1012] In certain embodiments, conductor-in-conduit heat source of desired length 8654, or any heat source (e.g., an insulated conductor heat source), may be installed such that the heat source is removable from opening 514. The heat source may be removable so that the heat source can be repaired or replaced if the heat source fails or breaks. In other instances, the heat source may be removed from the opening and transported and reused in another opening in the formation (or in a different formation) at a later time. Being able to remove, replace, and/or reuse a heat source may be economically favorable for reducing equipment and/or operating costs. In addition, being able to remove and replace an ineffective heater may eliminate the need to form wellbores in close proximity to existing wellbores that have failed heaters in a heated or heating formation.

[1013] In some embodiments, a conduit of a desired length may be placed into opening 514 before a conductor of the desired length. The conductor and the conduit of the desired length may be assembled in assembly facility 8650. The conduit of the desired length may be installed into opening 514. After installation of the conduit of the desired length, the conductor of the desired length may be installed into opening 514. In an embodiment, the conduit and the conductor of the desired length are coiled onto a spool in assembly facility 8650 and uncoiled from the spool for installation into opening 514. Components (e.g., centralizers 581, sliding connectors 583, etc.) may be placed on the conductor or conduit as the conductor is installed into the conduit and opening 514.

[1014] In certain embodiments, centralizer 581 may include at least two portions coupled together to form the centralizer (e.g., “clam shell” centralizers). In one embodiment, the portions are placed on a conductor and coupled together as the conductor is installed into a conduit or opening. The portions may be coupled with fastening devices such as, but not limited to, clamps, bolts, screws, snap-locks, and/or adhesive. The portions may be shaped such that a first portion fits into a second portion. For example, an end of the first portion may have a slightly smaller width than an end of the second portion so that the ends overlap when the two portions are coupled.

[1015] In some embodiments, low resistance section 584 is coupled to conductor-in-conduit heat source of desired length 8654 in assembly facility 8650. In other embodiments, low resistance section 584 is coupled to conductor-in-conduit heat source of desired length 8654 after the heat source is installed into opening 514. Low resistance section 584 of a desired length may be assembled in assembly facility 8650. An assembled low resistance conductor may be coiled onto a spool. The assembled low resistance conductor may be uncoiled from the spool and coupled to conductor-in-conduit heat source of desired length 8654 after the heat source is installed in opening 514. In another embodiment, low resistance section 584 is assembled as the low resistance conductor is coupled to conductor-in-conduit heat source of desired length 8654 and installed into opening 514. Conductor-in-conduit heat source of desired length 8654 may be coupled to a support after installation so that low resistance section 584 is coupled to the installed heat source.

[1016] Assembling a desired length of a low resistance conductor may include coupling individual low resistance conductors together. The individual low resistance conductors may be plate stock conductors obtained from a manufacturer. The individual low resistance conductors may be coupled to an electrically conductive material to lower the electrical resistance of the low resistance conductor. The electrically conductive material may be coupled to the individual low resistance conductor before assembly of the desired length of low resistance conductor. In one embodiment, the individual low resistance conductors may have threaded ends that are coupled together. In another embodiment, the individual low resistance conductors may have ends that are welded together. Ends of the individual low resistance conductors may be shaped such that an end of a first individual low resistance conductor fits into an end of a second individual low resistance conductor. For example, an end of a first individual low resistance conductor may be a female-shaped end while an end of a second individual low resistance conductor is a male-shaped end.

[1017] In another embodiment, a conductor-in-conduit heat source of a desired length may be assembled at a wellbore (or opening) in a formation and installed into the wellbore as the conductor-in-conduit heat source is assembled. Individual conductors may be coupled to form a first section of a conductor of desired length. Similarly, conduits may be coupled to form a first section of a conduit of desired length. The first formed sections of the conductor and the conduit may be installed into the wellbore. The first formed sections of the conductor and the conduit may be electrically coupled at a first end that is installed into the wellbore. The first sections of the conductor and conduit may, in some embodiments, be coupled substantially simultaneously. Additional sections of the conductor and/or conduit may be formed during or after installation of the first formed sections. The additional sections of the conductor and/or conduit may be coupled to the first formed sections of the conductor and/or conduit and installed into the wellbore. Centralizers and/or other components may be coupled to sections of the conductor and/or conduit and installed with the conductor and the conduit into the wellbore.

[1018] A method for coupling conductors or conduits may include a forge welding method (e.g., shielded active gas (SAG) welding). In an embodiment, forge welding includes arranging ends of the conductors and/or conduits that are to be interconnected at a selected distance. Seals may be formed against walls of the conduit and/or conductor to define a chamber. A flushing, reducing fluid may be introduced into the chamber. Each end within the chamber may be heated and moved towards another end until the heated ends contact each other. Contacting the heated ends may form a forge weld between the heated ends. The flushing, reducing fluid mixture may include less than 25% by volume of a reducing agent and more than 75% by volume of a substantially inert gas. The flushing, reducing fluid may inhibit oxidation reactions that can adversely affect weld integrity.

[1019] A flushing fluid mixture with less than 25% by volume of a reducing fluid (e.g., hydrogen and/or carbon monoxide) and more than 75% by volume of a substantially inert gas (e.g., nitrogen, argon, and/or carbon dioxide) may be non-explosive when the flushing fluid mixture comes into contact with air at elevated temperatures needed to form the forge weld. In some embodiments, the reducing agent may be or include borax powder and/or beryllium or alkaline hydrites. The flushing fluid mixture may contain a sufficient amount of a reducing gas to flush off oxidized skin from the hot ends that are to be interconnected. In some embodiments, the non-explosive flushing fluid mixture includes between 2% by volume and 10% by volume of the reducing fluid and between 90% by volume and 98% by volume of the substantially inert gas. In certain embodiments, the mixture includes about 5% by volume of the reducing fluid and about 95% by volume of the substantially inert gas. In one embodiment, a non-explosive flushing fluid mixture includes about 95% by volume of nitrogen and about 5% by volume of hydrogen. The non-explosive flushing fluid mixture may also include less than 100 ppm H₂O and/or O₂ or, in some cases, less than 15 ppm H₂O and/or O₂.

[1020] A substantially inert gas used during a forge welding procedure is a gas that does not significantly react with the metals to be forge welded at the pressures and temperatures used during forge welding. Substantially inert gas may be, but is not limited to, noble gases (e.g., helium and argon), nitrogen or combinations thereof.

[1021] A non-explosive flushing fluid mixture may be formed in-situ within the chamber. A coating on the conduits and/or conductors may be present and/or a solid may be placed in the chamber. When the conduits and/or conductors are heated, the coating and/or solid may react or physically transform to the flushing fluid mixture.

[1022] In an embodiment, ends of conductors or conduits are heated by means of high frequency electrical heating. The ends may be maintained at a predetermined spacing of between 1 mm and 4 mm from each other by a gripping assembly while being heated. Electrical contacts may be pressed at circumferentially spaced intervals against the wall of each conduit and/or conductor adjacent to the end such that the electrical contacts transmit a high frequency electrical current in a substantially circumferential direction in the segment between the electrical contacts.

[1023] To equalize the level of heating in a circumferential direction, each end may be heated by at least two pairs of electrodes. The electrodes of each pair may be pressed at substantially diametrically opposite positions against walls of the conduits and/or conductors. The different pairs of electrodes at each end may be activated in an alternating manner.

[1024] In one embodiment, two pairs of diametrically opposite electrodes are pressed at angular intervals of substantially 90° against walls of the conductors and conduits. In another embodiment, three pairs of diametrically opposite electrodes are pressed at angular intervals of substantially 60° against the walls of the conductors and conduits. In other embodiments, four, five, six or more pairs of diametrically opposite electrodes may be used and activated in an alternating manner to equalize the level of heating of the ends in the circumferential direction.

[1025] The use of two or more pairs of electrodes may reduce unequal heating of the pipe ends because of over heating of the walls in the direct vicinity of the electrode. In addition, using two or more pairs of electrodes may reduce heating of the pipe wall halfway between the electrodes.

[1026] In another embodiment, the ends may be heated by a direct resistance heating method. The direct resistance heating method may include transmitting a large current in an axial direction across the conduits and/or conductors while the conduits and/or conductors are pressed together. In another embodiment, the ends may be heated by induction heating. Induction heating may include using external and/or internal heating coils to create an electromagnetic field that induces electrical currents in the conduits and/or conductors. The electrical currents may resistively heat the conduits.

[1027] The heating assembly may be used to give the forge welded ends a post weld heat treatment. The post weld heat treatment may include providing at least some heating to the ends such that the ends are cooled down at a predetermined temperature decrease rate (i.e., cool down rate). In some embodiments, the assembly may be equipped with water and/or forced air injectors to increase and/or control the cool down rate of the forge welded ends.

[1028] In certain embodiments, the quality of the forge weld formed between the interconnected conduits and/or conductors is inspected by means of an Electro-Magnetic Acoustic Transmission weld inspection technique (EMAT). EMAT may include placing at least one electromagnetic coil adjacent to both sides of the forge welded joint. The coil may be held at a predetermined distance from the conduits and/or conductors during the inspection process. The absence of physical contact between the wall of the hot conduits and/or conductors and the coils of the EMAT inspection tool may enable weld inspection immediately after the forge weld joint has been made.

[1029] FIG. 91 shows an end of tubular 9150 around which two pairs of diametrically opposite electrodes 9152, 9153 and 9154, 9155 are arranged. Tubular 9150 may be a conduit or conductor. Tubular 9150 may be made of electrically conductive material (e.g., stainless steel). The first pair of electrodes 9152, 9153 may be pressed against the outer surface of tubular 9150 and transmit high frequency current through the wall of the tubular as illustrated by arrows 9157. An assembly of ferrite bars 9158 may serve to enhance the current density in the immediate vicinity of the ends of the tubular 9150 and of the adjacent tubular to which tubular 9150 is to be welded.

[1030] FIG. 92 depicts an embodiment with ends 9162, 9162A of two adjacent tubulars 9150 and 9150A. Tubulars 9150 and 9150A may be heated by two sets of diametrically opposite electrodes 9152, 9153, 9154, 9155 and 9152A, 9153A, 9154A and 9155A, respectively. Tubular ends 9162 and 9162A may be located at a few millimeters distant from each other during a heating phase. The larger spacing of current density arrows 9157 midway between electrodes 9152, 9153 illustrates that the current density midway between these electrodes may be lower than the current density adjacent to each of the electrodes. The lower current density midway between the electrodes may create a variation in the heating rate of the tubular ends 9162 and 9162A. To reduce a possible irregular heating rate, electrodes 9152, 9153 and 9152A, 9153A may be regularly lifted from the outer surface of tubulars 9150, 9150A while the other electrodes 9154, 9154A and 9155, 9155A are pressed against the outer surface of the tubulars 9150, 9150A and activated to transmit a high frequency current through the ends of the tubulars. By sequentially activating the two sets of diametrically opposite electrodes at each tubular end, irregular heating of the tubular ends may be inhibited (i.e., heating of the tubular ends may be more uniform).

[1031] All electrodes 9152–9155 and 9152A–9155A shown in FIG. 92 may be pressed simultaneously against tubular ends 9150 and 9150A if alternating current supplied to the electrodes is controlled such that during a first part of a current cycle the diametrically opposite electrode pairs 9152A, 9153A and 9154, 9155 transmit a positive electrical current as indicated by the “+” sign in FIG. 92, whereas electrodes 9152, 9153, and 9154A, 9155A transmit a negative electrical current as indicated by the “−” sign. During a second part of the alternating current cycle, electrodes 9152A, 9153A, and 9154, 9155 transmit a negative electrical current, whereas electrodes 9152, 9153, and 9154A, 9155A transmit a positive current into tubulars 9150 and 9150A. Controlling the alternating current in this manner may heat tubular ends 9162 and 9162A in a substantially uniform manner.

[1032] The temperature of heated tubular ends 9162, 9162A may be monitored by an infrared temperature sensor. When the monitored temperature has reached a temperature sufficient to make a forge weld, tubular ends 9162, 9162A may be pressed onto each other such that a forge weld is made. Tubular ends 9162, 9162A may be profiled and have a smaller wall thickness than other parts of tubulars 9150, 9150A to compensate for the deformation of the tubular ends when the ends are abutted. Profiling the tubular ends may allow tubulars 9150, 9150A to have a substantially uniform wall thickness at forge welded ends.

[1033] During the heating phase and while the ends of tubulars 9150, 9150A are moved towards each other, the tubular ends may be encased, both internally and externally, in a chamber 9168. Chamber 9168 may be filled with a non-explosive flushing fluid mixture. The non-explosive flushing fluid mixture may include more than 75% by volume of nitrogen and less than 25% by volume of hydrogen. In one embodiment, the non-explosive flushing fluid mixture for interconnecting steel tubulars 9150, 9150A includes about 5% by volume of hydrogen and about 95% by volume of nitrogen. The flushing fluid pressure in a part of chamber 9168 outside the tubulars 9150 and 9150A may be higher than the flushing fluid pressure in a part of the chamber 9168 within the interior of the tubulars such that throughout the heating process the flushing fluid flows along the ends of the tubulars as illustrated by arrows 9169 until the ends of the tubulars are forged together. In some embodiments, flushing fluid may flow through the chamber.

[1034] Hydrogen in the flushing fluid may react with oxidized metal on the ends 9162, 9162A of the tubulars 9150, 9150A so that formation of an oxidized skin is inhibited. Inhibition of an oxidized skin may allow formation of a forge weld with minimal amounts of corroded metal inclusions.

[1035] Laboratory experiments revealed that a good metallurgical bond between stainless steel tubulars may be obtained by forge welding with a flushing fluid containing about 5% by volume of hydrogen and about 95% by volume of nitrogen. Experiments also show that such a flushing fluid mixture may be non-explosive during and after forge welding. Two forge welded stainless steel tubulars failed at a location away from the forge weld when the tubulars were subjected to testing.

[1036] In an embodiment, the tubular ends are clamped throughout the forge welding process to a gripping assembly. Clamping the tubular ends may maintain the tubular ends at a predetermined spacing of between 1 mm and 4 mm from each other during the heating phase. The gripping assembly may include a mechanical stop that interrupts axial movement of the heated tubular ends during the forge welding process after the heated tubular ends have moved a predetermined distance towards each other. The heated tubular ends may be pressed into each other such that a high quality forge weld is created without significant deformation of the heated ends.

[1037] In certain embodiments, electrodes 9152–9155 and 9152A–9155A may also be activated to give the forged tubular ends a post weld heat treatment. Electrical power 9156 supplied to the electrodes during the post weld heat treatment may be lower than during the heat up phase before the forge welding operation. Electrical power 9156 supplied during the post weld heat treatment may be controlled in conjunction with temperature measured by an infrared temperature sensor(s) such that the temperature of the forge welded tubular ends is decreased in accordance with a predetermined temperature decrease or cooling cycle.

[1038] The quality of the forge weld may be inspected by a hybrid electromagnetic acoustic transmission technique which is 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. The EMAT technique makes use of an induction coil placed at one side of the welded joint. The induction coil may induce magnetic fields that generate electromagnetic forces in the surface of the welded joint. These forces may produce a mechanical disturbance by coupling to the atomic lattice through a scattering process. In electromagnetic acoustic generation, the conversion may take place within a skin depth of material (i.e., the metal surface acts as a transducer). The reception may take place in a reciprocal way in a receiving coil. When the elastic wave strikes the surface of the conductor in the presence of a magnetic field, induced currents may be generated in the receiving coil, similar to the operation of an electric generator. An advantage of the EMAT weld inspection technology is that the inductive transmission and receiving coils do not have to contact the welded tubular. Thus, the inspection may be done soon after the forge weld is made (e.g., when the forge welded tubulars are still too hot to allow physical contact with an inspection probe).

[1039] Using the SAG method to weld tubular ends of heat sources may inhibit changes in the metallurgy of the tubular materials. For example, the elemental composition of the weld joint may be substantially similar to the elemental composition of the tubulars. Inhibiting changes in metallurgy may reduce the need for heat-treatment of the tubulars before use of the tubulars. The SAG method also appears not to change the grain structure of the near-weld section of the tubulars. Maintaining the grain structure of the tubulars may inhibit corrosion and/or creep in the tubulars during use.

[1040] FIG. 93 illustrates an end view of an embodiment of a conductor-in-conduit heat source heated by diametrically opposite electrodes. Conductor 580 may be placed within conduit 582. Conductor 580 may be heated by two sets of diametrically opposite electrodes 9152, 9153, 9154, 9155. Conduit 582 may be heated by two sets of diametrically opposite electrodes 9172, 9173, 9174, 9175. Conductor 580 and conduits 582 may be heated and forge welded together as described in the embodiments of FIGS. 91–92. In some embodiments, two ends of conductors 580 are forged welded together and then two ends of conduits 582 are forged together in a second procedure.

[1041] FIG. 94 illustrates a cross-sectional representation of an embodiment of two sections of a conductor-in-conduit heat source before being forge welded. During heating of conductors 580, 580A and conduits 582, 582A and while the ends of the conductors and the conduits are moved towards each other, ends of the conductors and conduits may be encased in a chamber 9176. Chamber 9176 may be filled with the non-explosive flushing fluid mixture. Plugs 9178, 9178A may be placed in the annular space between conductors 580, 580A and conduits 582, 582A. In an embodiment, the plugs may be inflated to seal the annular space. Plugs 9178, 9178A may inhibit the flow of the flushing fluid mixture through the annular space between conductors 580, 580A and conduits 582, 582A. The flushing fluid pressure in a part of chamber 9176 outside the conduits 582, 582A may be higher than the flushing fluid pressure inside the conduits and outside conductors 580, 580A. Similarly, the flushing fluid pressure outside conductors 580, 580A may be higher than the flushing fluid pressure inside the conductors. Due to the pressure differentials throughout the heating process, the flushing fluid tends to flow along the ends of the tubulars as illustrated by arrows 9179 until the ends of the conductors and conduits are forged together.

[1042] FIG. 95 depicts an embodiment of three horizontal heat sources placed in a formation. Wellbore 9632 may be formed through overburden 540 and into hydrocarbon layer 516. Wellbore 9632 may be formed by any standard drilling method. In certain embodiments, wellbore 9632 is formed substantially horizontally in hydrocarbon layer 516. In some embodiments, wellbore 9632 may be formed at other angles within hydrocarbon layer 516.

[1043] One or more conduits 9634 may be placed within wellbore 9632. A portion of wellbore 9632 and/or second wellbores may include casings. Conduit 9634 may have a smaller diameter than wellbore 9632. In an embodiment, wellbore 9632 has a diameter of about 30.5 cm and conduit 9634 has a diameter of about 14 cm. In an embodiment, an inside diameter of a casing in conduit 9634 may be about 12 cm. Conduits 9634 may have extended sections 9635 that extend beyond the end of wellbore 9632 in hydrocarbon layer 516. Extended sections 9635 may be formed in hydrocarbon layer 516 by drilling or other wellbore forming methods. In an embodiment, extended sections 9635 extend substantially horizontally into hydrocarbon layer 516. In certain embodiments, extended sections 9635 may somewhat diverge as represented in FIG. 95.

[1044] Perforated casings 9636 may be placed in extended sections 9635 of conduits 9634. Perforated casings 9636 may provide support for the extended sections so that collapse of wellbores is inhibited during heating of the formation. Perforated casings 9636 may be steel (e.g., carbon steel or stainless steel). Perforated casings 9636 may be perforated liners that expand within the wellbores (expandable tubulars). 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. In an embodiment, perforated casings 9636 are formed by inserting a perforated casing into each of extended sections 9635 and expanding the perforated casing within each extended section. The perforated casing may be expanded by pulling an expander tool shaped to push the perforated casing towards the wall of the wellbore (e.g., a pig) along the length of each extended section 9635. The expander tool may push each perforated casing beyond the yield point of the perforated casing.

[1045] After installation of perforated casings 9636, heat sources 9638 may be installed into extended sections 9635. Heat sources 9638 may be used to provide heat to hydrocarbon layer 516 along the length of extended sections 9635. Heat sources 9638 may include heat sources such as conductor-in-conduit heaters, insulated conductor heaters, etc. In some embodiments, heat sources 9638 have a diameter of about 7.3 cm. Perforated casings 9636 may allow for production of formation fluid from the heat source wellbores. Installation of heat sources 9638 in perforated casings 9636 may also allow the heat sources to be removed at a later time. Heat sources 9638 may, for example, be removed for repair, replacement, and/or used in another portion of a formation.

[1046] In an embodiment, an elongated member may be disposed within an opening (e.g., an open wellbore) in an oil shale formation. The opening may be an uncased opening in the oil shale formation. The elongated member may be a length (e.g., a strip) of metal or any other elongated piece of metal (e.g., a rod). The elongated member may include stainless steel. The elongated member may be made of a material able to withstand corrosion at high temperatures within the opening.

[1047] An elongated member may be a bare metal heater. “Bare metal” refers to a metal that does 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 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 includes metal 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.

[1048] An elongated member may have a length of about 650 m. Longer lengths may be achieved using sections of high strength alloys, but such elongated members may be expensive. In some embodiments, an elongated member may be supported by a plate in a wellhead. The elongated member may include sections of different conductive materials that are welded together end-to-end. A large amount of electrically conductive weld material may be used to couple the separate sections together to increase strength of the resulting member and to provide a path for electricity to flow that will not result in arcing and/or corrosion at the welded connections. In some embodiments, different sections may be forge welded together. The different conductive materials may include alloys with a high creep resistance. The sections of different conductive materials may have varying diameters to ensure uniform heating along the elongated member. A first metal that has a higher creep resistance than a second metal typically has a higher resistivity than the second metal. The difference in resistivities may allow a section of larger cross-sectional area, more creep resistant first metal to dissipate the same amount of heat as a section of smaller cross-sectional area second metal. The cross-sectional areas of the two different metals may be tailored to result in substantially the same amount of heat dissipation in two welded together sections of the metals. The conductive materials may include, but are not limited to, 617 Inconel, HR-120, 316 stainless steel, and 304 stainless steel. For example, an elongated member may have a 60 meter section of 617 Inconel, 60 meter section of HR-120, and 150 meter section of 304 stainless steel. In addition, the elongated member may have a low resistance section that may run from the wellhead through the overburden. This low resistance section may decrease the heating within the formation from the wellhead through the overburden. The low resistance section may be the result of, for example, choosing a electrically conductive material and/or increasing the cross-sectional area available for electrical conduction.

[1049] In a heat source embodiment, a support member may extend through the overburden, and the bare metal elongated member or members may be coupled to the support member. A plate, a centralizer, or other type of support member may be located near an interface between the overburden and the hydrocarbon layer. A low resistivity cable, such as a stranded copper cable, may extend along the support member and may be coupled to the elongated member or members. The low resistivity cable may be coupled to a power source that supplies electricity to the elongated member or members.

[1050] FIG. 96 illustrates an embodiment of a plurality of elongated members that may heat an oil shale formation. Two or more (e.g., four) elongated members 600 may be supported by support member 604. Elongated members 600 may be coupled to support member 604 using insulated centralizers 602. Support member 604 may be a tube or conduit. Support member 604 may also be a perforated tube. Support member 604 may provide a flow of an oxidizing fluid into opening 514. Support member 604 may have a diameter between about 1.2 cm and about 4 cm and, in some embodiments, about 2.5 cm. Support member 604, elongated members 600, and insulated centralizers 602 may be disposed in opening 514 in hydrocarbon layer 516. Insulated centralizers 602 may maintain a location of elongated members 600 on support member 604 such that lateral movement of elongated members 600 is inhibited at temperatures high enough to deform support member 604 or elongated members 600. Elongated members 600, in some embodiments, may be metal strips of about 2.5 cm wide and about 0.3 cm thick stainless steel. Elongated members 600, however, may also include a pipe or a rod formed of a conductive material. Electrical current may be applied to elongated members 600 such that elongated members 600 may generate heat due to electrical resistance.

[1051] Elongated members 600 may generate heat of approximately 650 watts per meter of elongated members 600 to approximately 1650 watts per meter of elongated members 600. Elongated members 600 may be at temperatures of approximately 480° C. to approximately 815° C. Substantially uniform heating of an oil shale formation may be provided along a length of elongated members 600 or greater than about 305 m or, maybe even greater than about 610 m.

[1052] Elongated members 600 may be electrically coupled in series. Electrical current may be supplied to elongated members 600 using lead-in conductor 572. Lead-in conductor 572 may be coupled to wellhead 690. Electrical current may be returned to wellhead 690 using lead-out conductor 606 coupled to elongated members 600. Lead-in conductor 572 and lead-out conductor 606 may be coupled to wellhead 690 at surface 550 through a sealing flange located between wellhead 690 and overburden 540. The sealing flange may inhibit fluid from escaping from opening 514 to surface 550 and/or atmosphere. Lead-in conductor 572 and lead-out conductor 606 may be coupled to elongated members using a cold pin transition conductor. The cold pin transition conductor may include an insulated conductor of low resistance. Little or no heat may be generated in the cold pin transition conductor. The cold pin transition conductor may be coupled to lead-in conductor 572, lead-out conductor 606, and/or elongated members 600 by splices, mechanical connections and/or welds. The cold pin transition conductor may provide a temperature transition between lead-in conductor 572, lead-out conductor 606, and/or elongated members 600. Lead-in conductor 572 and lead-out conductor 606 may be made of low resistance conductors so that substantially no heat is generated from electrical current passing through lead-in conductor 572 and lead-out conductor 606.

[1053] Weld beads may be placed beneath centralizers 602 on support member 604 to fix the position of the centralizers. Weld beads may be placed on elongated members 600 above the uppermost centralizer to fix the position of the elongated members relative to the support member (other types of connecting mechanisms may also be used). When heated, the elongated member may thermally expand downwards. The elongated member may be formed of different metals at different locations along a length of the elongated member to allow relatively long lengths to be formed. For example, a “U” shaped elongated member may include a first length formed of 310 stainless steel, a second length formed of 304 stainless steel welded to the first length, and a third length formed of 310 stainless steel welded to the second length. 310 stainless steel is more resistive than 304 stainless steel and may dissipate approximately 25% more energy per unit length than 304 stainless steel of the same dimensions. 310 stainless steel may be more creep resistant than 304 stainless steel. The first length and the third length may be formed with cross-sectional areas that allow the first length and third lengths to dissipate as much heat as a smaller cross-sectional area of 304 stainless steel. The first and third lengths may be positioned close to wellhead 690. The use of different types of metal may allow the formation of long elongated members. The different metals may be, but are not limited to, 617 Inconel, HR120, 316 stainless steel, 310 stainless steel, and 304 stainless steel.

[1054] Packing material 542 may be placed between overburden casing 541 and opening 514. Packing material 542 may inhibit fluid flowing from opening 514 to surface 550 and to inhibit corresponding heat losses towards the surface. In some embodiments, overburden casing 541 may be placed in reinforcing material 544 in overburden 540. In other embodiments, overburden casing may not be cemented to the formation. Surface conductor 545 may be disposed in reinforcing material 544. Support member 604 may be coupled to wellhead 690 at surface 550. Centralizer 581 may maintain a location of support member 604 within overburden casing 541. Electrical current may be supplied to elongated members 600 to generate heat. Heat generated from elongated members 600 may radiate within opening 514 to heat at least a portion of hydrocarbon layer 516.

[1055] The oxidizing fluid may be provided along a length of the elongated members 600 from oxidizing fluid source 508. The oxidizing fluid may inhibit carbon deposition on or proximate the elongated members. For example, the oxidizing fluid may react with hydrocarbons to form carbon dioxide. The carbon dioxide may be removed from the opening. Openings 605 in support member 604 may provide a flow of the oxidizing fluid along the length of elongated members 600. Openings 605 may be critical flow orifices. In some embodiments, a conduit may be disposed proximate elongated members 600 to control the pressure in the formation and/or to introduce an oxidizing fluid into opening 514. Without a flow of oxidizing fluid, carbon deposition may occur on or proximate elongated members 600 or on insulated centralizers 602. Carbon deposition may cause shorting between elongated members 600 and insulated centralizers 602 or hot spots along elongated members 600. The oxidizing fluid may be used to react with the carbon in the formation. The heat generated by reaction with the carbon may complement or supplement electrically generated heat.

[1056] In a heat source embodiment, a bare metal elongated member may be formed in a “U” shape (or hairpin) and the member may be suspended from a wellhead or from a positioner placed at or near an interface between the overburden and the formation to be heated. In certain embodiments, the bare metal heaters are formed of rod stock. Cylindrical, high alumina ceramic electrical insulators may be placed over legs of the elongated members. Tack welds along lengths of the legs may fix the position of the insulators. The insulators may inhibit the elongated member from contacting the formation or a well casing (if the elongated member is placed within a well casing). The insulators may also inhibit legs of the “U” shaped members from contacting each other. High alumina ceramic electrical insulators may be purchased from Cooper Industries (Houston, Tex.). In an embodiment, the “U” shaped member may be formed of different metals having different cross-sectional areas so that the elongated members may be relatively long and may dissipate a desired amount of heat per unit length along the entire length of the elongated member.

[1057] Use of welded together sections may result in an elongated member that has large diameter sections near a top of the elongated member and a smaller diameter section or sections lower down a length of the elongated member. For example, an embodiment of an elongated member has two ⅞ inch (2.2 cm) diameter first sections, two ½ inch (1.3 cm) middle sections, and a ⅜ inch (0.95 cm) diameter bottom section that is bent into a “U” shape. The elongated member may be made of materials with other cross-sectional shapes such as ovals, squares, rectangles, triangles, etc. The sections may be formed of alloys that will result in substantially the same heat dissipation per unit length for each section.

[1058] In some embodiments, the cross-sectional area and/or the metal used for a particular section may be chosen so that a particular section provides greater (or lesser) heat dissipation per unit length than an adjacent section. More heat dissipation per unit length may be provided near an interface between a hydrocarbon layer and a non-hydrocarbon layer (e.g., the overburden and the hydrocarbon layer) to counteract end effects and allow for more uniform heat dissipation into the hydrocarbon layer. Higher heat dissipation per unit length may also be provided at a lower end of an elongated member to counteract end effects and allow for more uniform heat dissipation.

[1059] In certain embodiments, the wall thickness of portions of a conductor, or any electrically-conducting portion of a heater, may be adjusted to provide more or less heat to certain zones of a formation. In an embodiment, the wall thickness of a portion of the conductor adjacent to a lean zone (i.e., zone containing relatively little or no hydrocarbons) may be thicker than a portion of the conductor adjacent to a rich zone (i.e., hydrocarbon layer in which hydrocarbons are pyrolyzed and/or produced). Adjusting the wall thickness of a conductor to provide less heat to the lean zone and more heat to the rich zone may more efficiently use electricity to heat the formation.

[1060] FIG. 97 illustrates a cross-sectional representation of an embodiment of a heater using two oxidizers. One or more oxidizers may be used to heat a hydrocarbon layer or hydrocarbon layers of a formation having a relatively shallow depth (e.g., less than about 250 m). Conduit 6110 may be placed in opening 514 in a formation. Conduit 6110 may have upper portion 6112. Upper portion 6112 of conduit 6110 may be placed primarily in overburden 540 of the formation. A portion of conduit 6110 may include high temperature resistant, non-corrosive materials (e.g., 316 stainless steel and/or 304 stainless steel). Upper portion 6112 of conduit 6110 may include a less temperature resistant material (e.g., carbon steel). A diameter of opening 514 and conduit 6110 may be chosen such that a cross-sectional area of opening 514 outside of conduit 6110 is approximately equal to a cross-sectional area inside conduit 6110. This may equalize pressures outside and inside conduit 6110. In an embodiment, conduit 6110 has a diameter of about 0.11 m and opening 514 has a diameter of about 0.15 m.

[1061] Oxidizing fluid source 508 may provide oxidizing fluid 517 into conduit 6110. Oxidizing fluid 517 may include hydrogen peroxide, air, oxygen, or oxygen enriched air. In an embodiment, oxidizing fluid source 508 may include a membrane system that enriches air by preferentially passing oxygen, instead of nitrogen, through a membrane or membranes. First fuel source 6119 may provide fuel 6118 into first fuel conduit 6116. First fuel conduit 6116 may be placed in upper portion 6112 of conduit 6110. In some embodiments, first fuel conduit 6116 may be placed outside conduit 6110. In other embodiments, conduit 6110 may be placed within first fuel conduit 6116. Fuel 6118 may include combustible material, including but not limited to, hydrogen, methane, ethane, other hydrocarbon fluids, and/or combinations thereof. Fuel 6118 may include steam to inhibit coking within the fuel conduit or proximate an oxidizer. First oxidizer 6120 may be placed in conduit 6110 at a lower end of upper portion 6112. First oxidizer 6120 may oxidize at least a portion of fuel 6118 from first fuel conduit 6116 with at least a portion of oxidizing fluid 517. First oxidizer may be a burner such as an inline burner. Burners may be obtained from John Zink Company (Tulsa, Okla.) or Callidus Technologies (Tulsa, Okla.). First oxidizer 6120 may include an ignition source such as a flame. First oxidizer 6120 may also include a flameless ignition source such as, for example, an electric igniter.

[1062] In some embodiments, fuel 6118 and oxidizing fluid 517 may be combined at the surface and provided to opening 514 through conduit 6110. Fuel 6118 and oxidizing fluid 517 may be combined in a mixer, aerator, nozzle, or similar mixing device located at the surface. In such an embodiment, conduit 6110 provides both fuel 6118 and oxidizing fluid 517 into opening 514. Locating first oxidizer 6120 at or proximate the upper portion of the section of the formation to be heated may tend to inhibit or decrease coking in one or more of the fuel conduits (e.g., in first fuel conduit 6116).

[1063] Oxidation of fuel 6118 at first oxidizer 6120 will generate heat. The generated heat may heat fluids in a region proximate first oxidizer 6120. The heated fluids may include fuel, oxidizing fluid, and oxidation products. The heated fluids may be allowed to transfer heat to hydrocarbon layer 6100 along a length of conduit 6110. The amount of heat transferred from the heated fluids to the formation may vary depending on, for example, a temperature of the heated fluids. In general, the greater the temperature of the heated fluids, the more heat that will be transferred to the formation. In addition, as heat is transferred from the heated fluids, the temperature of the heated fluids decreases. For example, temperatures of fluids in the oxidizer flame may be about 1300° C. or above, and as the fluids reach a distance of about 150 m from the oxidizer, temperatures of fluids may be, for example, about 750° C. Thus, the temperature of the heated fluids, and hence the heat transferred to the formation, decreases as the heated fluids flow away from the oxidizer.

[1064] First insulation 6122 may be placed on lengths of conduit 6110 proximate a region of first oxidizer 6120. First insulation 6122 may have a length of about 10 m to about 200 m (e.g., about 50 m). In alternative embodiments, first insulation 6122 may have a length that is about 10–40% of the length of conduit 6110 between any two oxidizers (e.g., between first oxidizer 6120 and second oxidizer 6130 in FIG. 97). A length of first insulation 6122 may vary depending on, for example, desired heat transfer rate to the formation, desired temperature proximate the first oxidizer, and/or desired temperature profile along the length of conduit 6110. First insulation 6122 may have a thickness that varies (either continually or in step fashion) along its length. In certain embodiments, first insulation 6122 may have a greater thickness proximate first oxidizer 6120 and a reduced thickness at a desired distance from the first oxidizer. The greater thickness of first insulation 6122 may preferentially reduce heat transfer proximate first oxidizer 6120 as compared to a reduced thickness portion of the insulation. Variable thickness insulation may allow for uniform or relatively uniform heating of the formation adjacent to a heated portion of the heat source. In an embodiment, first insulation 6122 may have a thickness of about 0.03 m proximate first oxidizer 6120 and a thickness of about 0.015 m at a distance of about 10 m from the first oxidizer. In the embodiment, the heated portion of the conduit is about 300 m in length, with insulation (first insulation 6122) being placed proximate the upper 100 m portion of this length, and insulation (second insulation 6132) being placed proximate the lower 100 m portion of this length.

[1065] A thickness of first insulation 6122 may vary depending on, for example, a desired heating rate or a desired temperature within opening 514 of hydrocarbon layer 6100. The first insulation may inhibit the transfer of heat from the heated fluids to the formation in a region proximate the insulating conduit. First insulation 6122 may also inhibit charring and/or coking of hydrocarbons proximate first oxidizer 6120. First insulation 6122 may inhibit charring and/or coking by reducing an amount of heat transferred to the formation proximate the first oxidizer. First insulation 6122 may inhibit or decrease coking in conduit 6128 when a carbon containing fuel is in conduit 6128. First insulation 6122 may be made of a non-corrosive, thermally insulating material such as rock wool, Nextel®, calcium silicate, Fiberfrax®, insulating refractory cements such as those manufactured by Harbizon Walker, A.P. Green, or National Refractories, etc. The relatively high temperatures generated at the flame of first oxidizer 6120, which may be about 1300° C. or greater, may generate sufficient heat to convert hydrocarbons proximate the first oxidizer into coke and/or char if no insulation is provided.

[1066] Heated fluids from conduit 6110 may exit a lower end of the conduit into opening 514. A temperature of the heated fluids may be lower proximate the lower end of conduit 6110 than a temperature of the heated fluids proximate first oxidizer 6120. The heated fluids may return to a surface of the formation through the annulus of opening 514 (exhaust annulus 6124) and/or through exhaust conduit 6126. The heated fluids exiting the formation through exhaust conduit 6126 may be referred to as exhaust fluids. The exhaust fluids may be allowed to thermally contact conduit 6110 so as to exchange heat between exhaust fluids and either oxidizing fluid or fuel within conduit 6110. This exchange of heat may preheat fluids within conduit 6110. Thus, the thermal efficiency of the downhole combustor may be enhanced to as much as 90% or more (i.e., 90% or more of the heat from the heat of combustion is being transferred to a selected section of the formation).

[1067] In certain embodiments, extra oxidizers may be used in addition to oxidizer 6120 and oxidizer 6130 shown in FIG. 97. For example, in some embodiments, one or more extra oxidizers may be placed between oxidizer 6120 and oxidizer 6130. Such extra oxidizers may be, for example, placed at intervals of about 20–50 m. In certain embodiments, one oxidizer (e.g., oxidizer 6120) may provide at least about 50% of the heat to the selected section of the formation, and the other oxidizers may be used to adjust the heat flux along the length of the oxidizer.

[1068] In some embodiments, fins may be placed on an outside surface of conduit 6110 to increase exchange of heat between exhaust fluids and fluids within the conduit. Exhaust conduit 6126 may extend into opening 514. A position of lower end of exhaust conduit 6126 may vary depending on, for example, a desired removal rate of exhaust fluids from the opening. In certain embodiments, it may be advantageous to remove fluids through exhaust conduit 6126 from a lower portion of opening 514 rather than allowing exhaust fluids to return to the surface through the annulus of the opening. All or part of the exhaust fluids may be vented, treated in a surface facility, and/or recycled. In some circumstances, the exhaust fluids may be recycled as a portion of fuel 6118 or oxidizing fluid 517 or recycled into an additional heater in another portion of the formation.

[1069] Two or more heater wells with oxidizers may be coupled in series with exhaust fluids from a first heater well being used as a portion of fuel for a second heater well. Exhaust fluids from the second heater well may be used as a portion of fuel for a third heater well, and so on as needed. In some embodiments, a separator may separate unused fuel and/or oxidizer from combustion products to increase the energy content of the fuel for the next oxidizer. Using the heated exhaust fluids as a portion of the feed for a heater well may decrease costs associated with pressurizing fluids for use in the heater well. In an embodiment, a portion (e.g., about one-third or about one-half) of the oxygen in the oxidizing fluid stream provided to a first heater well may be utilized in the first heater well. This would leave the remaining oxygen available for use as oxidizing fluid for subsequent heater wells. The heated exhaust fluids tend to have a pressure associated with the previous heater well and may be maintained at that pressure for providing to the next heater well. Thus, connection of two or more heater wells in series can significantly reduce compression costs associated with pressurizing fluids.

[1070] Casing 541 and reinforcing material 544 may be placed in overburden 540. Overburden 540 may be above hydrocarbon layer 6100. In certain embodiments, casing 541 may extend downward into part or the entire zone being heated. Casing 541 may include steel (e.g., carbon steel or stainless steel). Reinforcing material 544 may include, for example, foamed cement or a cement with glass and/or ceramic beads filled with air.

[1071] As depicted in the embodiment of FIG. 97, a heater may have second fuel conduit 6128. Second fuel conduit 6128 may be coupled to conduit 6110. Second fuel source 6121 may provide fuel 6118 to second fuel conduit 6128. Second fuel source 6121 may provide fuel that is similar to fuel from first fuel source 6119. In some embodiments, fuel from second fuel source 6121 may be different than fuel from first fuel source 6119. Fuel 6118 may exit second fuel conduit 6128 at a location proximate second oxidizer 6130. Second oxidizer 6130 may be located proximate a bottom of conduit 6110 and/or opening 514. Second oxidizer 6130 may be coupled to a lower end of second fuel conduit 6128. Second oxidizer 6130 may be used to oxidize at least a portion of fuel 6118 (exiting second fuel conduit 6128) with heated fluids exiting conduit 6110. Un-oxidized portions of heated fluids from conduit 6110 may also be oxidized at second oxidizer 6130. Second oxidizer 6130 may be a burner (e.g., a ring burner). Second oxidizer 6130 may be made of stainless steel. Second oxidizer 6130 may include one or more orifices that allow a flow of fuel 6118 into opening 514. The one or more orifices may be critical flow orifices. Oxidized portions of fuel 6118, along with un-oxidized portions of fuel, may combine with heated fluids from conduit 6110 and exit the formation with the heated fluids. Heat generated by oxidation of fuel 6118 from second fuel conduit 6128 proximate a lower end of opening 514, in combination with heat generated from heated fluids in conduit 6110, may provide more uniform heating of hydrocarbon layer 6100 than using a single oxidizer. In an embodiment, second oxidizer 6130 may be located about 200 m from first oxidizer 6120. However, in some embodiments, second oxidizer 6130 may be located up to about 250 m from first oxidizer 6120.

[1072] Heat generated by oxidation of fuel at the first and second oxidizers may be allowed to transfer to the formation. The generated heat may transfer to a pyrolysis zone in the formation. Heat transferred to the pyrolysis zone may pyrolyze at least some hydrocarbons within the pyrolysis zone.

[1073] In some embodiments, ignition source 6134 may be disposed proximate a lower end of second fuel conduit 6128 and/or second oxidizer 6130. Ignition source 6134 may be an electrically controlled ignition source. Ignition source 6134 may be coupled to ignition source lead-in wire 6136. Ignition source lead-in wire 6136 may be further coupled to a power source for ignition source 6134. Ignition source 6134 may be used to initiate oxidation of fuel 6118 exiting second fuel conduit 6128. After oxidation of fuel 6118 from second fuel conduit 6128 has begun, ignition source 6134 may be turned down and/or off. In other embodiments, an ignition source may also be disposed proximate first oxidizer 6120.

[1074] In some embodiments, ignition source 6134 may not be used if, for example, the conditions in the wellbore are sufficient to auto-ignite fuel 6118 being used. For example, if hydrogen is used as the fuel, the hydrogen will auto-ignite in the wellbore if the temperature and pressure in the wellbore are sufficient for autoignition of the fuel.

[1075] As shown in FIG. 97, second insulation 6132 may be disposed in a region proximate second oxidizer 6130. Second insulation 6132 may be disposed on a face of hydrocarbon layer 6100 along an inner surface of opening 514. Second insulation 6132 may have a length of about 10 m to about 200 m (e.g., about 50 m). A length of second insulation 6132 may vary, however, depending on, for example, a desired heat transfer rate to the formation, a desired temperature proximate the lower oxidizer, or a desired temperature profile along a length of conduit 6110 and/or hydrocarbon layer 6100. In an embodiment, the length of second insulation 6132 is about 10–40% of the length of conduit 6110 between any two oxidizers. Second insulation 6132 may have a thickness that varies (either continually or in step fashion) along its length. In certain embodiments, second insulation 6132 may have a larger thickness proximate second oxidizer 6130 and a reduced thickness at a desired distance from the second oxidizer. The larger thickness of second insulation 6132 may preferentially reduce heat transfer proximate second oxidizer 6130 as compared to the reduced thickness portion of the insulation. For example, second insulation 6132 may have a thickness of about 0.03 m proximate second oxidizer 6130 and a thickness of about 0.015 m at a distance of about 10 m from the second oxidizer.