ELECTRICAL WELL HEATING SYSTEM AND METHOD

Abstract: A system for heating an underground hydrocarbon containing formation comprises an electrical conductor disposed within a heater well traversing the formation to provide radiant heat to at least a portion of the formation to initiate pyrolysis of hydrocarbons, wherein the electrical conductor comprises one or more elongate electrically conductive heater members which are each suspended in a fluid filled section of a heater well by a support member. The heater members may be bare metal strips or rods and may be arranged in an uncased section of the heater well in which a pressurized fluid, such as air, is circulated to remove hydrocarbon deposits from the heater members.

Description: ELECTRICAL WELL HEATING SYSTEM AND METHOD Background of the Invention The invention relates to a method and system for heating a hydrocarbon containing formation, such as a coal layer or an oil shale deposit, surrounding a heat injection well. Application of heat to oil shale formations is described in U.S. Patent Nos. 2,923,535 to Ljungstrom and 4,886,118 to Van Meurs et al. These prior art references disclose that electrical heaters transmit heat into an 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. U.S. Patent No. 2,548,360 describes an electrical 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. Patent No. 4,716,960 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. Patent No. 5,065,818 to Van Egmond describes an electrical heating element that is cemented into a well borehole without a casing surrounding the heating element. U.S. Patent No. 6,023,554 to Vinegar et al. describes an electrical 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 materia. 1 , which in turn conductively heats the formation. U.S. Patent No. 4,570,715 to Van Meurs et al. describes an electrical 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. U.S. Patent No. 5,060,287 to Van Egmond describes an electrical heating element having a copper-nickel alloy core . Disadvantages of the known electrical heaters is that they are expensive, require complex installation procedures and are prone to overheating, rupture and/or melting so that they may require frequent replacement in particular if they are used to heat a long interval of a downhole formation. It is an object of the present invention to provide an improved inexpensive downhole electrical well heating method and system which are configured to transmit a controlled amount of heat in a uniform manner into the formation over a long period of time. Summary of the Invention In accordance with the present invention a system for transmitting heat into a hydrocarbon containing formation surrounding a heat injection well comprises an electrical conductor configured to be disposed within a heater well traversing the formation to provide radiant heat to at least a portion of the formation during use, wherein the electrical conductor comprises a number of elongate electrically conductive heater members which are each suspended in a fluid filled section of a heater well by a support member. Optionally, at least one elongated electrically conductive heater member comprises an at least partly bare metal strip, rod, cable, conduit or other conductor. It is preferred that at least one metal heater member is suspended in an uncased lower section of a heater well by an elongate support member which extends from e.g. a wellhead at or near the earth surface into said uncased lower section of the heater well. The elongate support member may comprise an elongate tube on which a series of centralisers are mounted at selected intervals, which centralizers support a plurality of bare metal heater members . Optionally, one or more bare metal heater members are U-shaped and are each electrically connected at their upper ends to power supply conduits which supply electrical power from an electric source to the ends of the U-shaped bare metal heater member, and wherein the U-shaped bare metal heater member, power supply conduits and electric power source form an electric circuit. The tubular elongate support member may be provided with a series of orifices through which in use an oxidant is injected adjacent to at least part of the length of the metal heater members, which oxidant burns away hydrocarbons released by the surrounding hydrocarbon formation and deposited on the surface of the metal heater members. The tubular elongate support member may extend through a packer near a lower end of a cased upper section of the heater well to a wellhead. Optionally, the metal heater members and centralizers are located in an uncased lower section of the heater well, which lower section traverses a hydrocarbon containing formation. In an embodiment of the method according to the invention, the assembly of one or more heater members is configured to transmit in use a cumulative amount of radiant heat between 0.5 and 1.5 KW per meter length of the heater well into the formation, thereby heating hydrocarbons in the formation in the vicinity of the heater well to a temperature above 280 degrees Celsius and causing in-situ pyrolysis of hydrocarbons in the formation. If in use an oxidant, such as air, is injected into an uncased lower section of the heater well, the oxidant may at least partly burn away hydrocarbon deposits on the elongate heater members, thereby cleaning the heater members and reducing the risk of arcing and short- circuiting. Combustion gases may be vented to surface via an exhaust conduit extending through a cased upper part of the heater well, and that the fluid pressure in the uncased lower part is controlled such that transfer of combustion gases into the formation is inhibited and that transfer of pyrolised hydrocarbon fluids from the formation at a distance of more than a meter from the heater well into the uncased lower part of the heater well is also inhibited. Said fluid pressure is preferably maintained at a minimum level of 1.5 bar and is controlled in conjunction with an assessed temperature in a part of the formation in which hydrocarbons are pyrolysed. Control of the pressure may be desired in order to maintain conditions favourable to the formation of better quality hydrocarbons during pyrolysis. It may also be desired, during certain times, to maintain a minimum average pressure in the process zone, dependent on the thickness and weight of the overlaying. The selected pressure may contribute to supporting the weight of overlaying layers and thus may mitigate compaction and subsidence The hydrocarbon containing formation may be a coal, oil-shale, kerogen or a bitumen deposit. The method preferably comprises transmitting between 0.5 and 1.5 KW radiant heat per meter length of the heater well into a portion of the hydrocarbon containing formation surrounding the heater well thereby heating at least a portion of the formation surrounding the heater well to pyrolyze hydrocarbons within said portion of the formation surrounding the heater well, wherein a majority of pyrolysed hydrocarbons are induced to flow to a production well which is located at a selected distance of between 3 and 15 meter from the heat injection well. The assembly may consist of at least one and preferably more than one heat injector wells and it may also consist of one and preferably more than one production wells. An advantage of arranging the heater member (s) in an at least partly uncased section of the heater well is that the cost of installing and/or replacing a casing in the hottest section of the heater well is saved. Casings that are cemented or otherwise supported in the heater well are subject to high stresses as a result of the thermal expansion of the surrounding formation in combination with the radial and longitudinal expansion of the heated casing itself and the resulting high stresses may therefore limit the amount of heat that can be transmitted into the formation from the heater well. Description of preferred embodiments The invention will be described in more detail and by way of example with reference to the accompanying drawing, in which: Fig.l depicts a heater well in which an assembly of bare strip electrical conductors are suspended by means of a support member. As shown in Fig.l a number of elongate heater members 600 may be disposed within an opening 514 (e.g., an open heater wellbore) in a hydrocarbon containing formation 516. The opening 514 may preferably be an at least partly uncased opening in the hydrocarbon containing formation 516. The opening 514 may have a diameter of at least approximately 5 cm or, for example, approximately 8 cm. The diameter of the opening 514 may vary, however, depending on, for example, a desired heating rate in the formation. The elongate heater member (s) 600 may be a length (e.g., a strip) of metal or any other elongated piece of metal (e.g., a rod) and may be made of stainless steel. The elongated heater member (s) 600, however, may also include any conductive material configurable to generate heat to sufficiently heat a portion of the formation and to substantially withstand a corresponding temperature within the opening, for example, it may be configured to withstand corrosion at the temperature within the opening. The elongate heater member (s) 600 may comprise one or more bare metal heaters. "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 . Each elongate heater member 600 may have a length of about 650 meters. 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 elongate heater member (s) 600 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. 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 substantially electrically conductive material and/or increasing the cross-sectional area available for electrical conduction. Alternately, a support member 604 may extend through the overburden 540, and the bare metal elongated member or members may be coupled to a plate, centralizer or other type of support member near an interface between the overburden and the hydrocarbon formation. A low resistivity cable 606, such as a stranded copper cable, may extend along the support member and may be coupled to the elongated member or members. The copper cable may be coupled to a power source that supplies electricity to the elongated member or members. FIG. 1 illustrates an embodiment of a plurality of elongated members configured to heat a section of a hydrocarbon containing 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 be configured to provide a flow of an oxidizing fluid into opening 514. Support member 604 may have a diameter between about 1.2 cm to about 4 cm and more preferably about 2.5 cm. Support member 604, elongated members 600, and insulated centralizers 602 may be disposed in opening 514 in formation 516. Insulated centralizers 602 may be configured to maintain a location of elongated members 600 on support member 604 such that lateral movement of elongated members 600 may be substantially inhibited at temperatures high enough to deform support member 604 or elongated members 600. Insulated centralizers 602 may be a centralizer as described herein. Elongated members 600, in some embodiments, may be metal strips of about 2.5 cm wide and about 0.3 cm thick stainless steel. Elongate heater members 600, however, may also include a pipe or a rod formed of a conductive material. Electrical current may be applied to elongate heater members 600 such that the elongate heater members 600 may generate heat due to electrical resistance. The elongate heater members 600 may be configured to generate heat of approximately 650 watts per meter of elongated members 600 to approximately 1650 watts per meter of elongated members 600. In this manner, elongated members 600 may be at a temperature of approximately 480 °C to approximately 815 °C. Substantially uniform heating of a hydrocarbon containing formation may be provided along a length of elongated members 600 greater than about 305 m or, maybe, greater than about 610 m. A length of elongated members 600 may vary, however, depending on, for example, a type of hydrocarbon containing formation, a depth of an opening in the formation, and/or a length of the formation desired for treating The elongate heater 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 further configured as described herein. 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 member 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 substantially inhibit fluid from escaping from opening 514 to surface 550. Lead-in conductor 572 and lead-out conductor 606 may be coupled to elongate members using a cold pin transition conductor. The cold pin transition conductor may include an insulated conductor of substantially low resistance such that substantially no heat may generated by 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 any splicing or welding methods known in the art. The cold pin transition conductor may provide a temperature transition between lead-in conductor 572, lead-out conductor 606, and/or elongated members 600. The cold pin transition conductor may be further configured as described in any of the embodiments herein. Lead-in conductor 572 and lead-out conductor 606 may be made of low resistance conductors such that substantially no heat may be generated from electrical current passing through lead-in conductor 572 and lead-out conductor 606. Weld beads may be placed beneath the centralizers 602 on the support member 604 to fix the position of the centralizers. Weld beads may be placed on the 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. 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 m^y 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 area section of 304 stainless steel. The first and third lengths may be positioned close to the 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. Packing material 542 may be placed between overburden casing 541 and opening 514. Packing material 542 may be configured to inhibit fluid flowing from opening 514 to surface 550 and to inhibit corresponding heat losses towards the surface. Packing material 542 may be further configured as described herein. Overburden casing 541 may be placed in cement 544 in overburden 540 of formation 516. Overburden casing 541 may be further configured as described herein. Surface conductor 545 may be disposed in cement 544. Surface conductor 545 may be configured as described herein. Support member 604 may be coupled to wellhead 690 at surface 550 of formation 516. Centralizer 581 may be configured to maintain a location of support member 604 within overburden casing 541. Centralizer 581 may be further configured as described herein. 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 formation 516. An 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 to the elongated members. For example, the oxidizing fluid may react with hydrocarbons to form carbon dioxide, which may be removed from the opening. Openings 605 in support member 604 may be configured to provide a flow of the oxidizing fluid along the length of elongated members 600. Openings 605 may be critical flow orifices as configured and described herein. Alternatively, a tube may be disposed proximate to elongated members 600 to control the pressure in the formation as described in above embodiments. In another embodiment, a tube may be disposed proximate to elongated members 600 to provide a flow of oxidizing fluid into opening 514. Also, at least one of elongated members 600 may include a tube having openings configured to provide the flow of oxidizing fluid. Without the flow of oxidizing fluid, carbon deposition may occur on or proximate to elongated members 600 or on insulated centralizers 602, thereby causing 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 as described herein. The heat generated by reaction with the carbon may complement or supplement the heat generated electrically. In an embodiment, a plurality of elongated members may be supported on a support member disposed in the heater well or other opening. The plurality of elongated members may be electrically coupled in either a series or parallel configuration. A current and voltage applied to the plurality of elongated members may be selected such that the cost of the electrical supply of power at the surface in conjunction with the cost of the plurality of elongated members may be minimized. In addition, an operating current and voltage may be chosen to optimize a cost of input electrical energy in conjunction with a material cost of the elongated members. The elongated members may be configured to generate and radiate heat as described herein. The elongated members may be installed in opening 514 as described herein. In an 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, Texas) . 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 substantially the same amount of heat per unit length along the entire length of the elongated member. The use of different 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 7/8 inch (2.2 cm) diameter first sections, two 1/2 inch (1.3 cm) middle sections, and a 3/8 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 section 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. 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 containing formation) to counteract end effects and allow for more uniform heat dissipation into the hydrocarbon containing formation. A higher heat dissipation may also be located at a lower end of an elongated member to counteract end effects and allow for more uniform heat dissipation. A difference in heat dissipation into different sections may be required to cause favourable physicochemical differences in the pyrolysis process leading to more favourable hydrocarbon product quality. The electric heater may be configured to provide heat in addition to heat provided from a surface combustor. The electric heater may be configured to provide the additional heat to a hydrocarbon containing formation such that the hydrocarbon containing formation may be heated substantially uniformly along a selected depth of the heater well.

Claims: C L A I M S 1. A system configured to heat an underground hydrocarbon containing formation, comprising an electrical conductor configured to be disposed within a heater well traversing the formation to provide radiant heat to at least a portion of the formation during use, wherein the electrical conductor comprises a number of elongate electrically conductive heater members which are each suspended in a fluid filled section of a heater well by a support member. 2. The system of claim 1, wherein at least one elongated electrically conductive heater member comprises an at least partly bare metal strip, rod or conduit. 3. The system of claims 1 or 2, wherein at least one metal heater member is suspended in an uncased lower section of a heater well by an elongate support member which extends from a wellhead at or near the earth surface into said uncased lower section of the heater well . 4. The system of claim 3, wherein the elongate support member comprises an elongate tube on which a series of centralisers are mounted at selected intervals, which centralizers support a plurality of bare metal heater members . 5. The system of claim 4, wherein at least one of the bare metal heater members is U-shaped and is electrically connected at an upper end thereof to power supply conduits which supply electrical power from an electric source to the ends of the U-shaped bare metal heater member, and wherein the U-shaped bare metal heater member, power supply conduits and electric power source form an electric circuit. 6. The system of claim 4, wherein the tubular elongate support member is provided with a series of orifices through which in use an oxidant is injected adjacent to at least part of the length of the metal heater members, which oxidant burns away hydrocarbons released by the surrounding hydrocarbon formation and deposited on the surface of the metal strip heaters or its centralizers. 7. The system of claim 4, wherein the tubular elongate support member extends through a packer near a lower end of a cased upper section of the heater well to a wellhead. 8. The system of claim 7, wherein the metal heater members and centralizers are located in an uncased lower section of the heater well, which lower section traverses a hydrocarbon containing formation. 9. A method of heating a hydrocarbon containing formation with the system of any one of claims 1 to 8, wherein the assembly of heater members is configured to transmit in use a cumulative amount of radiant heat between 0.5 and 1.5 KW per meter length of the heater well into the formation, thereby heating hydrocarbons in the formation in the vicinity of the heater well to a temperature above 280 degrees Celsius and causing in-situ pyrolysis of hydrocarbons in the formation. 10. The method of claim 9, wherein the elongate heater elements are supported by a series of centralisers that are mounted at regular intervals on a tubular support pipe, which is provided with a series of orifices through which an oxidant, such as air, is injected into an uncased lower section of the heater well, which oxidant at least partly burns away hydrocarbon deposits on the elongate heater elements. 11. The method of claim 10, wherein the elongate heater elements are arranged in an uncased lower section of the heater well and combustion gases are vented to surface via an exhaust conduit extending through a cased upper part of the heater well. 12. The method of claim 11, wherein a the fluid pressure in uncased lower part is controlled such that transfer of combustion gases into the formation is inhibited and that transfer of pyrolised hydrocarbon fluids from the formation at a distance of more than a meter from the heater well into the uncased lower part of the heater well is also inhibited. 13. The method of claim 12, wherein the fluid pressure is maintained at a minimum level of 1.5 bar and is controlled in conjunction with an assessed temperature in a part of the formation in which hydrocarbons are pyrolysed. 14. The method of any one of claims 10 to 12, wherein the hydrocarbon containing formation is an underground coal, oil-shale, kerogen or a bitumen deposit.

Inventors: DE ROUFFIGNAC ERIC, HUNSUCKER BRUCE GERARD, KEEDY CHARLES ROBERT, VINEGAR HAROLD J, WELLINGTON SCOTT LEE, DE ROUFFIGNAC, ERIC, HUNSUCKER, BRUCE, GERARD, KEEDY, CHARLES, ROBERT, VINEGAR, HAROLD, J., WELLINGTON, SCOTT, LEE
Applicants: SHELL CANADA LTD, SHELL INT RESEARCH, SHELL CANADA LIMITED, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 2001
Main classification: E21B36
IPC codes: C09K8, E21B36, E21B41, E21B43