US20120103604A1

US20120103604A1 - Subsurface heating device

Info

Publication number: US20120103604A1
Application number: US12/915,530
Authority: US
Inventors: Sherif Hatem Abdulla Mohamed; Richard Blair Sheldon; Ahmed Mostafa Elkady; Andrei Tristan Evulet; Michael Francis Xavier Gigliotti, Jr.; James William Bray
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2010-10-29
Filing date: 2010-10-29
Publication date: 2012-05-03
Also published as: AR083540A1; CA2755755A1; RU2571120C2; BRPI1104237A2; ITMI20111884A1; RU2011143400A; CN102454386A

Abstract

In one aspect, the present invention provides a subsurface heating device comprising: (a) a combustion conduit casing defining a combustion conduit; (b) at least two combustors disposed within the combustion conduit casing; (c) at least one fuel supply conduit; d) at least one oxygen supply conduit configured to supply oxygen to at least one combustor; and (e) a combustion product gas outlet. The at least two combustors are characterized by an inter-combustor distance of at least one thousand feet and a combustion power of at least 3.41 million BTU per hour. The at least one fuel supply conduit is configured to supply a combustible fuel to at least one combustor. Also provided in another aspect of the present invention, is a method for heating a subsurface zone.

Description

BACKGROUND

There are extensive hydrocarbon reservoirs distributed throughout the world which, for the foreseeable future, represent key energy resources for the world's continued economic development. These reservoirs often contain a viscous hydrocarbon concoction, called “tar,” “heavy oil,” or “ultra heavy oil,” which typically has a viscosity in the range from about 3,000 to 1,000,000 centipoise when measured at around 37.5° C. Many hydrocarbon bearing geologic formations contain such hydrocarbon concoctions which do not permit a ready flow of the hydrocarbon content to a wellbore for extraction because of their high viscosity. In certain hydrocarbon reservoirs, for example, oil shale reservoirs, the hydrocarbon components must be thermally broken down into lower molecular weight compounds in order to effect their recovery from the reservoir. In certain instances, the reservoir must be heated to a temperature in excess of 300° C. in order to effect even the partial extraction of hydrocarbons from a hydrocarbon reservoir.
Three different types of processes are known to enhance hydrocarbon extraction from subterranean hydrocarbon reservoirs. These processes may be classified generally as thermal processes, chemical processes and miscible displacement processes.
A notable, known thermal process involves an “in situ” combustion technique in which the reservoir, serving as its own fuel source, is ignited through an injection well and a zone of combustion is propagated from the injection well towards a production well. The combustion can be somewhat controlled by the position of the injection well and the mode of delivery of the exogenous oxygen needed to effect combustion within the combustion zone. Because of the nature and complexity of the fuel involved, such in situ combustion techniques produce a complex variety of combustion product gases which must be carefully managed in order to prevent their uncontrolled release into the living environment.
Heat conduction phenomena within and around the reservoir may play a critical role in hydrocarbon recovery rates, and such rates may be further limited by a tendency of the hydrocarbon components of the reservoir to undergo coking. The heat transfer rate from a heat source to the reservoir may be limited by the coking temperature and the ambient temperature of the hydrocarbon bearing reservoir. Thus, methods involving heating of a hydrocarbon reservoir must balance the rate at which heat is introduced into the reservoir against the coking temperature of the hydrocarbon components of the reservoir and the rate at which the heat can be conducted from the heat source into the reservoir.
Therefore there is a need for subsurface heating devices which utilize clean fuels such as natural gas and effect a controlled delivery of substantial amounts of heat from the device to the reservoir such that coking may be minimized while maximizing the efficiency of hydrocarbon recovery.

BRIEF DESCRIPTION

In one aspect, the present invention provides a subsurface heating device comprising: (a) a combustion conduit casing defining a combustion conduit; (b) at least two combustors disposed within the combustion conduit casing; (c) at least one fuel supply conduit configured to supply a combustible fuel to at least one combustor; d) at least one oxygen supply conduit configured to supply oxygen to at least one combustor; and (e) a combustion product gas outlet. The at least two combustors are characterized by an inter-combustor distance of at least one thousand feet and a combustion power of at least 3.41 million BTU per hour.
In another aspect, the present invention provides a method for heating a subsurface zone, comprising: (i) creating an accommodation cavity for a subsurface heating device; (ii) installing the subsurface heating device; and (iii) operating the subsurface heating device. The subsurface heating device comprises (a) a combustion conduit casing defining a combustion conduit; (b) at least two combustors disposed within the combustion conduit casing; (c) at least one fuel supply conduit configured to supply a combustible fuel to at least one combustor; d) at least one oxygen supply conduit configured to supply oxygen to at least one combustor; and (e) a combustion product gas outlet. The at least two combustors are characterized by an inter-combustor distance of at least one thousand feet and a combustion power of at least 3.41 million BTU per hour.
In yet another aspect, the present invention provides a method for shale oil recovery comprising: (i) creating an accommodation cavity for a subsurface heating device; (ii) installing the subsurface heating device within the accommodation cavity; and (iii) operating the subsurface heating device. The subsurface heating device comprises (a) a combustion conduit casing defining a combustion conduit; (b) at least two combustors disposed within the combustion conduit casing; (c) at least one fuel supply conduit configured to supply a combustible fuel to at least one combustor; d) at least one oxygen supply conduit configured to supply oxygen to at least one combustor; and (e) a combustion product gas outlet. The at least two combustors are characterized by an inter-combustor distance of about 1000 feet and wherein a thermal output of a first combustor is about 1.5 to about 2.5 times that of a thermal output of a following combustor spaced at the inter-combustor distance of about 1000 feet.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic representation of a subsurface heating device in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

In the following specification and the claims, which follow, reference will be made to a number of terms, which shall be defined to have the following meanings.
The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.
“Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. Furthermore, whenever a particular feature of the invention is said to comprise or consist of at least one of a number of elements of a group and combinations thereof, it is understood that the feature may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Similarly, “free” may be used in combination with a term, and may include an insubstantial number, or trace amounts, while still being considered free of the modified term.
As discussed in detail below, embodiments of the present invention include a subsurface heating device comprising: (a) a combustion conduit casing defining a combustion conduit; (b) at least two combustors disposed within the combustion conduit casing; (c) at least one fuel supply conduit configured to supply a combustible fuel to at least one combustor; d) at least one oxygen supply conduit configured to supply oxygen to at least one combustor; and (e) a combustion product gas outlet. The at least two combustors are characterized by an inter-combustor distance of at least one thousand feet and a combustion power of at least 3.41 million BTU per hour.
In one embodiment of the present invention, as illustrated by FIG. 1, the subsurface heating device 10 includes a combustion conduit casing 12. The combustion conduit casing 12 typically defines a combustion conduit 30. In one embodiment, the combustion conduit casing 12 is comprised of at least one material selected from the group consisting of steel, stainless steel, inconel, and high corrosion resistant alloys. In another embodiment, the combustion conduit casing 12 is comprised of a steel pipe.
The combustion conduit casing 12 has disposed within it at least two combustors 14. The combustor 14 is at least one selected from electrical heaters, gas burners, flameless distributed combustors, natural distributed combustors, or hot-gas carrying conduits. In one embodiment, the combustor 14 is a low emission natural gas combustor such as those found in advanced gas turbines. In one embodiment, the at least two combustors 14 are coupled in series. In another embodiment, the at least two combustors 14 are coupled in a parallel. Typically, the at least two combustors 14 are separated by a vertical distance and allow for a larger coverage of the heated area. The subsurface heating device 10 includes at least two combustors which are disposed with the combustion conduit casing 12 and is characterized by an inter-combustor distance 16. In one embodiment, the inter-combustor distance 16 between the at least two combustors 14 is at least one thousand feet. In another embodiment, the inter-combustor distance 16 between the at least two combustors 14 is in a range from about 1000 feet to about 3000 feet. In yet another embodiment, the inter-combustor distance 16 between the at least two combustors 14 is about 2000 feet. In one embodiment, the at least two combustors 14 are independently movable with respect to the combustion conduit casing 12. In another embodiment, the combustors are attached to a moving platform for example a rail, wherein the moving platform is attached to an interior surface of the combustion conduit casing 12.
Typically, the combustor 14 has a combustion power of at least about 3.41 million BTU per hour. In another embodiment, the combustor 14 has a combustion power in a range of about 3.41 million BTU per hour to about 10.23 million BTU per hour. In yet another embodiment, each of the at least two combustors 14 have a combustion power of about 3.41 BTU per hour. In one embodiment, each of the at least two combustors may have similar combustion power. In an alternate embodiment, each of the at least two combustors may have different combustion power.
The subsurface heating device 10 includes a fuel supply conduit 18 configured to supply a fuel to least one combustor 14. The subsurface heating device 10 includes an oxygen supply conduit 20 that is configured to supply oxygen to at least one combustor 14. As illustrated in FIG. 1, in one embodiment, the fuel supply conduit 18 and the oxygen supply conduit 20 can be placed parallel to each other (for example a side by side type of arrangement). In another embodiment, the fuel supply conduit 18 and the oxygen supply conduit 20 may form a concentric pair. In some illustrative embodiments (FIG. 1), the fuel supply conduit 18 may be branched to supply fuel to the combustors 14 that are disposed in the combustion conduit casing 12.
Typically, the fuel employed for the subsurface heating device 10 is a combustible fuel that may be selected from natural gas, hydrocarbons such as methane, propane etc, synthesis gas (for example, a mixture that includes hydrogen and carbon monoxide), natural gas mixed with heavier components such as ethane, propane, butane, or carbon monoxide, a premix of methane and air, diesel, heating oil, kerosene type jet fuel. In one embodiment, the combustible fuel is a liquid fuel. In another embodiment, the liquid fuel is a jet fuel. In some embodiments, the fuel may further include a non-combustible gas such as nitrogen. In some embodiments, the fuel may further include products from a coal or heavy oil gasification process. Generally, after initiation of combustion of fuel and oxidant mixture in the combustor 14, the composition of the fuel may be varied to enhance operational stability of the combustor 14.
In one embodiment, the fuel supply conduit 18 provides natural gas to at least one combustor 14. In another embodiment, the fuel supply conduit 18 provides heating oil to at least one combustor 14. In one embodiment, the fuel is introduced in the fuel supply conduit 18 via a pump 26. The fuel supply conduit 18 may supply fuel to at least one combustor 14 disposed in the combustion conduit casing 12. In some illustrative embodiments (FIG. 1), the fuel supply conduit 18 may be branched, such that the branches of the fuel supply conduit 18 to supply fuel to the combustors 14 that are disposed in the combustion conduit casing 12. In some embodiments, a plurality of fuel supply conduits may be employed to enable the supply of fuel to be interrupted to one or more combustors without affecting the other combustors. Typically, multiple fuel supply conduit 18 may also help adjust the amount of fuel to be supplied to the combustor 14 during startup and when steady operation of the combustor is established. In one embodiment, the fuel supply conduit 18 may supply an equal amount of fuel to the plurality of combustors 14 disposed in the combustion conduit casing 12. In yet another embodiment, the fuel supply conduit 18 may supply varying amount of fuel to the plurality of combustors 14 disposed in the combustion conduit casing 12. In one embodiment, the amount of fuel supplied to the adjacent combustor of the plurality of combustors 14 may decrease from the first combustor to the adjacent second combustor and the like. In various embodiments, the fuel supply conduit 18 may further comprise one or more orifices (not shown) to selectively control the pressure loss along the fuel supply conduit 18.
The subsurface heating device 10 includes an oxygen supply conduit 20. In one embodiment, the combustion conduit casing 12 that defines a combustion conduit 30 is configured to serve as an oxygen supply conduit 20. Typically, the oxygen supply conduit 20 can further carry gas selected from air, inert gases such as argon, nitrogen, air enriched with oxygen, synthetic mixtures of oxygen and one or more gases. In another embodiment, the oxygen supply conduit 20 can carry gas that contains at least about 70 percent by weight of oxygen. In yet another embodiment, the oxygen supply conduit 20 can carry gas that contains at least about 90 percent by weight of oxygen. In an illustrative embodiment shown in FIG. 1 the oxygen supply conduit 20 is configured to receive an output from a compressor 24.
In one embodiment, the fuel supply conduit 18 is coupled to at least one fuel jet or fuel opening (not shown) that releases fuel in the combustor, and the oxygen supply conduit 20 is coupled to at least one oxygen (air) nozzle/oxygen opening (not shown) that releases an oxygen-containing gas (e.g. air, oxygen, or a synthetic mixture of oxygen and one or more gases in the combustor 14. In another embodiment, the at least one fuel jet or oxygen nozzle regulate the pressure inside the combustor 14, in addition to regulating the flow of the fuel and the oxygen respectively into the combustor 14. In one embodiment, the mixture of fuel and oxygen can be ignited by an igniter (not shown) disposed in the combustion conduit casing 12, for example the igniter may be a small open flame burner, an electrically heated wire, or a spark device. Once ignited the flame may propagate into a combustion/reaction zone of the combustor 14.
In one embodiment, each one of the combustors 14 are independently operable i.e. a combustor can be switched on or off independently without affecting the status of other combustors in the subsurface heating device 10. Thus, in one embodiment, during operation a first combustor located at a reference position denominated 1 along the axis defined by the combustion conduit casing 12 is “switched on” (i.e. the associated fuel jet and the oxygen (air) nozzle are open and the oxygen-fuel mixture emerging therefrom is burning) while a second combustor located adjacent to the first combustor along the axis defined by the combustion conduit casing 12 is “switched off” (i.e. the associated fuel jet and the oxygen (air) nozzle are closed). In yet another embodiment, the amount of heat produced at any given time at the at least one combustor 14 can be varied independently by varying parameters such as pressure of the combustible fuel, pressure of the oxygen, or varying the ratio of the oxygen to the combustible fuel. In one embodiment, the thermal output of a first combustor is about 1 to about 5 times that of the thermal output of a following combustor. In another embodiment, the thermal output of a first combustor is about 1.5 to about 2.5 times that of the thermal output of a following combustor.
In various embodiments, the at least one fuel jet and associated oxygen (air) nozzle is controlled such that they are open, partially opened or closed depending on need. Conventional control systems may be employed. In one embodiment, the mechanical components of the burners (e.g. the fuel jet, the associated oxygen (air) nozzles, and the burner igniter) and a set of operational sensors (flame on/off sensor, valve open/closed sensor, temperature sensor, pressure sensor, igniter on/off sensor) are linked to a controller via an insulated control cable arrayed along the axis of and within the combustion conduit casing 12. In one embodiment, the subsurface heating device 10 can further include a plurality of sensors (not shown). In one embodiment, the sensor is a temperature sensor. In one embodiment, the temperature sensor can be disposed within the subsurface heating device 10. In another embodiment, the temperature sensor can be disposed outside an outer surface of the combustion conduit casing 12 of the subsurface heating device 10. In another embodiment, the temperature sensor is configured to provide data to a control system.
In one embodiment, the combustor 14 can include three zones (not shown) that include a mixing zone, an ignition zone and a reaction zone. The reaction zone can also be referred to as a combustion zone as the combustion occurs at the reaction zone. In another embodiment, the three zone present in the combustor 14 can be easily distinguishable. In one embodiment, the oxygen and the fuel enter the mixing zone of the combustor 14. A combustible mixture of fuel and oxygen passes from mixing zone into a flame zone comprising the igniter which initiates the reaction of the fuel and oxygen to provide heat and combustion product gas. Typically, the combustion product gas thus produced may flow through the combustion conduit 30. In another embodiment shown in FIG. 1, the combustion product gas formed as result of the reaction in the combustor 14 is removed from the combustion conduit 30 via an exit pipe 34. In one embodiment, the combustion product gas may provide heat as it flows along the length of the combustion conduit casing 12. The heat provided by the combustion product gas along the length of combustion conduit casing 12, in addition to the heat produced in the combustor 14 may increase the amount of heat transferred to the formation from the combustion conduit casing 12 via the heat transmissive external housing. In one embodiment, the combustion conduit casing 12 is configured to accommodate a heat transfer substance such as an organic heat transfer liquid or, a molten salt that serves to transfer the heat produced in the combustor 14 more evenly to the formation. In various embodiments, the combustion product gas 34 after heat exchange in the combustion conduit casing 12 is directed via a combustion product gas outlet 22 in the exit pipe 34 to a gas treatment unit. In another embodiment, the combustion product gas outlet 22 is configured to deliver samples of combustion product gases 34 to a gas analyzer. Non limiting examples of gas analyzer includes gas chromatography and metal oxide sensor. In yet another embodiment, the combustion product gas outlet 22 is configured to return at least a portion of the combustion product gases 34 in a direction parallel to the combustion conduit casing 12.
As will be appreciated by those of ordinary skill in the art, the fuel and air tubes (i.e. the fuel supply conduit 18 and the oxygen supply conduit 20) may be in close proximity to the combustion zone of the subsurface heating device 10 and there is a tendency of heat to flow toward the center of the subsurface heating device 10 as well as being radiated outwardly from the subsurface heating device 10. As a result of the outward flow of the fuel and the oxygen containing gas from the fuel supply conduit 18 and the oxygen supply conduit 20 respectively, the temperature within each of the conduits can be maintained at relatively low temperature during operation of the subsurface heating device 10. Lower flow velocities and lower pressure losses are a result, of the relatively low temperatures prevailing within the fuel and oxygen containing gas supply conduits.
Another aspect of the invention provides a method for heating a subsurface zone, comprising: (a) creating an accommodation cavity for a subsurface heating device 10; (b) installing the subsurface heating device 10; and (c) operating the subsurface heating device 10.
In one embodiment, the accommodation cavity can be created in a hydrocarbon reservoir. As used herein the term “hydrocarbon” is defined as compounds comprising carbon and hydrogen. However, hydrocarbon-containing reservoirs may contain a host of components comprising elements other than carbon and hydrogen, for example halogens, nitrogen, oxygen, metals, sulfur, and selenium. Non-limiting examples of components which may be present in a hydrocarbon reservoir include, straight chain and branched hydrocarbons, for example eicosane (a C₂₀straight chain hydrocarbon) and phytane (a C₂₀branched hydrocarbon), bitumen, oil tars, minerals, asphaltites, kerogen. The hydrocarbon reservoir is typically contained within a geologic matrix, such as sedimentary rock, sands, silicilytes, carbonates, diatomites. In one embodiment, the hydrocarbon reservoir is a subterranean, viscous oil-containing formation. In one embodiment, the hydrocarbon reservoir is contained within a heavy oil tar sand formation. In another embodiment, hydrocarbon reservoir is contained within a shale oil formation. In one embodiment, the accommodation cavity can be subterranean, located under tundra, under sea or inland based wells. The methods provided by the present invention may be practiced in conjunction with a wide variety of hydrocarbon recovery techniques including vertical recovery, horizontal recovery, and steam assisted gravity drainage (SAGD) techniques. In another embodiment, the accommodation cavity can be created in a near-surface zone. Examples of applicable near-surface zones include but are not limited to construction activity zones, water containment zones, water transport zones (e.g. municipal water delivery and waste water removal), and water treatment zones such as municipal water treatment plants.
In one embodiment, the subsurface heating device 10 can be operated in a pressurized environment. In another embodiment, the can be operable at varying fuel and oxygen pressures over several thousands of feet in length. In one embodiment, the subsurface heating device 10 can be lowered in the accommodation cavity in a manner that is parallel to the surface of the earth. In some embodiments, the subsurface heating device 10 can be lowered in the accommodation cavity in a manner that is angled with respect to the surface of the earth. In another embodiment, the subsurface heating device 10 can be lowered in the accommodation cavity in a manner that is in a vertical position with respect to the surface of the earth.
In yet another aspect, the present invention provides a method for shale oil recovery comprising: (i) creating an accommodation cavity for a subsurface heating device; (ii) installing the subsurface heating device within the accommodation cavity; and (iii) operating the subsurface heating device. The subsurface heating device comprises (a) a combustion conduit casing defining a combustion conduit; (b) at least two combustors disposed within the combustion conduit casing; (c) at least one fuel supply conduit configured to supply a combustible fuel to at least one combustor; d) at least one oxygen supply conduit configured to supply oxygen to at least one combustor; and (e) a combustion product gas outlet. The at least two combustors are characterized by an inter-combustor distance of about 1000 feet and wherein a thermal output of a first combustor is about 1.5 to about 2.5 times that of a thermal output of a following combustor spaced at the inter-combustor distance of about 1000 feet.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

1. A subsurface heating device comprising:

(a) a combustion conduit casing defining a combustion conduit;

(b) at least two combustors disposed within the combustion conduit casing and characterized by an inter-combustor distance of at least one thousand feet and a combustion power of at least 3.14 million BTU per hour;

(c) at least one fuel supply conduit configured to supply a combustible fuel to at least one combustor;

(d) at least one oxygen supply conduit configured to supply oxygen to at least one combustor; and

(e) a combustion product gas outlet.

2. The subsurface heating device according to claim 1, wherein the combustion conduit serves as the oxygen supply conduit.

3. The subsurface heating device according to claim 2, wherein the oxygen supply conduit is configured to receive the output of a compressor.

4. The subsurface heating device according to claim 1, wherein the combustors are independently moveable with respect to the combustion conduit casing.

5. The subsurface heating device according to claim 4, wherein the combustors are attached to a rail attached to an interior surface of the combustion conduit casing.

6. The subsurface heating device according to claim 1, further comprising at least one sensor configured to provide data to a remote controller.

7. The subsurface heating device according to claim 1, wherein the combustors are independently controllable.

8. The subsurface heating device according to claim 1, wherein the combustion conduit casing is comprised of at least one material selected from the group consisting of steel pipe, stainless steel, inconel, and corrosion resistant alloys.

9. The subsurface heating device according to claim 1, wherein the combustion product gas outlet is configured to deliver samples of combustion product gases to a gas analyzer.

10. The subsurface heating device according to claim 1, wherein the combustion product gas outlet is configured to deliver combustion product gases to a gas treatment unit.

11. The subsurface heating device according to claim 1, wherein the combustion product gas outlet is configured to return at least a portion of the combustion product gases in a direction parallel to the combustion conduit casing.

12. A method for heating a subsurface zone comprising:

(i) creating an accommodation cavity for a subsurface heating device; said subsurface heating device comprising (a) a combustion conduit casing defining a combustion conduit; (b) at least two combustors disposed within the combustion conduit casing and characterized by an inter-combustor distance of at least one thousand feet and a combustion power of at least 3.41 million BTU per hour; (c) at least one fuel supply conduit configured to supply a combustible fuel to at least one combustor; (d) at least one oxygen supply conduit configured to supply oxygen to at least one combustor; and (e) a combustion product gas outlet;

(ii) installing the subsurface heating device within the accommodation cavity; and

(iii) operating the subsurface heating device.

13. The method according to claim 12, wherein the accommodation cavity is created in a hydrocarbon reservoir.

14. The method according to claim 12, wherein the accommodation cavity is created in a shale oil reservoir.

15. The method according to claim 12, wherein the accommodation cavity is created in a tar sands reservoir.

16. The method according to claim 12, wherein the fuel supply conduit provides liquid fuel to at least one combustor.

17. The method according to claim 12, wherein the fuel supply conduit provides natural gas to at least one combustor.

18. The method according to claim 12, wherein the fuel supply conduit provides heating oil to at least one combustor.

19. The method according to claim 12, wherein the combustion conduit serves as the oxygen supply conduit and provides compressed air to the combustors.

20. The method according to claim 12, wherein the thermal output of a first combustor is about 1.5 to about 2.5 times that of the thermal output of a following combustor.

21. A method for shale oil recovery comprising:

(i) creating an accommodation cavity for a subsurface heating device; said subsurface heating device comprising (a) a combustion conduit casing defining a combustion conduit, (b) at least two combustors disposed within the combustion conduit casing and characterized by an inter-combustor distance of about 1000 feet, and wherein the thermal output of a first combustor is 1.5-2.5 times that of the thermal output of a following combustor spaced at the inter-combustor distance of about 1000 feet, (c) at least one fuel supply conduit configured to supply a combustible liquid fuel to at least one combustor, (d) at least one oxygen supply conduit configured to supply oxygen to at least one combustor, and (e) a combustion product gas outlet;

(iii) operating the subsurface heating device.