Situ coal combustion heat recovery method

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be supplied from the surface of the coal bed, on both

STU COAL COMBUSTION HEATRECOVERY sides of the horizontal pipes, so that combustion can be METHOD carried on along the position of the horizontal pipes. This is a divisional of application Ser. No. 631,108, The horizontal pipes are supplied with water at the filed Nov. 12, 1975, and now U.S. Pat. No. 4,018,279. outcrop. The water will be heated and converted to BACKGROUND OF THE INVENTON steam, and can be carried to the surface through a verti cal pipe drilled from the surface, and adapted to inter 1. Field of the Invention sect the horizontal pipe or pipes. A plurality of such This invention lies in the field of energy recovery. horizontal pipes can be directed in different directions, More particularly, it is concerned with the in situ com 10 from the outcrop to a central vertical pipe, or the use of bustion in shallow thin beds of coal, at shallow depths several vertical pipes can be used. below the surface of the earth. Still more particularly, Another object of this invention is to provide a means the invention lies in the field of the combustion of the for utilizing the heat resulting from the combustion of coal by supplying air to the coal seam from the surface coal in situ in normally unminable coal veins, at a shal and utilizing the heat generated by the coal combustion 15 low depth below the surface. This is particularly useful to generate steam which is utilized at the surface of the where there is a water bearing formation directly ground, utilizing the earth (coal and surrounding rocks) above, and in contact with, the coal seam, or where as the furnace in the same manner as if it were geother there is a hydrocarbon bearing formation, where the oil mal. is of such high viscosity that it cannot be produced by In one application, this invention lies in the field of 20 ordinary means. Here, by causing an artificial combus outcropping seams of coal where horizontal conduits tion, in situ, in the coal seam, the heat of combustion can be drilled and placed within the coal seam to a passes upward into the overlying aquafer and the water specific point, where they join a vertical pipe. Water is heated and converted to steam, and is produced by supplied to the horizontal pipes at the outcrop is heated one or more vertical pipes drilled into the water sand within the pipes by the combustion of the coal around 25 from the surface. The natural flow of water in the the pipes, and steam thus formed is carried up the verti aquafer can replenish the water lost by steam, or addi cal pipe to be used to drive a turbine generator system. tional vertical boreholes can be provided for the intro A still further application lies in the field of utilizing duction of water under pressure into the aquafer. coal seams thin or thick, shallow or deep overlain by If the overlying bed above the coal seam carries a water sands or by hydrocarbon bearing formations 30 very viscous oil, then the heat of combustion, after due whereby fires are started within the coal seam, and the course of time for the transmission of the heat vertically heat of combustion is then communicated to the overly into the overlying formation, will cause the viscosity to ing aquifer or oil formation. If the overlying bed is an be reduced to a point where it can be produced by gas aquifer then steam will be formed which can be carried drive or water drive. If water is used then there will be to the surface to drive an electric generator system. If 35 a combination of water and steam drive to force the oil, the overlying bed contains viscous oil, then the heat will now of reduced viscosity, out through appropriate pro cause the oil to be reduced in viscosity sufficiently so ducing wells.

that it can be driven by a steam drive, or by pumping In a third embodiment, the hot gaseous products of water into the seam, which in view of the fire in the coal combustion are carried to a vertical borehole in which seam will turn to steam, which will drive the oil to the is installed a coiled pipe, through which water is passed surface to be collected. from the surface. The water is heated by heat transfer to Still a further application involves the burning of coal the pipe from the hot gases as they flow up the bore in situ and carrying the hot gaseous products of com hole. The water is converted to steam which flows to a bustion up a vertical borehole past a coiled pipe carry turbine, etc.

ing water. Heat is transferred to the water by convec 45 ; : BRIEF DESCRIPTION OF THE DRAWINGS tion, forming steam which is carried to a turbine, etc.

2. Description of the Prior Art These and other objects and advantages of this inven Subsurface combustion of coal has been carried on in tion and a better understanding of the principles and nature, by fires of unknown origin which were started details of the invention will be evident from the follow many years ago and are burning continuously to the 50 ing description taken in conjunction with the appended present date. Also oil companies, coal companies and drawings, in which:

government research laboratories have investigated FIGS. 1 and 2 illustrate in vertical cross-section and many ways of underground combustion of coal, so as to plan views, one embodiment of this invention. utilize the products of combustion directly, as a coal FIGS. 3 and 4 illustrate two views, in vertical section gasification system. However, none of these methods 55 and in plan view, of a second embodiment of this inven have become practical as of the present date because of tion.

one or more basic difficulties. FIGS. 5 and 6 illustrate two views of a third embodi ment.

SUMMARY OF THE INVENTION

It is a primary object of this invention to provide a DESCRIPTION OF THE PREFERRED rather specialized system for the utilization of the heat EMBODIMENTS of in situ combustion of coal in shallow, thin, veins, Referring now to the drawings, and in particular to seams or formations. The situations are specialized in FIGS. 1 and 2, there are shown two views of one em that they are not generally applicable, but are applicable bodiment of this invention. In FIG. 1 there is a vertical and are particularly useful in specialized areas, such as, cross-section through the earth, showing the surface of for example, where the thin, shallow coal beds outcrop. the earth 10, an upper portion of the earth 12, resting on The beds can be drilled and pipes inserted into the coal a thin, more or less horizontal seam or vein, of coal 14. bed, in a more or less horizontal direction. Air holes can Because of the surface topography, and the hillside 16,

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there will be an outcropping at 18 of the coal seam 14. Referring now to FIGS. 3 and 4, there is shown a Because of the availability of the outcrop 18, one or second embodiment of this invention which comprises a more horizontal boreholes 20 are drilled into the coal portion of the shallow surface of the earth containing a more or less horizontal coal seam 14 overlain by a po seam, parallel to the plane of the coal seam, to a selected point. There is a vertical borehole 22 drilled at said 5 rous water formation 13, and a portion of the earth 12 selected point, and a vertical pipe 26 is inserted into that up to the surface 10. If the porous zone 13 contains borehole, and is cemented into place around the inner water, then the combustion of the coal in bed 14 can end of pipe 20, inserted into the horizontal borehole. serve to heat the water in the zone 13 to form steam, This makes a continuous conduit from the outcrop 18, which can be utilized in a turbine, etc. For example, by means of the pipe or conduit 20 into the portion 28 of 10 boreholes 30 are drilled from the surface down to, and the borehole 22, and a pipe or casing 26 cemented into, the coal seam. The lower parts of the casing of around the inner end of the pipe 20 and around the these holes would be perforated or slotted for the pas vertical pipe 26. sage of air down the casing and into the coal seam. The The cementing of the joints between the horizontal coal would be ignited by methods well-known in the art and vertical pipes 20 and 26 in the region 28 permits the 15 and combustion would be carried out within the coal flow of water into the pipe 20 by means of a flow line 40 seam 14. The heat of combustion would be communi and arrows 42, for example, and the conversion of the cated vertically to the overlying porous zone 13, which water in the pipe 20 into steam in the region of the is filled with water. Because of the heat transmitted, the burning coal in the areas 46. The steam then flows along water would be converted to steam and steam would the pipe 20 and up the pipe 26 in accordance with ar 20 pass in accordance with arrows 58 from the porous sand rows 44 to the surface where the steam is supplied to a 13, up one or more casings 56 drilled from the surface of turbine electric system 34, 36 and the power output of the earth down into the water sand, and appropriately the generator goes by power line 38 to a point of appli cased, so that the steam would pass upward and into a cation. turbine 34 driving a generator 36 to provide power for A plurality of vertical boreholes 30 are drilled from 25 delivery by the power line 38.

the surface down to, and into, the coal seam 14 in the It may be desirable to provide additional cased holes vicinity of the horizontal pipes 20. As indicated by 54 drilled from the surface down into the zone 13, to arrows 19 air is supplied under pressure to pipes within supply water under pressure, in accordance with ar the boreholes 30, and the air passing down through the rows 56. This water would provide sufficient pressure pipes into the coal seam maintain the combustion of the 30 in the water zone 13, to create enough back pressure to coal. The heat of combustion of the coal serves to con maintain a high pressure and steam, for efficient turbine vert the water in the pipe 20 into steam. The borehole operation. No detail is supplied as to the turbine genera 30 will generally be lined with casing which passes into tor and power system, since that art forms no part of and through the coal seam. The lower part of the casing this invention.

will generally be perforated, or slotted, so as to permit 35 In FIG. 4 is shown a plan view of the surface 10 the air to pass out into the coal area, but to prevent indicating a plurality of pipes 56 representing the pipes clinker and other products of the combustion of the through which steam is supplied from the subsurface, to coal, from stopping up the cross-section of the casing. the turbine. Also shown are a plurality of spaced air As shown in FIG. 2, plurality of horizontal bores bore holes 30 through which is supplied air for the 20A, 20B, 200 etc. and corresponding pipes in these combustion of the coal. In addition, pipes.54 are pro bores, can be formed from different directions into or vided from the surface, to introduce water into the zone from the common vertical borehole 22 shown in FIG. 1. 13 to replenish the water supplied as steam to the tur However, it is possible also to have a separate vertical bine if necessary. Although not shown, it is possible to borehole 22 and casing 26 for each of the horizontal condense the steam issuing from the turbine and pump it pipes 20A, 20B, and 20C. The vertical air holes 30 are 45 back down into the formation 13 with the resulting shown clustered around the position of the pipes 20 so conservation of water.

as to completely burn all of the coal in the vicinity of In FIG. 3, it is further considered that if an oil zone or the pipes. an oil bearing formation 13 overlies the coal seam, and When all of the coal has been burned in the region if the oil is of a viscous nature, which is not normally around one of the pipes, an additional bore 20, and 50 produceable, it is possible, where there is coal below the corresponding pipe would be made in another portion oil formation, to burn the coal, and by means of the heat of the coal seam where the coal has not yet been burned, of combustion, to heat the oil, and thereby reduce its so that the combustion can be carried out in the new viscosity, so that it can be pumped to the surface. One area, to keep pace with the advance of the burning way of getting more fluid oil to the surface would be by front. Of course, the original air holes can be used and 55 means of pumping a gas down the pipes 54 to create a Mor additional air holes drilled as needed. flow pressure on the oil toward the center of the field. As shown in FIG. 1 no provision has been made for The oil would then flow up the pipes 56. Of course, in the passage out of the coal seam, of the products of the case of an oil formation the vertical pipes 56 would combustion. If this becomes necessary, it may be desir carry oil and would not be connected to a turbine gen able to drill one or more additional boreholes, for the 60 erator system as shown.

passage of the products of combustion from the fires in It is also possible to force the flow of thinned oil to the regions 46 to pass through the porosity of the coal the producing pipes 56 by flowing water into the pipes into the outlet boreholes, to the surface, where the hot 46 at the surface, and causing the water to convert to gases can be utilized in heat recovery devices, or boil steam due to the heat provided by the burning coal. ers. If there are any unburned combustibles in the exit 65 This steam would then provide a very effective sweep gases, they may be supplied, with the combustion air, in ing or driving fluid to carry the oil into the central a boiler so as to utilize the further heat value of the portion of the field and to the outlet pipes. 56. In this products of combustion. type of situation, it may be desirable to surround the

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plurality of oil pipes 56 with a ring of input wells 54 bustion then pass out through the borehole and deliver arranged in a circle around the pipes 56, for the flow of their heat to the pipes 76 and 78 by convection and then drive fluid into the oil formation to drive the oil toward pass out to the atmosphere or to some other heat recov the center of the field. ery means such as is well known in the art. Referring now to FIGS. 5 and 6, there is shown a FIG. 6 shows in planned view the spacial arrange third embodiment of this inventionin which there is a ment of the air holes in a pattern surrounding the central coal seam 14 at a selected depth in the earth. There are bore holes 72.

a plurality of vertical boreholes to which air is supplied Other elements of FIG. 5, which have not been spe under pressure 30 and which boreholes intersect the cifically described, are similar to the corresponding coal seam and the airflows in accordance with arrows 10 elements 82 so that after the coal is ignited it will continue to burn thereforehaving the same numbers in FIGS. 1 and 3 and need no further description.

and the hot products of combustion will pass through No detail has been provided on the methods or appa the pores of the coal seam and possibly through radial ratus required for drilling the horizontal and vertical bores 70 to a central relatively large diameter vertical borehole 72. This will be cased or lined with concrete 15 boreholes,

and for setting casing, etc. since all of this art in the oil industry. While we have talked with cement as is customary. There is a continuous of "more or less' conduit which consists of pipe 75 which passes con that the coal seamshorizontal can be coal seams, it will be clear tilted to any angle of dip in densed steam from the turbine 34 in accordance with which they may be found. It may be valuable in that arrows 79 downward through a first pipe 76 which joins case to design the drilling of the pipes a second vertical pipe 78 at the bottom of the borehole. 20 in such a direction as to take advantageand of the air holes the dip of the

The condensed steam goes down as hot water through coal seam.

pipe 76 and is heated and passes upward in accordance with arrows 80 through the second pipe 78 in which the in While it is old in the art, to generally burn coal in situ the earth, and to recover its products of combustion, water is heated to steam and the steam then goes into we have invented particular embodiments which utilize the turbine 34 where it drives the turbine and the steam 25 is then condensed and passes as hot water through the the(a)geological the coal environments in which:

seam crops out, and pipe 75 and back down into the borehole. (b) where the coal seam is overlain by either a water The hot product of combustion 84 pass up the bore formation or a formation containing very viscous hole in accordance with arrows 84 and 86 and by con oil, which normally cannot be produced except by vection heat the pipes 76 and 78 and thereby heat the 30 liquid water inside the pipes turning the water into being heated and then driven through the forma steam so as to drive the turbine. tion.

The products of combustion 86 then vent at the sur Although the physical requirements of these two face end of the borehole and are utilized in any way that embodiments are rather specific, they do have consider may be desired such as for example passing to a waste 35 able advantage in that:

heat boiler for further utilization of the heat content of (a) they eliminate the mining of the coal which would the products of combustion. be unprofitable, particularly for thin seams; The down going pipe 76 is shown as a sinuous pipe. It (b) they eliminate waste in the mining of coal; will undoubtedly be advantageous to coil that pipe 76 (c) they eliminate the dust pollution, etc.; possibly as helix surrounding the vertical pipe 78 thus (d) they eliminate the transportation of coal; providing a greater surface area of pipe for heat transfer (e) they can recover all available energy and usuable from the upwardly flowing gases to the pipe and to the gases;

water inside the pipe. No further detail of the construc (f) the methods are applicable to any depth of the coal tion of these pipes is needed since this is a common type seam, although the economics would indicate that of situation where such as in the convection section of a 45 they are best for relatively shallow coal seams; heater where hot gases are passed over a plurality of (g) they can utilize thin coal seams that would never pipes for heating a liquid in the pipe thereby recovering be mined;

the waste heat that still remains in the products combus (h) the burning of the coal can be regulated by the tion just prior to their issuance through a stack to the amount of air pumped to obtain all the energy that atmosphere. 50 can be captured with the described facilities; This embodiment as shown in FIGS. 5 and 6 is differ (i) the energy can be transmitted as electrical energy, ent from either of the other two body embodiments the cheapest form of energy transportation. shown in FIGS. 1 and 2 and in FIGS. 3 and 4. In the While the invention has been described with a certain former the pipe containing the water passes through the degree of particularity it is manifest that many changes zone where the coal is actually being burned and is 55 may be made in the details of construction and the ar therefore in a hotter environment and therefore there is rangement of components. It is understood that the a greater rate of heat transfer. In the embodiment invention is not to be limited to the specific embodi shown in FIGS. 3 and 4, the water is in a planar contact ments set forth herein by way of exemplifying the in over the surface of the coal seam and heat is transferred vention, but the invention is to be limited only by the from the hot coal into the water sand and from there to 60 scope of the attached claim or claims, including the full the water where it is converted to steam which passes to range of equivalency to which each element or step the turbine. Here in this embodiment the combustion of thereof is entitled.

the coal provides a continuous stream of products com What is claimed:

bustion which flows from the points of air inlet 82 1. In a coal seam in the earth, the method of burning through the burning zone and through a volume either 65 the coal in situ and recovering the heat of combustion, drilled or otherwise formed 70 or through porous re comprising:

gions which have previously been burned. The hot (a) forming at least one horizontal borehole into said products of hot gases which are products of the com seam, substantially parallel to the plane of said

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seam, to a selected point, and inserting a first casing said selected point, inserting a plurality of first casings, into said one horizontal borehole; and joining said first casings to said second casings. (b) forming from the surface at least one first vertical 3. The method as in claim 2 including the steps of borehole to intersect said one horizontal borehole drilling a plurality of vertical boreholes at a multiplicity at said selected point, and inserting a second casing of selected points, placing second casings in each verti into said one first vertical borehole, and joining cal borehole, and joining each second casing to one of said first and second casings; said first casings.

(c) forming a plurality of horizontally spaced second 4. The method as in claim 3 including the steps of supplying vertical boreholes from the surface into said coal 10 ing the steam water to each of said first casings and utiliz seam in the area of said one horizontal borehole, produced at said second casings. 5. The method as in claim 1 including the additional and supplying air under pressure through said steps of:

boreholes to said coal seam. (g) drilling at least one third vertical borehole from (d) igniting said coal in the vicinity of said air bore the surface into said coal seam in the area of said holes and continuously burning said coal; 15 second vertical boreholes, whereby the products of (e) flowing water into said first casing, whereby said combustion of said coal will flow up said third water will be converted to steam, and will flow up vertical borehole to the surface; and said second casing; and (h) utilizing said products of combustion. (f) flowing said steam into a steam power utilization 6. The method of claim 1 wherein said coal seam e2S, 20 crops out and said horizontal borehole extends from 2. The method as in claim 1 including the additional said outcrop into said seam.it k it steps of drilling a plurality of horizontal boreholes to