US2994374A - In situ combustion process - Google Patents

Description

Aug. B9 391m F. W. CQAWFOR@ E11-AL 2:99593@ IN SITU COMBUSTION PROCESS Filed 00i- 28, 3.957

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A? TORNEYS United States Patent 2,994,374 IN SITU COMBUSTION PROCESS Francis W. Crawford and Joseph C. Trantham, Bartlesville, Ckla., assignors to Phillips Petroleum Company, a corporation of Delaware Filed Oct. 28, 1957, Ser. No. 692,857 13 Claims. (Cl. 166-11) This invention relates to a process for initiating in situ combustion in a stratum containing carbonaceous material and to a process for the recovery of hydrocarbons from such a stratum.

ln situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceous material is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move thru the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial proportion of the hydrocarbon in the stratum drives out and usually upgrades a substantial proportion of the remaining hydrocarbon material. One of the problems involved in the recovery of hydrocarbons by in situ combustion lies in the ditculty in initiating combustion of the carbonaceous deposit in the stratum and establishing a combustion zone of substantial extent. Usually the section of a stratum adjacent an ignition borehole is heated for an extended period, such as several days or even weeks, so as to raise the temperature of the section of stratum to a combustion supporting temperature for the carbonaceous material therein and, while the stratum is at this temperature, air or other oxygen containing gas is injected into the stratum so as to initiate combustion of the carbonaceous deposit.

In starting in situ combustion of oil and tar sands, etc., there has been considerable difiiculty in establishing a self-sustaining combustion at the producing well bore by inverse injection because the high temperatures developed destroy the heaters and other well equipment before the combustion front becomes sufficiently well established to propagate into the formation.

An object of the invention is to provide an improved process for the production of hydrocarbons by in situ combustion in a stratum containing carbonaceous material. Another object is to provide an improved process for initiating combustion in a carbonaceous stratum. A further object is to provide a process for initiating in situ combustion in a carbonaceous stratum which avoids the use of a heater in the ignition borehole. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

The broadest aspect of the invention comprises passing a combustible mixture of fuel and free oxygen-containing gas thru a carbonaceous stratum into an ignition borehole therein and igniting the combustible mixture as it enters the -borehole so that the mixture is burned on the surface of the borehole within the carbonaceous stratum, thereby heating the same to ignition temperature when free-oxygen-containing gas is present and, while the stratum is at ignition temperature, injecting free-oxygen containing gas into the stratum so as to ignite the hot carboneceous material and thereby initiate in situ combustion therein. In order to effect this type of ignition technique, injection boreholes closely surrounding the ignition borehole are drilled in the stratum and these are spaced in the range of about 6" to several feet from the ignition borehole. After the stratum around the ignition borehole is raised in temperature to about 500 to 600 F., or higher, by burning the combustible mixture in the ignition borehole and in the formation surrounding the same, in situ combustion of the carbonaceous material is readily Patented Aug. 1, 1961 initiated by contacting the hot material with air, or air enriched with oxygen. In accordance with one embodiment of the invention, the concentration of oxygen in the combustible mixture is increased so that excess oxygen over the stoichiometric amount required for combustion of the injected fuel is present in the injected gas so that ignition of the carbonaceous material gradually takes place and, as the combustion of carbonaceous material increases in rate, the flow of fue] gas or the concentration of this component in the combustible mixture is decreased and cut olf entirely so that the combustion feeds entirely on the carbonaceous material in the stratum. It is also feasible to maintain the flow of fuel gas constant and gradually increase the flow of air until the combustion of carbonaceous material is well established before cutting olf the ow of fuel gas.

In accordance with another embodiment of the invention, ignition of the hot carbonaceous material is initiated by direct injection of air or other free-oxygen-containing gas into the ignition borehole after cutting olf the flow of combustible gas thru the stratum. In this method of operating, combustion gas is removed thru the injection boreholes until the combustion zone approaches the vicinity of the injection boreholes. Direct injection of air may be continued so as to drive the combustion zone beyond the injection boreholes by sealing the injection boreholes and opening more remote boreholes spaced farther from the ignition borehole than the injection boreholes, in which case the remote boreholes function as producing wells from which the product hydrocarbon are produced by conventional methods.

A preferred method of operation comprises injecting air thru the injection boreholes until the combustion front approaches the points of air injection and, thereafter, sealing the injection boreholes and utilizing the remote boreholes as injection boreholes so that the combustion zone is moved thru the stratum countercurrently to the flow of air until it arrives at the remote boreholes. At this time, with continued injection of air, the combustion zone reverses itself in direction and moves back thru the stratum, the combustion feeding on the carbonized residue remaining in the formation after the rst (inverse) combustion wave passes thru the stratum. This second combustion wave is known as reflection burning and is described and claimed in the U.S. application of J. C. Trantham et al., Serial Number 529,916.

A 'more complete understanding of the invention may be had by reference to the accompanying drawing of which FIGURE 1 is a plan view of an arrangement of boreholes or wells suitable for effecting the invention; and FIGURE 2 is a section thru a stratum showing a suitable arrangement of equipment for effecting the invention.

Referring to FIGURE 1, an ignition borehole 10 is surrounded by four closely spaced injection boreholes 12 in a typical S-spot pattern and more remote boreholes 14 are spaced around the inner boreholes in a similar pattern.

In FIGURE 2, borehole 10 is provided with a casing 16 and tubing or conduit 18 for injection or withdrawal of fiuids from the borehole. Boreholes 12 which are spaced from borehole 10 a distance in the range of about 6 to several feet are each provided with casings 20 and a tubing 22. An additional conduit 24 connects with conduit or tubing 22 above the well head, however, this conduit may connect with casing 20. Fuel is introduced thru conduit 24 and air thru conduit 22 so that these gases are mixed within conduit 22 and are then passed to the hydrocarbon bearing stratum. Alternatively, the air may be injected thru conduit 2A and the fuel thru tubing 22. Remote boreholes or wells 14 are provided with tubing 26 for passage of iuid to and from the producing stratum as hereinafter described. All of the boreholes extend thru stratum 28 which contains carbonaceous material, such aS petroleum, tar, or other carbonaceous deposit to lbe produced in the process. Boreholes 14 may be spaced a distance from borehole in the range of about 40 to 50 feet to 1A or 1%; of a mile depending upon the permeability of the stratum and character of the carbonaceous deposit.

In producing hydrocarbons from stratum 28 in accordance with the process of the invention a combustible mixture of fuel gas and air is introduced to permeable stratum 28 thru the conduits leading into boreholes 12, boreholes 14 being sealed and `borehole 10 being open so that the combustible mixture passes thru the narrow section of stratum 28 between boreholes 12 and borehole 10. The combustible mixture is ignited in borehole 10 as it emerges from the stratum by an electric spark. a squib, a flame or pilot light, or by any other suitable means. The rate of ow of the combustible mixture is adjusted so as to cause the burning to take place on the wall of the borehole .in stratum 28. When the section of stratum adjacent the borehole becomes sutliciently hot, ameless combustion of the mixture takes place within the permeable stratum and therefore heats the stratum to a substantial depth. When the temperature of the stratum in a section thereof around borehole 10 reaches combustionsupporting temperature of the carbonaceous material in the stratum in the presence of air, the flow of fuel is cut off so that the hot formation is contacted with free oxygen, thereby igniting the hot carbonaceous material and causing the resulting combustion front 30 to move thru stratum 28 toward boreholes 12 in an annular and cylindrical conformation around borehole 10. As the combustion zone 30 approaches boreholes 12, ow of air thru these boreholes is terminated, and air is injected into boreholes 14 thru conduits 26, the air thus injected passing thru stratum 28 to the hot zone 30 in which combustion has temporarily ceased. During movement of the combustion front from the area of boreholes 12 to the area of boreholes 14, production can be recovered thru boreholes 12 or thru boreholes 10 by sealing oil? boreholes 12. As the air again arrives in the hot section of the stratum, combustion is again initiated because the ternperature of the hot zone is above the ignition temperature of the carbonaceous material in the stratum and now the re-established combustion zone is moved countercur- -rently to the flow of air toward the remote injection boreholes 14. I

It is also feasible to utilize direct injection of air thru conduit 18 in borehole 10 to initiate the combustion of the stratum zone around ignition borehole 10, after heating the stratum as previously discussed, and to move the combustion zone 30 toward boreholes 12. In formations or strata which are not susceptible to plugging during direct injection of air, the combustion zone 30 may be driven thru the stratum to boreholes 14 by injection of air thru borehole 10. In this type of operation boreholes 14 serve as production boreholes with the produced hydrocarbons being recovered by conventional methods thru tubing 26. However, movement of the combustion zone 30 thru the stratum `by inverse air injection thru boreholes 14 is preferred and, during this type of operation, borehole 10 functions as a producing borehole with the produced hydrocarbons being recovered thru tubing 18.

In field operation it has been found necessary to inject air at a minimum rate of 50 s.c.f.h. per square foot of vertical cross section of stratum in order to maintain and advance the combustion zone in a carbonaceous stratum. For initiation of combustion around an ignition borehole, the area of the wall of the borehole is considered to be the initial area and the area to be considered thereafter is the area of the advancing combustion front, which is of course expanding in a successful drive. The upper limit for eicient operation has never been reached due to the restrictive effect of low permeability of the stratum and compressor limitations. However, flow rates of 300 to 400 s.c.f.h. per square foot of area are effective in maintaining and driving the combustion front thru more permeable strata.

Because of the high temperature in borehole 10 during ignition and the early phases of the combustion process, it is advisable to install tubing 18 in this ignition borehole after the temperature in the producing stratum adjacent the borehole has dropped so that damage to the equipment is avoided.

lIn carbonaceous strata of low permeability, fracturing and propping in conventional manner may be practiced in order to improve the permeability to gas flow therethru. In many strata the permeability is suiciently high that suitable gas flow rates result from injection gas pressures within the capacity of conventional compressors.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

We claim:

l. A process for initiating in situ combustion in a stratum containing combustible carbonaceous material which comprises injecting a combustible gaseous mixture containing free-oxygen and fuel into said stratum thru a ring of boreholes therein surrounding a central ignition borehole therein, said ring of boreholes being a distance in the range of 6 inches to several feet from said ignition borehole so as to pass said mixture thru said stratum to said ignition borehole and permeate a complete annulus of said stratum around said ignition borehole; igniting said mixture as it enters said ignition borehole so as to burn same' adjacent the wall thereof; continuing injection and burning so as to burn said mixture in said annulus and heat said annulus to the ignition temperature of said carbonaceous material in free-oxygen-containing gas; while said annulus is at said temperature, igniting the hot carbonaceous material therein by contacting same with free-oxygen-containing gas.

2. The process of claim l wherein the flow of fuel into said stratum is terminated and the ow of free-oxygencontaining gas is continued so as to ignite said carbonaceous material.

3. The process of claim 1 wherein flow of fuel is diminished thereby increasing the concentration of freeoxygen in said mixture so as to ignite said carbonaceous material.

4. The process of claim 1 wherein the concentration of free-oxygen-containing gas in said mixture is increased to ignite said carbonaceous material.

5. The process of claim 1 including the steps of terminating the flow of free-oxygen containing gas thru said ring of boreholes after in situ combustion has been established and injecting free-oxygen-containing gas into said stratum thru a more remote borehole so as to pass same to the combustion zone and advance said zone countercurrently to the ow of said gas.

6. The process of claim 1 wherein air is injected into said stratum thru said ignition borehole after heating said annulus to combustion supporting temperature to ignite said carbonaceous material and the resulting annulus combustion zone is driven radially outwardly from said ignition borehole.

7. The process of claim 1 wherein air is injected thru said ignition borehole to ignite said carbonaceous material.

8. A process for producing hydrocarbons from a stratum containing combustible carbonaceous material which comprises injecting a combustible mixture of fuel gas and free-oxygen-containing gas into said stratum thru a plurality of injection boreholes surrounding an ignition borehole and spaced therefrom a distance in the range of 6 inches to several feet so as to cause said mixture to ow to said ignition borehole; igniting said mixture as it enters said ignition borehole so as to burn same adjacent the wall thereof; continuing the injection and burning so as to cause burning of said mixture in an annulus of said stratum around said ignition borehole, thereby heating aannam said annulus to the ignition temperature of said carbon- -aceous material in said free-oxygen-containing gas; while said annulus is at said temperature, igniting the hot carbonaceous material therein by injecting free-oxygen-containing gas into same and continuing the injection so as to extend the resulting combustion zone radially outwardly from said ignition borehole to the area of said injection boreholes; thereafter passing free-oxygenontaining gas thru said stratum to the advanced combustion zone from a plurality of boreholes more remote from said ignition borehole than said injection boreholes; and recovering hydrocarbons driven from said stratum by said combustion.

9. The process of claim 8 wherein said free-oxygen-containing gas comprises air.

10. The process of claim 8 wherein the ignition of said carbonaceous material is effected by injecting air into said stratum thru said injection boreholes and produced hydrocarbons are recovered thru said ignition borehole.

11. The process of claim 8 wherein the ignition of the carbonaceous material in said annulus is effected by injecting air into said annulus thru said ignition borehole after heating said annulus to combustion supporting temperature.

12. A process for producing hydrocarbons from a stratum containing combustible carbonaceous material which comprises injecting a combustible mixture of fuel gas and free-oxygen-containing gas into said stratum thru a plurality of injection boreholes surrounding an ignition borehole and spaced therefrom a distance in the range o inches to several feet so as to cause said mixture to to said ignition borehole; igniting said mixture as it ent said ignition borehole so as to yburn same adjacent wall thereof; continuing the injection and burning u: a substantial section of said stratum around said ignit borehole is heated to the ignition temperature of s carbonaceous material in said free-oxygen-containing j and said mixture is burning in an annulus of said strati 10 around said ignition borehole; while said annulus is said temperature, igniting the hot carbonaceous mater therein by injecting free-oxygen-containing gas into sal thru said ignition borehole; continuing the injection said gas thru said ignition borehole so as to drive 1 resulting combustion zone thru said stratum to a plural of surrounding boreholes beyond said injection borehol and recovering hydrocarbons driven from said strati by the combustion thru the remote boreholes.

13. The process of claim 12 wherein said free-oxyge containing gas comprises air.

References Cited in the tile of this patent UNITED STATES PATENTS 2,642,943 Smith June 23, 19: 2,793,696 Morse May 28, 191 2,880,803 Parker Apr. 7, 19!

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