US3398793A - Process for rapid reignition of in situ combustion - Google Patents

Description

United States Patent "ice 3,398,793 PROCESS FOR RAPID REIGNITION OF IN SITU COMBUSTION Harry W. Milton, Jr., Denver, Colo., assignor t0 Marathon Oil Company, Findlay, Ohio, a corporation of Ohio No Drawing. Filed May 27, 1966, Ser. No. 553,317 8 Claims. (Cl. 166-39) ABSTRACT OF THE DISCLOSURE The present invention comprises a method for reigniting a combustion zone in an organic-bearing formation comprising in combination the steps of injecting into a formation having a burned-out zone and an unburned organic-bearing zone, a quantity of a flammable gaseous mixture comprising oxygen and a fuel at least sufficient to substantially fill said burned-out zone, thereafter igniting said flammable mixture through at least one well located in said burned-out zone and located at least 10 feet from the interface between said burned-out zone and said organic-bearing zone, so as to produce a gas combustion front which passes through a portion of the burned-out zone and reignites the carbonaceous material in place at the interface, between said burned-out zone and said organic-bearing zone.

The present invention relates to new methods for the reignition of the combustion of organic material in underground formations, and is particularly useful in connection with the recovery of oil by in situ combustion methods.

The well known in situ combustion process involves the ignition of carbonaceous material in a stratum with organic content around a wellbore as a result of heating and injection of air. A combustion front is established which moves radially out from the wellbore with continued air injection. Recovery of hydrocarbons and combustion gas products takes place through one or more other wells.

A difficulty with in situ combustion oil recovery operations is that the combustion front may be extinguished, either because of inadvertent reductions in the air supply, in homogeneities in the formation, or other reasons. When the combustion front is so extinguished, reignition can be extremely difficult. Previous attempts at solving this problem have included the flooding of the burned-out zone with flammable petroleum fractions, a process requiring large amounts of such flammable fluids with considerable resultant expense and delay. Another method is that of US. 2,793,697 which heats the zone adjacent to the injection well to a temperature of above 1000 F. then injects nonflammable gases to move the heat from the hot zone outward from the wellbore, then begins to inject flammable gases and immediately burns them close to the wellbore and continues this process cyclically so as to force a hot zone outward from the wellbore until this zone reaches the interface between the burned-out and the unburned-out portions of the formation at which point the oil in place is reignited. As stated in US. 2,793,697, this process requires many days to ignite an interface located within ten feet of the wellbore.

In contrast, the present invention permits the reignition of interfaces where combustion died out at much greater distances from the wellbore, and accomplishes this reignition -very rapidly without the need for the repititious cyclic operation described above. Further, the present invention utilizes gaseous hydrocarbons which can be natural gas or other gaseous fuels readily available in most oil fields.

The present invention involves the injection of a flam 3,398,793 Patented Aug. 27, 1968 mable mixture of oxygen-containing gas, preferably air, and fuel, preferably gaseous hydrocarbons, into the burned-out portion of the formation, preferably either through a well or wells originally used for ignition or an original producing well or Wells. When an amount of the combustible gaseous mixture which is at least suflicient to fill the burned'out area has been injected, the mixture is conventionally ignited. Ignition of such a flammable gaseous mixture in the burned-out portion of a forma tion according to the invention, causes a gas combustion front which can travel rapidly outward from the point of ignition toward the interface between the burned-out area and the nonburned out area. On reaching this interface the gas combustion front ignites the carbonaceous material in place and oil recovery may then be reinstituted according to the conventional in situ combustion techniques.

The preferred fuels for use with the present invention are the natural gas commonly present in oil and gas fields, methane, ethane, propane, butane, acetylene, ethylene, propylene and butylene.

While the point of ignition of the hydrocarbon gases must be in the burned-out portion of the formation, the flammable gas mixtures can be injected at any of a number of points. A convenient point for injection would be an existing air injection well if combustion cocurrent to the direction of gas flow is desired. An existing production well normally used for Withdrawal of production gases or recovered hydrocarbons would be the point of injection if countercurrent combustion is to be utilized. However, a special well may be drilled for the purpose where desired. Where the organic-bearing formation is sufficiently permeable to the flow of gas, a gas mixture may be injected at a point within the non-burned out portion of the formation.

The pressure under which the flammable gas mixture is injected will preferably be that normally used for injection of air into the formation. In most cases, the approximate volume of the burned-out area will be known and the quantity of gas necessary to fill the burnedout area and sustain the gas combustion front can be readily calculated. However, in some instances it may be necessary to try successively increasing quantities of flammable gas mixture and increasing gas flow rates until the oil in place has been reignited. Because of differences in the volumes, shapes, depths and reservoir characteristics involved in the formations in which reignition is to be accomplished, the various parameters necessary to cause movement of the combustion front according to the present invention will vary relatively widely. For example, the injection pressure used for injecting the flammable gas mixture will be dependent on the permeability of the formation and also will be limited by the strength of the overburden, i.e. the depth' of the well. As a rough indication, the pressure will generally be from one to about 2,000 atmospheres with pressures of from 10 to about 1,000 atmospheres being more preferred, and pres sures of from about 20 to about atmospheres being most preferred in the most commonly encountered types of reservoirs.

Also, flow rates of the flammable gas mixture will vary greatly according to the formation differences given above. Generally, fuel gas-air fluxes will be from about 10 to about 2,000, preferably from 100 to about 1,000 and most preferably from 200 to about 800 standard cubic feet per hour per square foot of reservoir formation. The exact composition of the flammable gaseous mixtures can also vary Widely according to the results desired and the reservoir involved. In general, however, a stoichiometric mixture of oxygen and combustible gas will be injected into the formation. Also for better control of the com- 3 bustion front, it may be desired to use a catalyst in con junction with process as described in US. 3,127,935.

The reignition of the oil in place can be determined by analysis of the combustion gases emitting from a production well.

As soon as reignition has been accomplished, normal air feed is resumed and hydrocarbon recovery proceeds as before the in situ combustion front was extinguished.

While the present invention may be employed to establish reignition at interfaces which are very close to the well through which reignition is established, its advantages will be more apparent when such interfaces are located more than ten feet or even more than one hundred feet away from the well used for reignition.

The following example will further illustrate the invention, but should not be taken as limiting it. The process of the invention is adaptable to a wide variety of modifications and variations which will be obvious to a person skilled in the art, from the reading of this disclosure, and all such obvious modifications and variations are to be taken as included within the scope of the claims appended hereto.

EXAMPLE In situ combustion is established in a strata of oil bearing sandstone and continues with normal injection of the air and production of hydrocarbons and combustion gases until the oil combustion front forms a roughly 100 foot radius circle concentric with the air-injection 'Well. At this time, due to an insufficiency in the air injected, the combustion front is extinguished. Rapid reignition of the oil combustion front is accomplished by injecting into former production wells a mixture of approximately 9.5% by volume methane with 90.5% by volume air. Injection is continued until the entire burned-out portion of the reservoir is filled with the flammable methane-air mixture. An electric igniter located at the bottom of the previous air injection well is then actuated causing ignition of the methane-air mixture. A gas combustion front radiates rapidly outward from the base of the air injection well, passing through the porous burned-out portion of the rock formation. On reaching the interface between the burnedout and the non-burned-out portions of the formation, the high temperatures of the gas combustion front reignites the carbonaceous material in place. When it is observed from the analysis of combustion products that the in situ oil combustion process has been reestablished, then the methane-air supply may be shut olf. Air injection is then continued as before, but perhaps at a somewhat greater flow rate and the production of hydro- 4 carbons and combustion gases from the withdrawal wells resumes normally.

What is claimed is:

1. A method for reigniting a combustion zone in an organic-bearing formation comprising in combination the steps of injecting into a formation having a burned-out zone and an unburned organic-bearing zone, a quantity of a flammable gaseous mixture comprising oxygen and a fuel at least sufiicient to substantially fill said burned-out zone, thereafter igniting said flammable mixture through at least one well located in said burned-out zone and located at least 10 feet from the interface between said burned-out zone and said organic-bearing zone, so as to produce a gas combustion front which passes through a portion of the burned-out zone and reignites the carbonaceous material in place at the interface, between said burned-out zone and said organic-bearing zone.

2. The process of claim 1 wherein the gaseous mixture consists essentially of a combustible mixture of air and hydrocarbons have from 1 to around 4 carbon atoms; and wherein flammable mixture continues to be fed into the burned-out zone after initial ignition of the flammable mixture.

3. The process of claim 2 wherein the hydrocarbons comprise a major portion of methane.

4. The process of claim 2 wherein the hydrocarbons comprise a major portion of ethane.

5. The process of claim 2 wherein the hydrocarbons comprise a major portion of acetylene, ethylene or propylene.

6. The process of claim 2 wherein the flammable gaseous mixture is injected through at least one well located in the burned-out portion of the formation.

7. The process of claim 2 wherein the flammable gaseous mixture is injected through at least one Well located in the organic-bearing non-burned-out portion of the formation.

8. The process of claim 1 wherein the organic-bearing formation is selected from the group consisting of tarsands, oil shale, and liquid petroleum-bearing formation.

References Cited UNITED STATES PATENTS 2,793,697 5/1957 Simm et a1. 166-39 2,901,043 8/1959 Campion et al. 166-11 3,055,422 9/1962 Schleicher et a1. 166-11 3,127,935 4/1964 Poettmann et a1. 166-11 STEPHEN I. NOVOSAD, Primary Examiner.