US3520363A - Recovery of hydrocarbons from a subterranean formation by a combination of in situ combustion and water flood - Google Patents

Description

United States Patent RECOVERY OF HYDROCARBONS FROM A SUB- TERRANEAN FORMATION BY A COMBINA- TION OF IN SITU COMBUSTION AND WATER FLOOD Charles L. Bauer, New York, N.Y., assignor to Texaco Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed June 19, 1968, Ser. No. 738,118

Int. Cl. E21b 43/24 U.S. Cl. 166251 8 Claims ABSTRACT OF THE DISCLOSURE Improved recovery of hydrocarbons from subterranean hydrocarbon-bearing formations is effected by a combination of in situ combustion and water injection. In situ combustion is caused to occur in random pockets in a subterranean formation, followed by water injection thereinto to scavenge the generated heat as part of a water drive and thereby produce additional hydrocarbons contained therein. During injection of a combustion-support ing gas into the formation, effluent gases are monitored at output wells until the carbon dioxide content of the effluent gases exceeds 4% and the percent of oxygen accounted for has increased to about 50% to indicate when in situ spontaneous ignition of hydrocarbons has occurred.

FIELD OF THE INVENTION This invention relates to an improved process for the recovery of hydrocarbons from subterranean hydrocarbon-bearing formations. More particularly, this invention relates to a method which exploits favorable recovery conditions produced by conducting an in situ combustion operation prior to water injection.

DESCRIPTION OF THE PRIOR ART One of the newer developments for the recovery of hydrocarbons from subterranean hydrocarbon-bearing formations has been by in situ combustion operation. In the conventional method of applying this process, the hydrocarbon-bearing formation is penetrated by an injection well and one or more offset wells. Air is provided to the formation via the injection well, and combustion of the hydrocarbons adjacent the well bore in the forma tion is initiated by any one of the many accepted means, whereby a combustion zone is created in the form of a narrow cylindrical zone immediately surrounding the well bore. The air injection is continued until the combustion zone is driven radially from the injection well toward one or more offset wells, and the resulting thermal drive forces the displaced hydrocarbons in the formation toward one or more offset wells by direct or reverse drive.

More recent developments have created a thermal drive based upon a combination comprising subjecting the formation surrounding the well bore to in situ combustion by conventional means and conducting the combustion front radially outward more or less uniformly into the hydrocarbon-bearing formation to allow the generated heat to be accumulated in the formation, whereupon the air injection is terminated causing the combustion to cease. The air injection well is then converted to a water injection well, making it possible to flood the formation with water. The water injected into the formation absorbs the residual heat in the formation behind the front, and because of the nature of the heat capacities and thermal conductivities of water, it becomes an efficient transfer medium of this heat to the rest of the formation. The thermal drive is therein moved a further distance outwardly resulting in displacing and forcing additional hydrocarbons toward the offset production wells.

3,520,363 Patented July 14, 1 970 One of the difficulties in utilizing the combination of the in situ combustion and water injection has been the relatively confined areas of the formations in which the subsequent water injection step can scavenge effectively and thus utilize the heat which has been generated in the formation. That transfer by conduction from the source of heat to a portion of the reservoir remote from the source of heat is relatively slow. For example, it has been estimated that the temperature of the formation may be increased only -l00 F. for a distance up to 30 feet from the zone of combustion by conduction alone in about 2 years.

Accordingly, it is an object of the present invention to overcome this limitation by providing a method whereby heat is generated more effectively and advantageously throughout the reservoir, which is then more effectively utilized by a water injection step.

SUMMARY This invention comprises subjecting the subterranean hydrocarbon-bearing formation to an in situ combustion wherein the in situ combustion is caused to occur spontaneously and randomly throughout the hydrocarbonbearing formation, which is then followed by a water injection step which takes advantage of the favorable recovery conditions created thereby.

DESCRIPTION OF THE PREFERRED EMBODIMENT More specifically, it has been found that an in situ combustion which has been spontaneously initiated, randomly in a plurality of sites within a hydrocarbon-bearing formation, causes heat to be generated more effectively within the hydrocarbon-bearing formation which can then be utilized in a subsequent water injection procedure.

The method comprises initiating combustion in a formation spontaneously by injecting a combustion-supporting gas until evidence that combustion has occurred to a significant extent is shown by both an increase in the carbon dioxide content and an increase in the percent of oxygen accounted for in the efiiuent gases from monitored offset wells, at which time the injection of the combustion-supporting gas is terminated, and water injection is undertaken. The percent of oxygen accounted for may be defined as the ratio in percent of the atomic oxygen appearing in the efiluent gases to the atomic oxygen in the injection gas.

Experience has shown that injection of a combustionsupporting gas, i.e. one which contains oxygen, through a formation containing hydrocarbons which are susceptible to auto'oxidation, can effect spontaneous ignition of the hydrocarbons contained therein. The sequence leading to initiating an in situ combustion by this means involves auto-oxidation followed by spontaneous ignition of the crude. Auto-oxidation, which results from the slow oxidation of hydrocarbons 'by the oxygen in the gas, occurs initially at a relatively low rate with an accompanying slow release of exothermic heat. With continued injection of the gas into the formation, however, autooxidation occurs to a significant extent and effects on appreciable rise in the temperature of the formation, resulting in spontaneous ignition of the hydrocarbons.

The manifestation of the reaction sequence and the rate at which it is occurring can be obtained by continuously monitoring and analyzing the efiiuent gases from offset wells. In particular, it is desirable to determine the carbon dioxide content and the percent of oxygen accounted for in the effluent gases. The determination of the carbon dioxide content may by made by any desired means, e.g. by employing an Orsat apparatus, and the percent of oxygen accounted for can be determined from appropriate calculations using stoichiometric principles.

When a hydrocarbon undergoes auto-oxidation, it has been observed that it is accompanied by an appearance of carbon dioxide and by a low value of percent of oxygen accounted for in the effluent gases with the accompanying rise in temperature within the formation, due to the exothermic oxidative reactions, the carbon dioxide content of the effluent gases will continue to rise and the percent of oxygen accounted for will increase. As the temperature of the formation rises, the ignition temperature of the hydrocarbons is approached and spontaneous ignition becomes imminent. The time at which this point in the reaction sequence is reached is approximately indicated when the carbon dioxide content in the effluent gases exceeds about 4%, and the percent of oxygen accounted for exceeds about 50%, although there is an indication that combustion is occurring when the percent oxygen accounted for is about 30%.

Experience has shown also that spontaneous ignition of a formation by this reaction sequence may occur randomly within the formation, and not necessarily in the immediate vicinity of the injection well bore, but rather in a plurality of sites or pockets wherever the concentration of the hydrocarbons, the concentration of the oxygen, and the formation characteristics produce optimum conditions for its occurrence. This randomness is evidenced by the differences in carbon dioxide content and the percent of oxygen accounted for observed in the effluent gases from the offset wells located throughout the field. Generally, when the sites at which spontaneous ignition is occurring are located in relatively close proximity to those offset wells, from which the effluent gases indicate an increasing carbon dioxide content and an increasing percent of oxygen accounted for, stimulation of fluid production is observed also. The location of these sites is affected by such factors as the character of the hydrocarbon, the oxygen content of the injected gases, the rate of injection, the formation pressure, and temperature.

In most formations wherein ignition has been accomplished spontaneously, about 8 to 10 weeks are required, although there are instances in which longer periods of time have been required.

The tendency of a hydrocarbon to undergo auto-oxidation leading to spontaneous ignition, and the rate at which it will occur can be measured in the laboratory by means of experiments in which a sample of the hydrocarbon is contacted with an oxygen-bearing gas. Measurements of oxidation rates can also be determined in the laboartory using samples of the representative hydrocarbon-bearing formation. Such measurements will give an indication of how long a time will be required before the injection of the combustion-supporting gas will result in spontaneous ignition.

The susceptibility of hydrocarbons to undergo spontaneous ignition varies, and appears to be greater in lower API gravity crudes, i.e. crudes whose API gravities are less than As will become apparent, the present invention is particularly applicable to field-wide operations, utilizing a plurality of wells, which are not necessarily in any definite pattern. However, it may be applied to a specific area within a field. The offset wells included in the operation of this method should be adaptable as monitor wells and production wells, and convertible to water injection wells for subsequent water flooding.

In accordance with the invention, there is provided a method which involves as its essential steps passing a combustion supporting gas through a formation containing hydrocarbons to effect spontaneous ignition of the hydrocarbons and thereby generate heat in the formation, monitoring the composition of the effluent gas to determine the extent to which spontaneous ignition has occurred within the formation, and thereafter passing an aqueous medium through the formation to scavenge the 4 in situ heat and drive the hydrocarbons to a production well.

Illustrative of the efficacy of this invention was its application to a producing field in Trinidad, of about 40 acres, in which the hydrocarbon-bearing formation contained an l8.4 API crude. The formation was traversed by one injection well and 18 offset wells, more or less, randomly spaced, and up to distances of about 1200 feet from the injection well. In this field, the air injection step was conducted for about 11 months until an estimated 4.5 x10 B.t.u.s of heat had been generated in situ. Air injection was then terminated and thereupon sea water was injected into the formation via the former air injection well. For the period of 22 months during which time approximately 75,000 barrels of sea water were injected, it was estimated that an additional 51,200 barrels of oil were produced with approximately 62% of the production occurring at wells located from 800 to 1200 feet away from the site of the original air injection well.

A procedure for the application of this invention to a hydrocarbon-bearing formation which is susceptible to spontaneous ignition would comprise the following steps:

(a) Inject a combustion-supporting gas at ambient temperatures or above into the formation via an injection well or wells, while monitoring eflluent gases from a plurality of offset wells. The efiluent gases would be analyzed for carbon dioxide content and the percent of oxygen accounted for would be determined by appropriate calculations. The period of gas injection would continue until evidence-that spontaneous ignition had occurred within the formation was shown by at least 4% of carbon dioxide appearing in the effluent gases of offset Wells together with an increase in the percent of oxygen accounted for;

(b) Continue injection of the combustion-supporting gas for a sufficient period of time to allow the in situ combustion thus initiated to heat a substantial portion of the formation to temperatures above the ignition temperatures of the hydrocarbons in the formation, i.e. above 450-500 F. The length of this period can be determined from calculations of the heat generated in situ, based on the air injection rates and the effluent gas compositions;

(c) Terminate injection of the combustion-supporting (d) Commence water injection. Either the former gas injection well or one or more of the offset wells may be used for Water injection. The selection of offset wells for conversion to water injection wells will depend upon the estimated combustion pattern and sites of combustion, the fluid movements, and the location of the prospective water injection wells in relation to the production wells. Water injection may be timed to control the amount of heat generated by the in situ combustion and the thermal energy left in the reservoir; and

(e) Continue water injection until sufficient amounts have been injected to have scavenged effectively the generated heat from in situ combustion. The amount of water can be calculated from the thermal properties of the formation and the fluids, and the in situ generated heat. The mechanics of the heat transfer are not known entirely, but it is postulated to involve successive vaporization and condensation of the water. Thus, the in situ heat is conserved and further utilized within the formation by transferringheat from the hot rock formation to the hydrocarbons to make the hydrocarbons more mobile and thereby more easily produced. The water used may be fresh water, brine, or water containing carbon dioxide.

It will be apparent from the foregoing description that the method is subject to other modifications without departing from the scope of the invention as defined in the following claims.

I claim:

1. A method of recovering hydrocarbons from a subterranean hydrocarbon-bearing formation penetrated by at least one injection well and offset wells, comprising the steps of:

(a) injecting into said formation through said injec tion well a combustion-supporting gas having oxygen to cause spontaneous ignition of said hydrocarbons in a random plurality of sites in said formation,

(b) monitoring efiluent gases from said offset wells and determining carbon dioxide content and percent of oxygen accounted for in said efiluent gases until the said carbon dioxide content of said effluent gases exceeds 4% and said percent of oxygen accounted for has increased to about 50%,

(c) continuing injection of said combustion-supporting gas into said formation through said injection well until a substantial portion of said formation in said random sites has undergone in situ combustion as determined by step (b); and thereafter terminating said injection of said combustion-supporting gas,

((1) thereupon, injecting into said formation an aqueous medium through said injection well, and producing said hydrocarbons from said offset wells, and

(e) continuing injecting said aqueous medium into said injection well to scavenge effectively said generated heat in said formation.

2. The method of claim 1 wherein the injected combustion-supporting gas is air.

3. The method of claim 1 wherein the injected aqueous medium is selected from the group consisting of fresh water, brine, water saturated with carbon dioxide, or mixtures thereof.

4. In the method of claim 3, injecting said aqueous medium into specified offset wells in close proximity to offset wells whereat the carbon dioxide content is greater than about 4% and said percent of oxygen accounted for is greater than about 5. In the method of claim 1, producing said hydrocarbons from specified offset wells wherein the carbon dioxide content in the effluent gases is greater than about 4%, and said percent of oxygen accounted for is greater than about 50%. r

6. In the method of claim 1, producing from specified offset Wells showing increased fluid production and initiating the injection of said aqueous medium into said offset wells in close proximity to said specified offset wells.

7. In the method of claim 1, said wells being included in any given pattern in a producing field, the steps of consecutively changing an offset well which has signs of high carbon dioxide content in said effluent gas to a production well and converting said adjacent offset wells to said aqueous medium injection wells.

8. In a method of claim 1, the steps of changing the function of said offset wells from monitoring wells to production wells when the carbon dioxide is greater than about 4% thereat.

References Cited UNITED STATES PATENTS 3,036,632 5/1962 Koch et al. 166256 3,064,728 11/1962 Gould 166261 3,072,185 1/1963 Bond et al. 166261 3,221,809 12/1965 Walton 166263 STEPHEN I. NOVOSAD, Primary Examiner U.S. Cl. X.R.