US 3126954 A
Description (OCR text may contain errors)
March 31, 1964 F. E. CAMPION 3,126,954
IN SITU COMBUSTION PROCESS Filed Dec. 30, 1959 INJECTION EL GASES FLUIDS BURNED ZONE UNBURNED ZONE FIG. I
INJECTION PRODUCED GASES FLUIDS UNBURNED ZONE BURNED ZONE UNBURNED ZONE FIG. 2
Francis E. Compioninventor B t. Attorney United States Patent Office 3,126,954 Patented Mar. 31, 1964 3,126,954 IN SITU COMBUTHQN PRUQESS Francis E. Campion, Tuisa, @ida, assignor to Jersey Production Research Company, a corporation of Delaware Filed Dec. 36, 1959, Ser. No. $62,943 8 Claims. (Cl. 166ll) The present invention relates to thermal methods for the recovery of petroleum from underground formations and more particularly relates to an improved process for recovering oil from partially depleted reservoirs by the in situ combustion of carbonaceous materials present in such reservoirs. In still greater particularity, the invention relates to an improved in situ combustion process wherein maximum utilization of oxygen in the input gas stream following breakthrough of the initial combustion front at the production Well is achieved by establishing and maintaining a secondary combustion front in the reservoir between the injection and production Wells.
In situ combustion is an attractive method for recovering oil from partially depleted underground reservoirs. This method in essence involves the establishment of a combustion front within the reservoir in the vicinity of one or more injection wells and the subsequent introduction of a combustion-supporting gas behind the combustion front in order to move it through the reservoir toward one or more production wells. As the combustion front advances, the heat liberated results in the vaporization of oil from a high temperature zone preceding the front. Cracking and the formation of coke which serves as fuel for the process also occur. The resulting oil vapors are carried forward with the combustion products and condensed in cooler portions of the reservoir. Heat transfer to cold oil in sections of the reservoir in front of the advancing high temperature zone leads to a reduction in viscosity of the oil and facilitates its displacement from the reservoir. A mixture of oil and gases is withdrawn from the reservoir at the production Will and the oil contained therein is subsequently recovered.
Although this process is a promising one, there are several difficulties associated with it. Not the least of these is the fact that the combustion front and associated high temperature zone often tend to finger through high permeability sections of the reservoir instead of advancing uniformly. This frequently leads to breakthrough of the combustion front into the production well at an early stage in the process. After such a breakthrough occurs, high temperature combustion products, oxygen and oil flow into the combustion well simultaneously, creating a danger of fire in the wellbore and in associated equipment. Process efficiency in terms of the utilization of injected oxygen falls off and the operation becomes less attractive from an economics standpoint. Despite this, the introduction of oxygen at the injection well and the withdrawal of fluids from the production well must be continued if additional oil is to be recovered from the reservoir.
Difficulties of the type described above are encountered in both Stratified and unstratified reservoirs. In Stratified reservoirs, some of the strata frequently have much greater permeability than others. Breakthrough of the combustion front at the producing well through a high permeability stratum results in a direct channel through which the injected oxygen may pass from the injection well to the production well without encountering significant quantities of unburned hydrocarbons. Recovery from the adjacent strata of relatively low permeability is slow and must be carried out in the face of undesirably high temperatures and free oxygen in the production Well.
In unstratified reservoirs, breakthrough generally occurs at the production well near the top of the producing formation due to overburning. Utilization of the injected oxygen thereafter depends primarily upon the diffusion of gaseous oxygen from the burned out area at the top of the reservoir down into the unburned zone. The distance through which the oxygen must diffuse increases as the thickness of the burned-out zone increases and hence the portion of the injected oxygen utilized to support combustion is apt to decline as the operation progresses. The result is a continual increase in the difiiculties occasioned by the presence of free oxygen in the production well.
The present invention provides a new and improved in situ combustion process which largely obviates the difficulties outlined above. In accordance with the invention, it has now been found that the problems encountered following breakthrough of the combustion front at the production well during in situ combustion can largely be avoided by establishing and maintaining a secondary combustion front in the reservoir at an intermediate point between the injection and production wells after breakthrough occurs. Such a secondary front results in substantially complete utilization of the injected oxygen within the reservoir. The heat liberated due to combustion at the secondary front is transferred to relatively impermeable sections of the reservoir more effectively than is otherwise possible. Total oil recovery is increased. The invention thus provides an improved in situ combustion process which is safer, more effective and more economical than in situ combustion processes employed in the past.
The secondary combustion front utilized in practicing the invention is established at an intermediate point in the reservoir between the injection and production wells after breakthrough of the initial combustion front by introducing a preselected mixture of fuel gas, oxygen and inert gas at the injection well. The mixture thus introduced flows through the high permeability, burnedout section of the reservoir traversed by the initial combustion front, picking up heat as it progresses. When the ignition temperature of the fuel gas in the mixture is reached, generally after the mixture has advanced a relatively short distance into the reservoir, combustion occurs. The oxygen and fuel gas injected thereafter are consumed at the secondary combustion front thus es tablished and hence never reach the initial combustion front. Combustion of coke at the initial front and the how of free oxygen into the production well cease. Heat liberated at the secondary front is in part transferred to the surrounding rock and in part carried to downstream sections of the reservoir by the combustion products and inert gases. This heat stimulates the production of oil from relatively impermeable zones by-passed by the initial front. Temperatures at the production Well are considerably lower than in operations not utilizing a sec ondary front.
The gaseous mixture introduced at the injection well in order to establish and maintain the secondary combustion front utilized in the practice of the invention will normally comprise a fuel gas, oxygen and one or more inert gases. Suitable fuel gases include methane, ethane, propane, butane, natural gas, mixed refinery gases and the like. Inert gases which may be employed as diluents include carbon dioxide, nitrogen and flue gases. A typical gaseous mixture may contain from about 1 to about 10 volume perment fuel gas, from about 3 to about 25 volume percent oxygen, and from about 65 to about 96 volume percent inert gas. It is generally preferred to mix the fuel gas with air to provide the necessary oxygen and to add sufiicient inert gas to give the desired mixture. The relative amount of each constituent utilized, of course, depend, to some extent upon the particular fuel 3 gas used and, as pointed out below, will also depend in part upon the point at which" the secondary front is to be maintained.
After the secondary combustion front has been esta lished, the composition of the gas stream introduced into the reservoir through the production well is controlled to secure high oil recovery rates. Since the heat transmitted to oil bypassed by the initial combustion front depends in part upon the point at which it is liberated within the reservoir, the recovery rate can be regulated by controlling the position of the secondary front. Studies and experimental work have shown that the position depends largely upont he oxygen-to-fuel ratio in the gaseous mixture introduced at the injection well. An increase in the ratio generally results in movement of the secondary front back toward the injection well; while a reduction normally causes it to move forward toward the production well. Proper adjustment of the ratio at intervals permits maintenance of a substantially stationary front. By thus varying the relative amounts of oxygen and fuel in the gas stream injected, the position of the secondary front can be controlled to obtain high recovery rates without encountering unduly high temperatures and free oxygen at the production well.
The oxygen-to-fuel ratio required to establish and maintain the secondary combustion front at an intermediate position between the injection and production wells will, as pointed out above, depend somewhat upon the particular fuel gas utilized. It will also depend upon the physical properties of the reservoir, the fluids present therein, and the course taken by the initial combustion front prior to breakthrough at the production well. These latter factors are extremely difficult to evaluate during an in situ combustion operation and hence it is impractical to attempt to prescribe the precise oxygen-to-fuel ratio which will be required in a particular situation. A more satisfactory method for controlling the operation is to initially establish the secondary front by injecting a combustible gaseous mixture, one containing theoretically equivalent amounts of fuel gas and air under atmospheric conditions for example, and thereafter to adjust the composition until a satisfactory recovery rate is attained. Any of the conventional methods of measuring the recovery rate may be utilized. There is generally an appreciable time.
lag between changes in the input gas composition and changes in the recovery rate and hence it is normally preferred that a day or more elapse between changes in composition so that the effect of each change can be fully assessed. Measurement of the temperature and composition of the gases produced at the production well serves to indicate that the secondary front has been established and that combustion at the initial front has ceased.
fter the secondary front has been established and a satisfactory oil recovery rate has been attained, it is preferred that the oxygen-to-fuel ratio be held constant until a decline in the recovery rate indicates the heat transferred to the oil-bearing section of the reservoir is too low for most effective recovery. At that point, the secondary front is moved forward in the reservoir by reducing the oxygen content of the input gas stream slightly. Such a change in the position of the front results in the liberation of heat nearer the oil-bearing region in the reservoir and, since more heat is transferred to the oil present there, produces an increase in the recovery rate. The position of the secondary front may be continuously controlled in this manner to obtain maximum oil recovery.
In addition to controlling the position of the secondary combustion front, it is generally desirable to regulate the mass rate at which the gases are injected into the reservoir. This provides a further control on the temperature of the fluids flowing into the production well. If the production well temperature is too high, the mass rate can be reduced to reduce the total heat input per unit of time. The energy carried to the production well as sensible heat in the product gases will therefore decline. if, on the other hand, an increase in the temperature of the product flowing into the production well is desired, the mass rate may be increased. The actual quantity of gas injected will, of course, partially depend upon the porosity and permeability of the reservoir and upon the reservoir pressure. In general, pressures between about pounds per square inch and pressures approaching the overburden pressure may be used.
The method of the invention is applicable to a variety of different oil-bearing reservoirs wherein breakthrough of the initial combustion front occurs at the production well before recovery of the oil contained in the reservoir is completed. It may be applied to processes wherein reverse burning is utilized as well as to conventional in situ combustion operations. The method is not limited to the use of a single injection well and a single production well and instead may be carried out using both multiple injection wells and multiple production wells. Any of the various systems conventionally utilized for spacing the injection and production wells may be utilized.
The nature and objects of the invention can be more"- fully understood by referring to the following detailed description of in situ combustion processes carried out in accordance therewith and to the accompanying drawings in which:
HS. 1 depicts an in situ combustion process utilizing a secondary combustion front wherein overburning occurred prior to breakthrough of the initial combustion front at the production well; and,
FIG. 2 represents a process wherein a secondary combustion front is used in a stratified reservoir after breakthrough of the initial front through a high permeability zone.
Referring now to FIG. 1, reference numeral 11 desig nates an injection well drilled through overburden 12: into an oil-bearing reservoir 13. Production well 14- has been drilled into the reservoir at a point removedfrom the injection well. The distance separating the injection and production wells will depend upon a number of factors, including the extent of the reservoir, the permeability andporosity of the subsurface strata, the reservoir pressure, and the recovery pattern utilized. This distance may vary widely but will generally range between about 300 feet and about 3,009 feet. The injection and production wells will normally be cased and perforated opposite the producing strata in the conventional manner but in some instances uncased wells may be used. Conventional f cilities for introducing air and other gases at the injection well and for recovering and separating oil and gaseous products from the production well are provided on the surface.
In situ combustion has been carried out in the reservoir shown in FIG. 1 of the drawing by establishing a combustion front in the vicinity of the injection well and thereafter injecting air in order to propel the front through the reservoir toward the production well. A number of methods for estaolishing such a front are well known, including the injection of high temperature combustion products into the reservoir, the use of electrical devices to ignite a mixture of fuel gas and oxygen in the injection well opposite the producing formation, and the injection of pyrophoric materials and a stream of combustion-supporting gas into the reservoir through the injection well. As shown in FIG. 1, overburning occurred as the combustion front thus established progressed through the reservoir. The upper portion of the reservoir was substantially burned out and thus depleted of oil. The lower section of the reservoir, on the other hand, was relatively unaffected by passage of the combustion front and hence still contains appreciable quantities of oil. Reference numeral 15 designates the boundary between the burned and unburned sections. Breakthrough of the combustion front occurred in the production well near the top of the producing zone. As a result of the overburning and premature breakthrough of the combustion front, continued injection of air or oxygen into the reservoir will result in combustion near or in the production well. A substantial portion of the injected oxygen will flow through the high permeability burned-out zone at the top of the reservoir and will not be utilized for the generation of heat within the reservoir. In situ combustion in the section of the reservoir below the burned-out zone will depend largely upon the diffusion of oxygen downwardly into the cooler region at the bottom of the reservoir. Since this occurs only to a limited extent, utilization of oxygen and overall efficiency of the process will be poor.
In carrying out the process of the invention, the injection of air or oxygen at injection well 11 is discontinued upon breakthrough of the combustion front at production well 14. Imminent breakthrough of the combustion front can often be detected by observing the production well temperature. A substantial rise in temperature over a relatively short period of time generally indicates that combustion is occurring in the area immediately surrounding the production well. The appearance of oxygen in the product gases indicates that breakthrough has occurred. After air injection has been halted following breakthrough of the initial front, a secondary combustion front is established in the hot reservoir at an intermediate point between the injection and production wells by introducing a gas stream containing fuel gas, oxygen and one or more inert gases through injection well it. The gaseous mixture utilized to establish the secondary front may comprise, for example, about 3.6 volume percent methane, 260 volume percent air, and 70.4 volume percent nitrogen. As pointed out previously, a variety of fuel gases and diluents may be employed.
The gaseous mixture introduced into the reservoir as described above will flow toward the production well, absorbing heat from the formation, until it has reached the ignition temperature. As that point, the fuel gas in the mixture will ignite and combustion will occur. The secondary combustion front which results is indicated by reference numeral 16 in FIG. 1. Under normal operating conditions this secondary front will generally occur at a point where the injected gases have traversed about one-fifth of the reservoir volume swept by the previous front. The heat transfer characteristics in oil reservoirs during most in situ combustion operations are such that the gases will usually flow through about 20 percent of the swept volume before the fuel ignition temperature is reached. Other factors also affect the position of the secondary front, however, and hence its precise location cannot ordinarily be predetermined.
The gases employed to establish and maintain the secondary front will generally be injected into the reservoir at temperatures between the ambient temperature and about 400 F. The temperature should obviously be somewhat below the ignition temperature of the fuel gas in the mixture. Heat economies can usually be effected by recycling a portion of the combustion products recovered from the production well to the injection well as diluent gases. The pressures at which the gases are introduced may range from values slightly in excess of the formation pressures up to values approaching pressures at which fracturing of the reservoir occurs. Pressures between about 100 p.s.i.g. and about 1,000 p.s.i.g. are generally preferred.
Following establishment of the secondary combustion front at an intermediate point within the reservoir as shown in FIG. 1 of the drawing, the composition of the input gas stream is adjusted until a high oil recovery rate is attained at the production well. Heat liberated due to combustion of the fuel gas is carried toward the production well be conduction of the hot gases. The temperature of the portion of the reservoir down stream from the combustion front through which the gases flow rises. Overlying and underlying portions of the reservoir are heated by conduction. As heat diffuses into the oil-sat- 5 urated rock below the burned zone, the viscosity of the oil is reduced. The oil moves toward the producing Well by the imposed pressure gradient between the wells and by the hydrostatic pressure of the oil.
Maximum reservoir temperatures in an in situ combustion process carried out in accordance with the invention occur within a relatively narrow zone at the leading edge of the secondary combustion front. The temperature beyond this zone declines to a relatively low value at the production well. The temperature at the production well can readily be controlled by regulating the volume of gases injected into the reservoir. An increase in the injection rate results in an increase in tem perature at the production well; while the temperature declines with a decrease in the injection rate. A change in the injection rate will frequently result in some move ment of the combustion front even though the composition of the input gas stream remains unchanged. Adjustment of the composition to maintain a stationary combustion front may therefore become necessary.
A decline in the rate at which oil is recovered from production well 14 after the secondary combustion front has been held stationary for some time indicates that the front should be moved forward in order to increase the amount of heat transferred to sections of th ereservoir located some distance from the injection well. This can be accomplished by reducing the oxygen content of the gas stream introduced into the reservoir through injection well 11. Since it is generally preferred to maintain the input gas volume constant while changing the oxygen content, a portion of the oxygen in the input stream will normally be replaced by inert gas. Using a mixture of methane, air and nitrogen for example, the air content might be reduced from about 26.0 volume percent to about 23.0 volume percent while increasing the inert gas content from about 70.4 volume percent to about 73.4 volume percent. Such a change causes the combustion front to move in the direction of production well 14. After an increase in the oil recovery rate is noted at well 14, the composition is again held constant until a change in recovery rate is observed. This incremental movement of the secondary front through the reservoir is continued until the reservoir has been depleted of oil.
The system depicted in FIG. 2 of the drawing is similar to that shown in FIG. 1 except that in FIG. 2 the reservoir is a stratified one. Injection well 20 and production well 21 extend downwardly through overburden 22 and oil-bearing zones 23, 24 and 25. Zone 24 has greater permeability than adjacent zones 23 and 25 and hence the initial combustion front employed in the reservoir rapidly moved through zone 24 and broke through at production well 21 before recovery in the adjacent zone had been completed. Establishment of a secondary combustion front in the high permeability zone at a point between the injection and production wells as indicated in FIG. 2 by reference numeral 26 results in the complete utilization of oxygen injected into the reservoir and improves heat transfer to oil-bearing zones 23 and 25. The secondary front may be moved forward in the reservoir at intervals in order to obtain maximum oil recovery rates in the manner described in conjunction with FIG. 1 above.
It will be recognized that the method of the invention is applicable to a variety of in situ combustion operations and is not limited to the specific systems described above. It may, for example, be utilized following break through of the initial combustion front in a reverse burning operation wherein air is injected in one well and flows through the reservoir to a second well at which combustion is initiated. The method may be utilized in conjunction with operations for the recovery of oil from shale deposits wherein the deposit is first fractured and in situ combustion is thereafter utilized to recover oil. Other applications will be apparent to those skilled in the art.
What is claimed is:
1. An improved oil recovery process which comprises establishing an initial combustion front in a subsurface oil-bearing reservoir in the vicinity of a first well penetrating said reservoir; injecting a combustion-supporting gas into said reservoir to advance said initial combustion front toward a second well penetrating said reservoir; recovering oil and gases from said reservoir; discontinuing the injection of said combustion-supporting gas upon breakthrough of said initial combustion front at said second well; establishing a secondary combustion front in the burned-out section of said reservoir intermediate said first and second wells by injecting a combustible mixture of a fuel gas, oxygen and an inert gas into the portion of said reservoir traversed by said initial combustion front while the temperature of at least part of said burned-out section remains above the ignition temperature of said combustible mixture; adjusting the composition of said combustible mixture to maintain said secondary combustion front essentially stationary within said burned-out section and recovering additional oil and gases from said reservoir.
2. A method for improving oxygen utilization during an in situ combustion oil recovery operation which comprises establishing an initial combustion front in a subsurface oil-bearing reservoir adjacent an injection well penetrating said reservoir; injecting a combustion-supporting gas through said injection well to advance said initial combustion front toward a production well penetrating said reservoir; producing oil and gases from said production well; halting the injection of said combustionsupporting gas at said injection well upon breakthrough of said initial combustion front at said production well; injecting a combustible mixture of a fuel gas, oxygen and an inert gas through said injection well while the temperature of at least part of the burned-out zone in said reservoir exceeds the ignition temperature of said combustible mixture to establish a secondary combustion front in said reservoir intermediate said injection and production well; adjusting the composition of said combustible mixture to maintain said secondary combustion front at an essentially stationary position within the burned-out zone in said reservoir and producing additional oil and gases from said production well.
3. A method for reducing the influx of free oxygen into the production well during an in situ combustion oil recovery operation which comprises heating a subsurface oil-bearing reservoir adjacent an injection well to a temperature sufficient to establish an initial combustion front in said reservoir; injecting a combustion-supporting gas into said reservoir through said injection well to move said initial combustion front toward a production well; producing oil and gases from said production well; discontinuing the injection of said combustion-supporting gas at said injection well upon breakthrough of said initial combustion front at said production Well; injecting a combustible mixture of a fuel gas, oxygen and an inert gas through said injection well while the temperature of at least part of said reservoir remains above the ignition temperature of said combustible mixture to establish a secondary combustion front in said reservoir intermediate said injection and production wells; adjusting the fuel-oxygen ratio in said injection mixture to maintain said secondary combustion front at an essentially stationary position within the burned-out section of said reservoir; and producing additional oil and gases from said reservoir through said production well.
4. A process as defined by claim 3 wherein said mixture injected into said reservoir to establish said secondary combustion front contains from about 1 to about 10 volume percent of fuel gas, from about 3 to about 25 volume percent of oxygen, and from about to about 96 volume percent of inert gas.
5. A process as defined by claim 3 wherein the oxygenfuel ratio in said injection mixture is adjusted at intervals following establishment of said secondary combustion front.
6 A process as defined by claim 3 wherein the gas injection rate is varied to control the production well temperature following establishment of said secondary combustion front.
7. In an oil field secondary recovery operation wherein a combustion front is established in a subterranean oilbearing reservoir in the vicinity of a first well and propagated through said reservoir toward a second well and wherein overburning and the formation of a burned-out zone near the upper boundary of said reservoir occur, the improvement which comprises injecting a combustible mixture of oxygen, a fuel gas and an inert gas into said reservoir after said combustion front reaches said second Well and while the temperature of at least part of said burned-out zone is still above the ignition temperature of said combustible mixture to establish a secondary combustion front Within said burned-out zone, adjusting the composition of said combustible mixturev to maintain said secondary combustion front essentially stationary within said burned-out zone, and recovering oil from said reservoir as a result of the generation of heat at said secondary combustion front.
8. In an oil field secondary recovery operation wherein a combustion front is established in a subterranean oilbearing reservoir in the vicinity of a first well and propagated through said reservoir toward a second Well and wherein said combustion front advances through a highly permeable zone of said reservoir, producing a burned-out zone bounded by a low permeability zone containing substantial quantities of oil, the improvement which comprises injecting a combustible mixture of oxygen, a fuel gas and an inert gas into said reservoir after said combustion front reaches said second well and while the temperature of at least part of said burned-out zone is still above the ignition temperature of said combustible mixture to establish a secondary combustion front within said burned-out zone, adjusting the composition of said combustible mixture to maintain said secondary combustion front essentially stationary within said burned-out zone adjacent said low permeability zone, and recovering oil from said low permeability zone as a result of the generation of heat at said secondary combustion front.
References Cited in the file of this patent UNITED STATES PATENTS 2,793,697