Production And Upgrading Of Heavy Viscous Oils

Abstract

Production and upgrading of viscous oils is effected by heating an underground formation for a period of time sufficient to heat said formation to a temperature of at least about 550.degree. F. and form a substantial heat bank therein. A viscous oil is then flowed into said heated formation and maintained therein for a period of time sufficient to effect a significant reduction in viscosity of the oil. Heating of said formation can be effected by in situ combustion or injection of a heat-bearing fluid, e.g., steam.

Description

This invention relates to the production of crude oils. In one aspect this invention relates to the production of heavy viscous crude oils.

Heavy crude oils, e.g. crude oils having a low API gravity and/or high viscosity are very difficult to produce by ordinary methods of primary and secondary production. In many fields containing such oils only a few percent of the oil can be produced by ordinary methods of production. Waterflooding and similar methods of secondary production are not very effective in producing such oils because of adverse mobility and fingering of the driving fluid.

Another problem associated with the production and use of such heavy oils is that, after production, such oils frequently require upgrading even before transferring same to a refinery or other ultimate use. For example, such oils are difficult to pump in a pipeline or load into tankers or other conveyance for transportation. Thus, it is desirable that a permanent reduction in viscosity be effected on such oils. These heavy crude oils frequently have a high sulfur content. It is desirable to effect a reduction in the sulfur content before processing such oils in expensive refining equipment and subjecting said equipment to corrosion by sulfur. Such oils can be rendered much more valuable if, when produced, the viscosity and sulfur content thereof is lowered.

The present invention provides an improved method of producing heavy viscous crude oils which overcomes or at least mitigates the above-described problems. Broadly speaking, the present invention comprises heating an underground formation for a period of time sufficient to heat same to a temperature of at least about 550.degree. F. and form a substantial heat bank therein. A heavy viscous oil is then flowed into the heated formation and maintained therein for a period of time sufficient to effect a significant reduction in the viscosity of said oil. A reduction in sulfur content usually occurs concomitantly with said reduction in viscosity.

An object of this invention is to provide an improved method for recovering hydrocarbons from subterranean formations containing same. Another object of this invention is to provide an improved method for producing heavy viscous crude oil from subterranean formations containing the same. Another object of this invention is to provide improved methods of producing heavy viscous crude oils from subterranean formations whereby said crude oils are upgraded in quality. Another object of this invention is to provide improved methods for producing two or more spaced apart oil-bearing strata. Another object of this invention is to provide an improved method for producing underground oil-bearing formations of varying permeability. Other aspects, objects, and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.

Thus, according to the invention, there is provided a method of recovering hydrocarbons from a subsurface formation containing same, comprising the steps of: (a) providing communication between the surface of the earth and said formation through at least one well; (b) heating said formation for a period of time sufficient to heat a substantial portion thereof to a temperature of at least 550.degree. F. and form a substantial heat bank within said formation; (c) terminating heating of said now heated formation; (d) flowing a viscous hydrocarbon oil into said heated formation; (e) maintaining said hydrocarbon oil in said heated formation for a period of time sufficient to significantly reduce the viscosity thereof; and (f) recovering from said formation a hydrocarbon oil having a reduced viscosity which is less than that of said oil of step (d).

The methods of the invention are applicable to a wide variety of formations containing a wide variety of crude oils. Said methods are particularly applicable to formations containing heavy viscous crude oils having an API gravity of not more than about 15 and/or a Saybolt Furol viscosity at 122.degree. F. of at least 100 seconds. However, the invention is also applicable to the production of crude oils having an API gravity greater than 15 and a Saybolt Furol viscosity of less than 100 seconds which, for one reason or another, are difficult to produce by conventional methods. The tabulation given below in Table I illustrates that the methods of the invention are applicable to a wide variety of crude oils. ##SPC1##

In the above Table I the first four oils were heated for one day at 662.degree. F. in a manner similar to that described in the example given hereinafter. The last two oils in said Table I were heated for 14 days at 600.degree. F., similarly as described in said example given hereinafter. The data show that in all cases there was a significant reduction in viscosity, resulting in a definitely upgraded oil.

The tabulation given in Table II below is based on a series of test runs carried out on Morichal Gp. II crude oil in which said oil was heated to various temperatures for varying periods of time. ##SPC2##

The date in the above Table II show that permanent viscosity reduction by mild thermal cracking or visbreaking, as practiced in the invention, is sensitive to both temperature and time. For example, for a heating period of 10 days, at 500.degree. F. there is only a 2 percent reduction in viscosity, at 550.degree. F. the reduction in viscosity is 17 percent, and at 600.degree. F. the reduction in viscosity is 72 percent. Thus, in the practice of the invention it is desirable to heat the oil to a temperature of at least about 550.degree. F., preferably at least about 600.degree. F., for a period of time sufficient to obtain a significant reduction in viscosity. Preferably, said period of time will be at least about 10 days. As used herein and in the claims, unless otherwise specified, a significant reduction in viscosity is considered to be at least 10, preferably at least 25, percent of the original viscosity of the oil.

FIG. 1 is a vertical cross section through a formation comprising an upper less permeable oil-bearing sand and a lower more permeable oil-bearing sand and illustrates an arrangement of wells for carrying out one embodiment of the invention.

FIG. 2 is a vertical cross section through a formation comprising an upper more permeable oil-bearing sand and a lower less permeable oil-bearing sand and illustrates an arrangement of wells for carrying out another embodiment of the invention.

FIGS. 3 and 4 are vertical cross sections through an oil-bearing sand and illustrates an arrangement of wells for carrying out another embodiment of the invention.

FIG. 5 is a vertical cross section through an oil-bearing formation and shows a combination injection-production well for carrying out another embodiment of the invention.

FIG. 6 is a vertical cross section through a plurality of permeable oil-bearing sands separated by impermeable layers of shale or other impermeable barrier and illustrates an arrangement of wells for carrying out still another embodiment of the invention.

The drawings illustrate diagrammatically various methods of producing and upgrading heavy viscous oils in accordance with the invention. It will be understood that many valves, pumps, compressors, and other items of apparatus not necessary for explaining the invention to those skilled in the art have been omitted for the sake of brevity. In one embodiment illustrated in FIG. 1 the oil-bearing sand or formation is preferably at least 20 feet thick and comprises a more permeable lower portion 10 disposed below a less permeable upper portion 12. Preferably, said more permeable lower portion 10 is at least 5 feet thick and is at least 25, more preferably at least 50, percent more permeable than said upper portion 12. In addition, said sand or formation will have vertical permeability.

At least two wells, an injection well 14 and a production well 16, are drilled into the formation to provide communication between the surface of the earth and said formation. Said wells can be spaced apart any suitable distance, depending upon the characteristics of the formation and the oil contained therein. Usually, said wells will be spaced apart a distance within the range of from about 10 to 1,000 feet. In some instances, the preferred well spacing will be in the range of 50 to 300 feet. Air or other oxygen-containing gas is injected into the formation at a high rate through said injection well 14 for establishing and maintaining a combustion front 18 and drive said front out into the formation a suitable distance. Said formation is ignited at the injection well 14 in any conventional manner known to the art. For example, a downhole burner or a charcoal pack can be employed. Frequently, the formation will ignite spontaneously upon injection of air. Also, if desired, a spontaneously ignitable fuel can be injected into the formation to accelerate ignition. Said distance the combustion front is moved out into the formation can be any suitable distance depending upon the characteristics of the formation and the oil contained therein. However, for economic reasons it is usually desirable to stop the injection of oxygen-containing gas before the rate of heat loss by conduction reaches more than about 25 percent of the heat generation rate. During said injection of oxygen-containing gas and movement of the combustion front, the formation is heated to a temperature of about 1,000.degree. F., or greater, depending upon the gas injection rate and the oxygen content of the injected gases. As illustrated in FIG. 1, only lower section 10 is open to the injection of air. This is a presently preferred method of operation. However, it is within the scope of the invention for the upper section 12 of the formation to be open to air injection. In such instances, the combustion front 18 will advance farther into the formation in the more permeable lower section 10 thereof with the establishment of a greater heat bank in said lower section. The injection rate for the air or other oxygen-containing gas should be high enough to substantially overcome any tendency for gravity segregation of said gas into the upper portion of the formation during the injection period. Upon cessation of injection of oxygen-containing gases, oil from the unburned upper portion of the formation will drain back into the burned out lower portion of the formation as indicated by the arrow 20. The natural heating of the oil in the upper portion of the formation by conduction and convection will lower the viscosity of said oil and accelerate its flow from the upper portion to the lower portion of the formation. The oil drained into the hot lower portion of the formation is permitted to remain therein and "soak" for a period of time which is at least sufficient to significantly reduce the viscosity of said oil, or until the formation temperature has decreased to about 550 to 600.degree. F. The temperature of the heated formation can be calculated by methods known to those skilled in the art. The formation temperature can also be determined and/or monitored by temperature observation wells drilled thereinto between the injection well and the production well. Preferably, depending upon formation temperature and oil characteristics, said oil drained into the heated lower portion of the formation will be maintained therein for an average period of time of about 10 days.

After said oil has soaked for the desired period of time, the injection of oxygen-containing gases at injection well 14 is resumed to displace the heated and visbroken oil into the production well 16 from which it can be produced in any suitable manner, as by pumping. The formation is again ignited and the combustion front driven farther into the formation toward said production well. Injection of oxygen-containing gases and movement of the combustion front through the formation are again continued until the rate of heat loss by conduction reaches about 25 percent of the rate of heat generation. At this time, injection of oxygen-containing gases is terminated, oil is again permitted to flow by gravity from the upper section to the hot lower section of the formation, is permitted to soak or remain therein for a period of time sufficient to significantly reduce the viscosity of the oil, and is then displaced by again resuming injection of oxygen-containing gases. The described alternate injection of oxygen-containing gases with burning, and oil draining and soaking periods, will produce the maximum amount of visbroken and upgraded oil. This method of production will also result in increased total oil recovery due to heating of the portions of the formation surrounding the portion of the formation which is actually burned.

Another embodiment of the invention, illustrated in FIG. 2, is particularly applicable to those formations wherein the oil-containing formation or sand comprises a more permeable upper section 10' disposed above a less permeable lower section 12'. In this embodiment of the invention, one method for injection of oxygen-containing gases is through annulus 15 of injection well 14 into more permeable section 10' of the formation, said formation is ignited at well 14, and gas injection is continued until the combustion front 18' has proceeded out into the formation a suitable distance as described above. After terminating injection of oxygen-containing gases, water is injected into the lower portion 12' of said formation through tubing 17 to displace oil therefrom up into the heated portion of the formation as indicated by the arrow 22. Any suitable arrangement of tubing, casing, and packers 19 can be employed for so injecting said water. The oil so displaced into the heated portion of the formation is maintained therein for a period of time sufficient to significantly reduce the viscosity thereof. At the end of the soaking period injection of oxygen-containing gases is resumed to displace the visbroken oil to the production well from which it is produced in any suitable manner, the formation is again ignited, and the combustion front 18' moved farther into the formation to reheat same, similarly as described above. In this embodiment of the invention injection of oxygen-containing gases with combustion can be alternated with the water injection.

The above-described technique of alternate combustion and water displacement is also applicable where the oil-bearing sand or formation is homogenous. Field experience has shown that in such formations the fire flood will proceed through the top portion, e.g., 8 to 12 feet, of the sand only. With the burned out section of the formation at the top or in the middle thereof, water injection can be employed to displace oil from the lower to the upper portion of the sand as described above.

In another embodiment of the invention illustrated in FIG. 3, a direct drive in situ combustion is initiated at injection well 14 and carried out in conventional manner until the combustion front 18 has proceeded out into the formation 24 from the injection well a suitable or desired distance. At this time injection of oxygen-containing gases is terminated. Gas pressure on the injection well 14 is then vented which causes heavy oil from the heated but uncombusted portion of the formation to be sucked into the heated portion of the formation, as indicated by the arrow 23. Said formation 24 is preferably at least 10 feet thick. After a suitable soaking period as described above, injection of oxygen-containing gases is resumed through well 14 to displace the visbroken oil to the production well 16 from which it is produced in any suitable manner, the formation is ignited again, and the front again driven farther out into the formation. These periods of alternate gas injection with combustion and then pressure venting can be repeated, similarly as described above, to move the front farther into the formation until it reaches the production well 16.

In another embodiment of the invention illustrated in FIG. 4, two or more wells, at least one injection well 14 and at least one production well 16, are completed in an oil-bearing sand or formation 26 which is preferably at least 10 feet thick. Oxygen-containing gases are injected through the injection well 14, the formation is ignited at well 14, and a combustion front 18 moved out into the formation a suitable or desired distance to form a sizable heat bank in said formation. At this time injection of oxygen-containing gases is terminated and a previously produced dewatered crude oil is introduced through the injection well 14 and pumped through said heat bank to the production well 16 from which the oil is pumped in conventional manner. The rate of injection of said produced crude oil and movement thereof through the heat bank will be such as to maintain a residence time for said oil in said heat bank which is sufficient to significantly reduce the viscosity of said oil. Preferably, said residence time will be at least about 10 days. Pumping of said produced oil through the heat bank is continued until the temperature of the heat bank drops below about 600.degree. F. At this time the injection of oxygen-containing gases through well 14 is resumed, the formation is reignited and the combustion front moved farther into said formation. Injection of oxygen-containing gases is stopped after a suitable period of time, determined as described above, and injection of produced dewatered crude repeated. This alternate burning and injection of produced dewatered crude not only will furnish a method for upgrading previously produced crudes but will also markedly increase oil production from the formation, particularly around the production well when the heated oil from the heat bank reaches said production well. Said previously produced crude oil can be any oil which can be upgraded in accordance with the invention, e.g., an oil previously produced from the formation being heated or an oil from a different formation or field.

In another embodiment of the invention illustrated in FIG. 5, only an injection well 14 is employed. One or more of said wells can be employed. In this embodiment oxygen-containing gases are introduced through the well 14, and the formation 28 ignited to form a combustion front and move same out into the formation a suitable or desired distance, determined as described above. Injection of oxygen-containing gases is then terminated and a dry produced oil introduced into the formation until the temperature of the heated portion of the formation decreases to a temperature below about 600.degree. F. This can be computed from heat transfer calculations by methods known to those skilled in the art, or can be observed in a special temperature observation well drilled for that purpose. When sufficient oil has been introduced to decrease the temperature of the heated formation to less than about 600.degree. F., the well is returned to production. Oil is produced from the well, as by pumping or by formation pressure, until the production rate and/or temperature of the produced oil indicates that the heating portion of the cycle should be repeated. This process can be repeated, employing alternate periods of heating, injection of dry produced oil, and production of oil, as desired. This process will provide considerable stimulation of production from the formation surrounding the injection well in addition to the visbreaking of the injected previously produced oil. The process is particularly advantageous after a number of cycles have been run and the heat from the heat bank has penetrated to adjacent sands. The process avoids the production of hot air and hot combustion gases which can damage a well when the well is returned to production immediately after termination of injection oxygen-containing gases. The injection of the previously produced oil will also "kill" the well and facilitate the running of temperature surveys or other well work which are sometimes performed prior to production.

It is not uncommon for a heavy oil reservoir to comprise a plurality or series of permeable oil-bearing sands separated by shale or other impermeable barrier. The Morichal field in Venezuela is an example of such a field. FIG. 6 illustrates diagrammatically such a field and one presently preferred method for producing same in accordance with the present invention. In this embodiment of the invention one or more injection wells 14 are completed open to all of the penetrated sands 30, 32, 34, and 36. One or more production wells 16 are completed, but said production well or wells are open to only one sand, e.g., 32, hereinafter referred to as a visbreaking sand. Said visbreaking sand will usually be the sand having the highest permeability. However, in some cases it may be desirable to choose the sand having the highest capacity (permeability times thickness), or to choose a sand near the middle of the reservoir (vertically speaking) if the permeabilities of the adjacent sands are not greatly different.

In this embodiment of the invention a presently preferred procedure is to initiate injection of oxygen-containing gases through injection well 14, ignite the formations, and move a combustion front out into the formations a desired or suitable distance, determined as described above, so as to form a sizable heat bank. Since the visbreaking sand 32 is usually the one of highest permeability, and the fact that this sand is open to the production well, the major portion of the introduced oxygen-containing gases will go into this sand to produce the largest heat bank therein. After the combustion front has moved out into the formation 32 a suitable or desired distance, injection of oxygen-containing gases is terminated, and the other sands 30, 34, and 36 in the reservoir are produced by permitting oil therefrom to flow, as by formation pressure, through the hot visbreaking sand 32 to the production well. Flow of said oil through the visbreaking sand 32 is controlled, by controlling the rate of production from said production well, so as to maintain said oil in the hot visbreaking sand for a period of time sufficient to significantly reduce the viscosity thereof. Flow of said oil into and through the visbreaking sand 32 is continued until the temperature of the sand decreases to about 600.degree. F. At this time, injection of oxygen-containing gases is resumed and the formation ignited to again heat the formation. The process can be repeated in cycles, as described, and the period or frequency of injection of oxygen-containing gases can be adjusted, taking into consideration heat transfer calculations and the desired time-temperature relationship for visbreaking of the oil so as to maximize heat utilization for maximum economic benefits. This embodiment of the invention is particularly applicable when employing relatively close well spacings, e.g., in the 50 to 200 foot range, more preferably 50 to 100 foot range.

A number of advantages are obtained in this method of producing plural sands in reservoirs. Included among these advantages is the fact that the heat generated in the visbreaking sand by combustion is used repeatedly to visbreak oil flowing through said sand. Another advantage is that the heat conducted to adjacent sands from the visbreaking sand is picked up by the counterflow of oil in said adjacent sands. Thus, said conducted heat is not lost and serves to improve productivity from the region around the injection well, as well as the region around the production well, during the production cycle of the process. Still another advantage is that the upper part of the injection well is not used for oil production. This reduces the explosion hazard and well work when switching from the oxygen-containing gas injection step of the process to the production step of the process. Another advantage is that since the injection of oxygen-containing gases is intermittent, most of the production at the production well will be at a low gas-oil ratio, thus further increasing productivity.

It may be desirable in some instances to inject an inert gas or a limited amount of water at the end of the oxygen-containing gas injection period so as to displace the oxygen-containing gases to the fire front. In a commercial application of this embodiment of the invention, it is desirable to employ a plurality of air injection wells so as to employ the air compressors continuously. The number of injection wells per production well will depend upon the characteristics of the formation and the oil contained therein and the time-temperature relationship in the visbreaking step of the invention. In many instances, a 1 to 1 ratio of injection wells to production wells is desirable.

All of the above-described embodiments of the invention afford the advantage of heat conservation. The heat stored in the heat bank is used repeatedly in visbreaking the heavy oil flowed into said heat bank, as the heat bank is moved farther and farther into the formation. Even when the temperature of the heat bank decreases below the visbreaking temperature, less heat is required to reheat the formation than in other methods of thermal production. Another advantage which is obtained in all of the methods of the invention is that the heat arriving at the production well with the visbroken oil side in the primary recovery of oil because said oil heats the oil in the formation around the production well.

While the various embodiments of the invention have been described with particular reference to employing in situ combustion as the method of heating the heavy oil containing sands or formations, the invention is not limited to employing in situ combustion as the method of heat generation. In all the above-described embodiments of the invention the injection of a heat-containing fluid such as steam or superheated steam can be employed for heating the formation if the formation pressure is high enough, or can be increased sufficiently by steam injection, to heat the formation to a temperature of at least 500.degree., preferably at least 600.degree. F.

The invention is not limited to any particular rate for the injection of oxygen-containing gases in the in situ combustion heating steps. In the practice of the various embodiments of the invention, any suitable injection rate for the oxygen-containing gases commonly employed in in situ combustion processes can be employed. Usually, said rate will be within the range of from about 500,000 to 10,000,000 scf per day, depending upon the characteristics of the formation being produced. However, it is within the scope of the invention to employ injection rates of oxygen-containing gases which are outside said ranges. Generally speaking, low injection rates for the oxygen-containing gases are undesirable because of excessive heat losses. As indicated above, high rates are especially desirable for the embodiment of the invention illustrated in FIG. 1. Lower gas injection rates can be employed in the other embodiments of the invention. The injection of oxygen-containing gases will be carried out for a sufficient period of time to burn out or move the combustion front out into the formation any suitable or desired distance. The time of injection is frequently determined by heat losses. Generally speaking, it is not desirable to continue heating of the formation after the sum of the heat losses by conduction to adjacent sands is more than 25 percent of the heat generation rate.

Air is the oxygen-containing gas most commonly used in the practice of the invention. However, it is within the scope of the invention to use air enriched with oxygen or air diluted with an inert gas, e.g., CO.sub.2 or combustion gases. The oxygen content of the oxygen-containing gases can range from about 5 to about 60 percent oxygen, by volume, or higher. It is also within the scope of the invention to vary the oxygen content of the injected oxygen containing gases so as to control the temperature of the heat bank.

Similarly, the invention is not limited to using any particular rate for the injection of steam in those embodiments of the invention where the heat bank is generated by the injection of steam. Any suitable rate can be employed. Usually, said steam injection rate will be within a range sufficient to supply from 5,000,000 to 100,000,000 BTU per hour, depending upon the characteristics of the formation. In all instances it is preferred that the steam injection rate be such that the heat input to the formation is at least twice the heat losses in the well bore and the formation. The times of heating when using steam injection can be determined similarly as described above when injecting oxygen-containing gases.

The following example will serve to further illustrate the invention.

EXAMPLE

A sample of Morichal Group II crude oil was heated to 600.degree. F. in a one-liter glass flask and maintained as said temperature for 14 days at atmospheric pressure. Said flask was equipped with suitable liquid traps for collection of oil and liquid products boiling below 600.degree. F. and a gas collection bulb for collection of gas. The results of this test are set forth in Table III below. ##SPC3## There was also produced 6720 ml of gas (nitrogen free) having a composition including H.sub.2, C.sub.1 to C.sub.6 hydrocarbons, H.sub.2 S, COS, CO.sub.2, and O.sub.2.

The results of the above test show there was a reduction in viscosity of more than 79 percent and a reduction in sulfur content of almost 22 percent for the oil which remained in the flask. For the aliquot blend of flask oil and distilled oil the reduction in viscosity was more than 94 percent and the reduction in sulfur content was more than 24 percent. These reductions in viscosity and sulfur content represent marked upgrading in the oil treated.

While the invention has been described and illustrated in several embodiments as employing one injection well and one production well, it will be understood that in all embodiments of the invention any number of injection wells and any number of production wells, arranged in any suitable pattern, can be employed in the practice of the invention. Similarly, the well spacings set forth above in connection with the embodiment illustrated in FIG. 1 are applicable to the other embodiments of the invention.

While the invention has been described primarily with reference to producing heavy viscous crude oils of lower gravity, the invention is also applicable to the recovery of higher gravity crude oils. The invention can also be employed to recover oil from reservoirs which have already been subjected to primary and/or secondary recovery methods. The invention can also be employed in recovering high gravity oil from low energy reservoirs, e.g., reservoirs which contain very little, if any, dissolved gas, and thus very little, if any, reservoir pressure.

While certain embodiments of the invention have been described for illustrative purposes, the invention is not limited thereto. Various other modifications of the invention will be apparent to those skilled in the art in view of this disclosure. Such modifications are within the spirit and scope of the disclosure.

Claims

I claim:

1. A method of recovering hydrocarbons from a subsurface formation containing same, said formation comprising at least two permeable oil-bearing sands penetrated by at least one injection well and at least one spaced apart production well which provide communication between the surface of the earth and said formation, at least one of said sands being open to said injection well, but only one of said sands also being open to said production well, said method comprising the steps of:

a. heating said sand which is open to both said injection well and said production well for a period of time sufficient to heat a substantial portion thereof to a temperature of at least 550.degree. F. and form a substantial heat bank therein;

b. terminating heating of said now heated sand;

c. flowing a viscous hydrocarbon oil from another sand of said formation into said heated sand;

d. maintaining said hydrocarbon oil in said heated sand for a period of time sufficient to significantly reduce the viscosity thereof; and

e. resuming heating of said previously heated sand and displacing said hydrocarbon oil therefrom into said production well and recovering from said production well a hydrocarbon oil having a reduced viscosity which is less than that of said oil of step (c).

2. A process according to claim 1 wherein: said heating of said formation in step (a) is accomplished by the injection of an oxygen-containing gas to effect an in situ combustion in said heated sand; and termination of said heating in step (b) is effected by terminating injection of said oxygen-containing gas.

3. A process according to claim 2 wherein: said formation comprises a more permeable lower sand open to said injection well and said production well and disposed below a less permeable upper sand not open to said injection well and said production well; at least the major portion of said oxygen-containing gas is injected into and at least a major portion of said in situ combustion is effected in said more permeable lower sand of said formation; after termination of heating as per step (b), said hydrocarbon oil of step (c) flows from said upper sand of said formation into said combusted lower sand of said formation; and said oil of reduced viscosity in step (e) is recovered by resuming injection of said oxygen-containing gas to displace said reduced viscosity oil into said production well.

4. A process according to claim 3 wherein said oil from said upper sand of said formation is maintained in said combusted lower sand of said formation for an average period of time of at least about 10 days.

5. A process according to claim 2 wherein: said formation comprises a more permeable upper sand open to said injection well and said production well and disposed above a less permeable lower sand not open to said injection well and said production well; at least the major portion of said oxygen-containing gas is injected into and at least a major portion of said in situ combustion is effected in said more permeable upper sand of said formation; after termination of heating as per step (b), said hydrocarbon oil of step (c) is flowed by fluid drive displacement from said lower sand of said formation into said combusted upper sand of said formation; and said oil of reduced viscosity in step (e) is recovered by resuming injection of said oxygen-containing gas to displace said reduced viscosity oil into said production well.

6. A process according to claim 5 wherein said oil from said lower sand of said formation is maintained in said combusted upper sand of said formation for an average period of time of at least about 10 days.

7. A process according to claim 2 wherein: said formation comprises at least two permeable oil-bearing sands, separated from each other by a stratum of shale or other impermeable barrier, open to said injection well, but with only one of said sands being open to said production well; the major portion of said oxygen-containing gas is injected into and a major portion of said in situ combustion is effected in said sand open to said production well; after termination of said heating as per step (b), said viscous hydrocarbon oil of step (c) flows from said sand which is not open to said production well into said combusted sand; and said oil of reduced viscosity in step (e) is recovered by resuming injection of said oxygen-containing gas to displace said reduced viscosity oil into said production well.

8. A process according to claim 7 wherein said oil of step (c) is maintained in said combusted sand for an average period of time of at least about 10 days.

9. A process according to claim 1 wherein: said formation comprises at least two permeable oil-bearing sands, separated from each other by a stratum of shale or other impermeable barrier, open to said injection well, but with only one of said sands being open to said production well; said heating of said formation in step (a) is accomplished by injecting steam into said formation; the major portion of said steam is injected into and a major portion of said heating of said formation is effected in said sand open to said production well; termination of said heating in step (b) is effected by terminating said steam injection; after termination of said heating as per step (b), said viscous hydrocarbon oil of step (c) flows from said sand which is not open to said production well into said combusted sand; and said oil of reduced viscosity in step (e) is recovered by resuming injection of said steam to displace said reduced viscosity oil into said production well.

10. A process according to claim 9 wherein said oil of step (c) is maintained in said combusted sand for an average period of time of at least about 10 days.