US3047063A - Recovery of petroleum oil - Google Patents

Description

July 31, 1962 c. CONNALLY, JR, ET AL 3,047,063

RECOVERY OF PETROLEUM OIL Filed July 17, 1959 3 Sheets-Sheet l v 44114 V E 5555555555 5 5 5 55;

\I3 \II 5 5 F I G. 2

CARL CONNALLY, JR. JOHN J. JUSTEN INVENTOR.

ATTORNEY July 31, 1 62 c. CONNALLY, JR, ET AL 3,047,063

RECOVERY OF PETROLEUM OIL Filed July 17, 1959 5 Sheets-Sheet 2 CARL CONNALLY,JR. JOHN J. JUSTE'N INVENTOR.

ATTORNEY July 31, 196 c. CONNALLY, JR, ET AL 3,047,063

RECOVERY OF PETROLEUM OIL Filed July 17, 1959 3 Sheets-Sheet 3 FIG. 5

GAS OIL RATIO OIL PRODUCTION RATE Bbls Day CUMULATIVE OIL PRODUCTION BbIs.

CARL CONNALLY,JR. JOHN J. JUSTEN 1N VENTOR. 1 54.0.4

ATTORNEY United States E atenterl July 31, 1962 3,647,663 RECDVERY 8F PETRGLEUM fill.

Carl (Ionnally, .lin, Dallas, Ten and John J. Justen, Qalgary, Alberta, Qanada, assignors, by direct and mesne assignments, to Socony Mobil Oil Company, Inc, New York, N.Y., a corporation of New York Filed July 17, 1959, Ser. No. 327,823 7 Claims. (Cl. tee-ate) This invention relates to recovery of petroleum oil from a subterranean formation and relates more particularly to secondary recovery of petroleum oil from a subterranean formation by miscible drive methods.

Petroleum oil is usually recovered initially from most subterranean formations as a result of gas pressure or natural water drive forcing the oil from the petroleum bearing formation, or reservoir, to a producing well and then to the surface of the earth. As recovery of petroleum oil from the formation continues, the reservoir energy gradually decreases. A major portion of the petroleum oil still remains in the formation and at any time after the reservoir energy begins to decline secondary recovery methods may be initiated to increase the recovery of this remaining oil. Among these methods are those which have been termed miscible drive or miscible flood methods.

In miscible drive methods, a fluid phase is passed through the subterranean formation from an input well to one or more output wells by means of a driving gas forced under pressure through the formation. The material forming the fluid phase is one which is miscible not only with the petroleum oil in the formation but also with the driving gas. As a result of being miscible with the petroleum oil and with the driving gas, the fluid phase, as it passes through the formation, defines a zone varying in composition from a solution of the fluid phase in petroleum oil at its leading edge, to the fluid phase 1 per so at its central portion, to a solution of the fluid phase and the driving as at its trailing edge. Thus, in these methods, the miscible fluid phase provides a more or less distinct zone of homogeneous flowing fluid. in its passage through the formation, this fluid phase effectively displaces the petroleum oil and moves it to an output well from which it can be recovered.

While miscible drive methods are highly satisfactory for recovery of petroleum oil, they suffer from difficulties arising from non-uniformity of the formations with re spect to permeability. Variations in the relative permeability of the formation introduce the problem of obtaining a uniform sweep of the formation by the miscible fluid phase. Where the formation has zones of different relative permeability, the miscible fluid phase, and the driving gas, preferentially pass through the zone or zones of higher relative permeability. The miscible fluid phase and the driving gas passing through the zone or zones of higher relative permeability reach an output well before the miscible fluid phase and the driving gas passing through a zone or zones of lesser relative permeability. The breakthrough, i.e., arrival of the miscible fluid phase and the driving gas at the output well, results in by-passing by the driving gas of the zone of lesser relative permeability. As a result, the pertroleum oil in the zone of lesser relative permeability is unrecovered except through the employment of further measures which involve added costs. Moreover, the effluent from an output well has a high ratio of driving gas to petroleum oil requiring separa. tion of large amounts of driving gas from the petroleum oil. Thus, the extent to which uniform sweep of the formation by the miscible fluid phase and the driving gas is obtained is a measure of the efficiency of petroleum oil recovery.

It is an object of this invention to increase the recovery of petroleum oil from a subterranean formation. It is another object of this invention to obtain a uniform sweep of a petroleum-containing formation subjected to a miscible. drive procedure. it is another object of this invention to increase the efficiency of a miscible drive procedure. It is another object of this invention to reduce the ratio of driving gas to petroleum oil in the effluent from a subterranean formation subjected to a misible drive procedure. Other objects of the invention will become apparent from the following detailed description.

In accordance with the invention, petroleum oil is reu from an oil-containing subterranean formation ed with an input well and at least one output well by a cedure which involves among other steps the step of injecting water through the input Well to establish a water phase in the formation. In this procedure, the first step involves establishing within the formation a fluid phase, which fluid phase is formed of a material which is miscible with the petroleum oil in the formation. A second step involves injecting a driving gas into the input well and forcing the fluid phase through the formation into the direction of an output well, which driving gas is formed of a gas which is miscible with the material forming the fluid phase. The driving gas is injected into the input well until the driving gas enters the output well. A third step involves, subsequent to the time the driving gas enters the output well, discontinuing injection of the driving gas into the input well. At this point there is carried out the step of injecting the water through the input well to establish the water phase in the formation. A final step involves thereafter injecting a driving fluid into the input well to force the water phase through the formation into the direction of the output well.

FIGURE 1 is a schematic diagram illustrating a subterranean formation at one stage of the process of the invention.

FEGURE 2 is a schematic diagram illustrating the subterranean formation at another stage of the process of the invention.

FEGURE 3 is a schematic diagram illustrating the subterranean formation at still another stage of the process of the invention.

FIGURE 4 is a schematic diagram illustrating the subterranean formation at a later stage of the process of the invention.

FIGURE 5 is a graph illustrating oil production rate, gas-oil ratio, and cumulative petroleum oil production of a subterranean formation treated in accordance with the invention.

Referring to FIGURE 1, a subterranean formation, indicated generally by the numeral 1, is penetrated by an input well 2 and a plurality of output wells, of which, for purposes of simplicity of illustration, only well 3 is shown. The formation, also for purposes of simplicity, is indicated as consisting of only two zones, namely, upper zone 4 and lower zone 5, each of which has diflerent relative permeabilities to the flow of fluids. Zone 4 has a higher relative permeability to the flow of fluids than zone 5. Further, for purposes of simplicity of illustration, zones 4 and 5 are of equal thickness. Originally, each of the zones contains petroleum oil. While two zones have been illustrated, it will be understood that formation 1 may contain more than two zones and adjoining zones will have different relative permeabilities. Each of the zones will constitute a flow path through the formation from the input well to the output well.

A fluid phase formed of a material which is miscible with the petroleum oil in the formation is established in formation ll. This is accomplished by injecting the material forming the fluid phase into the formation through input Well 2. The material forming the miscible fluid phase which is injected through Well 2 will distribute itself in zones 4 and 5 in amounts which are proportional to the product of their thicknesses and their relative permeabilities to the material forming the miscible fluid phase. Since zone 4 has a higher relative permeability to the material forming the miscible fluid phase than zone 5, but an equal thickness, a greater volume of the material forming the miscible fluid phase will enter zone 4 than zone 5.

Following establishment of the miscible fluid phase in zones 4 and 5, the miscible fluid phase is passed through the formation in the direction of the output well 3. Passage of the miscible fluid phase through the formation is effected by injecting driving gas into input well 2. The driving gas will distribute itself in zones 4 and 5 in amounts proportional to the product of the thicknesses and the relative permeabilities to the driving gas of zones 4 and 5. Accordingly, a greater volume of driving gas per unit thickness of the zone will enter Zone 4 than will enter zone 5. In this connection, also, for any given rate of injection of the driving gas into the formation from input well 2 the driving gas will enter the more permeable zone 4 at a rate per unit thickness of the zone greater than it will enter the less permeable zone 5. Therefore, after establishment of the miscible fluid phase in formation 1 and injection of the driving gas, the miscible fluid phase in zone 4 will have advanced a greater distance through the formation into the direction of output well 3, than the miscible fluid phase in zone 5. FIGURE 1 illustrates this stage of the procedure. Miscible fluid phase in zone 4 is greater in volume than miscible fluid 11 in zone 5. Further, driving gas 12 which has entered zone 4 is greater in amount than driving gas 13 which has entered zone 5 and, thus, miscible fluid phase 10 has advanced through the formation from the input well a greater distance than miscible fiuid phase 11. As a consequence, a greater amount of petroleum oil per unit thickness of formation will have been displaced from zone 4 into output well 3 than has been displaced from zone 5 and the petroleum oil 14 remaining in zone 4 will be less in amount per unit thickness .of formation than the petroleum oil 15 remaining in zone 5.

With continued injection of driving gas into formation 1 from input well 2, miscible fluid phases 1t} and '11 continue to advance through formation 1 to output well 3 displacing the petroleum oil in the formation and moving it before them in the direction of the output well. Eventually, miscible fluid phase 19 displaces all of the oil from zone 4 into input well 3 and the miscible fluid phase 10 enters the output well. During the time that the petroleum oil in zones 4 and 5 is being advanced into output well 3, the eflluent recovered at the surface of the earth from the output well is petroleum oil and gas and water naturally present with the oil in formation 1. At the arrival of miscible fluid phase 10 at the output well and entrance of the miscible fluid phase into the output well, the effluent will consist of the miscible fluid phase and the petroleum oil, and gas and water with which the oil was naturally associated in the formation, from zone 5. Subsequently, when all of the miscible fluid phase along any flow path has passed into the output well, driving gas will also enter the output well and the character of the effluent from the output well will again change. This change will be characterized by the appearance in the effluent of the driving gas. Where the driving gas is the same as the gas naturally associated with the petroleum oil in the formation 1, the entry of the driving gas into the output well will be manifested by an increase in the amount of this gas relative to the amount of the petroleum oil. In any case,

breakthrough of the driving gas into the output well will be manifested by an increase in the eflluent of the ratio of the amounts of gas to petroleum oil.

At breakthrough of the driving gas from zone 4 into output well 3, conditions within formation 1 will be as illustrated in FIGURE 2. The driving gas 12 will have occupied all of zone 4 and will be passing through zone 4 from input well 2 to output well 3. Except for minor amounts of petroleum oil 15 picked up from zone 5, the passage of the driving gas 12 through zone 4 is not accompanied by advance of petroleum oil to output well 3. Miscible fluid phase .11 will have advanced through the formation in the direction of the output well and this advance will have resulted in movement of petroleum oil 15 into the output well.

Subsequent to breakthrough of the driving gas into output well 3, as manifested by a change in the ratio of the amount of gas to the amount of petroleum oil in the efl luent from the well, injection of driving gas into formation 1 from input well 2 is discontinued. The injection of driving gas can be discontinued at the time the driving gas enters the output well. However, the injection of driving gas can be discontinued at any time thereafter. For example, engineering or economic considerations may indicate the desirability of continued injection of driving gas after breakthrough at the output well. Eventually, as the driving gas from more and more flow paths enters the output well, the ratio of gas to petroleum oil in the eflluent will reach a sufliciently high point that the previous considerations indicating desirability of continuing the injection of driving gas are no longer valid. At this point, injection of driving gas is discontinued. Ordinarily, injection of driving gas is discontinued when the ratio of gas to petroleum oil in the effluent from the output well is more than 500 cubic feet of gas per barrel of oil above the ratio established before breakthrough of the driving gas. In most instances, the injection of driving gas is discontinued when the ratio of gas to petroleum oil in the effluent from the output well becomes more than 10,000 cubic feet of gas per barrel of oil above the ratio established before breakthrough of the driving gas.

Following discontinuance of injection of the driving fluid, water is injected into formation 1 through input well 2 to establish a water phase in the formation. The water injected through the input well and entering the formation will distribute itself between zones 4 and 5 in amounts in proportion to the product of the thicknesses and the relative permeabilities to water of the two zones. For any given amount of water injected into the formation, or any given rate of injection of water, the greater amount of the water per unit thickness of formation will enter zone 4 than zone 5.

After the step of injection of water into the formation, a driving fluid is injected into the formation through the input well 2. The driving fluid will distribute itself between zones 4 and 5 in amounts proportional to the product of the thicknesses and relative permeabilities of the two zones. FIGURE 3 illustrates the formation at this stage of the process. Water phase 20 in zone 4 is greater in volume than water phase 21 in zone 5. The driving fluid 22 in zone 4 is in greater amount than the driving fluid 23 in zone 5. Consequently, water phase 29 will have advanced through formation 1 toward output well 3 a greater distance than water phase 21. Miscible fluid phase 11 will have advanced further into the formation in the direction of the output well 3 and petroleum oil 15 still remains in zone 5..

With continued injection of the driving fluid into the formation through the input well, the fluid phases will advance through the formation into the direction of the output well. However, as a result of the injection of the water, and the presence of the moving water phase in the formation, the rate of advance of the fluid phases in the formation, for any given pressure differential between the input well and the outputwell, will have been reduced. Further, the extent to which the rate of advance of the fluid phases will have been reduced will be greater in the more permeable formation 4 than in the less permeable formation 5. Thus, the rate at which driving gas 12 and petroleum oil 15 enter the output well 3 will be reduced. However, the rate at which the driving gas enters the output well will have been anemone reduced to a greater extent than the rate at which the petroleum oil enters the output well. As a result, the ratio of driving gas to petroleum oil in the eflluent from output well 3 is decreased.

As the water phase advances through the formation, portions of the Water phase will be trapped in residual pores or dead-end channels of the formation or will be otherwise removed from the advancing water phase. Accordingly, as the water phase advances through the formation, it erodes or depletes and becomes progressively smaller in volume. While we do not wish to limit our invention to the consequences of any theory, it is believed that the proportionately greater decrease in the rate of advance of the fluid phases in the more permeable zone is due to two facts, namely, (1) depletion of the water phase as it advances through the formation and (2) the entrance of a relatively greater amount of water per unit thickness into the more permeable Zone.

With the injection into the formation of the water phase, the relative permeability of the formation to the subsequently injected driving fluid is reduced. The water phase in zone is originally smaller in amount per unit thickness than the water phase in zone 4. The factors causing depletion of the advancing water phase will be substantially the same in zones 4 and 5. Accordingly, for a given distance of travel of the water phases in the two zones, the water phase in zone 5 will deplete to a proportionately greater extend than the water phase in zone 4. The advancing water phase in zone 5 will eventually deplete to the point of disappearance while the advancing water phase in zone 4-, although reduced in volume, will remain. Simultaneously with depletion of the advancing water phase, its effect on the relative permeability of the formation to the driving fluid becomes less. Upon disappearance of the advancing water phase,

its blocking effect on the flow of the driving gas is substantially reduced. Accordingly, the blocking efiect of the advancing water phase in zone 5 will be reduced to a greater extent than the blocking effect of the advancing water phase in zone 4.

The final stage of the process of the invention is as illustrated in FIGURE 4. All of petroleum oil of FIGURE 3 has been produced through output well 3. The miscible fluid phase 11 in zone 5 has advanced to output well 3 and is entering the output well. The advancing water phase 21 of FIGURE 3 has disappeared. The driving fluid 23 is adjacent to the driving gas 13. In zone 4, the advancing water phase remains although it has diminished in volume and it has approached output well 3.

It is an essential step of the invention that Water be injected one time only into the formation. With injection of water into the formation, a greater amount of the injected Water per unit thickness will enter the more permeable zone and a lesser portion will enter the less permeable zone, as indicated. The amount which will enter each zone, also as indicated, is proportional to the relative permeabilities to water of each zone. Thus, at least a portion of the injected water will enter the less permeable zone. This water entering the less permeable zone of the formation decreases the rate of advance in this zone. Desirably, the rate of advance in the less permeable formation should not be lowered and, therefore, water should not be injected into the less permeable zone. On the other hand, such result is generally not attainable in practical operations. However, the amount of water injected into the less permeable formation can be kept at a minimum. This is done by injecting all of the water at one time into the formation. With one injection of water, the greater portion will enter the more permeable zone. As a result, the rate of advance in the more permeable zone is reduced to a greater extent than in the less permeable zone. Stated otherwise, the permeabilities in the two zones are changed relative to each other. With the permeabilities thus changed, a

subsequent injection of water would result in a relatively larger portion entering the less permeable zone and a relatively lesser portion entering the more permeable zone. As a result, the permeability of the less permeable zone would be reduced to a relatively greater extent than that of the more permeable zone. Thus, entrance of a minimum amount of water into the less permeable zone with minimum reduction in relative permeability of this zone is effected by injection of water at only one time.

The miscible fluid phase may be formed of any material heretofore employed for this purpose in miscible flood operations. The material may be a liquid or a gas. Preferably, the material is a liquefied normally gaseous hydrocarbon. Hydrocarbons which may be employed for this purpose include propane, butane, and pentane. Mixtures of these hydrocarbons may also be employed. A preferred mixture of liquefied normally gaseous hydrocarbons is the mixture commonly termed liquefied petroleum gas, or LPG. This mixture ordinarily consists predominantly of propane and butane with minor amounts of ethane and pentane. Heavier, normally liquid hydrocarbons, such as naptha, may also be employed. Additionally, materials other than hydrocarbons may be em ployed. For example, oxygen-containing compounds such as alcohols, ketones, dioxane, andn carbon dioxide may be employed.

The pressures employed for injecting the material forming the miscible fluid phase in the formation may be as desired. Ordinarily, ressures at which the material forming the miscible liquid phase is injected into the formation will be between 1,000 and 5,000 pounds per square inch gauge. be employed. 7

The amount of material forming the miscible liquid phase to be employed may be determined by examination of the formation by core analysis or may be determined by the use of models of the formation, as known in the art. The amount to be employed depends to some extent upon the distance, and thus the area, of the formation to be swept between the input well and an output well. Ordinarily, the amount of material to be employed will be between 1 and 10 percent of the hydrocarbon pore volume of the formation between the input well and the output well. By hydrocarbon pore volume is meant the gore volume of the formation occupied by hydrocarbon uid.

The driving gas may be any gas which is miscible with the material forming the miscible fluid phase. Where the miscible fluid is, under the pressure conditions employed and the temperature of the formation, in the gaseous phase, any gas may be employed since gases are miscible in all proportions. Where the material forming the miscible fluid phase is, under the pressure conditions employed and the temperature of the formation, a liquid phase, the driving gas must be a gas miscible with the liquid phase. For example, where the liquid phase is a hydrocarbon, the driving gas can be a hydrocarbon gas. A suitable hydrocarbon gas is methane. The driving gas need not consist entirely of methane but may contain minor proportions of other constituents. A suitable driving gas is separator gas which consists predominantly of methane with some ethane and minor amounts of propane and higher molecular weight hydrocarbons. Where the material forming the miscible fluid phase is an oxygen-containing material such as an alcohol, a ketone, or dioxane, the driving gas may be carbon dioxide.

The amount of water to be employed in the water injection step will depend upon the permeabilities to water of the various zones of the formation relative to each other. As indicated, it is preferred that the amount of Water injected into the zone of lesser permeability be at a minimum. Thus, Where the ratio of the permeabilities of the zones is large, a larger amount of water will be However, higher and lower pressures may ace-zoos employed than where the ratio is lower, since the greater proportion of this water will enter the zone of higher permeability. Generally, satisfactory results are obtained Where the amount of water injected into the formation is at least as great as five percent of the total hydrocarbon pore volume previously swept by the driving gas. However, amounts of water as high as 40 percent of the total hydrocarbon pore volume previously swept by the driving gas may be employed.

The driving fluid injected into the formation through the input well following injection of the water may be any type of nonaqueous fluid heretofore employed as a driving fluid in secondary recovery operations. Suitably, the driving fluid is the same material employed for the driving gas. Thus, the driving fluid can be a hydrocarbon such as methane or a gas containing methane such as separator gas. The gas may also be carbon dioxide.

Miscible flood operations can be characterized broadly into three methods depending upon the means by which the miscible fluid phase is developed within the formation. In one method, the miscible fluid phase is developed in-situ by injection of a normally gaseous material, such as a gas containing a large amount of methane, into the formation. The injection is carried out at pressures which may be above about 3,000 pounds per square inch gauge and the gas will dissolve in the condensable hydrocarbons in the petroleum oil to form a liquid phase. This method is known as the high pressure gas, miscible flood method. The second method is similar to the first.

method but involves the injection of the gas at a lower pressure. To effect development of a miscible liquid phase in-situ at lower pressures, the injected gas is enriched with hydrocarbons heavier than methane such as propane and minor amounts of butane and pentane. By virtue of the presence of the heavier hydrocarbons in the gas, the gas, after picking up low boiling hydrocarbons from the petroleum oil in the formation, will condense at the lower pressures employed. Pressures employed in this second method will be above 1,000 pounds per square inch gauge but will be lower than those required where the injected gas consists predominantly of methane. This method istermed the enriched gas or condensed miscible flood method. In the third method, the miscible fluid phase is developed by injecting a condensable hydrocarbon, such as liquefied petroleum gas, propane, butane, or naphtha, under pressures such that the injected gas Will be in the liquid phase within the formation. The pressures employed in .this method are usually about 1,000 pounds per square inch gauge. This method is known as the miscible plug method. The procedure of the present invention is applicable in connection with any of these methods, as well as other methods, of effecting recovery of petroleum oil by miscible drive.

The following example will be illustrative of the invention. Miscible drive was to be employed for secondary recovery of petroleum oil from a subterranean formation. The formation was penetrated by an input well and by four production wells located equidistantly from each other and equidistantly from the input well. Each of the wells penetrated. the same zones in this formation. The formation contained a large number of zones and the Zones'varied in permeability from about one to 200 millidarcies as determined by laboratory tests on core samples taken during drilling of the wells. 7

As a first step, a quantity of liquefied petroleum gas was injected under pressure into the input well. The liquefied petroleum gas consisted of the following hydrocarbons in the amounts indicated: propane-48%, isobutane-12%, normal butane-39%, and isopentane- 1%. The injection pressure was 2,000 pounds per square inch gauge and the amount of liquefied petroleum gas injected was equal to five percent of the hydrocarbon pore volume of the formation between the input well and the four output wells.

As a second step, a driving fluid was injected into the input well and into the formation. The driving gas was separator gas and contained the following hydrocarbons in the amounts indicated: methane84%, ethane7%, propane6%, butane2%,and pentane and higher hydrocarbonstrace. During injection of the separator gas, petroleum oil was recovered from the four production wells. At the beginning of the injection of the separator gas, the average gas-oil ratio of the effluent from the four production wells was approximately 500 cubic feet of gas per barrel of oil. Also, the rate of oil production from the four production wells at the beginning of the injection of the separator gas was 150 barrels of oil per day. Referring to FIGURE 5, the initial rate of oil production is indicated at 30 and the initial gas-oil ratio is indicated at 31.

With continued injection of driving fluid, the gas-oil ratio of the eflluent from the four production Wells gradually increased as the driving fluid from different flow paths to each output well reached the output well. Further, the rate of oil production from the four production wells also increased. Eventually, the oil production rate reached a maximum of 600 barrels per d y, as indicated at 33, but thereafter the ratev of production gradually declined. The gas-oil ratio remained substantially constant as the rate of oil production increased. However, the gas-oil ratio began to increase at the same timethat the rate of oil production decreased.

The decrease in the rate of oil production and the increase in the gas-oil ratio were evidence that the driving fluid from various flow paths had broken through into the output wells. However, injection of separator gas as a driving fluid was continued from the standpoint of economy since the decline in oil production rate was not excessive and the gas-oil ratio was not excessively high. Eventually, the oil production rate declined to a point which was below its initial rate. This occurred, as indicated at 35, when the cumulative oil production was about 56,000 barrels of oil. At this point, it was calculated that about 68 percent of the oil in the formation between the input well and the four production wells had been produced. Further, at this point, as indicated at 40, the gas-oil ratio was about 10,500 cubic feet of gas per barrel of oil.

With the gas-oil ratio at the production wells being 10,500 cubic feet per barrel, as a third step of the process, injection of the driving fluid was discontinued. Water was then injected into the formation, to establish a water phase within the formation, as a fourth step of the process. About 7,500 barrels of water were injected into the formation. This amount of water was equal to about 11 percent of the formation pore volume occupied by the injected separator gas. Prior to the injection of the water, the rate of oil production was about barrels per day, as indicated at .35. During the time that the water phase was being established in the formation, the oil production rate decreased to about 50 barrels per day, as indicated at 41. Further, the gas-oil ratio decreased to about 5,900 cubic feet per barrel.

Following injection of the water, a driving fluid was injected into the formation, as indicated at 42, as a fifth step of the process. This driving fluid was the same separator gas employed as a driving gas in the second step of the procedure. With injection of the driving fluid, the oil rate increased to about barrels per day, as indicated at 43, and the gas-oil ratio increased slightly, as indicated at 44. Thereafter, the gas-oil ratio remained constant at about 6,300 cubic feet per barrel, and the oil production rate remained constant at about 150 barrels per day.

The decrease in the gas-oil ratio from 10,500 cubic feet per barrel to 6,300 cubic feet per barrel was the effect of the decrease in the rate of flow through the more permeable zones of the formation by the injection of the water.

phase into the formation. With the establishment of the water phase in the formation, the effective permeability of.

the formation to the driving fluid thereafter injected was decreased. Thus, the rate at which the driving gas entered the output well was reduced. Simultaneously, there was an increase in the oil production rate from about 100 barrels per day to about 150 barrels per day. Thus, a greater portion of the driving fluid was entering the less permeable zones of the formation which still contained oil and this oil was being produced at the output wells.

Having thus described our invention, it will be understood that such description has been given by way of illustration and example and not by way of limitation, reference for the latter purpose being had to the appended claims.

We claim:

1. In the process for recovering petroleum oil from a subterranean formation containing Zones of different permeabilities and provided with an input well and at least one output well, the steps comprising:

(1) injecting into said formation through said input well a material miscible with said petroleum oil in said formation to establish a fluid phase within said formation in a flow path between said input well and said output well,

(2) injecting a driving gas, said driving gas being a gas miscible with said fluid phase, into said formation through said input well and forcing said fluid phase through said formation into the direction of said output well whereby petroleum oil is displaced from within said formation and enters said output well, said driving gas distributing itself within each of said zones of different permeabilities in said formation in amounts proportional to the product of the thickness and permeability of each of said zones and said fluid phase advancing into the direction of said output well at a higher rate in a zone of higher permeability than in a zone of lower permeability,

(3) recovering petroleum oil from said formation through said output well,

(4) continuing injection of said driving gas into said input well at least until said driving gas enters said output well from a zone of higher permeability and the ratio of said driving gas to said petroleum oil entering said output well increases,

(5) subsequent to the time said driving gas enters said output well from a zone of higher permeability in said formation discontinuing injection of said driving gas through said input well,

(6) thereafter injecting water into said formation through said input Well to establish a water phase in said formation whereby said water distributes itself within each of said zones of different permeabilities in said formation in amounts proportional to the product of the thickness and permeability of each of said zones and reduces the permeability of said zones,

(7) discontinuing injection of said water through said input well, and

(8) thereafter injecting a non-aqueous driving fluid into said formation through said input well and forcing said water phase through said formation into the direction of said output well whereby the rate of advance of fluid through said zones of different permeabilities will be reduced to a relatively greater extent in a zone of higher permeability than in a zone of lower permeability and flow of said driving gas into said output well is reduced to a relatively greater extent from a zone of higher permeability than a zone of lower permeability and the ratio of said driving gas to said petroleum oil entering said output well decreases.

2. The process of claim 1 wherein said fluid phase is established within said formation by injecting into said formation through said input well said material in the amount of 1 to 10 percent of the hydrocarbon pore volume of said formation between said input well and said output well.

3. The process of claim 1 wherein said fluid phase is formed of a liquefied normally gaseous hydrocarbon.

4. The process of claim 1 wherein said fluid phase is formed of liquefied petroleum gas.

5. The processof claim 1 wherein said water is injected into said formation through said input well in an amount equal to 5 to 40 percent of the hydrocarbon pore volume swept by said driving gas.

6. The process of claim 1 wherein said non-aqueous driving fluid is the same material as said driving gas.

7. In the process for recovering petroleum oil from a subterranean formation containing zones of different permeabilities and provided with input means including at least one input well and output means including at least a one output well, the steps comprising:

(1) injecting into said formation through said input means a material miscible with said petroleum oil in said formation to establish a fluid phase Within said formation in a flow path between said input means and said output means,

(2) injecting a driving gas, said driving gas being miscible with said fluid phase, into said formation through said input means and forcing said fluid phase through said formation into the direction of said output means whereby petroleum oil is displaced from within said formation and enters said output means, said driving gas distributing itself within each of said zones of different permeabilities in said formation in amounts proportional to the product of the thickness and permeability of each of said zones and said fluid phase advancing into the direction of said output means at a higher rate in a zone of higher permeability than in a zone of lower permeability,

(3) recovering petroleum oil from said formation through said output means,

(4) continuing injection of said driving gas into said input means at least until said driving gas enters said output means from a zone of higher permeability and the ratio of said driving gas to said petroleum oil entering said output means increases,

(5) subsequent to the time said driving gas enters said output means from a Zone of higher permeability in said formation discontinuing injection of said driving gas through said input means,

( 6) thereafter injecting water into said formation through said input means to establish a water phase in said formation whereby said water distributes itself within each of said zones of different permeabilities in said formation in amounts proportional to the product of the thickness and permeability of each of said zones and reduces the permeability of said zones,

(7 discontinuing injection of said water through said input means, and

(8) thereafter injecting a non-aqueous driving fluid into said formation through said input means and forcing said water phase through said formation into the direction of said output means whereby the rate of advance of fluid through said zones of different permeabilities will be reduced to a relatively greater extent in a zone of higher permeability than in a zone of lower permeability and flow of said driving gas into said output means is reduced to a relatively greater extent from a zone of higher permeability than a zone of lower permeability and the ratio of said driving gas to said petroleum oil entering said output means decreases.

References Cited in the file of this patent UNITED STATES PATENTS (Other references on following page) 11 if FOREIGN PATENTS V Talash, A. W'.,-et ale: Miscible Displacement After 696,524 Great Britain Sept 2, 1953 Waterflooding, Petroleum Engineer, September 1957,

pp. B-27-B-30. I OTHER REFERENCES Clark, N. J., et al.: Latest Oil Recovery Idea; Petro- Trends in Oil Recovery, R. A. Morse, Producers 5 leum Engineer, September 1957 pp. B-21-B26. Monthly, February 1960.

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