US3823776A

US3823776A - Oil recovery method by oxidation and forming surfactants in situ

Info

Publication number: US3823776A
Application number: US00354823A
Authority: US
Inventors: B Holmes
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1973-04-26
Filing date: 1973-04-26
Publication date: 1974-07-16
Anticipated expiration: 1991-07-16

Abstract

This specification discloses a method of producing oil from an oil-containing subsurface formation penetrated by an injection well and a production well. An oxygen-containing gas is injected into the formation to rapidly oxidize a portion of the oil therein and establish a stabilized in situ combustion zone. The oxidation of the oil forms organic acids within and surrounding the in situ combustion zone. An aqueous caustic solution is injected into the formation to quench the combustion within the in situ combustion zone and to react with the organic acids within and surrounding the in situ combustion zone and form surfactants in situ. Oil is produced from the formation facilitated by the surfactants and the heat of the in situ combustion zone.

Description

Holmes 11] 3 ,823,776 [451 July 16, 1974 OIL RECOVERY METHOD BY OXIDATION AND FORMING SURFACTANTS IN SITU [75] Inventor: Billy G. Holmes, Lancaster, Tex.

[73] Assignee: Mobil Oil Corporation, New York,

[22] Filed: Apr. 26, 1973 [2]] Appl. No.: 354,823

[52] 11.8. C1. 166/261, 166/270 [51] Int. Cl E2lb 43/22, E2lb 43/24 [58] Field of Search 166/256, 261, 270

[56] References Cited UNITED STATES PATENTS I 1,651,311 11/1927 Atkinson 166/275 2,288,857 7/1942 3,036,631 5/1962 3,047,062 7/1962 3,115,929 12/1963 3,387,655 6/1968 3,638,727 2/1972 Primary Examiner-Stephen J. Novosad Attorney, Agent, or FirmA. L. Gaboriault; Henry L. Ehrlich [5 7] ABSTRACT This specification discloses a method of producing oil from an oil-containing subsurface formation penetrated by an injection well and a production well. An oxygen-containing gas is injected into the formation to rapidly oxidize a portion of the oil therein and establish a stabilized in situ combustion zone. The oxidation of the oil forms organic acids within and surrounding the in situ combustion zone. An aqueous caustic solution is injected into the formation to quench the combustion within the in situ combustion zone and to react with the organic acids within and surrounding the in situ combustion zone and form surfactants in situ. Oil is produced from the formation facilitated by the surfactants and the heat of the in situ combustionzone.

6 Claims, 2 Drawing Figures EPAIENTEDJUL 1 arm FIG.I

FIG.2

1, OIL RECOVERY METHOD BY OXIDATION AND FORMING SURFACTANTS IN SITU BACKGROUND OF THE INVENTION This invention relates to the recovery of oil from a subsurface oil-containing formation and more particularly relates to the recovery of heavy and highly viscous oil from subsurface formations.

Subsurface formationsthat contain oil or hydrocarbons trapped therein are generally referred to as reservoirs. Such reservoirs have associated therewith naturally occurring forces which may be used in the production of the oil from the formation via wells communicating therewith. These naturally occurring forces include: (I) the expanding force of high pressure gas, (2) the buoyant force of encroaching water, and (3) the force of gravity. Initial recovery techniques which employ principally these naturally occurring forces for producing oil from a subsurface formation are generally referred to as primary recovery techniques. Primary recovery techniques which employ principally the naturally occurring forces in the reservoir are generally characterized by a high residual oil content remaining in the reservoir at the end of primary production. In order to recover a portion of this high residual oil content, secondary recovery techniques are often employed, which techniques utilize forces applied from which effects emulsification of the petroleum. The emulsifying agent may be formed by a reaction between the components present in the petroleum and in extraneous energy sources to supplement the naturally occurring forces in .the reservoir. These secondary forcesmay result from, for example, gas. injection, steam injection, water injection, or in situ combustion. It is not necessary that the primary forces of the reservoir be exhausted before secondary recovery be initiated; In fact, good reservoir engineering practices many times dictate that secondary recovery be begun early in the primary recovery cycle, though this is sometimes called pressure maintenance rather than secondary recovery.

Waterflooding techniques have long been employed as secondary recovery techniques for recovering oil from subsurface formations. Generally, in accordance with these techniques, at least an injection well and a production well are provided which communicate with the reservoir and water is injected via the injection well into the reservoir and oil is produced via the production well. Many modifications of the basic Waterflooding techniques have been employed. These modifications include the use of various chemicals and materials in the water injected into the formation to improve the recovery of oil therefrom. Materials have been employed in the water to thicken the water and thereby improve the efficiency of the water in driving the oil from the formation. Also, surfactants have been included in the water to reduce the surface tension of the oils in the formation and thereby facilitate their production therefrom.

Alkaline or caustic waters have been described for flooding certain types of reservoirs. For example, in US. Pat. No. 1,651,31 lthere is described a method for recovering oil from an oil-bearing formation by injecting a strong alkali, preferably in .a saturated aqueous solution, into the formation. In US. Pat. No. 2,288,857 there is disclosed a technique for recovering petroleum from subsurface formations by injecting into the formation an aqueous solution which aids in stripping the petroleum from, the formation and depressing the interfacial tension between the petroleum and the water and the aqueous solution. Thus, when using aqueous solutions containing caustic alkali, such as sodium, and potassium hydroxides or ammonium hydroxide or alkali salts, an emulsifying soap may be formed by the reaction between the alkali and the organic acids naturally present in the petroleum. This type of emulsifying agent is useful mainly with restricted types of asphaltic crudes having sufficient acids of the proper molecular weight to yield an emulsifying agent on reaction with alkali. In US. Pat. No. 3,036,631, there is described another method of producing oil from a reservoir by means of a caustic waterfiood. An oxygen-containing gas is injected into the oil-containing formation under controlled conditions to oxidize the oil therein while avoiding ignition of the oil in the formation. The controlled oxidation partially oxidizes the hydrocarbon molecules of the oil to produce organic acids, alcohols, ethers, aldehydes, ketones, etc. These organic compounds exhibit surface tension depressing properties, especially in the presence of caustic. After the oxidation step, production is resumed and an aqueous caustic solution is injected through the injection wells and the solution is driven through the reservoir by the injection of ordinary water.

Such Waterflooding techniques are effective in recovering oil from many types of oil-containing formations. However, in the recovery of heavy and highly viscous oils, Waterflooding techniques leave much to be" desired. Oneapproach to the recovery of heavy oils from formations is to use heat' to reduce the viscosity of the oil and thereby facilitate theirrecovery. Heat may be supplied, for example, by initiating in situ combustion in a formation and burning a'portion of the oil therein to generate heat to lower the viscosity of the oil in the formation. Water is sometimes used in conjunction with in situ combustion processes. For example, in US.

Pat. No. 3,163,216, an oil-bearing formation is ignited and water is injected into the burning zone to transmit heat to a greater volume of the formation. It is there pointed out that the injection of water directly into the formation immediately after the latter has been ignited does not extinguish combustion but serves to diffuse the heat over a larger volume of .the formation. This renders the heated zone less susceptible to a drop in temperature below that required to sustain combus-- tion.

In US. Pat. No. 3,196,945, there is described a method for carrying out a forward combustion process under conditions requiring less air than is necessary for conventional methods. The reservoir is first ignited after which air or equivalent oxygen-containing gas is injected in an amount sufficient to establish a definite combustion zone or front. Thereafter, water or other suitable condensable fluid isv injected into the formation and contactsthe front, thereby cooling the front and vaporizing the condensable .fluid- The resulting vapors are forced ahead of the front and push oil along with them. These vapors in moving forward of the front contact cold reservoir rock and condense, thereby heating that portion of the reservoir. Air or other oxy- 1 aforesaid condensation. Water injectionis then resumed and air injection is stopped.

SUMMARY OF THE INVENTION This invention is directed to a method of producing oil from an oil-containing subsurface formation penetrated by an injection well and a production well. An oxygen-containing gas is injected via the injection well into the formation in a sufficient amount and at a sufficient pressure to rapidly oxidize the oil and form organic acids and establish a stablized in situ combustion zone in the formation. An aqueous caustic solution is injected via the injection well into the formation in a sufficient amount to quench the in situ combustion and flow liquid aqueous caustic solution through the in situ combustion zone and into contact with the oil on the other side thereof. During the course of flow the aqueous caustic solution reacts with the organic acids present and forms surfactants which lower the interfacial tension between the oil and the aqueous solution.

Thereafter, oil is produced via the production well from the formation. In an embodiment there is injected a sufficient amount of aqueous caustic solution to quench and cool the in situ combustion zone to a temperature of no more than 150 F. and thereafter there is injected via the injection well into the formation a polymer solution to drive the aqueous caustic solution and surfactants through the formation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic vertical section of a reservoir penetrated by an injection well and a production well illustrating the method of this invention.

FIG. 2 is'the same schematic vertical section illustrating another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is directed to a method of producing oil from an oil-containing subsurface formation penetrated by an injection well and a production well and is particularly applicable to producing heavy oils having low acid values. r

In accordance with this invention there is described a process for producing oil from an oil-containing for-' mation that ispenetrated by an injection well and a production well. An oxygen-containing gas, air being the most common such gas, is injected via the injection well into the formation in a sufficient amount and at a sufficient pressure to rapidly oxidize the oil. The injection of the oxygen-containing gas is continued until there is established in the formation a stabilized in situ combustion zone. Thereafter, an aqueous caustic solution is injected via the injection well into the formation in a sufficient amount to quench the combustion in the in situ combustion zone and flow liquid aqueous caustic solution through the in situ combustion zone and into contact with the oil in the formation on the other side thereof and oil is produced via the production well y from the formation.

such-things as aldehydes, ketones, ethers, and alcohols. The injection of the oxygen-containing gas is continued and there is established a stabilized in situ combustion zone in the formation. A characteristic of a stabilized in situ combustion zone is that the temperature in the zone will increase to a temperature of at least 500 F. Thereafter, injection of the oxygen-containing gas is interrupted and an aqueous caustic solution such as a solution of sodium hydroxide or potassium oxide is injected via the injection well into the formation. Initially the aqueous caustic solution that enters the in situ combustion zone is vaporized, and thereby forms steam and leaves behind in the in situ zone the salts which were the solutes in the aqueous caustic solution. The steam passes through the in situ combustion zone and is condensed by the cool formation in advance of the in situ combustion zone. The injection of the aqueous caustic solution is continued until the combustion taking place in the in situ combustion zone is quenched and the temperature in the zone is reduced sufficiently to allow the aqueous caustic solution to be flowed through the zone as a liquid. Normally this reduced temperature will be no greater than about the boiling temperature of the aqueous caustic solution at the pressure of the subsurface formation. The aqueous caustic solution that is flowed in liquidform through the in situ combustion zone and into contact with the oil on the other side thereof reacts with the organic acids and forms surfactants which lower the interfacial tension between the oil and the aqueous solution. This reduction in int'erfacial tension greatly facilitates the recovery of oil via the production well via the formation;

After establishing a stabilized in situ combustion zone in the formation and prior to quenching the zone with the aqueous caustic solution, injection of the oxygen-containing gas may be continued to propagate the in situ combustion into the formation to form an in situ combustion zone of a predetermined size. The most desirable size will vary with the particular formation and oil-therein but normally will be no greater than 10 percent of the pore volume of the formation. The. term pore volume is used to mean the pore volume within the drainage pattern that is related to the injection and production wells.

In accordance with an embodiment, the aqueous caustic solution is injected via the injection well into the formation in a sufficient amount to cool the formation to a temperature sufficiently low that a thickened water may be injected into the formation without deterioration that is without the loss of viscometric properties of the thickened water. The temperature to which the formation should be cooled will depend upon properties of the thickened water. It is desirable that the formation be cooled to a temperature of no more than about F. to ensure that the thickened water employed is not deteriorated. By thickened water is meant a water having thickening agents added thereto to increase the viscosity of the water and thereby improve the efficiency of the water as a driving agent. Any thickening agent which is compatible with the aqueous caustic solution, the formation, and the temperature of the formation may be employed. Many such thickening agents are known and include such diverse compositions as the natural gums, the water-soluble cellulose compounds such as carboxymethylcellulose and carboxymethylhydroxyethylcellulose, polymeric thickening agents such as the polyvinyl toluene sulfonates, and the partially hydrolyzed polyacrylamides. Particularly suitable thickening agents include the heteropolysaccharides prepared by employing bacteria of the genus Xanthomonas. A suitable polysaccharide B-1459 is commercially available under the trade name Kelzan from the Kelco Company, San Diego, California. The thickened water is then injected via the injection well into the formation and driven through the formation to displace the oil therefrom. The steps of rapidly oxidizing the oil and establishing a stabilized in situ combustion zone and thereafter quenching the zone with an aqueous caustic solution may be repeated until the in situ combustion zone breaks into the production well.

The actual amount of the aqueous caustic solution that is required to be injected into the in situ combustion zone to ensure that at least a portion of the aqueous caustic solution passes through the in situ combustion zone in liquid form may vary with the formation and the size of the in situ combustion zone. Normally the amount of aqueous caustic solution will be at least one barrel (42 gallons) of aqueous caustic solution per 1,000 cubic feet of air (measured at standard conditions) injected into the formation, after the stabilized in situ combustion was established, to propagate the in situ combustion zone into the formation and form an in situ combustion zone of predetermined size.

This invention has been described in the preferred sequence of steps of injecting an oxygen-containing gas into the formation to establish stabilized in situ combustion in the formation and thereafter injecting an aqueous caustic solution into the formation. These steps may be reversed, however, and the aqueous caustic solution may be injected into'the formation and thereafter an oxygen-containing gas injected into the formation to establish an in situ combustion zone. When such a sequence is followed, the amount of aqueous caustic solution injected into the formation should be sufficiently large to ensure that at least a portion of the aqueous caustic solution will remain in liquid form in the formation after the stabilized in situ combustion zone is established in the formation. The injection of the oxygen-containing gas into the formation oxidizes the hydrocarbons making up the oil andforms organic acids both within the portion of the formation where the in situ combustion zone is later established and surrounding this portion of the formation. Thus, the aqueous caustic solution that was previously injected into the formation reacts 'with these formed'organic acids as well as those organic acids which may already be present in'the oil and forms surfactants in situ.

In establishing a stabilized in situ combustion zone in a subsurface formation, there is a tendency for the in situ combustion zone to advance more rapidly near the upper part of the formation than near the lower part,

thus resulting in the in situ combustion zone overriding a portion of the formation. The liquid aqueous caustic solution that is injected into the formation has a greater density than the heated fluids in the in situ combustion zone and the oil within the formation and thus tends to underride the formation, that is to advance more rapidly in the lower portion of the formation. In accordance with an embodiment of this invention there is injected via the injection well into the formation a slug of aqueous caustic solution that is tailored to counteract this overriding tendency of the in situ combustion zone. By tailoring the size of the slug of aqueous caustic solution that is injected into the formation, the overriding tendencies of the in situ combustion zone is counteracted and the outer edge of the in situ combustion zone is kept more uniform in a vertical direction. It is desirable that the slug of the aqueous caustic solution be sufficiently large so as to advance this slug a greater horizontal distance along the lower portion of the formation than the in situ combustion zone advanced in the upper portion of the formation. The oil in the lower portion of the formation and the surfactant formed in situ are thus forced to override the aqueous caustic solution and be pushed into the primary path which will be taken by the next cycle of in situ combustion. In this manner the outer edge of the in situ combustion zone is kept more uniform and the oil is displaced upward into the path of the in situ combustion zone, thereby further improving the efficiency of the recovery of oil from the subsurface formation.

A more detailed description of this invention is given by reference to the drawings. In FIG. 1 there is shown an injection well I and production well 3 which extend from the surface of the earth 5 through the overburden 7 and communicate with an oil-containing formation 9. Air or other suitable oxygen-containing gas is injected via the injection well 1 and into the oil-containing formation 9 to rapidly oxidize the oil therein and form organic acids which increase the acid value of the oil. This rapid oxidation may take the form of a stabilized burn (stabilized in situ combustion) in the formation 9. Injection of an oxygen-containing gas via the injection well 1 is continued and the stabilized in situ combustion is propagated into the formation 9 until a stabilized in situ combustion zone 11 is formed which occupies in volume up to 10 percent of the pore volume of the formation. The oxygen-containing gas alsofflows beyond the in situ combustion zone 11 and into the formation 9 and oxidizes the oil surrounding the in situ combustion zone 11, thereby forming organic acids in this portion of the formation. The in situ combustion zone 11 tends to override the formation as illustrated by the front 13 of the in situ combustion zone. When the in situ combustion zone 11 has been extended to the desired size, the injection of the oxygen-containing gas is interrupted and an aqueous caustic solution is injected via injection well 1 into the formation 9. The aqueous caustic solution is injected in a sufficient amount to quench the'in situ combustion and cool the in situ combustion zone 11 and flow liquid aqueous caustic solution through the zone 11 and into the formation 9 beyond the zone I] as indicated by the dotted line 15. I

The dotted line 15 represents the front of a bank or slug of the liquid aqueous caustic solution that was flowed as a liquid through the in situ combustion zone 11 and positioned thereabout. The liquid aqueous caustic solution which was passed through the in situ combustion zone 11 reacts with the organic acids in the oil suraqueous fluids are injected via injection well 1 into the in situ combustion zone 11 to cool the zone to a temperature sufficiently low that a conventional thickened water may be injected into the formation to drive the surfactant zone through the formation 9 and produce the oil into the production well 3 and thence to the surface of the earth 5.

In accordance with another embodiment, the overriding tendencies of the in situ combustion zone 1 1 are counter-balanced by injecting into the formation an additional amount of aqueous caustic solution to underride the formation 9 as illustrated in FIG. 2. The aqueous caustic solution being more dense than the oil in the subsurface formation 9 tends to underride the oil and flow along the lower portion of the formation 9. Thus, as illustrated in FIG. 2, it is desirable to inject a sufficient amount of aqueous caustic solution via the injection well 1 to underride the formation 9 beyond the forward edge of the in situ combustion front 13 by about the same amount that the forward edge of the in situ combustion front 13 overrides the formation. This sufficient amount of aqueous caustic solution will vary somewhat depending upon the characteristics of the particular formation but normally will be at least 1 barrel of aqueous caustic solution per 1,000 cubic feet of oxygen-containing gas that was required to propagate the in situ combustion and form the in situ combustion zone 11. in accordance with another embodiment, after the injection of this sufficient amount of aqueous caustic solution, the injection of the aqueous caustic solution is interrupted and injection of an oxygencontaining gas is reinitiated and another in situ combustion zone 19 is formed. Because of the previous steps, the front 21 of the in situ combustion zone 19 is more nearly vertical than the front 13 of the initial in situ combustion zone 11. These steps may be repeated as desired in producing the oilsfrom the oil-containing formation 9 until the front of the in situ combustion zone reaches the production well 3.

I claim:

1. A method of producing oil from an oil-containing subsurface formation penetrated by an injection well and a production well, comprising the steps of:

a. injecting an oxygen-containing gas via said injection well into said formation to rapidly oxidize said oil and establish a stabilized in situ combustion zone in said formation;

b. injecting an aqueous caustic solution via said injection well into said formation to quench said in situ combustion zone and flow liquid aqueous caustic solution through said in situ combustion zoneand into said formation on the other side of said in situ combustion zone from said injection well whereby said aqueous caustic solution during the course of flow reacts with organic acids present to form surfactants; and

c. producing oil via said production well from said formation.

2. The method of claim 1 wherein injection of air is continued after said oil is ignited in step (a) and said in situ combustion zone is propagated into said formation to occupy up to percent of the pore volume of said 8 formation.

3. The method of claim 2 wherein in step (b) a sufficient amount of aqueous caustic solution is injected into said formation to quench and cool said in situ combustion zone to a temperature sufficiently low that a thickened water will not be deteriorated thereby and thereafter injecting via said injection well into said formation a thickened water to drive said aqueous caustic solution and said surfactants through said formation.

4. The method of claim 2, following step (b), further comprising repeating alternately steps (a) and (b).

5. A method of producing oil from an oil-containing subsurface formation penetrated by an injection well and a production well, comprising the steps of:

a. injecting an oxygen-containing gas via said injection well into said formation in a sufficient amount and at a sufficient pressure to rapidly oxidize said oil and establish a stabilized in situ combustion zone having a temperature of at least 500 F. in said formation;

b. injecting a slug of air via said injection well into said formation, said slug of air being of a sufficient size to propagate said in situ combustion zone through up to 10 percent of the pore volume of said formation;

c. injecting a slug of aqueous caustic solution via said injection well into said formation to quench said in situ combustion zone and flow liquid aqueous caustic solution through said in situ combustion zone, said slug of aqueous caustic solution being formed of at least one barrel of aqueous caustic solution per 1,000 cubic feet of air in said slug of air in step d. repeating alternately steps (a), (b), and (c); and

e. producing oil via said production well from said formation;

6. A method of producing oil from an oil-containing subsurface formation penetrated by an injection well and a production well, comprising the steps of: I

a. injecting an oxygen-containing gas via said injection well into said formation in a sufficient amount and at a sufficient pressure to rapidly oxidize said oil and to establish a stabilized in situ combustion zone that overrides a portion of said formation;

b. injecting an aqueous caustic solution via said injection well into said formation in a sufficient amount to quench said in situ combustion zone and flow liquid aqueous caustic solution through said in situ combustion zone and into contact with said oil in said formation to form a surfactant zone about said in situ combustion zone;

c. injecting an additional amount of an aqueous liquid via said injection well into said formation to drive said aqueous caustic solution and said surfactant zone into said formation and to underride said formation; q

d. repeating alternately steps (a), (b), and (c) to produce another in situ combustion zone having a more nearly vertical front; and

e. producing oil via said production well from said formation.

- Patent No.

5 UNITED STATES PAT N OFFICE CERTIFICATE OF CORRECTIQN July 16, 1974 3,823 776 Dated Billy G. Holmes Inventofls) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the drawings, U. S. Patent No. "3,833,776

should be --3,823,776-.

Column 3, line 11, 'stablized" should be --stabilized--.

Signed and sealed this 8th day of October 1974,

(SEAL) Attest:

MCCOY M. GIBSON JR. I (.3c MARSHALL DANN Commissioner of Patents Attesting Officer USCOMM'DC 6O376-P69 U.S. GOVERNMENT PRINTING OFFICE: I969 0-366-334 FORM PO-105O (10-69)

Claims

2. The method of claim 1 wherein injection of air is continued after said oil is ignited in step (a) and said in situ combustion zone is propagated into said formation to occupy up to 10 percent of the pore volume of said formation.
3. The method of claim 2 wherein in step (b) a sufficient amount of aqueous caustic solution is injected into said formation to quench and cool said in situ combustion zone to a temperature sufficiently low that a thickened water will not be deteriorated thereby and thereafter injecting via said injection well into said formation a thickened water to drive said aqueous caustic solution and said surfactants through said formation.
4. The method of claim 2, following step (b), further comprising repeating alternately steps (a) and (b).
5. A method of producing oil from an oil-containing subsurface formation penetrated by an injection well and a production well, comprising the steps of: a. injecting an oxygen-containing gas via said injection well into said formation in a sufficient amount and at a sufficient pressure to rapidly oxidize said oil and establish a stabilized in situ combustion zone having a temperature of at least 500* F. in said formation; b. injecting a slug of air via said injection well into said formation, said slug of air being of a sufficient size to propagate said in situ combustion zone through up to 10 percent of the pore volume of said formation; c. injecting a slug of aqueous caustic solution via said injection well into said formation to quench said in situ combustion zone and flow liquid aqueous caustic solution through said in situ combustion zone, said slug of aqueous caustic solution being formed of at least one barrel of aqueous caustic solution per 1,000 cubic feet of air in said slug of air in step (b); d. repeating alternately steps (a), (b), and (c); and e. producing oil via said production well from said formation.
6. A method of producing oil from an oil-containing subsurface formation penetrated by an injection well and a production well, comprising the steps of: a. injecting an oxygen-containing gas via said injection well into said formation in a sufficient amount and at a sufficient pressure to rapidly oxidize said oil and to establish a stabilized in situ combustion zone that overrides a portion of said formation; b. injecting an aqueous caustic solution via said injection well into said formation in a sufficient amount to quench said in situ combustion zone and flow liquid aqueous caustic solution through said in situ combustion zone and into contact with said oil in said formation to form a suRfactant zone about said in situ combustion zone; c. injecting an additional amount of an aqueous liquid via said injection well into said formation to drive said aqueous caustic solution and said surfactant zone into said formation and to underride said formation; d. repeating alternately steps (a), (b), and (c) to produce another in situ combustion zone having a more nearly vertical front; and e. producing oil via said production well from said formation.