High Conformance Oil Recovery Process

This invention concerns a process for use in subterranean formations containing two or more zones which differ from one another in permeability to such a degree that water flooding or other enhanced oil recovery processes cannot effectively deplete both zones, resulting in poor vertical conformance. More specifically, the process involves injecting a fluid into the more permeable zone, after it has been depleted by water flooding or other supplemental oil recovery process, which fluid has relatively low viscosity at the time of injection but forms a high viscosity, coarse emulsion with the residual hydrocarbon in the depleted zone to reduce the permeability of that zone to subsequently injected fluids.

It is well recognized by persons skilled in the art of petroleum recovery that only a small fraction of the petroleum originally present in a formation can be recovered by primary production, e.g., by allowing the oil to flow to the surface of the earth as a consequence of naturally occuring energy forces, or by so called secondary recovery processes which comprise injecting water into the formation by one or more wells to displace petroleum laterally through the formation toward one or more spaced apart production wells and then to the surface of the earth. Although water flooding is an inexpensive supplemental oil recovery process, water does not displace oil effectively even in those portions of the formation through which it passes, because water and oil are immiscible and the interfacial tension between water and oil is quite high. This too has been recognized by persons skilled in the art of oil recovery and many surface active agents or surfactants have been proposed for addition to the flood water, which materials reduce the interfacial tension between the injected aqueous fluid and the formation petroleum thereby increasing the microscopic displacement efficiency of the injected aqueous fluid. Surfactants which have been disclosed in the prior art for such purposes include alkyl sulfonates, alkylaryl sulfonates, petroleum sulfonates, alkyl or alkylarylpolyalkoxy sulfates, alkyl- or alkylarylpolyalkoxyalkylene sulfonates, nonionic surfactants such as polyethoxylated aliphatic alcohols or alkanols, and polyethoxylated alkylphenols, and numerous mixtures thereof.

Even if the surface tension between the injected aqueous fluid and the petroleum present in the subterranean reservoir can be reduced by incorporating surface active agents into the injected fluid, the total oil recovery efficiency of the process is frequently poor because many subterranean petroleum-containing reservoirs are comprised of a plurality of layers of widely differing permeabilities. When any oil recovery fluid is injected into such a heterogeneous reservoir, the fluid passes primarily through the most permeable zones and little or none of the fluid passes through the lower permeability zones. If the ratio of permeabilities of the zones is as high as 2:1, essentially all of the injected fluid passes through the more permeable zone to the total exclusion of the less permeable zone. Furthermore, the situation described immediately above causing poor vertical conformance of the injected fluid in a heterogeneous reservoir is aggravated by application of the supplemental oil recovery process itself. If water or any other oil displacing fluid is injected into a heterogeneous multi-layered petroleum reservoir, the fluid passes principally through the most permeable zone and displaces petroleum therefrom, and as a consequence further increases the permeability of that zone. Accordingly, the difference between the permeability of the most permeable zone and the less permeable zone or zones is increased as a consequence of applying a fluid displacement oil recovery process thereto, including water flooding, surfactant flooding, etc.

The above described problem of poor vertical conformance in water flooding operations has also been recognized by persons skilled in the art, and numerous processes have been described in the prior art for treating the formation to correct the problems encountered when injecting an oil-displacing fluid into a formation having two or more zones of significantly different permeabilities. Many of the these processes involve the use of solutions of hydrophilic polymers including partially hydrolyzed polyacrylamide, copolymers of acrylamide and acrylic acid or water soluble acrylates, polysaccharides, etc. Unfortunately, the fluids employing these hydrophilic polymers are substantially more viscous than water at the time of injection, and so injection into the zones is difficult and there is little assurance that they will invade the same zones as would water or another aqueous fluid having about the same viscosity as water. Accordingly, the effectiveness of these prior art processes has been restricted to reducing the permeability of only the most permeable flow channels in a zone, and only affects the permeability distribution of the near wellbore zone of the formation, e.g. that portion of the most permeable zone in a formation immediately adjacent to the injection well, because of the difficulty of injecting viscous fluids through large portions of the formation.

In view of the foregoing discussion of the problems associated with poor vertical conformance in heterogeneous formations, it can be appreciated that there is a substantial need for a method of treating such formations to achieve indepth reduction of the permeability of the very high permeability zones to force subsequently injected oil displacing fluids to pass into those zones which were originally of lower permeability, and so were not invaded by the first injected fluids.

Numerous references suggest formulating viscous emulsions on the surface, and injecting the emulsion into a subterranean formation for the purpose of decreasing the permeability of a zone which was originally more permeable than other zones. These include U.S. Pat. Nos. 3,149,669; Re. 27,198 (original U.S. Pat. No. 3,443,636); U.S. Pat. No. 3,502,146 (1970); and U.S. Pat. No. 3,866,680 (1975). U.S. Pat. Nos. 3,827,497; and 3,890,239 relate to the use of interfacial tension reducing mixtures of organic sulfonate and ethoxy sulfonate surfactants in an oil displacing fluid.

We have discovered a process applicable to subterranean, petroleum-containing formations containing two or more zones, at least one of which has a permeability at least 50 percent greater than the other zone, which will permit application of enhanced oil recovery processes such as water flooding or surfactant flooding in both zones. The process involves first injecting water or other aqueous displacing fluid into the formation to pass through the more permeable zone, displacing petroleum therefrom, until the ratio of injected fluid to formation petroleum of fluids being recovered from the formation reaches a predetermined or economically unsuitable level. This further increases the ratio of the permeability of the most permeable zone to the permeability of the lesser permeable zone or zones. Thereafter an aqueous emulsifying fluid is injected into the formation, which fluid flows substantially exclusively into and through the most permeable, previously water flooded zone. Injection into the zone may be by means of the well utilized as the injection well initially, or by means of the production well, or by both wells simultaneously or sequentially. The fluid has a viscosity not substantially greater than the viscosity of water, and contains a surfactant combination which readily emulsifies the residual oil present in the previously water flooded zone. The surfactant mixture present in the injected treating fluid must be one which forms an emulsion with the residual formation petroleum in the zone being treated at a salinity about equal to the salinity of the aqueous fluid present in the previously flooded, high permeability zone, and should also be relatively stable with changes in salinity since there are normally variations in water salinity from one point in a subterranean formation to another. The emulsion formed should also be stable for a long period of time at the temperature of the formation, in order to maintain the desired reduction of permeability within the treated zone. The surfactant employed in the process of our invention comprises at least two components: (1) an organic sulfonate such as a water soluble salt, preferably a sodium, potassium or ammonium salt of an alkyl or alkylaryl sulfonate having from 6 to 25 and preferably from 8 to 18 carbon atoms in the alkyl chain, which may be linear or branched, or a water soluble salt, preferably a sodium, potassium or ammonium salt of petroleum sulfonate having a median equivalent weight from 325 to 475, and (2) a solubilizing co-surfactant, preferably a water soluble salt of an alkyl or alkylarylpolyalkoxyalkylene sulfonate having the following formula:

wherein R is an alkyl, linear or branched and having from 8 to 22 and preferably from 10 to 18 carbon atoms, or an alkylaryl such as benzene, toluene or xylene having attached thereto an alkyl, linear or branched, containing from 8 to 15 and preferably from 9 to 13 carbon atoms in the alkyl, R' is ethylene or a mixture of ethylene and higher alkylene such as propylene with relatively more ethylene than higher alkylene, preferably at least 65% ethylene, R" is ethylene, propylene, hydroxy propylene or butylene, n is a number from 2 to 20 and preferably from 4 to 12, and M is a monovalent cation, preferably sodium, potassium or ammonium.

The equivalent weight of the organic sulfonate, the balance between R and x in the above formula and the weight ratio of solubilizing co-surfactant are all chosen based on experimentation using petroleum and brine from the formation into which the fluid will be injected, on the basis of exhibiting the best combination of the following:

(1) Optimum emulsifying property;

(2) Maximum emulsion viscosity;

(3) Emulsion brine tolerance;

(4) Maximum stability of emulsion at formation temperature and salinity;

(5) Tolerance for changes in aqueous fluid salinity within the range of variations expected in the formation and which may be caused by subsequently injecting fluids having greater or less salinity than the salinity of the aqueous phase of the emulsion.

A low molecular weight alcohol, e.g. a C.sub.2 to C.sub.7 alkanol or an alkyl-substituted phenol having from 1 to 5 carbon atoms in the alkyl chain, may be required or used advantageously for use in the emulsifying fluid, to enhance the emulsification-forming properties or to enhance the stability of the emulsion.

In a slightly different preferred embodiment of the process of our invention, a small amount of hydrocarbon is added to the emulsifying liquid to form an emulsion, micellar dispersion or microemulsion. The hydrocarbon may be crude oil, as well as kerosene, naphtha, gasoline or other commercially available mixtures, as well as C.sub.6 to C.sub.24 hydrocarbons, preferably saturated hydrocarbons. The amount of hydrocarbon which is incorporated in the emulsion is determined experimentally from the aqueous emulsifying fluid by preparing samples containing various concentrations of hydrocarbon over the range of 0.05 to about 15.0 and preferably 0.5 to 5.0 percent by volume, and determining the viscosity of each sample. The maximum amount of hydrocarbon which can be incorporated in the fluid without causing the viscosity of the fluid to exceed the viscosity of water or brine at formation conditions by more than 500% and preferably by no more than 200%, is selected. By incorporating this small, critical volume of hydrocarbon in the fluid, the desired viscosity development in the formation to be treated will be obtained much more rapidly than if an oil free liquid were injected. Maintaining the level below the above described limit ensures that the fluid may be injected easily and into the same flow channels of the formation as would be invaded by water.

FIG. 1a illustrates a subterranean oil formation containing three strata or zones of different permeabilities, illustrating the interface between an injected fluid and the petroleum in each zone at a time near the economic end of a water flood process.

FIG. 1b illustrates the same subterranean formation, after it has been subject to the treatment of the process of this invention, and then subjected to additional water flood.

Briefly, the process of our invention comprises a method of treating a subterranean petroleum-containing formation made up of at least two zones or strata whose permeabilities are sufficiently different that a fluid injected into a well in communication with both zones will pass primarily through the more permeable zone. Ordinarily, for example, if the permeability to the flow of the injected fluid in one stratum is at least 50 percent greater than and certaining if it is 100 percent greater than the other stratum, fluid injected into wells in fluid communication with both strata will pass almost exclusively into the more permeable stratum. For example, in a water flood applied to such a formation, water will pass into the more permeable stratum exclusively or at least predominantly, and will displace petroleum toward the production well, with substantially no oil displacement occuring in the less permeable stratum. After oil has been displaced through the more permeable stratum and oil recovery has proceeded to the point at which water breakthrough has occurred at the production well, continued injection of water into the well in communication with both strata will accomplish substantially no additional oil recovery since all of the water injected into the formation passes through the high permeability stratum, even though the oil saturation in the lesser permeable stratum may be substantially the same as it was before commencing water flood or other supplemental oil recovery operations.

Attempts to treat a situation such as that described above by techniques taught in the prior art have been only partially successful at best for a variety of reasons. Injecting a viscous fluid, which may be either a viscous emulsion formed on the surface for the purpose of plugging the more permeable zone, or an aqueous solution of a hydrophilic polymer such as polyacrylamide, partially hydrolyzed polyacrylamide, copolymers of acrylamide and acrylates, polysaccharides, etc., are generally not entirely satisfactory because the more viscous fluid only invades the largest flow channels of the formation, and so does not invade all of the flow channels which would be invaded by a fluid whose viscosity was more nearly equal to the viscosity of water. Furthermore, emulsions formed according to prior art teachings by, for example, adding caustic and water to crude oil are not particularly stable with respect of time and are also not stable with respect to changes in the salinity of fluid with which they may be brought into contact. Thus a viscous emulsion which effectively plugs the larger flow channels of a high permeability zone, including one which has previously been water flooded, may be broken later either as a consequence of the passage of time, or as the emulsion contacts pockets of water having greater or lesser salinity, which frequently are found in most subterranean reservoirs. Moreover, there are problems associated with adsorption of hydrophilic polymers, and furthermore many of the hydrophilic polymers are not sufficiently temperature stable to allow them to be used in even moderate temperature formations.

The fluid injected into the formation according to the process of our invention is either an aqueous oil-free solution or a low viscosity, minimum oil-containing emulsion containing two or more surfactants, or surface-active agents, which are carefully chosen on the basis of displaying optimum emulsification characteristics. Surfactants which are effective for this purpose, e.g. for forming gross macroemulsions capable of plugging the flow channels of the formation, are not suitable for use in recovering oil by low surface tension flooding operations, and will not produce optimum oil displacement in a formation if utilized in a surfactant water flooding process. The reason the surfactants suitable for use in the process of this invention are ineffective for water flooding operations is believed to be be that when a stable emulsion such as is employed in our process, is formed, essentially all of the surface active agents which participate in the emulsification reaction, are concentrated at the interface between the discontinuous and continuous emulsion phases, and so little surfactants remain in the aqueous solution, and so cannot reduce the interfacial tension between formation petroleum and the aqueous fluid present in the flow channels as is necessary to achieve efficient low surface tension displacement of petroleum.

It is necessary that the surfactants utilized in the process of this invention be stable and effective for emulsification at the formation temperature in an aqueous fluid having a salinity about equal to the average salinity of the aqueous fluid present in the flow channel of the high permeability zone, e.g. the zone into which the treating fluid is to be injected. Preferably, the surfactants should be identified by tests utilizing actual fluids from the formation, including brine and formation petroleum, since particular characteristics of any of these fluids will affect the efficiency of these surfactants for emulsification of formation petroleum.

The aqueous emulsifying treating fluid injected into the high permeability zone in practicing the process of our invention contains the following surfactants. (1) An organic sulfonate such as a water soluble salt, preferably a sodium, potassium or ammonium sulfonate having a median equivalent weight from 325 to 475, or a water soluble salt, preferably a water soluble sodium, potassium or ammonium salt of an alkyl sulfonate containing from 8 to 22 and preferably 10 to 18 carbon atoms, or alkylaryl sulfonate having from 8 to 15 and preferably from 9 to 13 carbon atoms in the alkyl chain, and (2) a water soluble salt of an alkyl or alkylarylpolyalkoxyalkylene sulfonate having the following formula:

wherein R is an alkyl, linear or branched and having from 8 to 22 and preferably from 10 to 18 carbon atoms, or an alkylaryl such as benzene, toluene or xylene having attached thereto an alkyl, linear or branched, containing from 8 to 15 and preferably from 9 to 13 carbon atoms in the alkyl chain, R' is ethylene or a mixture of ethylene and propylene with relatively more ethylene than propylene, preferably at least 65% ethylene, R" is ethylene, propylene, hydroxy propylene or butylene, x is a number from 2 to 20 and preferably from 4 to 12, and X is a monovalent cation, preferably sodium, potassium or ammonium.

The equivalent weight of the organic sulfonate, the balance between R and x in the above formula and the weight ratio of solubilizing co-surfactant are all chosen based on experimentation using petroleum and brine from the formation into which the fluid will be injected, on the basis of exhibiting the best combination of the following:

(1) Optimum emulsifying property;

(2) Maximum emulsion viscosity;

(3) Emulsion brine tolerance;

(4) Maximum stability or emulsion at formation temperature and salinity;

(5) Tolerance for changes in aqueous fluid salinity within the range of variations expected in the formation and which may be caused by subsequently injecting fluids having greater or less salinity than the salinity of the aqueous phase of the emulsion.

A low molecular weight alcohol, e.g. a C.sub.2 to C.sub.7 alkanol or an alkyl-substituted phenol having from 1 to 5 carbon atoms in the alkyl chain, may be required or advantageous for use in the emulsifying fluid, to enhance the emulsification-forming properties or to enhance the stability of the emulsion.

In one preferred embodiment of our process, the emulsifying fluid injected into the formation is a substantially oil-free aqueous solution, and the residual oil present in the formation is used exclusively to form the emulsion.

In a slightly different preferred embodiment of the process of our invention, a small amount of hydrocarbon is added to the emulsifying liquid. The hydrocarbon may be crude oil, as well as kerosene, naphtha, gasoline or other commercially available mixtures, as well as C.sub.6 to C.sub.24 hydrocarbons, preferably saturated hydrocarbons. The amount of hydrocarbon which is incorporated in the emulsion is determined experimentally from the aqueous emulsifying fluid by preparing samples containing various concentrations of hydrocarbon over the range of 0.05 to about 15.0 and preferably 0.5 to 5.0 percent by volume, and determining the viscosity of each sample. The maximum amount of hydrocarbon which can be incorporated in the fluid without causing the viscosity of the fluid to exceed the viscosity of water or brine at formation conditions by more than 500% and preferably by no more than 200%, is selected. By incorporating this small, critical volume of hydrocarbon in the fluid, the desired viscosity development in the formation to be treated will be obtained much more rapidly than if an oil free liquid were injected. Maintaining the level below the above described limit ensures that the fluid may be injected easily and into the same flow channels as water.

The balance between the oil soluble and water soluble groups, e.g. the number of carbon atoms in R and the value of n, the number of ethoxy groups, in the above formula, is slightly different for an optimum emulsifying surfactant for use in our process, compared to an optimum surfactant for use in a low surface tension surfactant flooding processes.

Although it is preferred that the concentration of all of the surfactants be determined experimentally as stated above, the following general guideline will be helpful for establishing the ranges for initial experimentation.

The concentration of the alkyl or alkylaryl sulfonate or petroleum sulfonate surfactant will ordinarily be in the range of from about 0.01 to about 10 and preferable from about 0.5 to about 4.0 percent by weight. The concentration of the alkyl or alkylarylpolyalkoxyalkylene sulfonate surfactant, will ordinarily be from about 0.1 to about 5.0 and preferably from about 0.4 to about 2.0 percent by weight. The ratio of organic sulfonate surfactant to the alkyl or alkylarylpolyalkoxyalkylene sulfonate will ordinarily be from about 0.5 to about 5.0, depending on the salinity of the fluid in which it is formulated, which in turn is usually about equal to the salinity of the fluid present in the subterranean formation.

The volume of treating fluid to be injected into the formation when applying the process of our invention is ordinarily from about 1.0 to about 100 and preferably from 10 to 50 pore volume percent, based on the pore volume of the high permeability zone or zones to be contacted by the treating fluid. It is important to note that the pore volume on which these numbers are based relate to the pore volume of the high permeability zone to be treated, not the pore volume of the whole formation.

A near well bore treatment may be effective in oil reservoirs having impermeable layers separating the oil zones such as shale layers. In such reservoirs, the volume of treating fluid is ordinarily from 50 to 100 pore volume percent within the zone radius to be treated.

This fluid and process is suitable for use in formations which contain water whose salinity is from 10,000 to 200,000 parts per million total dissolved solids, and whose temperature is as high as 150.degree. F. (66.degree. C.).

In another, slightly different embodiment of the process of our invention, the emulsion forming liquid is saturated or near saturated with a noncondensible gas at the injection pressure. Preferred gases suitable for use in this embodiment are nitrogen, air, carbon dioxide, methane, ethane and natural gas. The amount of gas should be at least slightly greater than the maximum amount which can be dissolved in the injected liquid at reservoir conditions. The gas-saturated liquid is injected at substantially higher pressure than formation pressure, preferably at the maximum injection pressure which is safely below the fracture pressure of the overburden formation. After the desired amount of liquid has been injected into the zone or layer of the formation, the pressure is reduced to a value about equal to the normal formation pressure. This causes small gas bubbles to break out of solution in the injected emulsifying liquid and/or emulsion formed in the zone. The breakout of gas bubbles contributes at least two beneficial effects to our process: (1) mixing energy is made available to assist in the forming of the emulsion, and in forming dispersed phase cells which are smaller and thus maximizing the viscosity of the emulsion. (2) At least a portion of the gas bubbles are trapped in the viscous emulsion in the zone, forming a stable foam which increases both the viscosity and the volume of the emulsion formed in the pore spaces, thus enhancing the effectiveness of the permeability reduction obtained from application of our process.

The procedural steps involved in applying the process of our invention to a subterranean formation are best understood by referring to the attached drawing, to which the following description of a field example applies.

A subterranean, petroleum-containing formation is located at depth of about 6200 feet, and it is determined that the total thickness of the formation is 35 feet. The formation is not homogeneous in terms of permeability, however; rather, the formation is made up of three separate strata or layers. The oil saturation in all three layers is approximately 30 percent. Layer 1, the top layer in the formation, has a permeability of about 6 md and is approximately 10 feet thick. Layer 2, the middle zone of the formation, has a permeability of about 46 md and is about 15 feet thick. Layer 3, which occupies the lower portion of the formation, is approximately 10 feet thick and has an average permeability of about 15 md. The formation temperature is 105.degree. F. (40.6.degree. C.).

Water is injected into injection well 5 which is in fluid communication with the full vertical thickness of the formation, i.e., with all three layers of the formation. Since the permeability of layer 2 is substantially greater than the permeability of either layer 1 or layer 3, water flows much more readily into layer 2, and substantially all of the oil production obtained as a consequence of water injection is in fact derived from layer 2. It should be noted that this is not necessarily apparent to operators on the surface of the earth, however. Water injection continues and an interface is formed in each layer between the injected water flood and an oil bank that is formed as a consequence of the water flood, which are designated as 6 in layer 1, 7 in layer 2 and 8 in layer 3. At a time just before water breakthrough at the production well 4, the position of interfacial zones 6, 7 and 8 is shown in FIG. 1A. It can be seen that water breakthrough is about to occur at production well 4 from layer 2. After water breakthrough has occurred, further injection of water into well 5 will not recover any significant amount of additional oil from any of the three layers. All of the water injected after breakthrough of water at production well 4 will pass into and through layer 2, and essentially no additional water will pass into layers 1 and 3. Thus interfaces 6 and 8 will remain approximately where they are shown in FIG 1a after breakthrough of water into the production well at layer 2, no matter how much additional water is injected into the injection well and flowed through the reservoir. At this time oil production drops off rapidly and the amount of water being produced increases rapidly until further water injection and oil production are no longer economically feasible, even though large amounts of oil remain in layers 1 and 3.

The water that has been utilized for water flooding is itself from the same formation, and so the salinity of the water being injected into the formation and the salinity of water naturally present in the formation is about the same, and it is determined that in this example the salinity of this water is approximately 180,000 parts per million total dissolved solids including 9800 parts per million divalent ions, principally calcium and magnesium. It is desired to formulate a treating fluid suitable for use in this high salinity environment, and the surfactant is chosen by a series of laboratory experiments employing actual samples of field water and petroleum from the formation into which the treating fluid is to be injected. After a series of laboratory tests, essentially similar to those to be described later hereinafter below, it is determined that a preferred emulsifying fluid for use in reducing the permeability of layer 2 contains (1) 1.5 percent by weight of an ammonium petroleum sulfonate having a median equivalent weight of 380 and (2) 0.8 percent by weight of a sodium dodecylbenzenepolyethoxyethylene sulfonate containing an average of 3.5 ethoxy groups per molecule, plus 2.0 percent by weight of iso-pentanol.

Since the wells are 150 feet apart, and the formation to be treated is principally layer 2, which is 15 feet thick, and since it is determined that the swept area in a simple two-spot pattern such as this is 11,200 square feet, the volume of formation (30% porosity) to be treated is (11,200)(15)(0.30) =50,400 cu. ft.

A 20 percent pore volume slug is chosen for use in treating the above identified zone. Accordingly, the volume of the solution necessary to treat layer 2 in this example is approximately 10,080 cubic feet (2133 cubic meters) or 75,398 gallons.

The above described emulsifying fluid is injected into injection well 5. Because the permeability of layer 2 is substantially greater than the permeability of layers 1 and 3 at that time, the difference being substantially greater than it was existed at the time water flooding was initiated, it is not necessary to isolate layer 2 from the other layers for the purpose of selectively injecting the fluid into layer 2. Substantially all of the fluid injected into well 5, which is in fluid communication with all of the formation, will pass into layer 2. Injection of the treating fluid into layer 2, which causes an emulsion to form in layer 2, reducing the permeability of the layer and additionally recovering some additional oil therefrom, reduces the oil saturation in layer 2 to only 4 percent.

After injecting 10 pore volume percent of the emulsifying liquid into the formation, 1.5 percent by volume of crude oil is added to the formation, which only increases the viscosity of the fluid by 75% over the viscosity of the liquid without the oil, but increases the rate of formation of the emulsion in situ in the flow channels. Water injection is then again resumed into the formation. Since the permeability of layer 2 has been decreased substantially, water injected into well 5 will now flow principally into layers 1 and 3, and so will continue pushing the interface between the injected water and the formation petroleum toward the production well. If water from layer 3 breaks through at producing well 4 before it does in layer 1, it may be necessary to treat layer 3 in about the same fashion as was used to treat layer 2 in the procedural steps described above. After this has been accomplished, water injection may again be resumed, with essentially all of the water passing into layer 1. Water injection is then continued until water again breaks through at well 4, signifying that substantially all of the formation has been swept by water flooding.

After completion of the above described multi-step water flood with intermittent treatment to alleviate the adverse permeability distribution problem, the formation may thereafter be subjected to additional supplemental oil recovery processes such as, for example, surfactant flooding, since the permeability of the formation has now been made more homogeneous and there still remains a substantial amount of petroleum in layers 1 and 3 sufficient to justify the injection of an efficient, low surface tension oil displacing fluid into layers 1 and 3.

For the purpose of illustrating the types of fluids suitable for use in the process of our invention, and illustrating the results obtainable from application thereof, a series of laboratory experiments were performed.

A series of emulsification tests were conducted to illustrate how slight changes in surfactant molecular characteristics affect emulsification effectiveness of the surfactant. These tests comprised mixing together 5 cc's of oil and 30 cc's of the one percent surfactant solution in an 85 kilogram/meter.sup.3 (85,000 ppm) brine. The solutions were heated to a temperature of 109.degree. F. (42.8.degree. C.) and shaken periodically over an eight hour period. The solutions were then allowed to equilibrate for several days, and the volume of emulsion and total volume of fluid including the emulsion, the oil and the aqueous phase, were observed. The figures reported in Table I below under volume ratio represents the volume of emulsion divided by the total volume of fluid, including emulsion and separate phases of the field brine and any unemulsified oil that may have been present. It can be seen that a change in the number of ethoxy groups of only .+-.0.2 causes a very significant change in the emulsification effectiveness of the surfactant.

Laboratory equipment was especially constructed for core flood tests, and comprised essentially two separate formation earth core samples of significantly different permeabilities encased in holders and arranged for flooding, with the two cores being placed in parallel to simulate the situation similar to that described above, in which an injection well contacts two earth strata of substantially different permeabilities. Fluids injected into the apparatus will pass predominantly through the highest permeability core to the exclusion of the other core. In all of the experiments described below, the cores were separately water flooded to an irreducable oil saturation prior to being connected in parallel for the purpose of studying the effect of the adverse permeability distribution-correcting treatment of our invention.

In the first experiment of this series, run 7, core A was a fresh Berea sandstone core having a permeability of 704 millidarcy. The core was 5.08 cm in diameter and 15.8 cm in length and had a total pore volume of 73 cubic centimeters. The porosity was 23 percent. The residual oil saturation after water flooding was 25 percent. Core B utilized in Run 7 was a similar size core having pore volume of 65 cubic centimeters and a porosity of 20 percent, but a much lower permeability, only 139 millidarcy. The residual oil saturation of Core B after water flooding was 35 percent. After the cores were flooded to irreducable water saturation and mounted in parallel, water injection into the cores at a flow rate of 0.9 cc per minute resulted in a receptivity ratio (the ratio of the volume of fluid injected into core A divided by the volume of fluid injected into core B during the same period, when the cores are connected in parallel) of approximately 5.8. During the treatment procedure the receptivity ratio declined to 4.7 and levelled off at 4.0 during the subsequently applied water flood operation. A quantity of petroleum sulfonate solution was then injected, and during the surfactant flood portion of the test, the receptivity declined still further to 2.4. A polymer mobility control buffer was then injected into the system, and the receptivity ratio increased to 4.2 after 0.2 pore volumes of the polymer solution had been injected, and then rose to 5.6 after 1 pore volume of polymer had been injected. It is believed that the increase in receptivity ratio resulting from the fact that the polymer was dissolved in fresh water, which broke the emulsion formed in the course of the treatment procedure described above. Nevertheless, Run 7 clearly illustrates how treatment of two cores in a parallel arrangement, which cores have widely different permeabilities, can reduce the permeability deviation between the two cores and improve the receptivity ratio from 5.8 to 2.4, which is substantially less than half of the original receptivity ratio.

Run 8 was performed to verify that in situ emulsification was the mechanism responsible for the improvement in receptivity noted in experiment 7 above. In Run 8, two packs of crushed formation core material were formulated and cleaned. Pack C was saturated with crude oil and pack D was not. Pack C was water flooded to an irreducable oil saturation prior to the treatment. Both packs were treated with 13 pore volume percent of a 30 kilogram/meter.sup.3 solution of dinonylphenolpolyethoxyethyl sulfonate (3.8 ethoxy groups per molecule average) and finally flooded with field brine. In this experiment, the packs were not flooded in parallel as was the case in Run 7 above but rather were independently flooded after treatment with the emulsifying fluid. The pressure differential across the packs was determined during the course of the treatment and subsequent water flood as an indication of increasing resistance to fluid flow through the packs. The pack which was originally saturated with oil, water flooded and then treated, experienced a four-fold increase in the pressure required to flood with water in a constant rate flood whereas the pack which contained essentially no oil prior to the treatment experienced less than a 50 percent increase in differential pressure during the course of approximately 3 pore volumes of water flood. This clearly illustrates that oil must be present in the treated formation for the injectivity-reducing emulsification phenomena to be achieved, which is necessary for the treatment described herein to accomplish the desired objective of reducing the permeability of the high permeability zone.

Experiment 9 was comparable to experiment 7, except the treating solution contained 13.6 kg/m.sup.3 dodecylbenzene (3.0) polyethoxyethylene sulfonate with 7.6 kg/m.sup.3 3.0 mole ethylene oxide adduct of dodecylphenol and packs were formulated from crushed formation core material. Pack E had 96 millidarcy permeability and Pack F had 20 millidarcy permeability. After the packs were each flooded to irreducable water saturation and mounted in parallel, water injection into the cores at a flow rate of 1.0 cm.sup.3 per minute in a receptivity ratio (Pack E/Pack F) of 4.6. During the treatment procedure, the receptivity ratio decline to 2.8 and levelled off at 1.0 during the subsequently applied water flood operation. A receptivity ratio of 1 was maintained during injection of petroleum sulfonate solution and the ratio fluctuated between 1.6 and 0.6 during a polymer solution injection. Experiment 9 clearly illustrates that the sulfonate-nonionic mixture can be used to reduce the permeability deviation between two packs.

While the above discloses mixing the two essential surfactants in a single fluid, two or more fluids each containing only one component can be injected sequentially so as to achieve mixing in the formation. In certain applications, there is an advantage to this embodiment in that emulsification is delayed and greater in depth treatment of the high permeability zone is achieved.

Thus we have disclosed and demonstrated how it is possible to treat a formation containing two or more strata of substantially different permeabilities so as to reduce the permeability of the more permeable strata, by injecting an emulsifying fluid thereinto which forms a gross macroemulsion with residual oil remaining in the flow channels of the flooded portion of a formation after water flooding, thereby reducing the permeability difference between the strata, after which water or other oil displacing fluids may be injected into the formation with substantially improved vertical conformance over that which would be obtained without the permeability adjusting treatment of our invention.

While our invention has been described in terms of a number of illustrative embodiments, it is clearly not so limited since many variations thereof will be apparent to persons skilled in the art of oil recovery without departing from the true spirit and scope of our invention. It is our desire and intention that our invention be limited only by those limitations and restrictions appearing in the claims appended immediately hereinafter below.