GB2220687A - Method of enhanced oil recovery using a stabilized polymer combination in chemical flood - Google Patents

Abstract

A method is disclosed for recovering oil from a subterranean formation comprising injecting an aqueous composition into said formation and displacing said oil toward one or more production wells, said aqueous composition comprising: (A) from about 0.01% to about 5% by weight of at least one anionic or amphoteric surfactant; (B) from about 0.02% to about 0.3% by weight of at least one polymeric thickener; and (C) from about 0.005% to about 0.1% by weight of at least one polymeric material having a flexible backbone and a higher polarizability than component (B). i

Description

Title: METHOD OF ENHANCED OIL RECOVERY USING A STABIL IZED POLYMER COMBINATION IN CHEMICAL FLOOD Technical Field This invention relates to the recovery of oil from subterranean oil reservoirs and, more particularly, to improved waterflooding operations involving the use of waterfloods containing relatively low concentrations of at least one surfactant in combination with at least one polymeric - thickener and at least one supplemental polymeric material to stabilize the waterflood.

Background of the Invention In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. A variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.

It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected-water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. It has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or alter the wettability characteristics of the reservoir rock. Processes which involve the injection of aqueous surfactant solutions are commonly referred to as surfactant waterflooding or as low tension waterflooding, the latter term having reference to the mechanism involving the reduction of the oil-water interfacial tension.Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.

Many waterflooding processes have employed anionic surfactants. One problem encountered in waterflooding with certain of the anionic surfactants such as the petroleum sulfonates is the lack of stability of these surfactants in so-called "hard water" environments. These surfactants tend to precipitate from solution in the presence of relatively low concentrations of divalent metal ions such as calcium and magnesium ions. For example, divalent metal ion concentrations of about 50-100 ppm and above usually tend to cause precipitation of the petroleum sulfonates.

Nonionic surfactants, such as polyethoxylated alkyl phenols, polyethoxylated aliphatic alcohols, carboxylic esters, carboxylic amides, and polyoxy ethylene fatty acid amides, have a somewhat higher tolerance of polyvalent ions such as calcium or magnesium than do the more commonly utilized anionic surfactants. While it is technically feasible to employ a nonionic surfactant solution to decrease the interfacial tension between the injected aqueous displacing medium and petroleum contained in some limestone formations, such use is generally not economically feasible for several reasons. Nonionic surfactants are not as effective on a per mole basis as are the more commonly used anionic surfactants and, additionally, the nonionic surfactants generally have a higher cost per unit weight than do the anionic surfactants.Nonionic surfactants usually exhibit a reverse solubility relationship with temperature and become insoluble at temperatures of above their cloud points making them ineffective in many oil formations. Nonionic surfactants that remain soluble at elevated temperatures are generally not effective in reducing interfacial tension. Moreover, nonionic surfactants hydrolyze at temperatures above about 750C.

The use of certain combinations of anionic and nonionic surfactant to combat hard water formations has also been suggested. For example, U.S. Patent 3,811,505 discloses the use of alkyl or alkylaryl sulfonates or phosphates and polyethoxylated alkyl phenols. U.S.

Patent 3,811,504 discloses the use of three component mixture including an alkyl or alkylaryl sulfonate, an alkyl polyethoxy sulfate and a polyethoxylated alkyl phenol. U.S. Patent 3,811,507 discloses the use of a water-soluble salt of a linear alkyl or alkylaryl sulfonate and a polyethoxylated alkyl sulfate.

Cat ironic surfactants such as quaternary ammonium salts, and derivatives of fatty amines and polyamines, have also been used. However, these compounds have the disadvantage of substantivity or attraction, especially towards silicate rock, and they lose their activity by adsorption.

The use of certain amphoteric surfactants which function as cationics in acid media and become anionic when incorporated in alkaline systems has been suggested. For example, U.S. Patent 3,939,911 discloses a surfactant waterflooding process employing a threecomponent surfactant system. This surfactant system includes an alkyl or alkylaryl sulfonate such as an ammonium dodecyl benzene sulfonate, a phosphate ester sulfonate, and a sulfonated betaine such as a C12 C24 alkylamido C1-C5 alkane dimethylammonium propane sulfonate.

The use of thickening agents to increase the viscosity of injected water, normally to a value of at least equal to that of the reservoir oil, in order to arrive at a favorable mobility ratio between the oil and water and increase the macroscopic displacement efficiency of waterflood is known. Examples of such thickeners or mobility control agents are polysaccharides such as xanthan gum, which are available from Kelco Company under the tradename "Kelzan", and partially hydrolyzed polyacrylamides available from the Dow Chemical Company under the tradename "Pusher".

U.S. Patent 4,554,974 discloses an enhanced oil recovery method employing betaine amphoteric surfactants in combination with high molecular weight homopolysaccharide gum thickeners in a waterflood. The waterflood can be followed by a thickened buffer slug and then an aqueous flooding medium to displace the oil toward a production well.

A paper by W.R. Foster entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, Feb. 1973, pp. 205-210, describes a technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a a water-soluble biopolymer.

This thickened water slug is then followed by a field brine driving fluid which is injected as necessary to carry the process to conclusion.

A number of pilot plant studies and field tests employing polymeric thickeners in surfactant waterflooding have been reported in the literature. M.S. French et al and H.J. Hill et al report the use of 1.3% by weight petroleum sulfonate and 0.5% by weight sodium tripolyphosphate a partially hydrolyzed polyacrylamide in a 9250 ppm total dissolved salts (TDS) brine solu- tion. M.S. French et al, "Field Test of an Aqueous Surfactant System for Oil Recovery, Benton Field, Illinois", J. Pet. Tech., February, 1973, pp. 195-204; and H.J. Hill et al, "Aqueous Surfactant Systems for Oil Recovery", J. Pet. Tech., February, 1973, pp. 186-194.

H.H. Ferrell et al report pilot studies using 2.5% alkyl benzene sulfonate, 3% isobutyl alcohol and a biopolymer in a 1.0% NaCl brine solution. H.H. Ferrell et al, "Analysis of Low-Tension Pilot at Big Muddy Field, WY.", SPE/DOE 12683. H.H. Ferrell et al report field tests using 3.0% alkyl benzene sulfonate, 5% isobutyl alcohol and partially hydrolyzed polyacrylamide in about 6000 ppm TDS brine solution. H.H. Ferrell et al, "Progress Report: Big Muddy Field Low-Tension Flood Demonstration Project With Emphasis on Injectivity and Mobility", SPE/DOE 12682. J.R. Bragg et al report pilot studies using 2.3% hardness tolerant surfactant and biopolymer in formation brine. J.R. Bragg et al, "Loudon Surfactant Flood Pilot Test", SPE/DOE 10862. D.L.Taggart et al report the use of 3.86% petroleum sulfonate, 1.25% isobutyl alcohol and partially hydrolyzed polyacrylamide in a 1.2% NaC1 brine solution. D.L. Taggart et al, "Sloss Micellar/Polymer Flood Post Test Evaluation Well", SPE/DOE 9781. See also, S.P. Gupta, "Composition Effects on Displacement Mechanisms of the Micellar Fluid Injected in the Sloss Field Test", SPE 8827; S.P. Gupta, "Dispersive Mixing Effects on the Sloss Field Micellar System", SPE/DOE 9782; J.L. Wanosik et al, "Sloss Micellar Pilot: Project Design and Performance", SPE 7092; P.B. Basan et al, "Important Geological Factors Affecting the Sloss Field Micellar Pilot Project", SP 7047.

U.S. Patent 4,360,061 discloses the use of polyethylene oxide or polyvinylpyrrolidone in combination with a surfactant to form a microemulsion for use in an enhanced oil recovery process.

While many surfactant waterflooding methods have been proposed, there is a substantial, unfulfilled need for a surfactant waterflooding method that is useful in recovering oil from subterranean formations wherein an effective, stable combination of surfactant(s) and other chemicals is employed at sufficiently low concentration levels to render the method economically feasible. It would be desirable if the chemicals employed in this method were suitable for use in environments characterized by high salinities and high concentrations of divalent metal ions such as are found in sea water.

Summary of the Invention The present invention provides for a method for recovering oil from a subterranean formation comprising injecting an aqueous composition into said formation and displacing said oil towards one or more production wells, said aqueous composition comprising: (A) from about 0.01% to about 5% by weight of at least one anionic or amphoteric surfactant; (B) from about 0.02% to about 0.3% by weight of at least one polymeric thickener; and (C) from about 0.005% to about 0.1% by weight of at least one polymeric material having a flexible backbone and a higher polarizability than component (B).

Description of the Preferred Embodiments The term "hydrocarbyl", when used throughout this specification and in thewappended claims, denotes hydrocarbon groups and groups having a predominantly hydrocarbon character within the context of the invention. Such hydrocarbyl groups include the following: 1. Hydrocarbon groups, that is, aliphatic, (e.g., alkyl or alkenyl), alicyclic (e.g., cycloalkyl or cycloalkenyl), aromatic, aliphatic- and alicyclic-substituted aromatic, aromatic-substituted aliphatic and alicyclic materials, and the like.

2. Substituted hydrocarbon groups, that is, groups containing non-hydrocarbon substituents which, in the context of this invention, do not alter the predominantly hydrocarbon character of the group. Those skilled in the art will be aware of suitable substituents; examples are: halo, nitro, cyano, R*O-,

R*2N-,

(R* being hydrogen or a hydrocarbon group).

3. Hetero groups, that is, groups which, while predominantly hydrocarbon in character within the context of this invention, contain atoms other than carbon present in a chain or ring otherwise composed of carbon atoms. Suitable hetero atoms will be apparent to those skilled in the art and include, for example, oxygen and nitrogen.

The surfactants that are used in accordance with the method of the present invention can be any anionic or amphoteric surfactant that is normally used in enhanced oil recovery operations.

Useful anionic surfactants can be represented by the formula RO-tR')nR"X-Y+ wherein R is a hydrocarbyl group of about 8 to about 24 carbon atoms, preferably about 10 to about 20 carbon atoms, more preferably about 12 to about 18 carbon atoms. R is preferably an alkyl, alkenyl, alkaryl or aryl group, more preferably an alkyl or an alkenyl group. R can be straight chained or branch chained.

R'is ethoxy, propoxy or a mixture thereof. R" is a divalent hydrocarbyl group of 1 to about 20 carbon atoms, more preferably from 1 to about 10 carbon atoms, more preferably from about 2 to about 4 carbon atoms, more preferably about 2 to about 3 carbon atoms. R" can be an alkyl, alkenyl, alkaryl or aryl group. R" is preferably an alkyl group. X is a sulfate, sulfonate or carboxylate group. Y is a cation, preferably an alkali metal (preferably Na+ or K+), an alkaline earth metal (preferably Ca++ or Mg++) a nitrogen-containing group represented by the formula

wherein R2, R3, R4 and R5 are independently hydrogen or methyl, preferably R2, R3,. R4 and R5 are each hydrogen. n is a number in the range of 1 to about 20, more preferably in the range of about 1 to about 10, more preferably in the range of about 2 to about 6.It should be understood that these surfactants are typically not pure substances in the strict sense, but rather are mixtures of components distributed such that n is the resulting average value.

These surfactants can be prepared in a number of ways using known procedures. Suitable precursors include C10-24 linear or branched-chain alcohols.

Preferably the linear or branched-chained alcohol contains from about 10 to about 20 carbon atoms, more preferably about 12 to about 18 carbon atoms. If R' is ethoxy, the alcohol is reacted with ethylene oxide to yield a hydrocarbyl oxyethylene ether. The average number of ethylene oxide units can vary from 1 to about 20, more preferably from 1 to about 10, more preferably from 1 to about 6. If R' is propoxy, the alcohol is reacted with propylene oxide to yield a hydrocarbyl oxypropylene ether. The average number of propylene oxide units can also vary from 1 to about 20, more preferably from 1 to about 10, more preferably from 2 to about 6.

If R' is a mixture of ethoxy and propoxy groups, the alcohol is reacted initially with either ethylene oxide or propylene oxide, and then the resulting intermediate product is reacted with the other. The sequence of reaction of the alcohol with ethylene oxide or propylene oxide is dependent upon the desired structure of the surfactant. For example, if it is desired to have ethylene oxide attached to the RO group, the alcohol is initially reacted with ethylene oxide and then the resulting intermediate product is reacted with propylene oxide. On the other hand, if it is desired to have propylene oxide attached to the RO group, the alcohol is initially reacted with propylene oxide and then the resulting intermediate product is reacted with ethylene oxide. Any combination of ethoxy and propoxy groups in the surfactants of the invention is useful. Preferably the total number of ethoxy and propoxy groups ranges from 1 to about 20, more preferably from 1 to about 1?, more preferably from 2 to about 6.

Methods of alkoxylation are well known to those skilled in the art. The alkoxylation reaction can be achieved using a strong base or Lewis acid catalyst such as NaOH, KOH, BF3 or SnC12. Examples of other suitable bases include sodium phenol ate and alkali metal alkoxides such as sodium methoxide or propoxide. Other suitable acids include BF3-etherate, p-toluene sulfonic acid, fluorosulfonic acid, aluminum butyrate and perchloric acid.

This invention applies to both sulfated and sulfonated surfactants. When the surfactant is a sulfate, X is OS03-; when the surfactant is a sulfonate, X is S03-. Sulfates and sulfonates are made somewhat differently but they behave similarly in this invention. Alkoxylated surfactants that are sulfates can be prepared in a number of ways well known to those skilled in the art. The techniques of sulfonat ion which may be used to prepare such surfactants are discussed in the U.S. Patent 4,203,428, which is incorporated herein by reference. Alkoxylated surfactants that are sulfonates can also be prepared in a number of ways. For example, they may be prepared in the same way as alkoxylated sulfates with sulfonation substituted for the sulfation step, as described in U.S. Patent 4,293,428, which is incorporated herein by reference.

The sulfated or sulfonated material can then be neutralized using, for example, an alkali metal, alkaline earth metal or ammonium to form the sulfated or sulfonated surfactant.

A carboxylate group may be incorporated into the alkoxylated material by any number of well-known methods. For example, the alkoxylated material may be reacted with a halogen carboxylic acid to result in a carboxylic acid. Alternatively, the alcoholic end group -CH2OH can simply be oxidized under well known conditions to yield a carboxylic acid. The resulting product is then neutralized using, for example, an alkali metal alkaline earth metal or ammonium to form the carboxylate surfactant.

Useful anionic surfactants can also be represented by the formula R6-X-M+ wherein R6 is a hydrocarbyl group of preferably from about 10 to about 28 carbon atoms, more preferably from about 16 to about 22 carbon atoms. X is a sulfate, sulfonate or carboxylate group. M is a cation. R6 is preferably an alkyl or an alkenyl group, and internal olefins are particularly preferred. M is preferably an alkali metal (e.g., Na+, K+) alkaline earth metal (e.g., Ca++, Mg++), or an ammonium ion. An example of a commercially available material that is useful in the inventive method is Enoroet IOS 1720 (a product of Shell identified as a sulfonated C17-20 internal olefin, sodium salt).

Hydrocarbyl-substituted polyoxyalkylene sulfonates which are useful as the surfactant (A) are disclosed in U.S. Patents 3,916,994; 4,181,178; 4,23i,427; 4,269,271; 4,270,607; 4,296,812; 4,307,782; 4,316,809; 4,485,873; and 4,478,281. These patents are incorporated herein by reference.

Useful amphoteric surfactants include compounds represented by the formula

wherein Z is S03- or COO-, R7 is a hydrocarbyl group of 1 to about 14 carbon atoms, R8 is a hydrocarbyl group of 1 to about 5 carbon atoms, R9 is a divalent hydrocarbyl group of 1 to about 3 carbon atoms which can be unsubstituted or hydroxy-substituted (e.g., -CH2-, -CH2-CH2-CH2

and R10 is a hydrocarbyl group of about 8 to about 28 carbon atoms, preferably about 16 to about 22 carbon atoms or is

wherein Rll is a hydrocarbyl group of about 6 to about 26 carbon atoms and R12 is a divalent hydrocarbyl group of about 1 to - about 6 carbon atoms. R7 is preferably methyl when R8 is methyl. R7 and/or R8 can also be hydroxy-substituted, (e.g., -CH2CH20H).

R9 is preferably methylene. R10 can be an alkyl, dialkyl, alkenyl, alkylaryl or alkyleneoxy. When RiO is alkenyl, there is generally one ethylenic (-C=C-) linkage which is preferably near the middle of the chain. When R10 is alkylaryl, it generally has the formula

wherein R13 is hydrogen or a hydrocarbyl group of 1 to about 20 carbon atoms and R14 is a divalent hydrocarbyl group of 1 to about 20 carbon atoms. R10 can also contain a nitrogen atom in a ring or chain. When R10 is alkyleneoxy, it is generally represented by the formula R15-t R160 + y wherein R15 is a hydrocarbyl group of about 6 to about 26 carbon atoms, R16 is a divalent hydrocarbyl group of about 2 to about 18 carbon atoms, preferably about 2 to about 3 carbon atoms, and y is an integer of from 1 to about 4.

The sulfonate derivatives wherein Z is S03- may be prepared by any suitable technique such as the reaction of a tertiary amine with a sulfone as disclosed in Parris et al, "Soap-Based Detergent Formulations. V.

Amphoteric Lime Soap Dispersing Agents", Journal of the American Oil Chemist Society, Vol. 50, pp. 509-512 (1973), and U.S. Patents 3,280,179 and 3,660,470, which are incorporated herein by reference. An alternative synthesis procedure involves the reaction of a tertiary amine with a halogenated alkane sulfonate such as chloroethane sulfonate as disclosed in the above U.S.

Patent 3,660,470.

An alternative procedure for the preparation of quaternary ammonium methane sulfones of the type useful in accordance with the process of the present invention involves the reaction of the appropriate tertiary amine with aqueous formaldehyde and sulfur dioxide. This reaction is carried out by treating a mixture of tertiary amine and formaldehyde with gaseous sulfur dioxide. The formaldehyde and amine may be dissolved in any suitable solvent such as ethanol, methanol or isopropyl alcohol.

The carboxylate derivatives wherein Z is COOmay be prepared by quaternizing a tertiary amine with a chlorinated ester to form a cationic quaternary ammonium salt and then hydrolyzing the ester constitutent to form the amphoteric carboxylate.

Examples of the preparation of amphoteric surfactants that are useful with the method of the present invention are disclosed in U.S. Patents 3,939,911; 4,076,743; 4,090,969; 4,130,491; 4,193,452; 4,216,097; and 4,259,191. The disclosures of these patents are incorporated herein by reference.

Examples of commercially available amphoteric surfactants which are useful in accordance with the present invention include Lonzaine 16S, a product of Lonza, Inc., identified as cetyl betaine, and Schercotaine PAB, a product of Scher Chemicals, Inc., identified as palmityl amido betaine.

The surfactants (A) of the invention are preferably present in the aqueous compositions used in accordance with the inventive method at a level in the range of about 0.01% to about 5% by weight, more preferably from about 0.1% to about 3% by weight, more preferably from about 0.1% to about 1% by weight, and still more preferably from about 0.1% to about 0.5% by weight of said aqueous composition.

The polymeric thickeners (B) can be any thickening agent that is stable under the anticipated operating conditions. These thickeners are preferably employed in the aqueous compositions of the invention at a level in the range of from about 0.02% to about 0.3% by weight, more preferably from about 0.05% to about 0.15% by weight. A preferred class of polymers suitable for use in carrying out the present invention include polysaccharides such as those available commercially which are produced by fermentation of carbohydrates by bacteria of the genus Xanthomonas. Examples of such polysaccharides are those produced by action of Xanthomonas Campestris, Xanthomonas Begonia, Xanthomonas Phaseoli, Xanthomonas ~ Hederpe, Xanthomonas Incanae, Xanthomonas Carotae, and Xanthomonas Translucens.

Preferred species are xanthan gums which are commercially available from Pfizer under the tradenames "Flocon 4800", "Flocon 4800M", and "Flocon 4800 MT".

Another preferred species is ionic polysaccharide B-1495, which is available under the tradenames "Kelzan" and "Biozan" from Kelco Company. Production of useful polysaccharides 'is described in Smiley, K.L. "Microbia Polysaccharide-A Review", Food Technology 29,9:112-116 (1966) and in Moraine, R.A., Rogovin, S.P., and Smiley, K.L. "Kinetics of Polysaccharide B-1459 Synthesis", J.

Fermentation Technology, 44, pp. 311-132 (1966), which are incorporated herein by reference.

A class of thickeners that is particularly useful includes the homopolysaccharide gum thickeners represented by the formula.

wherein m is a number that is high enough to provide the desired molecular weight. These thickeners are typically nonionic and have a molecular weight that is greater than about one million, preferably in the range of about 1 to about 3.5 million. The polymer structure is preferably a linear chain of anhydroglucose units linked beta (1-3). In a particularly advantageous embodiment about 30% to about 35% of the linear chain units bear single appended anhydroglucose units linked beta (1-6). This thickener is preferably cell-free and produced by selected fungus strains of the genus Schlerotium in a pure culture fermentation. An example of a commercially available thickener of this type is Actigum CS-ll, a product of Jetco Chemicals identified as scleroglucan.

Another preferred class of polymeric thickeners that is useful with the inventive method includes the commercially available, water-soluble high molecular weight, unhydrolyzed or partially hydrolyzed polyacrylamides having molecular weights in the range of above about 200,000, preferably from about 500,000 to about 40,000,000, more preferably from about 3,000,000 to about 10,000,000. Copolymers of acrylamide and acrylic acid within the same molecular weight range, may also be used. If the polymer employed is a partially hydrolyzed polyacrylamide, up to about 70% and preferably from about 12% to about 45% of the carboxylamide groups are hydrolyzed to carboxyl groups. A number of partially hydrolyzed polyacrylamides and/or co-polymers of acrylamide and acrylic acid are available commercially and commonly employed in enhanced oil recovery.These include, for example, materials marketed by the Dow Chemical Company under the trade name "Pusher" and by American Cyanamid under the tradename "Cyanatrol".

Naturally occurring polymers may also be employed as the polymeric thickeners in the inventive method. Included in this class of effective materials are Guar gum, Locus Bean Gum, natural starches and derivatives thereof, cellulose and its derivatives including hydroxy ethyl cellulose.

-The polymeric material (C) can be any polymer having a flexible backbone and a higher polarizability than component (B). Preferred polymeric materials include polyvinyl alcohol, polyoxyethylene and polyvinylpyrrolidone. Polyvinylpyrrolidone is particularly preferred. These polymeric materials typically have molecular weights in the range of about 20,000 to about 6,000,000. The polyvinylpyrrolidones preferably have molecular weights in the range of about 20,000 to about 600,000. The polyoxyethylenes preferably have molecular weights in the range of about 50,000 to about 6,000,000.

These polymeric materials are preferably present in the aqueous compositions used in the inventive method at a level in the range of about 0.005% to about 0.18 by weight, more preferably from about 0.01% to about 0.05% by weight.

The inventive method may be carried out utilizing injection and production systems as defined by any suitable arrangement of wells. One well arrangement commonly used in waterflooding operations and suitable for use in carrying out the method of the present invention is an integrated five-spot pattern of the type illustrated in U.S. Patent 3,927,716 which is incorporated herein by reference. Other well arrangements may be used in carrying out the present invention, examples of some of which are also disclosed in the foregoing '716 patent.

The term "pore volume" is used herein to mean that volume of the portion of the formation underlying the well pattern employed, as described in greater detail in the '716 patent.

The aqueous composition containing components (A), (B) and (C) that is injected in accordance with the inventive method can be referred to as a surfactant slug. In a typical operation, the surfactant slug is injected into the formation through one or more injection wells using standard techniques known in the art, then, depending on the design of the process, a buffer slug may be injected, and finally an aqueous flooding medium is injected after the surfactant slug (or if a buffer slug is used after the buffer slug) to drive the oil toward one or more production wells. The buffer slug contains an effective amount of a thickener to increase the viscosity of the buffer slug to the same level or a level above that of the surfactant slug, and thereby provide stable propogation of the mobilized oil toward the production wells.The thickened employed in the buffer slug can be the same or different than component (B), but is preferably the same.

The size of the surfactant slug typically ranges from about 0.2 to about 3 pore volumes. The size of the buffer slug is preferably in the range of up to about 3 pore volumes.

The drive fluid or aqueous flooding medium is injected into the reservoir in sequential order after the surfactant slug, or if a buffer slug is used after the buffer slug. This flooding medium is preferably water and can be any source of water, such as sea water, that is readily available.

An advantage of the method of the present invention is that it can be practiced in subterranean formations having temperatures ranging from about 150C to about 1200C and above, high pressures (e.g., up to about 4000 psi), and the flood water can have a high salinity (e.g., TDS values as high as about 200,000 ppm such as when the floodwater is sea water), and high concentrations of divalent metal ions (e.g., up to 20,000 ppm). These conditions are typically encountered under various circumstances at Prudhoe Bay, the North Sea, the Persian Gulf, the Gulf of Mexico, as well as other major oil fields.

Another advantage of this invention is that effective levels of oil recovery can be achieved with relatively low levels of surfactant utilization. This result provides this invention with significant economic advantages over the prior art due to the fact that surfactants are typically costly materials. While not wishing to be bound by theory, it is believed that this is achieved in part due to a synergistic interaction between the surfactant (A) and the polymeric thickener (B). It is further believed that this phenomenon is due to enhanced stabilization of the surfactant (A) and polymeric thickener (B) that is achieved by the addition of the supplemental polymer (C).

Owing to the chemical nature of the surfactants (A) and polymeric thickeners (B), these constituents often cause each other to associate in an aqueous composition in the form of flocculation, or the polymeric thickeners (B) often exclude the surfactant (A) as a separate phase. Both phenomenon can disable these constituents from exhibiting their desired aqueous composition enhancement properties. While not wishing to be bound by theory, it is believed that stabilization of the surfactant (A) and polymeric thickener (B) is effected when the supplemental polymer (C) is adsorbed on the surface of surfactant self-assemblies (i.e., spherical or cylindrical micelles or vehicles). For purposes of simplicity, these surfactant self-assemblies can be referred to as micelles.Segments of the polymer (C) wrap around the interface of a micelle further shielding a part of the hydrocarbon core of the micelle from contact with water. Each polymer molecule (C), depending on its size, can have more than one micelle bound to it. The characteristics of polymer (C) that are believed to permit such adsorption are its flexible backbone, and relatively high polarizability when compared to polymeric thickener (B). When two polymer (C) bound micelles approach one another such that the adsorbed layers begin to interpenetrate, a repulsive force is generated. This repulsion between polymerbound micelles, i.e., steric stabilization, stabilizes otherwise unstable mixtures of surfactant (A) and polymeric thickener (B).

The following example is provided for the purpose of further illustrating the inventive method.

Unless otherwise indicated, in the following example as well as throughout the specification and in the appended claims, all parts and percentages are by weight, and all temperatures are in degrees centigrade.

Example Oil displacement tests were conducted at a temperature of 93.30C using dead crude oil and Berea sandstone cores. The sandstone cores had dimensions of 1.5"x24", average porosities of 22% and average permeabilities of 420 md. Each core was first degassed under vacuum and then saturated with a brine having a TDS concentration of 24,000 ppm, including 250 to 300 ppm multivalent cations. The cores were thereafter flooded with stock tank oil until no further brine could be produced. The cores were then flooded with seawater until no further oil could be produced. The average residual oil level in each core was about 37% of the core pore volume. The aqueous compositions being tested, which are identified in the Table below, were then injected into the cores until approximately 2 to 3 pore volumes of such aqueous compositions had been injected.Sea water was used in formulating these aqueous compositions. The sandstone cores were then flooded with 2 to 5 pore volumes of seawater.

The seawater that was used had about 30,000 ppm TDS level and contained the following major cations: Cation Na+ 9200-9300 Mg++ 1080-1100 Ca++ 330-340 K+ 330-350 Sr++ 5-10 Interfacial tension (IFT) measurements were determined using the following procedure. The aqueous phase and the oil phase were first pre-equilibrated by mixing one part by volume of the oil phase with one part by volume of the aqueous phase. The aqueous phase consisted of the aqueous compositions being tested. The oil phase and aqueous phase were brought to thermal equilibrium at 93.30C. The mixture was then shaken by hand for about 30 seconds and allowed to equilibrate until there was no change in the volume of each phase.

IFT values for these systems were determined at 800C using a spinning drop interfacial tensiometer. These IFT values were determined in the following way: the interfacial tension of a system was measured as a function of time, usually for 2 hours. If the values for the last 20 minutes agreed to within 1-2%, the 2-hour measurement was reported; if not, the measurements were continued until the 1-28 agreement for a 20-minute period was obtained.

Turbidity was measured at 250C using the procedure described in "Procedures for Water and Wastewater Analysis", Hach Company, P.O. Box 389, Loveland, Colorado, 80539, U.S.A., 1984, which is incorporated herein by reference. The procedure was calibrated using formazin polymer as the turbidity reference standard. The readings are in terms of formazin turbidity units (FTU) which are equivalent to nephelometric units (NTU). Actigum CS-ll was solubilized in sea water. The resulting polymer solution was filtered through 1.2 Fm filter paper and had a turbidity value of 2.6 NTU. The solution was clear. When the surfactants identified in the Table below were added to the polymer solution, the turbidity increased to 210 NTU. When polyvinylpyrridone K-30 was added, the turbidity dropped to 49 NTU.

Example 1, which is reported in the Table below, is illustrative of the invention. Examples 1A and 1B, which are also reported in the Table, are provided for purposes of comparison. In the Table, the numerical values for concentrations of surfactants, polymeric thickener and supplemental polymer are in percent by weight. Sea water having the above-indicated characteristics was used for each example.

TABLE Example No.

1 lA 1B Surfactants Linear alkyl propoxy ethoxy sul- 0.15 0.15 0.15 fonate, sodium salt, represented by formula C16H330(C3H60)4(C2H40)2C3H6S03Na Shell Enordet IOS-1720, a product 0.2 0.2 0.2 of Shell identified as a sulfonated C17-20 internal olefin, sodium salt Polymeric Thickener Actigum CS-ll, a product of Jetco 0.15 0.15 Chemicals identified as scleroglucan gum Supplemental Polymer Polyvinylpyrrolidone R-30, a 0.02 product of Fluka Chemical Corp.

identified as nonionic polyvinylpyrrolidone having a molecular weight of 40,000.

Test Results Oil Displacement, % of ROIP 92 IFT, (Dynes/Cm) xl0-3 5 5.6 10 Turbidity, NTU 49 210 Example 1 indicates that significant levels of oil recovery can be achieved using the inventive method and in doing so employing relatively low concentrations of surfactant in the waterflood. A comparison of Example 1 with Example 1A indicates significant improvement in turbidity when a small amount of polyvinylpyrrolidone is added. A comparison of Example 1 with Example 1B indicates a significant reduction of IFT values is achieved when a polymeric thickener and supplemental polymer are used in combination with a surfactant as opposed to when the polymeric materials were not used.

While the invention has been explained in relation to its preferred embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading this specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims.

Claims (31)

Claims

1. A method for recovering oil from a subterranean formation comprising injecting an aqueous composition into said formation and displacing said oil toward one or more production wells, said aqueous composition comprising: (A) from about 0.01% to about 5% by weight of at least one anionic or amphoteric surfactant; (B) from about 0.02% to about 0.3% by weight of at least one polymeric thickener; and (C) from about 0.005% to about 0.1% by weight of at least one polymeric material having a flexible backbone and a higher polarizability than component (B).

2. The method of claim 1 wherein the concentration of (A) in said aqueous composition is from about 0.18 to about 3% by weight.

3. The method of claim 1 wherein the concentration of (A) in said aqueous composition is from about 0.1% to about 1% by weight.

4. The method of claim 1 wherein the concentration of (A) in said aqueous composition is from about 0.1% to about 0.5% by weight.

5. The method of claim 1 wherein the concentration of component (B) in said aqueous composition is from about 0.05% to about 0.15% by weight.

6. The method of claim 1 wherein the concentration of component (C) in said aqueous composition is from about 0.01% to about 0.05% by weight.

7. The method of claim 1 wherein component (A) is at least one compound represented by the formula RO- (R' ) nR"X-Y+ wherein: R is a hydrocarbyl group of from about 10 to about 24 carbon atoms; R' is ethoxy, propoxy or a mixture thereof; R" is a divalent hydrocarbyl group of 1 to about 20 carbon atoms; X is a sulfate, sulfonate or carboxylate group; Y is a cation; and n is a number in the range of 1 to about 20.

8. The method of claim 7 wherein R is a hydrocarbyl group of about 10 to about 20 carbon atoms.

9. The method of claim 7 wherein R is a hydrocarbyl group of about 12 to about 18 carbon atoms.

10. The method of claim 7 wherein R is an alkyl or an alkenyl group.

11. The method of claim 7 wherein R" is a divalent hydrocarbyl group of 1 to about 10 carbon atoms.

12. The method of claim 1 wherein component (A) is a hydrocarbyl-substituted polyoxyalkylene sulfonate.

13. The method of claim 1 wherein component (A) is a compound represented by the formula R6-X-M+ wherein R6 is a hydrocarbyl group of about 10 to about 28 carbon atoms, X is a sulfate, sulfonate or carboxylate group, and M is a cation.

14. The method of claim 1 wherein component (A) is a compound represented by the formula

wherein: R7 is a hydrocarbyl group of 1 to about 14 carbon atoms; R8 is a hydrocarbyl group of 1 to about 5 carbon atoms; R9 is a divalent hydrocarbyl group of 1 to about 3 carbon atoms; and RlO is a hydrocarbyl group of about 8 to about 28 carbon atoms or represented by the formula

wherein Rll is a hydrocarbyl group of about 6 to about 26 carbon atoms, R12 is a divalent hydrocarbyl group of about 1 to about 6 carbon atoms; and Z is SO3- or COO-.

15. The method of claim 14 wherein R7 and R8 are methyl.

16. The method of claim 14 wherein R9 is -CH2- or -CH2-CH-CH2-.

OH

17. The method of claim 14 wherein R10 has about 16 to about 22 carbon atoms.

18. The method of claim 14 wherein R10 is an alkyl or an alkenyl group.

19. The method of claim 14 wherein RlO is represented by the formula

wherein R13 is hydrogen or a hydrocarbyl group of 1 to about 20 carbon atoms and R14 is a divalent hydrocarbyl of 1 to about 20 carbon atoms.

20. The method of claim 14 wherein Rl0 is represented by the formula R15 tR160t wherein Rl5 is a hydrocarbyl group of about 6 to about 26 carbon atoms, R16 is a divalent hydrocarbyl group of about 2 to about 18 carbon atoms, and y is an integer of 1 to about 4.

21. The method of claim 1 wherein component (B) is a polysaccharide.

22. The method of claim 1 wherein component (B) is a partially hydrolyzed polyacrylamide.

23, The method of claim 1 wherein component (B) is a xanthan gum.

24. The method of claim 1 wherein component (B) is a homopolysaccharide.

25. The method of claim 1 wherein component (B) is represented by the formula

wherein m is an integer that is sufficiently high enough to provide a molecular weight of above about one million.

26. The method of claim 1 wherein component (C) is polyvinyl alcohol.

27. The method of claim 1 wherein component (C) is polyoxyethylene.

28. The method of claim 1 wherein component (C) is polyvinylpyrrolidone.

29. The method of claim 1 wherein said aqueous composition comprises a surfactant slug, said method further comprising the steps of injecting into said formation a buffer slug to follow said surfactant slug, said buffer slug comprising an aqueous solution containing an effective amount of at least one thickener to provide said buffer slug with a viscosity exceeding the viscosity of said surfactant slug, then injecting into said formation an aqueous flooding medium.

30. The method of claim 1 wherein said aqueous composition comprises a surfactant slug, said method further comprising the step of injecting into said formation an aqueous flooding medium.

31. The method of claim 1 wherein the water employed in said aqueous composition is sea water.