GB2213850A - Enhanced oil recovery process - Google Patents

Abstract

Oil recovery from a subterranean oil-bearing formation is accomplished by injection under pressure of an aqueous solution of a water-soluble terpolymer having the formula: <IMAGE> wherein R1 is a C6 to C22 straight, chained or branched alkyl or cycloalkyl group; R2 is hydrogen or a C6 to C22 straight, chained or branched alkyl or cycloalkyl group or a C1 to C3 straight, chained or branched alkyl group; and M<+> is an alkali metal or ammonium cation, wherein x is about 60 to 98 mole percent, y is about 2 to about 40 mole percent, and Z is about 0.1 to about 10.0 mole percent

Description

ENHANCED OIL RECOVERY PROCESS DETAILED DESCRIPTION OF THE INVENTION This invention relates to a process for recovering oil from a subterranean oil-bearing formation. It entails the use of an aqueous treating media which comprises a hydrophobically associating terpolymer of (meth)acrylamide, a salt of (meth)acrylic acid, and an N-alkyl(meth)acrylamide.

The aqueous treating solution will generally contain some salts compatible with the reservoir fluids.

The treating solution may also contain 0.1 to 5.0 weight percent of surfactants or cosurfactants to lower the interfacial tension (0.1 dyne/cm.) with the resident crude oil. In addition, oil may be present to compatibilize the surfactants and polymer.

The relative amounts of the monomers comprising the terpolymers used in the process of this invention are critically chosen to provide a balance between aqueous solubility, brine tolerance, viscosification efficiency, and mechanical stability. In addition, the composition of these polymers will also influence their adsorption onto reservoir rock and interaction with surfactants.

The water soluble polymers used in the process of this invention are characterized by the formula:

wherein R1 is preferably a C6 to C22 straight, chained or branched alkyl or cycloalkyl group, more preferably a C6 to C20, and most preferably a C6 to C18; R2 is the same or different alkyl group as R1 or hydrogen or C1 to C3 straight, chained or branched alkyl group; and M+ is an alkali metal or ammonium cation. Typical but non-limiting examples of preferred alkyl groups are hexyl, octyl, decyl, dodecyl and hexadecyl groups. Typical but non-limiting examples of preferred cations are sodium, potassium and ammonium. The mole percentage of acrylamide, x, is preferably 60 to 98, more preferably 65 to 95, and most preferably 70 to 90.

The mole percentage of the salt of acrylic acid, y, is preferably 2 to 40, more preferably 5 to 35, and most preferably 10 to 30. The mole percentage of the hydrophobic group, z, is preferably 0.1 to 10.0, more preferably 0.2 to 5.0, and most preferably 0.2 to 2.0.

The molecular weight of the water soluble terpolymers of this invention is sufficiently high so that they are efficient viscosifiers of water or brine, but not so high that the polymer molecules are readily susceptible to irreversible shear degradation. Thus, the weight average molecular weights are preferably 200,000 to 25,000,000, more preferably 500,000 to 20,000,000, and most preferably 1,000,000 to 15,000,000. The intrinsic viscosity of these polymers as measured in 2% sodium chloride solution is preferably greater than 1 dl/g but less than 40 dl/g.

The terpolymers may be synthesized by a variety of processes. Two of the most preferred processes rely on dispersing the water insoluble or hydrophobic monomer on a very fine scale into an aqueous solution of the water soluble monomer. The product in both cases is substantially free of microgel or particulates of insoluble polymer. One process relies on cosolubilizing the water insoluble or hydrophobic N-alkylacrylamide monomer into a predominantly aqueous media containing acrylamide monomer and perhaps acrylic acid or a monovalent salt of acrylic acid by the use of a special mixture of surfactant, cosurfactant and hydrocarbon oil.

The resultant fluid has the water insoluble monomer dispersed on almost a molecular scale. This isotropic, translucent to transparent, homogeneous fluid is called a microemulsion. Further details as to the type and level of surfactants, cosurfactants, hydrocarbon oil and monomers can be found in U. S.

Patent No. 4,521,580, which is hereby incorporated by reference. When an anionic monomer, such as acrylic acid or salt of acrylic acid, is used only anionic or nonionic surfactants can be used.

Cationic surfactants will generally complex with the anionic monomer which may cause precipitation and, thus, are not suitable for preparing the compositions of this invention.

An alternative process for dispersing the water insoluble or hydrophobic monomer into a predominantly aqueous phase containing the dissolved water soluble monomers, such as acrylamide and acrylic aci1 or a salt or acrylic acid, makes use of a single surfactant or mixture of surfactants with no hydrocarbon oil. In order to prevent the formation of undesirable particulates of insoluble polymer the surfactant must be chosen to be one that is capable of solubilizing the water insoluble monomer on an extremely fine scale so that the resulting mixture is isotropic, clear and homogeneous. Thus, the solubilization of the hydrophobic monomer must take place in the micelles. The surfactant is dissolved into the water at concentrations above the critical micelle concentration.

Further details of this polymerization technique can be found in U. S. Patent No. 4,528,348, which is herein incorporated by reference. The critical aspect is that the micellar reaction mixture of monomers permits a uniform polymerization to occur, such that the resultant polymer does not contain particulates or latices of water insoluble polymer.

The surfactants which may be used in the polymerization process may be one of the water soluble surfactants, such as salts of alkyl sulfates, sulfonates, and carboxylates, or alkyl arene sulfates, sulfonates or carboxylates. Preferred are sodium or potassium salts of decyl sulfate, dodecyl sulfate or tetradecyl sulfate. For these ionic surfactants the Krafft point, which is defined as the minimum temperature for micelle formation, must be below the temperature used for the polymerization. Thus, at the conditions of polymerization, the desired surfactant will form micelles which solubilize the water insoluble monomer.

Nonionic surfactants can also be used for preparing the polymers of this invention. For example, ethoxylated alcohols, ethoxylated alkyl phenols, ethoxylated dialkyl phenols, ethylene oxide-propylene oxide copolymers and polyoxyethylene alkyl ethers and esters can be used. Preferred nonionic surfactants are alkoxylated alcohols or alkyl phenols, such as ethoxylated nonyl phenol with 5 to 20 ethylene oxide units per molecule, ethoxylated dinonyl phenol containing 5 to 40 ethylene oxide units per molecule and ethoxylated octyl phenol with 5 to 15 ethylene oxide units per molecule.

Surfactants which contain both nonionic and anionic functionality, e.g., sulfates and sulfonates of alkoxylated alcohols and alkyl phenols, can also be used. Combinations of anionic and nonionic surfactants can also be used as long as the surfactants solubilize the hydrophobic monomer into an aqueous phase containing the water soluble monomers. The surfactant or mixtures of surfactants are used at concentrations above the critical micelle concentration and preferably at concentrations such that only one or at most a few hydrophobic monomers are associated with a surfactant micelle. Thus, the actual concentration of surfactant for a given polymerization depends on the concentration of oil soluble or hydrophobic monomers employed.

Polymerization of the water soluble and water insoluble monomers is effected in an aqueous micellar solution containing a suitable free radical initiator. Examples of suitable water soluble free radical initiators include peroxides, such as hydrogen peroxide, and persulfates, such as sodium, potassium or ammonium persulfate. Suitable oil soluble initiators are organic peroxides and azo compounds, such as azobisisobutylonitrile. Water soluble initiators, such as potassium persulfate, are preferred. Redox initiation, involving an oxidant such as potassium persulfate and a reductant such as sodium metabisulfite, can also be used to initiate polymerization, particularly at low temperatures. Polymerizing at lower temperature results in the formation of higher molecular weight polymers which are desirable from the standpoint of efficient aqueous viscosification.Typically it is desired to employ from 0.01 to 0.5 weight percent of initiator based on the weight of monomers. The polymerization temperature is preferably 200C to 900C, more preferably 250C to 800C and most preferably 300C to 700C.

The hydrophobically associating terpolymers used in the enhanced oil recovery process of this invention can be prepared by the micellar free radical copolymerization process which comprises the steps of forming a micellar surfactant solution of the oil soluble or hydrophobic alkylacrylamide in an aqueous solution of acrylamide; deaerating this solution by purging with nitrogen or additionally applying a vacuum; raising the temperature to the desired reaction temperature; adding sufficient free radical initiator to the reaction solution; and polymerizing for a sufficient period of time at a sufficient temperature to effect polymerization. Base can be added to the polymerized reaction mixture to convert some of the acrylamide to acrylic acid groups.This hydrolysis reaction can be performed with a stoichiometric amount of base at a temperature of preferably 300C to 900C, more preferably 400C to 800C and most preferably 450C to 700C for 1 to 10 hours. Higher amounts of base can be employed to accelerate the hydrolysis which then could be run for either a shorter time or at a lower temperature. The resulting terpolymer of acrylamide, a salt of acrylic acid and a hydrophobic N-alkylacrylamide can be isolated from the reaction mixture by any of a variety of techniques which are well known to one skilled in the art. For example, the polymer may be recovered by precipitation using a non-solvent, such as acetone, methanol, isopropanol or mixtures thereof.

The precipitated polymer can then be washed and oven dried to provide a product in the form of a free flowing powder. Alternatively, the polymer solution may be used as is by diluting with the desired aqueous solvent to the concentration of use.

An alternative method for preparing the terpolymers used in this invention is to use acrylic acid monomer or a monovalent salt of acrylic acid, such as sodium or potassium acrylate, along with acrylamide and the micellar dispersion of the hydrophobic N-alkyl acrylamide in the initial reaction mixture. Similar polymerization and isolation conditions could be used as described above without the need for a post hydrolysis reaction. Further details on the methods for preparing these hydrophobically associating polymers can be found in EP-A-O 228 798.

The hydrophobically associating polymers described above have been found to impart many desirable characteristics to the mobility control fluids used in the oil recovery process of the present invention. To prepare these thickened mobility control fluids, an amount of the terpolymer thickening agent is dissolved in the aqueous fluid by agitation using any of a number of techniques well known in the art. For example, a marine impeller operating at relatively low speed can be used to first disperse and then dissolve these hydrophobically associating terpolymers. It is desirable to use relatively low agitation conditions since these polymers have a tendency to cause and stabilize foams which can be difficult to break.

The aqueous solutions may be relatively fresh water or contain high concentrations of electrolyte, such as in hard water or brine. Monovalent inorganic salts, such as sodium chloride, and divalent salts, such as calcium or magnesium chloride, or sulfate can be present in the brine in substantial amounts.

A preferred method for preparing the thickened brine solutions involves first preparing a concentrated solution of the polymer in relatively fresh water and then adding a concentrated brine solution to obtain the desired final thickened brine solution.

The amount of polymeric thickening agent needed to produce a desired level of viscosification will depend on the composition of the electrolytes in the aqueous reservoir fluid and the temperature of the reservoir. In general, more polymer will be required as the electrolyte concentration increases and as the temperature increases. Viscosification of about 2 to about 100 times or more that of the solvent can readily be achieved with the terpolymers of this invention. Preferably 0.01 to 2.0 weight percent, more preferably 0.05 to 1.0 weight percent, and most preferably about 0.1 to 0.5 weight percent polymer based on the aqueous medium will provide the desired level of thickening efficiency.

The thickening efficiency of a given polymer is influenced by the amount of anionically charged acrylate groups, the level and type of hydrophobic groups and the molecular weight. The addition of anionic acrylate groups improves polymer solubility and enhances thickening efficiency due to repulsion of charges along the backbone, which tends to open the polymer coil and increase hydrodynamic volume. In addition, the presence of these groups tends to reduce adsorption of the polymer onto the reservoir rock during enhanced oil recovery operations. The hydrophobic groups decrease polymer solubility and associate in solution to reversibly bridge polymer molecules, creating greater resistance for flow and, hence, increased viscosity. The more insoluble the hydrophobic group is in the solvent, the less that is needed to create the associations in solution.For example, less dodecylacrylamide is needed in a polymer to create the same viscosification as a larger amount of octylacrylamide in a similar polymer. In addition, it is possible to have too much association, in which case the polymer becomes insoluble in the solvent and cannot be used as a viscosifier. During enhanced oil recovery operations, too much hydrophobe in the polymer can lead to increased polymer adsorption and, in extreme cases, to plugging. Thus, the amount of hydrophobic groups present in the polymer must be critically controlled. Fortunately, the solubility and rock adsorption characteristics of the acrylate and hydrophobic groups go in opposite directions and, thus, the addition of more acrylic acid can be used to counter balance the addition of hydrophobic groups.Increasing both acrylate and hydrophobic groups can result in a synergistic enhancement of thickening efficiency and, in turn, mobility control.

Molecular weight of the polymer is also an important consideration. High molecular weight polymers incorporating both anionically charged acrylate groups and hydrophobic groups can provide significantly improved viscosification of water based fluids. All other things being equal, the higher the molecular weight the less soluble the polymer. Thus, as molecular weight is increased, the amount of hydrophobic groups should be reduced and the amount of acrylate groups increased. It is desirable that the resulting polymer in an aqueous solution not be susceptible to irreversible mechanical degradation under the shear or elongational stress experienced during injection in reservoir formations. This places an upper limit on polymer molecular weight to minimize loss of viscosification during injection. This depends on polymer composition, injection fluid composition, injection rate and rock properties, such as permeability and porosity. Control of molecular weight is achieved by adjusting polymerization conditions, such as the concentration of monomers, the type and level of initiator and the reaction temperature. As is well known in the art, the molecular weight is increased by increasing the monomer level and decreasing the initiator level and reaction temperature. A redox catalyst system, such as ammonium or potassium persulfate and triethylamine, can be used at low temperature to provide high molecular weight polymers.

To evaluate and characterize the unique and useful properties of the hydrophobically associating polymers of this invention dilute solution viscometric measurements were made. These measurements are particularly useful for evaluating the effect of composition and polymerization process conditions on the hydrodynamic size per unit weight of the polymer in solution and the influence of associating groups. The hydrodynamic size is measured by the intrinsic viscosity, which is related to some power of the viscosity average molecular weight. To determine the intrinsic viscosity, the reduced viscosity is first evaluated at several polymer concentrations in the dilute regime. The reduced viscosity is defined as the incremental viscosity increase of the polymer solution relative to the pure solvent normalized with respect to the pure solvent viscosity and the polymer concentration.A plot of reduced viscosity versus polymer concentration should yield a straight line at sufficiently low polymer concentrations.

The intercept of this reduced viscosity plot at zero polymer concentration is defined as the intrinsic viscosity, while the slope is the Huggins' interaction coefficient times the square of the intrinsic viscosity. The Huggins' constant is a measure of polymer-solvent interactions. For hydrophobically associating polymers, it is characteristically greater than the 0.3 to 0.7 value normally observed for non-associating polymers, such as polyacrylamides.

Measurements of the dilute solution viscosity were made with conventional Couette or capillary viscometers. A set of Ubbelohde capillary viscometers were used in this study. Shear rate effects were found to be negligible in the concentration range of interest. However, since the terpolymers contained anionically charged groups, a polyelectrolyte effect was observed in dilute solution. This polyelectrolyte effect resulted in an increase in reduced viscosity with decreasing polymer concentration and tended to mask the effect of hydrophobic associations. The addition of salts, such as sodium chloride or sodium sulphate, shields the charge repulsion, causing the polyelectrolyte effect and resulted in the desired linear reduced viscosity versus concentration plot. The dilute solution measurements were thus made on solutions containing 2.0 weight percent sodium chloride.

The solution viscosity of associating polymers in the semi-dilute concentration regime is dramatically different from conventional water soluble polymers. Viscosities of these solutions were measured by means of a Contraves low shear viscometer, model LS 30, using a No. 1 cup and No. 1 bob. Temperatures were controlled to +O.loC and measurements were made at a variety of rotational speeds corresponding to shear rates from 1.0 sec-l to 100 sec-l. In contrast to conventional water soluble polymers and relatively low molecular weight weakly associating polymers, the terpolymers of this invention can exhibit significant relaxation times which result in slow equilibration. To determine steady state viscosity values at a given stress or shear rate, relatively long measurements times were employed.This effect was most evident at higher polymer concentrations, higher polymer molecular weights and in regions of strong intermolecular hydrophobic associations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples illustrate the present invention without, however, limiting the same hereto.

Example 1 Synthesis of HRAM Polymer An HRAM terpolymer of acrylamide, sodium acrylate and N-n-octylacrylamide was synthesized using the micellar polymerization technique. In a 5 liter glass reactor, equipped with stainless steel baffles, turbine impellers and nitrogen sparge, 75.0 g of acryl amide (AM), 80 g of sodium dodecyl sulfate (SDS), 2.0 g (1.0 mole percent) of N-n-octylacrylamide and 2,425 g of distilled water were mixed to form a homogeneous, transparent solution. The reaction fluid was deoxygenated by sparging with nitrogen (N2) for 2 hours while heating to bring the temperature up to 500C. At temperature, the initiator, 0.05 g of potassium persulfate (K2S2Og) in 10 ml of deoxygenated water, was added. The reaction was maintained at 50 C with gentle agitation for 18 hours.While maintaining the reaction temperature at 500C, 200 ml of 50% sodium hydroxide (0.40 moles of NaOH) was added and allowed to react for 1 hour. The resulting terpolymer was isolated by precipitating in 2 volumes of methanol. The swollen polymer mass was ground in a Waring blender, washed with methanol and vacuum oven dried at 400C for 16 hours. This HRAM polymer, designated Example 1, was a white friable material with about 3 weight percent moisture as determined by weight loss following 24 hours of vacuum oven drying at 110 C. Nitrogen and sodium analysis, along with potentiometric titration showed that the polymer contained 18.4 mole percent sodium acrylate groups.

Comnarative Example 1 PreParation of HPAM Polvmer A partially hydrolyzed polyacrylamide (HPAM) copolymer was synthesized using a similar procedure as described in Example 1. Using the same reactor setup, 75 g of acrylamide, 80 g of SDS and 2,425 g of distilled water were mixed and sparged with nitrogen for 2 hours while heating to SOOC.

The initiator, 0.05 g of K2S2O8, was then added.

The reaction was maintained at 50 C with gentle agitation for 18 hours and then 200 ml of 50% NaOH (0.40 moles) was added and allowed to react for 1 hour. The resulting HPAM polymer was then isolated by precipitation in 2 volumes of methanol, ground and washed with methanol in a Waring blender and vacuum oven dried at 400C for 16 hours. This partially hydrolyzed polyacrylamide, designated Comparative Example 1, was a white friable material with 4% moisture and 18.2% sodium acrylate groups.

Examples 2 to 8 Effect of Hydrolysis Conditions A series of copolymers consisting of 99 mole percent acrylamide and 1.0 mole percent N-n-octylacryl amide were prepared using the recipe and procedures described in Example 1. At the end of copolymerization, different hydrolysis reaction conditions were used to prepare terpolymers with different levels of anionically charged sodium acrylate groups. Prior studies had indicated that hydrolysis was very slow at 400C and, thus, temperature was held at 500C. The amount of base in terms of the moles of sodium hydroxide per mole of acrylamide and the reaction time were varied as shown in Table I. The degree of hydrolysis was a monotonic increasing function of the amount of added base and, thus, was the major variable for controlling the charge content in the terpolymer. The composition of the resulting HRAM Polymers are also given in Table I with the mole percent sodium acrylate determined by titration and/or sodium analysis.

Examples 9 and 10 Different Hydrophobe Contents Using the recipe and polymerization procedures described in Example 1, terpolymers were prepared with 0.75 and 1.25 mole percent N-n-octyl acrylamide monomer. These are designated as Examples 9 and 10, respectively. Both polymers were hydrolyzed under the same conditions as used for Example 1 as shown in Table I. An interesting observation was made by comparing Examples 1, 9 and 10. As the amount of hydrophobe increased the level of hydrolysis or sodium acrylate content decreased slightly. This indicated improved hydrolytic stability of the hydrophobically associating terpolymers.

Examples 11 and 12 Polvmerization Conditions The polymerization procedure and recipe in these Examples were similar to that used for Example 10, with the following changes: the reaction temperature was reduced from 500C to 450C and the hydrolysis reaction time increased from 60 to 90 minutes (Example 11); polymerization at 450C and the K2S2O8 initiator was reduced from 0.05 to 0.03 g (Example 12). Both the reduction in polymerization temperature and initiator level should result in increased polymer molecular weight, .lich, in turn, should result in polymers with increased aqueous fluid thickening efficiency. However, as shown in Table I, the compositions of these polymers were essentially identical, although the molecular weight, molecular weight distribution and sequence distribution might be considerably different.

Examples 13 and 14 HvdroDhobic Monomer Structure The structure of the hydrophobic monomer is an important consideration in controlling the viscosification efficiency of the terpolymers of this invention. First, the monomer must be essentially insoluble in water. This precludes the use of alkyl groups with four carbons or less. To evaluate the influence of longer chain lengths a decylacrylamide and a dodecyl acrylamide monomer were used with the synthesis conditions described in Example 1. In place of 2.0 g of N-n-octylacrylamide in the recipe of Example 1, 2.3 g (1.0 mole percent) N-n-decylacrylamide was used to form the polymer of Example 13. Similarly, 2.6 g (1.0 mole percent) of N-n-dodecylacrylamide was used as the hydrophobic monomer to form the polymer of Example 14.The resulting polymers had extremely high viscosity and were essentially solid gels at 3.0 weight percent solids in the reactor. This compares to aqueous fluids of 10,000 cP at similar concentrations using the octyl hydrophobe in place of the decyl or dodecyl. Thus, as the chain length on the hydrophobic monomer increased the amours of hydrophobe used for the polymerization should be decreased to maintain comparable solubility and viscosification characteristics.

Example 15 Preparation of High Molecular Weight HRAM Into a two liter polymerization vessel, 73.56 g of acrylamide (twice recrystallized from methanol), 1.44 g of octylacrylamide, and 23.78 g of sodium dodecyl sulfate were added. The vessel was purged of oxygen by alternately pulling a vacuum and flushing with nitrogen that had been bubbled through a basic pyrogallol solution. Then 750 g of boiled, deionized water was added and the purging process carried out six more times. The monomers were dissolved by stirring and heating to 350C for 20 minutes. It was then cooled back to 200C and nitrogen bubbled through the solution for two hours.

The initiator, 0.0028 g of triethylamine and 0.020 g of potassium persulfate, each dissolved separately in 1 ml of water, was then added. Within 30 minutes the solution had noticeably thickened, and the polymerization was carried out for an additional 18 hours. The polymer was then removed from the flask as a solid, rubbery gel. To carry out the hydrolysis of the copolymer, 400 g of the gel was macerated into small pieces and placed in 2,000 ml of deionized water. After stirring for 150 minutes, 13 g of sodium hydroxide was added and kept at a temperature of 500C for 90 minutes. While still at 50 C the polymer was precipitated by the addition of an excess of methanol. It was washed with a Waring blender for ten seconds, the methanol filtered off and the polymer was rinsed twice in methanol. It was then dried in a vacuum oven overnight at room temperature.The resulting polymer was designated Example 15.

The degree of hydrolysis of the polymer was determined by a potentiometric titration of a small sample that had been passed through both anionic and cationic ion exchange resins and found to be 16%. The intrinsic viscosity in brine containing 3 weight percent NaCl and 0.3 weight percent CaCl2 was 19 dl/g.

Example 16 Dilute Solution Properties A series of terpolymers of acrylamide, sodium acrylate and hydrophobic monomers of alkyl acrylamide, described in Examples 1 to 15, and a copolymer of acrylamide and sodium acrylate, described in Comparative Example 1, were dissolved in a 2.0% NaCl solution at 2,000 ppm. Dilutions of these solutions were made in the same solvent and Ubbelohde viscosities were obtained at 250C. Measurements were made at different polymer concentrations to obtain 5 solutions with viscosities from 1.1 to 2.0 times the solvent viscosity. Plots of reduced viscosity versus polymer concentration were analyzed with the following equation: urged = [fl) + kh[)2c to yield the intrinsic viscosity and Huggins' interaction coefficient, kh, as shown in Table II.

The intrinsic viscosity is a function of polymer molecular weight, amount of charged anionic acrylate groups and hydrophobic groups. As the amount of sodium acrylate groups in the polymer increased, the intrinsic viscosity also increased. This is illustrated in Table II for Examples 2, 7, 8, 3, 1 and 6, which have sodium acrylate content increasing from 0.3 to 34.0 mole percent. The intrinsic viscosity increased from 3.4 to 9.0 dl/g respectively. This indicates that the hydrodynamic size per unit mass of the polymer increased due to the charge repulsion created by the anionic groups. A measure of the degree of association of the hydrophobic groups was provided by the Huggins' interaction coefficient. As shown in Table II, the Huggins' constant decreased as the amount of sodium acrylate groups increased.

Looking at the same set of polymers as above, the Huggins' constant decreased from about 2.5 to 0.71 as the amount of acrylate groups increased from 0.3 to 34 mole percent, respectively. This suggests that the associations decreased or solvent character improved with increasing acrylate content. The influence of acrylate on the intrinsic viscosity and Huggins' coefficient can be counterbalanced by increasing the amount of hydrophobic groups as shown by Example 10 in Table II. Increasing the amount of hydrophobic octylacrylamide groups reduced the intrinsic viscosity and increased the Huggins' constant. The increase in the Huggins' coefficient seems to be a good indicator of the presence of hydrophobic groups.As shown in Table II, an HPAM polymer (Comparative Example II) had the lowest Huggins' constant (0.42), typical of most water soluble polymers; while the hydrophobe-containing polymers have values from 0.7 to 4.6. A higher Huggins' constant indicates that the solution viscosity will increase faster with polymer concentration. At concentrations in the semi-dilute regime, the viscosity will be significantly higher than an otherwise comparable solution with a low Huggins' constant. The data in Table II indicates that the presence of hydrophobic groups had a measurable beneficial effect on the dilute solution properties given at concentrations as low as 50-ppm.

Also, high molecular weight HRAM polymers can be obtained as indicated by the intrinsic viscosity of Example 15.

Example 17 Solution Viscometrics To evaluate the ability of the terpolymers of this invention to control aqueous solution of viscometric properties, solutions were prepared with several of the polymers from Examples 1 to 15. The solvent was 2.0% NaCl and polymer concentrations were 1,000 and 2,000 ppm. Solution viscosities were determined using a Contraves LS 30 rotational viscometer at 1.28 sec'l shear rate and 250C. The solution shear viscosity is a measure of the resistance of the fluid to being deformed by shear. A somewhat different type of viscosity is extensional viscos'ty, which measures the resistance of a fluid to being extended. A technique which attempts to measure this latter effect is the screen factor.

The measurement consists of determining the flow time of a polymer solution through a set of five 200 mesh screens relative to the flow time of the pure solvent. Solutions which have a high degree of elasticity will exhibit high screen factors. Table III shows the solution viscosities and screen factors determined on polymer solutions in 2.0% NaCl. As shown, increasing polymer concentration from 1,000 to 2,000 ppm resulted in a significant increase in viscosity and screen factor, particularly for the terpolymers containing hydrophobic groups. Solutions of the terpolymer of Example 1 showed approximately a five-fold increase in viscosity and a three-fold increase in screen factor when polymer concentration increased from 1,000 to 2,000 ppm.In contrast, solutions of comparative Example 1 showed only a two-fold and about a 20% increase in viscosity and screen factor, respectively, over the same concentration range. Comparing the terpolymer solution of Example 1 and Comparative Example 1 at 2,000 ppm, the terpolymers used in the process of this invention showed more than a three-fold increase in viscosity and more than a two-fold increase in screen factor relative to a similarly prepared polymer except without the hydrophobically associating groups. Some of the other terpolymers in Table III containing lower amounts of sodium acrylate groups and/or higher amounts of octylacrylamide groups, exhibited even a more dramatic enhancement in viscosity and screen factor.The low acrylate-containing polymer of sample 2 showed over a 20-fold improvement in solution viscosity relative to the HPAM polymer of Comparative Example 1. Increasing the hydrophobic content from 1.0 to 1.25 mole percent as in Example 10 resulted in more than a four-fold enhancement in solution viscosity and over an order of magnitude improvement relative to the corresponding HPAM of Comparative Example 1.

Example 18 Solution Viscometrics in Brine Solutions of several of the polymers pre viously described were prepared at concentrations of 1,000, 1,500, and 2,000 ppm in a brine consisting of 3.0% NaCl and 0.3% CaCl2. Viscosity was determined at shear rates of 1.3 sec'l and 11 sec-l. As shown in Table IV, the terpolymer of Example 1 exhibited considerably higher viscosity than the corresponding copolymer of Comparative Example 1 at all concentrations and shear rates tested. The enhancement of viscosity of the hydrophobe containing polymers used in the oil recovery process of this invention increased as the polymer concentration increased and as the amount of hydrophobic groups increased (see Example 12 relative to Example 1).

The viscosity of solutions of hydrophobically associating polymers can exhibit quite unusual shear rate dependence. As shown in Table IV, these solutions can be essentially Newtonian, with viscosity independent of shear rate, as in Examples 1 and 11; pseudoplastic, with viscosity decreasing with shear rate, as in Example 2; and dilatant, with viscosity increasing with shear rate, as in Examples 10 and 12. This versatility in the shear rate response of viscosity can be controlled by the amount of acrylate and hydrophobic groups in the terpolymer. The ability of a solution to exhibit different viscosity-shear rate profiles could be quite useful in regulating flow properties at different flow conditions and exemplifies a unique characteristic of the hydrophobically associating terpolymers used in this invention.

Example 19 Salt Sensitivity One of the major deficiencies of aqueous viscosifiers based on polymers containing ionic groups is the salt sensitivity of the viscosity. To assess this sensitivity, the viscosity of a polymer solution in distilled water was divided by the viscosity of the same solution containing salt to give "a viscosity ratio". Solutions at two polymer concentrations (i.e., 1,000 and 2,000 ppm) and three salt contents (i.e., 0, 0.5, and 2.0% NaCl) were prepared and their viscosity determined at two shear rates (i.e., 1.3 and 11.0 sec-1). As shown by the data in Table V, all of these variables have an effect on the viscosity ratio. In general, the HRAM terpolymers are significantly less sensitive than the HPAM copolymer (Comparative Example 1) to the salt content of the solution.For example, comparing these polymers at 1,000 ppm, 1.3 sec-l shear rate and 2.0% NaCl, the viscosity ratio is 52 and 98 for the HRAM terpolymer and corresponding HPAM copolymer, respectively. This indicated that at these conditions the HRAM polymer had approximately half the salt sensitivity as the HPAM polymer. As the amount of hydrophobic octylacrylamide groups increased (Example 10) the salt sensitivity was reduced considerably. In fact, the HRAM polymer of Example 10 at 2,000 ppm 11 sec-l shear rate and 2.0% NaCl had a viscosity ratio less than 1.0. This indicated that the addition of salt to the polymer solution in water resulted in an increase in solution viscosity. The date in Table V suggests that for certain limited ranges of conditions HRAM polymers can be designed to provide solutions with salt insensitive viscosity. This could be of significant benefit in applications such as enhanced oil recovery where a fixed viscosity level independent of salt content is desired.

Example 20 Mechanical Stability A critical parameter of polymer systems used in petroleum recovery processes is the viscosity-stability of the polymer solution.

Mechanical degradation of polymer solutions can occur during mixing and injection operations by using high shear rates. Additionally, after injection mechanical degradation can occur during flow through the pores within the reservoir. Partially hydrolyzed polyacrylamides are susceptible to viscosity losses and represent a problem regarding their usefulness as fluids for chemically enhanced oil recovery applications.

The mechanical stability of the polymer solutions was monitored by measuring the viscosity of the fluid after flow through Berea sandstone as a function of flux (ft/day) or flow rate (cc/sec).

The viscosity-stability was measured by forcing fresh polymer solution of 1,500 ppm concentration in a 3.0% NaCl and 0.3% CaC12 brine through a 0.5 inch diameter disk of Berea sandstone. These disks had a length between about 0.2 inches to 0.5 inches and permeability, K, between 300 to 400 millidarcies (md). The core disk was cut from a Brea sandstone rod (which was previously epoxy coated to prevent fluid loss from the sides) with a diamond saw. The cutting media was a brine solution similar to that used in the subsequent flow test. The disk was briefly sonnicated to remove sandstone fines. After drying it was placed into a core holder equipped to measure the pressure drop across the core during flow by means of calibrated pressure transducers.

The permeability was determined by measuring the pressure drop with a constant flow rate pump and using Darcy's Law, KAVP Q= fl L where Q = flow rate, cc/sec, K = permeability, darcies, A = disk area, cm2, n = fluid viscosity, cP, L = length of the disk and P = pressure drop, atmospheres. Polymer solution was then injected through the Brea core at various flow rates and the extent of induced degradation was monitored by measuring the effluent viscosity by the Contraves Low Shear Rheometer at shear rates ofl.28 and 11.0 sec'l. The extent of induced degradation was determined from the loss of original viscosity. As shown by Examples 11, 12 and 15 in Table VI with a commercial HPAM system, studied under similar conditions, the HRAM systems showed a greater resistance to mechanical degradation.This (viscosity-stability) is an important property for enhanced oil recovery since decreased viscosity results in increased fluid mobility and poor sweep efficiency. Thus, using hydrophobically associating terpolymers for mobility control results in improved oil recovery.

Example 21 Low Flux Resistance Factor In order to assess polymer performance at fluxes typical of those used during oil recovery (about 1 ft/day), low flux core evaluations were performed. This was accomplished by injecting previously sheared (at a 1,200 psi/ft pressure gradient) polymer solution into a fresh Berea core at low flow rates. After obtaining the permeability of the Berea core using the brine, the polymer solution was injected. The mobility information obtained with the polymer solution was used to calculate the polymer solution resistance factor.

The polymer resistance factor is a comparison of the brine solvent and polymer solution mobilities (Mp) calculated by: R =- Mw/Mp = (Kw/nw)/(Kp/np) where the subscripts w and p refer to water (or brine) and polymer solution. Higher values for the polymer resistance factor indicate increased mobility control. As shown in Table VII, the average low flux resistance factor for an HRAM system, Example 15, was about 29 for fluxes between 0.03 and 1.3 ft/day. In comparison, a commercial HPAM polymer had an average resistance factor of 8.5 at similar ranges of flux. This 3.5 fold increase in resistance factor for the hydrophobic polymer system indicates a substantially improved sweep efficiency in a reservoir at the typical flux of 1 ft/day.

The information provided by these Examples illustrates the unique viscosity enhancing characteristics of the hydrophobically associating HRAM terpoly mers used in the secondary or tertiary oil recovery processes of this invention. These polymers viscosify at lower polymer concentrations, impart viscosity charac teristics with unique shear rate response, and give improved salt tolerance.

Table I HRAM Polymers Hydrolysis Conditions Composition Example NaOH Temp. Time COONa C8AM No. Moles C Min. Mole % Mole * 1 0.40 50 60 18.4 1.0 2 - - - 0.3 1.0 3 0.25 50 60 13.1 1.0 4 0.70 50 60 22.0 1.0 5 0.70 50 120 24.0 1.0 6 1.0 50 60 34.0 1.0 7 0.15 50 60 7.6 1.0 8 0.30 50 60 12.7 1.0 9 0.40 50 60 18.0 0.75 10 0.40 50 60 15.2 1.25 11 0.40 50 90 19.5 1.25 12 0.40 50 90 18.4 1.25 13 0.40 50 60 - 1.0(1) 14 0.40 50 60 - 1.1(2) 14 0.40 50 90 16.0 0.75 (1) Decylacrylanide (2) Dodecylacrylamide Table II Dilute Solution Properties Composition Mole % Intrinsic Huggins' Example Sodium Hydrophobe Viscosity Coefficient No.Acrylate Level dl/g Kh 1 18.4 1.0 8.0 1.0 Comp. 1 18.2 0.0 9.6 0.42 2 0.3 1.0 3.4 2.5 3 13.1 1.0 7.1 1.3 6 34.0 1.0 9.0 0.71 7 7.6 1.0 - 4.3 3.1 8 12.7 1.0 5.9 1.9 9 18.0 0.75 10 15.2 1.25 4.9 2.4 11 - - 5.1 3.1 12 - - 4.7 4.6 15 16.0 0.75 19.0 0.7 Table III Solution Viscometrics Polymer Solution Example Concentrations Viscosity Screen No. ppm cP Factor 1 1,000 3.4 4.3 2,000 15.0 12.0 Comp. 1 1,000 2.4 4.5 2,000 4.6 5.5 2 1,000 2.1 2,000 95.0 3 1,000 3.3 3.8 2,000 23.0 19.0 4 1,000 3.1 4.2 2,000 10.0 10.0 5 1,000 3.2 3.3 2,000 11.0 7.0 6 1,000 3.0 . 3.9 2,000 8.0 8.0 7 1,000 2.9 3.5 2,000 15.0 11.0 8 1,000 3.2 3.3 2,000 15.0 9.0 10 1,000 3.1 3.0 2,000 65.0 30.0 15 1,000 16.0 2,000 89.0 Table IV Solution Viscometrics in Brine Polymer Example Concentration Solution Viscosity, cP No. ppm 1.3 sec-l 11.0 sec -1 1 1,000 3.0 2.5 1,500 5.3 5.0 2,000 14.0 12.0 Dump. 1 1,000 2.0 2.0 1,500 3.0 3.0 2,000 4.0 4.0 2 1,000 2.1 1.6 1,500 14.0 11.0 2,000 95.0 43.0 10 1,000 3.5 3.4 1,500 7.0 6.5 2,000 30.0 122.0 11 1,000 3.0 3.0 1,500 13.0 11.0 12 1,000 3.2 3.1 1,500 23.0 98.0 15 1,000 16.3 12.5 1,500 60.0 64.0 Table V Salt Polymer Shear Concentration Example Concentration Rate $ Viscosity No. ppm sec -1 NaC1 Ratio* 1 1,000 1.3 0.5 40 11.0 2.0 52 0.5 19 2.0 26 2,000 1.3 0.5 28 2.0 14 11.0 0.5 28 2.0 14 Comp. 1 1,000 1.3 0.5 61 2.0 98 11.0 0.5 24 2.0 38 2,000 1.3 0.5 49 2.0 85 11.0 0.5 20 2.0 33 10 1,000 1.3 0.5 23 2.0 32 11.0 0.5 14 2.0 20 2,000 1.3 0.5 5 2.0 2 11.0 0.5 4 2.0 .5 *Viscosity Ratio is the ratio of copolymer solution vis cosity in water relative to salt.

TABLE VI Mechanical Stability Properties Example Flux Viscosity, cP Viscosity, cP No. Ft/Day 1.3 sec-1 11.0 sec-l 11 56 10.8 10.0 103 10.8 9.7 160 10.9 9.8 310 10.2 8.9 536 9.6 8.5 930 8.2 7.-5 1,557 8.3 7.3 12 55 15.0 70.0 102 14.7 83.0 157 15.0 57.0 312 14.0 18.0 502 13.0 13.0 902 11.0 10.0 1,495 9.5 8.1 15 59 31.0 25.0 106 20.8 16.0 173 19.2 12.5 331 11.0 9.3 529 7.3 7.2 982 5.4 5.3 1,572 4.5 4.6 Commercial 51 8.2 HPAM 110 5.9 149 4.9 299 4.0 512 3.5 982 2.8 TABLE VII Resistance Factors Example Flux Resistance No. Ft/Day Factor 15 0.033 31.9 0.062 28.1 0.124 25.1 0.293 26.5 0.589 27.7 1.233 31.7 Commercial 0.03 to HPAM 1.3 8.5

Claims (11)

CLAIMS:

1. A water flooding process for the secondary recovery of oil from a production well comprising injecting an aqueous solution under pressure to force oil to the production well, said aqueous solution comprising: (a) water; and (b) about 100 to about 5,000 ppm of a water soluble terpolymer having the formula:

wherein R1 is a C6 to C22 straight, chained or branched alkyl or cycloalkyl group; R2 is hydrogen or a C6 to C22 straight, chained or branched alkyl or cycloalkyl group or a C1 to C3 straight, chained or branched alkyl group; and M+ is an alkali metal or ammonium cation, wherein x is about 60 to 98 mole percent, y is about 2 to about 40 mole percent, and z is about 0.1 to about 10.0 mole percent.

2. A process according to claim 1 wherein M+ is a sodium cation.

3. A process according to claim 1 or claim 2 wherein R1 is an octyl group.

4. A process according to claim lor claim 2 wherein R1 is a dodecyl group.

5. A process according to claim 1 or claim 2 wherein R1 is a decyl group.

6. A process for recovering oil from a production well comprising injecting an aqueous solu tion under pressure to force oil to the production well, said aqueous solution comprising: (a) water; (b) 0.1 to 5.0 weight percent of a surfactant; and (c) 100 to 5,000 ppm of a water soluble terpolymer having the formula:

wherein R1 is a C6 to C22 straight, chained or branched alkyl or cycloalkyl group; R2 is hydrogen or a C6 to C22 straight, chained or branched alkyl or cycloalkyl group or a C1 to C3 straight, chained or branched alkyl group; and M+ is an alkali metal or ammonium cation, wherein x is 60 to 98 mole percent, y is 2 to 40 mole percent, and z is 0.1 to 10.0 mole percent.

7. A process according to claim 6 wherein M+ is a sodium cation.

8. A process according to claim 6 or claim 7 wherein R1 is an octyl group.

9. A process according to claim 6 or claim 7 wherein R1 is a dodecyl group.

10. A process according to claim 6 or claim 7 wherein R1 is a decyl group.

11. A process according to any preceding claim and substantially as herein described.