GB2199354A - Enhanced oil recovery process - Google Patents

Abstract

Oil recovery from a production well is enhanced by injection of aqueous solution of a hydrophobically associating, water-soluble polymers containing one or more water soluble monomers and a water insoluble monomer group. The water-insoluble group is a higher alkylacrylamide and the water soluble groups are (a) acrylamide and (b) a salt of (i) acrylic acid or (ii) a salt of an ethylenically unsaturated sulfonic acid. The polymers have the general formulae: <IMAGE> where R1 is a C6 to C22 alklylcycloalkyl group; R2 is hydrogen or a C6 to C22 alkyl or cycloalkyl group, or a C1 to C3 alkyl group; A is SO3-M???, phenyl-SO3-M??? or CONHC(CH3)2 CH2 SO3-M??? M??? is an alkali metal or ammonium cation; X is 5-98 mole per cent, y is 2-95 mole per cent and Z is 0.1-10 mole per cent or <IMAGE> where R1, R2, A & M are as defined above and W to 1-80 mole per cent, X is 10-90 mole per cent Y is 2-40 mole per cent and 2 is 0.1-10 mole per cent

Description

ENHANCED OIL RECOVERY PROCESS SUMMARY OF THE INVENTION This invention relates to a method for enhanced recovery of petroleum from a subterranean oil-bearing formation. More particularly, this invention relates to secondary or tertiary recovery of oil employing a polymer thickened aqueous drive fluid. The polymeric viscosifier for the drive fluid is selected from a class of hydrophobically associating water soluble polymers containing one or more water soluble monomers and a water insoluble monomer or group. The water soluble groups are acrylamide, N-vinyl pyrrolidone and a salt of acrylic acid and the water insoluble group is a higher alkyl acrylamide. These tetrapolymers, when dissolved in an aqueous brine solution, have the ability to substantially increase the viscosity of the solution.The control of displacement fluid mobility results in more uniform sweep efficiency and improved oil recovery. In addition, aqueous solutions of these hydrophobically associating tetrapolymers exhibit enhanced viscosification, reduced salt sensitivity and other desirable rheological properties found useful in chemically enhanced oil recovery processes. The polymeric viscosifier for the drive fluid is also selected from a class of hydrophobically associating water soluble polymers containing one or more water soluble monomers and a water insoluble monomer or group. The water soluble groups are (meth)acrylamide and a salt of an ethylenically unsaturated sulfonated monomer and the water insoluble group is a N-alkyl(meth)acrylamide. These polymers, when dissolved in an aqueous brine solution, have the ability to substantially increase the viscosity of the solution.

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 polymer of (meth)acrylamide, N-vinyl pyrrolidone, a salt of (meth)acrylic acid, and an N-alkyl(meth)acrylamide. It entails the use of an aqueous treating media which comprises a hydrophobically associating terpolymer of (meth)acrylamide, a salt of an ethylenically unsaturated sulfonic 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 surfactants or cosurfactants to lower the interfacial tension 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, hydrolytic and mechanical stability. In addition, the composition of these polymers will also influence their adsorption 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 about C6 to C18 and 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 nonlimiting, examples of preferred alkyl groups are hexyl, octyl, decyl, dodecyl and hexadecyl groups. Typical, but nonlimiting, examples of preferred cations are sodium, potassium and ammonium. The mole percentage of acrylamide, x, is preferably 10 to 90, more preferably 20 to 80, and most preferably 30 to 70. The mole percentage of the salt of acrylic acid, y, is preferably 2 to 40, more preferably 4 to 35, and most preferably 5 to 25. The mole percentage of the hydrophobic group, z, is preferably 0.1 to 10.0, more preferably 0.15 to 5.0, and most preferably 0.2 to 3.0. The mole percentage of the N-vinyl pyrrolidone group, w, is preferably 1 to 80, more preferably 5 to 75, and most preferably 10 to 70.

The molecular weight of the water soluble tetrapolymers of this invention is sufficiently high 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 20 million, more preferably 500,000 to 15 million, and most preferably 1 million to 10 million. The intrinsic viscosity of these polymers as measured in 2 percent sodium chloride solution is preferably greater than 1 dl/g but less than 40 dl/g.

The tetrapolymers may be synthesized by a variety of processes. The most preferred process relies on dispersing the water insoluble or hydrophobic monomer on a very fine scale into an aqueous solution of the water soluble monomer. The product is substantially free of microgel or particulates of insoluble polymer. This is achieved by dispersing the water insoluble or hydrophobic monomer into a predominantly aqueous phase containing the dissolved water soluble monomers such as acrylamide, N-vinyl pyrrolidone and acrylic acid or a salt of 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 that form when the surfactant is dissolved into the water at concentrations above the critical micelle concentration. Further details of this polymerizaton technique can be found in U.S. Patent 4,528,348 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 lattices 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 tetradecylsulfate. 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.

Although NVP monomer will dissolve substantial quantities of long chain N-alkylacrylamide monomers, the addition of water and acrylamide monomer often leads to a cloudy, nonhomogeneous reaction mixture. Addition of sodium dodecylsulfate (SDS) clarifies the solution and solubilizes the hydrophobic monomer in the aqueous phase, thereby improving the process. Also, the use of SDS leads to a more gel-free product as judged by visual inspection.

Polymerization of the water soluble and water insoluble monomers is effected in an aqueous micellar solution containing a suitable free radical initiator. Suitable water or oil soluble free radical initiators for the free radical terpolymerization of the acrylamide monomer, N-vinylpyrrolidone, and the N-alkylcrylamide monomer are selected from the group consisting of azo compounds, peroxides and persulfates. However, the preferred initiators are azo compounds, such as 2,2'-azobisisobutyronitrile (AIBN) (e.g., DuPont's Vazo-64R, 2,2' azobis (2-amidopropane) hydrochloride (Wako's V-5 4), 2-t-butylazo-2-cyanopropane.Most preferred initiators are low temperature azo initiators, such as 2,2'azobis (2,4-dimethyl-4-methoxyvaleronitrile) (DuPont's Vazo-3 * . The use of low temperature initiation and polymerization leads to higher polymer molecular weights. The concentration of the free radical initiator is 0.001 to 2.0 grams per 100 grams of total monomer, more preferably 0.01 to 1.0, and most preferably 0.05 to 0.1. The polymerization temperature is preferably 10 C to 90 C, more preferably 10'C to 70 C, and most preferably 200C to 60 C for a period of time of 1 to 24 hours, more preferably 2 to 10 hours, and most preferably 3 to 8 hours.

The hydrophobically associating tetrapolymers 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 alkyl acrylamide in an aqueous solution of acrylamide and N-vinyl pyrrolidone; deaerating this solution by purging with nitrogen or additionally applying a vacuum; adjusting 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 30"C to 90 C, more preferably 40"C to 80 C, and most preferably 45"C to 70"C 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 polymer of acrylamide, N-vinyl pyrrolidone, 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 nonsolvent 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 polymers of this invention is to use acrylic acid monomer or a monovalent salt of acrylic acid such as ammonium, sodium or potassium acrylate along with acrylamide, N-vinyl pyrrolidone 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 method for preparing these hydrophobically associating polymers can be found in U.S. Patent No. 4,663,408.

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

A = S03M+, phenyl-S03-M+, C0NHC(CH3)2CH2S03-M+ wherein R1 is preferably a C6 to C22 straight chained or branched alkyl or alkylcycloalkyl group, more preferably a C6 to C20, and most preferably a C6 to C18, and 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 nonlimiting, examples of preferred alkyl groups are hexyl, octyl, decyl, dodecyl and hexadecyl groups. Typical, but nonlimiting, examples of preferred cations are sodium, potassium and ammonium.The mole percentage of acrylamide, x, is preferably 5 to 98, more preferably 10 to 90, and most preferably 20 to 80.

The mole percentage of the salt of the sulfonate containing monomer, y, is preferably 2 to 95, more preferably 5 to 90, and most preferably 10 to 80.

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 3.0.

The molecular weight of the water soluble terpolymers of this invention is sufficiently high 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 10 million, more preferably 500,000 to 8 million, and most preferable 1 zillion to 7 million. The intrinsic viscosity of these polymers as measured in 2 percent sodium chloride solution is preferably greater than 1 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 micro-gel or particulates of insoluble polymer. The process for synthesizing these terpolymers relies on solubilizing the water insoluble monomer into a predominantly aqueous media by the use of a suitable water soluble surfactant, such as sodium dodecyl sulfate. When mixed with an aqueous solution of the water soluble acrylamide monomer and the water soluble sulfonate monomer, the surfactant solution can disperse the water insoluble monomer on an extremely fine scale so that the reaction mixture is isotropic, clear, and homogeneous. These micellar reaction mixtures are free of visible oil droplets or particulates of the water insoluble monomer. The terpolymerization can, therefore, be initiated by water insoluble initiators to yield terpolymers which are substantially free of visible particulates. The resultant reaction mixture remains homogeneous throughout the course of the reaction without the need for agitation with external mixers or stirrers. Further details of this polymerization technique can be found in copending European Patent Application No. 87308740.7.

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 lattices 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 for 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 non ionic 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 of hydrophobic monomers employed.

Polymerization of the water soluble and water insoluble monomers is effected inan 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 azobisisobutyronitrile. 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 20"C to iOeC, more preferably 25"C to 80"C, and most preferably 30"C to 70 e C 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 concentrate increases and as the temperature increases. Viscosification of 2 to 100 times or more that of the neat solvent can readily be achieved with the polymers used in the process of this invention. Preferably 0.01 to 2.0 weight percent, more preferably 0.02 to 1.0 weight percent, and most preferably 0.05 to 0.5 weight 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 sulfonate groups, the level and type of hydrophobic groups and the molecular weight. The addition of anionic sulfonate containing 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 hydrphobic 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 N-dodecylacrylamide is needed in a polymer to create the same viscosification as a larger amount of N-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.

Molecular weight of the polymer is an important consideration. High molecular weight polymers incorporating both anionically charged acrylate groups or sulfonate 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 or sulfonate groups increased. It is desirable that the resulting polymer in an aqueous solution not be susceptible to irreversible mechanical degradation under shear or elongated 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 monomers level and decreasing the initiator level and reaction temperature. The use of a low temperature initiator such as the azo compound, Vazo-3, provides high molecular weight problems.

To evaluate and characterize the unique and useful properties of the hydrophobically associating polymers used in the process 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 nonassociating 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 polymers contain anionically charged groups, a polyelectrolyte effect was observed in dilute solution. 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 than 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 +0.1 C 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 polymers 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 measurement times were employed.This effect is most evident at higher polymer concentrations, higher polymer molecular weights and in regions of strong intermolecular hydrophobic associations.

An important property of polymers used in secondary or tertiary processes for additional petroleum recovery is the viscosity retention of the polymer solution. Mechanical degradation of polymer solutions is caused by the high shear or elongational stress the polymer molecules experience during injection into reservoir rock. The resultant permanent loss in viscosity is a result of a reduction in the polymer molecular weight. This, in turn, decreases the sweep efficiency of the polymer solution within the reservoir resulting in decreased petroleum recovery. Partially hydrolyzed polyacrylamide is known to mechanically degrade depending on the polymer molecular weight, polymer composition, mixing and injection rates, and the reseivoir rock porosity and permeability.

Equally important, a polymer solution must be expected to withstand harsh environmental conditions, such as elevated temperatures for weeks or months and perhaps (depends on field size) several years. It is well known that polyacrylamides have a limited lifetime at elevated temperatures due to thermally promoted hydrolysis. Hydrolysis is undesirable because it can lead to viscosity changes and phase separation (precipitation) of the polymer by divalent cations, such as calcium and magnesium ions present in the reservoir. The resultant loss in viscosity gives a simultaneous loss of mobility control which decreases the sweep efficiency of the polymer solution in a reservoir.

It is an object of this invention to overcome loss of viscosity, by thermal or mechanical means, of water soluble polymers of the prior art used in secondary - or tertiary oil recovery processes. It is a further object to improve the salt tolerance and viscosification efficiency of brine drive solutions used for mobility control during secondary or tertiary petroleum recovery processes.

Yet another object of this invention is to provide a water soluble additive for use in rheological control during secondary or tertiary oil recovery operations. -Thus, this invention relates to a method to improve the process for the recovery of petroleum from a subterranean oil-bearing formation.

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

Examples 1 to 8 Svnthesis of NVP-HRAM Tetragolymers An NVP-RAM terpolymer of acrylamide, NVP, and N-n-octyl acrylamide was synthesized using the micellar polymerization technique. A 1 liter resin flask was equipped with a condenser, thermometer, stirrer (electric) and nitrogen inlet and outlet.

Acrylamide (17.47 g), N-octylacrylamide (0.65 g), and NVP (11.88 g) in 470 g water were polymerized with 0.123 g AIBN (Vazo-64 in the presence of 1.7 g sodium dodecyl sulfate (SDS) at 45"C for 18 hours. This corresponds to a [M]/[I]1/2 = 60 (monomer to initiator ratio) and total solids content of 6 percent in water. Although NVP helps solubilize octylacrylamides, the SDS further homogenizes and clarifies the reaction mixture. The resulting polymer solution was a soluble gel. It was passed through a meat grinder and diluted further with water to a 1 percent concentration.

The polymer solution was precipitated in 6 liters of acetone. Copolymers with various ratios of Cg hydrophobe were prepared and their composition is given in Table I, and their viscometrics compared in Table II, Examples 1 to 4. The viscosification efficiency of the copolymers compared with the composition containing no hydrophobe (Example 1) is clearly evident. Five gram quantities of the solid polymers were dissolved in 500 cc of water and heated to 600C to facilitate dissolution. They were then cooled to 40"C and mixed with a solution of NaOH (0.55 g/300 g water) for 18 hours. Tetrapolymers with about 7 percent hydrolysis and various ratios of Cg hydrophobe were prepared. Their compositions are given in Table I and viscometrics compared in Table II, Examples 5 to 6.The NVP-HRAM tetrapolymers are superior viscosifiers compared with a NVP-HPAM system containing no hydrophobe, Example 5, and to NVP-RAM containing no salt of acrylic acid, Example 1.

Examples 9 to 16 Synthesis of NVP-HRAM -Tetraolvmers (Low TemPerature) A series of NVP-HRAM tetrapolymers consisting of 69.25 mole percent acrylamide., and 30 mole percent NVP, and 0.75 mole percent N-n-octylacrylamide were prepared using the recipe and procedures described in Examples 1 through 8, with the exception of initiator type and reaction temperature. The low temperature initiator employed was Vazo-3 g at polymerization temperature between 20 and 30"C. Different hydrolysis reaction conditions were used to prepare the tetrapolymers with levels of anionically charged sodium acrylate groups ranging from about 5 to 10 mole percent.Prior studies had indicated that hydrolysis was very slow at 40"C and thus temperature for hydrolysis was performed at 55"C. The degree of hydrolysis is a monotonic increasing function of the amount of added base and thus is the major variable for controlling the charge content in the tetrapolymer. These polymers were shown to be efficient brine viscosifiers with even better solution quality/clarity than the polymers prepared in Examples 1 to 8. The composition of the resulting NVP-HRAM tetrapolymers are also given in Table I with the mole percent sodium acrylate determined by titration and sodium analysis. The solution viscometrics of these tetrapolymers are shown in Table III where modest amounts of shear thickening is also exhibited.

Example 17 Hydrolytic Stability Under Basic Conditions The rate of base (OH-) catalyzed hydrolysis of NVP-RAM terpolymers compared to NVP/AM copolymers and PAM (polyacrylamide homopolymers) at 40"C is shown in Table IV. Surprisingly, the NVP-RAM polymers with varying amounts of Cg (0.5 to 1.0 percent Cg) show less hydrolysis than both RAM and PAM polymers known in the art as shown in Table IV. Base catalyzed hydrolysis is both a means of converting terpolymers to tetrapolymers and a measure of the hydrolytic stability of the hydrophobically associating tetrapolymers.

Example 18 Hydrolytic Stability Under Neutral Conditions Polymer solutions were prepared containing 0.2 weight percent polymer in a 3.5 percent NaCl brine solution. Samples of these solutions were aged at constant elevated temperatures. Periodically, samples were withdrawn for measurement of the extent of conversion of acrylamide to acrylate functionality via hydrolysis.The method for the determination of the degree of hydrolysis consisted of: 1. dialysis to remove or reduce the concentration of extraneous salt; 2. treatment of the dialyzate with cation and anion exchange resins to remove all extraneous salt and convert carboxylate groups on the hydrolyzed polymer to carboxylic acid groups; 3. gravimetric determination of the polymer concentration in the ion exchange resin-treated solution; and 4. titration of the ion exchange resin-treated solution to determine acid content.

As can be seen from the results presented in Table V, hydrolytic stability of acrylamide based polymers was significantly improved by the incorporation of the NVP monomer. In addition1 resistance to hydrolysis increased as the NVP monomer content increased as shown by the reduced level of hydropyrolysis with time for an NVP/AM copolymer with 52 mole percent NVP, Example 16, as compared to Example 15.

Example 19 Solution Viscometrics Polymer solutions were prepared by the slow addition of a weighed polymer sample to rapidly stirred purified water obtained from a millipore water system. Upon complete polymer addition, stirring was decreased and dissolution was allowed to progress for about 24 hours or until solutions were homogeneous and clear. For characterization in 2 percent NaCl, a concentrated NaCl solution was used which, when added in proper amounts, gives the 2 percent brine and desired polymer concentration.

For characterization in a final brine solution containing divalent cations, a concentrated brine solution comprised of NaCl and CaC12 was used. When added in proper amounts, the final desired brine solution was a mixture of 3 percent NaCl and 0.3 percent CaC12, designated as 3.3 percent, also giving the desired polymer concentration, typically 1500 ppm.

Viscosities of these solutions were measured by means of a Contraves Low Shear 30 Rheometer, using a No. 1 bob and cup. Temperatures were controlled to +0.10C and measurements were made at a variety of rotational speeds from about 1.0 to 100. s-l. Intrinsic viscosities were determined using capillary viscometers at 25"C. Measurements were made at different polymer concentrations to obtain 5 solutions with viscosities from 1.1 to 2.0 times the 2 percent NaCl solvent viscosity. Plots of the reduced viscosity versus polymer concentration were analyzed with the following equation: 7?red = [n] + kh[71) 2c to yield the intrinsic viscosity and Huggins' interaction coefficient, kh, as shown in Tables II and III.The intrinsic viscosity is a function of polymer molecular weight, amount of charged anionic acrylate groups and hydrophobic groups. The influence of hydrophobic associations and NVP incorporation on the solution rheological properties is shown in Tables III and IV. The solution viscosities were measured on solutions containing 1500 ppm polymer in 3.3 percent brine at several shear rates including 1.3 and 11 sec'l which typifies the range of shear rates encountered in porous media at frontal advance rates of about l/ft/day. 10 sec-1 The presence of the hydrophobic group, N-n-octylacrylamide, has increased the viscosity significantly as compared to a non-hydrophobic system (Examples 1 and 15).These pronounced enhancements in solution viscosity are due to changes in solubility and hydrophobic associations and have little to do with molecular weight. This is indicated by similar values of the intrinsic viscosity, which is a measure of molecular weight.

The presence of associations is evidenced by the increase in the Huggins' coefficient from about 0.4 for nonassociating to about 4., for the associating tetrapolymers. A higher Huggins' coefficient indicates that the solution viscosity will increase faster with polymer concentration.

Example 20 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 polymers solution in distilled water was divided by the viscosity of the same solution containing salt to give a viscosity ratio. Solutions at a polymer concentration of 2000 ppm and several salt contents (i.e. O to 1.0, and 2.0 percent 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 VI, all of these variables have an effect on the viscosity ratio.In general, the NVP-HRAM tetrapolymer of this invention, illustrated by Example 14, is significantly less sensitive than a commercial HPAM polymer system to the salt content of the solution. The NVP-HPAM system, Example 15, without the hydr6phobe and the commercial HPAM, are more salt sensitive than the NVP-HRAM tetrapolymer.

Thus, it is evident that the hydrophobe incorporation reduces the salt sensitivity. For example, comparing these polymers at 2000 ppm1 1.3 sec-l shear rate and 2.0 percent NaCl, the viscosity ratio is 7 and 23 for the NVP-HRAM tetrapolymer and corresponding HPAM copolymer, respectively. This indicates that at these conditions, the HPAM polymer is approximately 3 times as salt sensitive as the NVP-HRAM polymer. At a salt concentration of 1 and 2 percent, the NVP-HRAM tetrapolymer showed a further decrease in salt sensitivity, as demonstrated by the viscosity ratio,at 11. secl as compared to the 1.3 sec'l shear rate. This was a result of an increase in the solution viscosity.

This could be of significant benefit in applications where one desires a fixed viscosity level tolerant of variation in salt content.

Example 21 Mechanical Stability The mechanical stability of the NVP tetrapolymer system, Example 14, along with a partially hydrolyzed polyacrylamide system, was monitored by determining the viscosity of the effluent polymer solution after passage through Berea sandstone having a nominal porosity of 0.2.

Fresh polymer solution of 1500 ppm concentration in 3.3 percent brine was pumped through a 0.5 inch diameter Berea sandstone disk by means of a dual piston constant flow rate pump. The disk had a nominal length of 0.5 inch and permeability under one darcy for the systems studied. The disk was cut to the above dimensions from a Berea sandstone rod, which was epoxy coated to prevent fluid loss from the disk sides during the flow process. Cutting of the Berea disk was accomplished with a diamond saw blade using 3.3 percent brine as the cutting fluid.

The disk was briefly sonnicated to remove sandstone fines from the disk faces followed by vacuum drying.

Subsequently, the disk was placed into a stainless steel holder equipped to measure the pressure drop across the disk by means of calibrated pressure transducers. The permeability was determined by flowing -the 3.3 percent brine solvent, measuring the pressure drop, flow rate, and using Darcy's Law, KAAP a7L where Q = flow rate, cc/sec, K = permeability, darcies, A = disk area, cm2, n = fluid viscosity, cP, L = disk length, cm, and AP = pressure drop, atmospheres.Polymer solution was then injected using various flow rates, and the extent of mechanical degradation was monitored by measuring the effluent viscosity by the Contraves Low Shear Rheometer at shear rates of 1.3 and 11.0 sec'l. The higher shear rate, 11.0 sec'l, typifies flux (flow) of 1 ft/day of the polymer fluid through the reservoir.

As shown in Table VII, the mechanical stability of the NVP tetrapolymer, Example 14, was superior to that of. a commercial partially hydrolyzed polyacrylamide (HPAM). Prior to the mechanical degradation studies, the NVP tetrapolymer and HPAM system had similar solution viscosities at the 1.3 and 11.0 sec'l. However, the NVP-HRAM system, containing 30 mole percent NVP and 0.7 mole percent hydrophobe, maintained 50 percent of its original viscosity up to a flux of about 675 ft/day.

In contrast, the HPAM system maintained only 50 percent of original viscosity up to about 50 ft/day flux. The increase in viscosity retention for the NVP-HRAM system represents a significant improvement in mechanical stability as compared to the hydrolyzed polyacrylamide.

Example 22 Resistance Factor In conjunction with monitoring the mechanical stability of the NVP-HRAM polymer system, Example 14, the polymer resistance factor was determined. The polymer resistance factor is a comparison of the brine solvent and polymer solution mobilities (Mp) calculated by: R = Mw/Mp = (Kw/Fw)/(Sp/0p) where the subscripts w and p refer to water (or brine) and polymer solution. In an oil-bearing formation, oil typically has a higher viscosity than the water phase, therefore, to improve the recovery of oil, the mobility ratio needs to be increased.

This is accomplished by increasing the driving fluid (polymer solution) viscosity, which would increase the polymer resistance factor. As shown in Table VIII, the resistance factor for the NVP-HRAM polymer system, Example 14, shows a pseudoplastic behavior with increasing flux up to about 90 ft/day, where a more newtonian resistance factor behavior is evident, up to the fluxes examined. In comparison with a commercial HPAM system, the resistance factor is significantly lower at low flux and reaches a plateau at about 10 ft/day flux, where a pseudoplastic response begins and continues to the highest fluxes studied. At the typical flux of 1 ft/day in a reservoir, the higher resistance factor for the NVP-HRAM polymer indicates an improvement in sweep efficiency which can lead to a more efficient oil recovery process.Thus, less polymer is needed to achieve comparable mobility control.

The information provided by these examples illustrate the unique viscosity enhancing charac teristics of the hydrophobically associating NVP-HRAM tetrapolymers of this invention. These polymers viscosify at lower polymer concentrations, and give improved salt tolerance, properties desirable for secondary and tertiary petroleum recovery.

Table I NVP-HRAM TETRAPOLYMERS SDS Rx Example Initiator Conc. Temp. NVP C8AM COONa No. Type Wt% C Mole% Mole% Mole% 1 Vazo-64 0.3 45 30 - 2 Vazo-64 0.3 45 30 0.5 3 Vazo-64 0.3 45 30 0.75 4 Vazo-64 0.3 45 30 1.0 5 Vazo-64 0.3 45 30 - 7.0 6 Vazo-64 0.3 45 30 0.5 6.7 7 Vazo-64 0.3 45 30 0.75 6.7 8 Vazo-64 0.3 45 30 1.0 6.9 9 Vazo-33 1.0 30 30 0.75 10 Vazo-33 1.0 24 30 0.75 7.7 11 Vazo-33 1.0 20 30 0.75 8.5 12 Vazo-33 1.0 24 30 0.75 5.6 13 Vazo-33 1.0 20 30 0.75 9.6 14 Vazo-33 1.0 20 30 0.75 9.2 15 Vazo-33 1.0 20 30 - 9.3 16 Vazo-33 1.0 20 52 - Table II Solution ProDerties of NVP-HRAM Tetrapolymers Prepared with Vazo-64 (High Temperature) Int.

Example C8AM COONa Viscosity, cP(1) Visc.(2) Huggins' No. Mole% Mole% 1.3 s-1 11 s - dl/a Coeff.

1 - - 3.1 - 7.0 1.1 2 0.5 - 5.3 - - 3 0.75 - 6.8 - - - 4 1.0 - 17.5 - - 5 - 7.0- 3.5 3.5 12.3 0.4 6 0.5 6.7 6.7 5.1 8.2 3.0 7 0.75 6.7 11.2 - 11.6 1.4 8 1.0 6.9 600-1267 - 8.1 3.7 Table III Solution Properties of NVP-HRAM Tetraoolvmers Prepared with Vazo-33 (Low Temperature) Int.

Rx Visc.

Example Temp. C8AM COONa Viscosity, cP(1) (2) Huggins' No. C Mole% Mole% 1.3 s-1 11 s-1 69 s-1 d1/g Coeff.

9 30 0.75 - 9.5 8.9 10.8 - 10 24 0.75 7.7 20.0 25.0 44.0 - 11 20 0.75 8.5 16.7 22.7 33.0 - 12 24 0.75 5.6 10.7 9.6 13.8 - 13 20 0.75 9.6 16.8 17.4 29.0 - 14 20 0.75 9.2 16.3 14.0 22.0 6.4 3.2 15 20 - 9.3 4.5 4.7 4.4 10.7 0.4 NVP level at 30 mole%.

(1) Polymer concentration at 1500 ppm in 3.3% brine.

(2) Intrinsic viscosity in 2% NaCl.

Table IV Hydrolysis Rate vs. Time for NVP Terpolymers (1) Hydropyrolysis % Hydropyrolysis Polymer TvPe Time Hrs. m.% Acrylic Acid Polyacrylamide 3 22.0 6 22.5 12 24.0 RAM (1 m.% C8AM) 3 14.0 6 15.5 12 17.0 NVP-RAM (1 m.% C8AM) 3 6.9 6 6.4 12 7.4 (1) At 40 C, Ratio of moles of NaOH/moles of Polymer = 1/3.

Table V Hydrolytic Stability of NVP-AM Copolymers (1 Hydrolysis Increase, NVP Time, Mole% Acrylic Acid Example No. Mole% Days 80 C 93 C Polyacrylamide 0 20 - 24.0 40 - 45.0 60 - 62.0 100 - 78.0 15 30 20 5.0 12.0 40 7.0 15.0 60 10.0 17.0 100 13.0 23.0 16 52 20 - 2.0 40 - 4.0 60 - 5.0 100 - 5.0 (1) 2000 ppm, 3.5% NaCl.

Table VI Effect of Salt Concentration on NVP-HRAM Solution Viscometrics Polymer Salt Level Viscosity Ratio(1 Example No. Conc., ppm % NaCl 1.3 s-1 11 s-1 14 2000 0.01 2.0 1.5 2000 0.10 5.0 3.0 2000 1.0 7.0 2.0 2000 2.0 7.0 1.5 15 2000 0.01 2.0 2.0 2000 0.10 11.0 6.0 2000 1.0 17.0 16.0 2000 2.0 36.0 18.0 Commercial 2000 0.5 10.0 5.0 HPAM 2000 1.0 16.0 7.0 2000 2.0 23.0 9.0 (1) The viscosity of the polymer solution divided by the viscosity of the polymer in salt water.

Table VII Mechanical Stability Properties of NVP-HRAM Tetraoolvmer Viscositcp Example No. Flux, ft/day 1.3 s-1 11 s 14 1.8 14.3 13.0 4.6 12.4 11.5 9.3 13.1 11.4 13.9 12.6 11.1 18.6 12.9 10.8 47.1 13.7 11.1 93.5 13.4 10.8 140.0 13.2 11.1 170.5 12.1 11.3 339.3 12.2 10.6 674.0 9.7 8.8 825.0 9.7 8.4 1018.0 8.3 7.7 Commercial 1.9 15.0 HPAM 4.9 15.0 10.0 14.0 29.9 11.0 51.0 8.0 77.0 7.0 149.0 5.0 384.0 4.0 683.0 3.0 982.0 3.0 1500 ppm, 3.3% brine Table VIII resistance Factor of NVP-HRAM Tetrapolymer Example No.Flux, ftddav Resistance Factor 14 1.8 1039.0 4.6 436.0 9.3 213.0 13.9 121.0 18.6 88.0 47.1 46.0 93.5 36.0 140.0 34.0 170.5 32.0 339.3 38.0 674.0 36.0 825.0 35.0 1018.0 33.0 Commercial 1.9 70.0 HRAM 3.1 124.0 4.9 213.0 10.0 264.0 13.0 241.0 29.9 210.0 51.0 155.0 77.0 120.0 149.0 79.0 384.0 45.0 683.0 32.0 982.0 27.0 1500 ppm, 3.3% brine Examples 23 to 40 Micellar Polvmerization with Sulfonate Containing Monomers Radical Initiation A 1 liter Morton style kettle, fitted with a chilled water condenser, thermometer, nitrogen sparger, and mechanical stirrer, was charged with 500 ml of purified water. - The water was refluxed for i hour with a nitrogen purge and -then cooled to room temperature. Acrylamide, 8.11 g (0.11 mole), 6.62 g (0.023 mole) of AMPS 0.265 g of N-octylacrylamide and 15 g of sodium dodecyl sulfate (SDS) were charged into the flask.

The reaction solution was heated to 50"C and 0.0047 g potassium persulfate was added. After 22.75 hours at 50"C and 300 rpm stirring, the viscous solution was slowly poured into 3 1 of methanol and the precipitated polymer was isolated by filtration. The polymer was then masticated in a Waring blender with 2 1 of methanol for 30 seconds, filtered and dried under vacuum at room temperature. The yield of polymer was 10.73 g. A variety of terpolymers were prepared using similar procedures but with different amounts of acrylamide, AMP, N-octylacrylamide, SDS and initiator levels, as shown in Table IX.

Examples 41 to 63 Micellar Polvmerization with Sulfonate Containing Monomers Redox Initiation A solution of 15.0 g of SDS in 500 ml of purified, deoxygenated water was prepared and 0.298 g of N-octylacrylamide, 12.24 g of acrylamide and 9.96 g of AMPSX were added. The resulting clear solution was placed into a 2 liter Morton style resin kettle fitted with a chilled water condenser, thermometer, nitrogen sparger and mechanical stirrer. The solution was purged with nitrogen for 0.5 hours at 25.0 C, then 0.0114 g of potassium persulfate and 0.0075 g 6f sodium metabisulfite were added. After 16 hours of stirring at 300 rpm and 25.0 C, the reaction mixture was slowly poured into 3 1 of methanol.

The precipitated polymer was isolated and masticated with 1 L of methanol in a Waring blender for 30 seconds, filtered and dried under vacuum at 30"C. The yield of polymer was 15.4 g. A variety of terpolymers were prepared using similar low temperature redox initiation procedures but with different amounts of acrylamide, AMP N-octylacrylamide, SDS and initiator levels, as shown in Table X.

Example 64 Solution Viscometrics Polymer solutions were prepared by the slow addition of a weighed polymer sample to rapidly stirred 2 percent NaCl solution. Upon complete addition, the stirring was stopped and the flask was sealed under nitrogen. Dissolution was allowed to progress with mild agitation for 24 hours or longer, until solutions were homogeneous and clear. For characterization in brines containing divalent cations a mixture of 3.0percent NaC1 and 0.3 percent CaC12 was used and designated as 3.3 percent brine. To prepare these solutions polymers were initially hydrated in water, followed by addition of concentrated brine solution to give the final polymer concentration of 1500 ppm in 3.3 percent brine.

Viscosities of these solutions were measured by means of a Contrave low shear viscometer, model LS30, using a No. 1 cup and No. 1 bob. Temperatures were controlled to +0.1 C and measurements were made at a variety of rotational speeds corresponding to shear rates from about 1.0 sec'l to about 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 measurement times were employed.This effect is most evident at higher polymer concentrations, higher polymer molecular weights and in regions of strong intermolecular hydrophobic associations.

Intrinsic viscosity was determined using Ubbelohde capillary viscometers. The solvent for these measurements was 2 percent NaCl solutions.

The influence of hydrophobic associations and sulfonate monomer content on solution rheological properties is illustrated in Table XI.

The solution viscosities were measured at shear rates of 1.3 and 11 sec'l on sclutions containing 1,500 ppm polymer in 3.3 percent brine. The presence of only 0.75 mole percent octylacrylamide has increased the low shear viscosity by more than an order of magnitude, as observed by comparing Example number 20 and 21 in Table III. A further increase in viscosity is noted by simultaneously raising both the AMPSX level and hydrophobe content. These significant enhancements in solution viscosity are due to changes in solubility and hydrophobic associations and have little to do with polymer molecular weight. This is indicated by the approximately constant value of the intrinsic viscosity, which is a measure of molecular weight.

The presence of associations is evidenced by the jump in the Huggins' coefficient from 0.4, for the nonassociating polymers, to about 1.5, for the associating polymers.

The synthesis conditions can have a dramatic effect on polymer molecular weight.

Increasing reactor monomer concentration brought about significant increases in solution viscosity, as shown in Table XII, for a series of terpolymers containing 30 mole percent AMPS and 0.75 mole percent C8AM. A linear response of solution viscosity was observed as the monomer concentration was increased from 4.5 to 9 weight percent.

Doubling the monomer concentration from 4.5 to 9 weight percent resulted in a five-fold increase in viscosity at 11 sec-l.

Example 65 Effect of AMPSR Content on Micellar Polymerization The effect of the amount of AMPSR in the terpolymer on the solution viscosity in brine was to decrease the viscosity with increased AMP as shown in Table XIII. The experiments were done at a constant concentration (4.5 weight percent) of total monomers and C8AM charge (0.75 mole percent).

The loss of viscosification efficiency with increasing AMPSR content could be explained on the basis of decreased molecular weight and associa-tions. Although either cause is plausible, the effect still needs to be overcome. As described in Examples 64 and 66, this loss of viscosification can be compensated for by adjustment of reactor monomer concentration and hydrophobe level, respectively.

Example 66 Effect of Hydrophobic Monomer Concentration The influence of hydrophobe level on polymer solution viscosity can be seen from the data in Table XIV for two series of polymers con taining 30 and 40 mole percent AMP 4, respectively.

The total monomer concentration was held at 4.5 weight percent and the hydrophobe, N-l-octylacrylamide, concentration was varied from 0 to 1.5 mole percent. The maximum response in viscosity occurred at 1.0 percent for 30 mole percent AMP t and 1.25 percent for the 40 mole percent AMP ss series. The viscometric data further indicates that the increase in viscosification occurs at a relatively narrow level of hydrophobic groups which depends on the level of sulfonate monomer in the polymer. This is unexpected based on the prior art.

Example 67 Effect of Surfactant Level The concentration of surfactant used during micellar polymerization can have a significant effect on the resultant hydrophobe-containing polymer. The solution viscosity data in Table XV are for a series of 40 mole percent AMP ss terpolymers at two levels of hydrophobe, 0.75 and 1.0 mole percent. At 0.75 mole percent C8AM maximum viscosity was achieved at a sodium dodecyl sulfate (SDS) concentration of 2 weight percent.

Increasing the hydrophobe level required 3 weight percent SDS to achieve maximum viscosity.

The solution -clarity of hydrophobically associating polymers can be used as a measure of polymer solubility. Thus, low concentration of surfactant used during micellar polymerization result in polymers with poor solubility in brine.

The brine solutions of these polymers are turbid and less viscous. As the surfactant concentration is increased during polymerization, the brine solutions of the resultant polymers become clearer.

It can also be seen that there is an optimum concentration of surfactant at which the maximum solution viscosity is attained. The optimum surfactant concentration is a function of the hydrophobe content of the polymer, the optimum surfactant concentration increases as the hydrophobe concentration increases. In addition, the optimum type and content of surfactant used in the micellar polymerization is a function of the type and amount of sulfonate monomer.

Example 68 Hydrolytic Stability of SRAM Polymers Polymer solutions were prepared containing 0.2 weight percent polymer in a 3.3 percent brine (3 weight percent NaCl and 0.3 weight percent CaC12). Samples of these solutions were aged at constant elevated temperatures. Periodically samples were withdrawn for measurement of the extent of conversion of acrylamide to acrylate functionality via hydrolysis.The method for the determination of the degree of hydrolysis consisted of: 1. dialysis to remove or reduce the concentration of extraneous salts; 2. treatment of the dialyzate salts with ion exchange resin to convert carboxylate groups on the hydrolyzed polymer to carboxylic acid groups; 3. gravimetric determination of the polymer concentration in the ion exchange resin-treated solution and 4. titration of the ion exchange resin-treated solution to determine the acid content. The sulfonate content of the original polymer must be accounted for in the determination since it was titrated along with the carboxylate functionality resulting from hydrolysis.

As can be seen from the results presented in Table XVI, hydrolytic stability of acrylamide based polymers was significantly improved by the presence of the sulfonate containing AMP ss monomer.

In addition, resistance to hydrolysis increased as the sulfonate monomer content increased as shown by the reduced level of hydrolysis with time for the polymer with 40 mole percent AMP ffi (Example 41) as compared to the 20 mole percent AMP z polymer (Example 42). The improvement in hydrolytic stability translates into an increased upper use temperature for mobility control in high temperature reservoirs.

Example 69 Mechanical Stability The mechanical stability of the SRAM polymer systems along with a partially hydrolyzed polyacrylamide system was monitored by determining the viscosity of the effluent polymer solution after passage through Berea sandstone having a nominal porosity of 0.2 and permeability of 500 ml.

Fresh polymer solution of 1500 ppm concentration in 3.3 percent brine was pumped through a 0.5 inch diameter Berea sandstone disk by means of a dual piston constant flow rate pump. The disk had a nominal length of 0.5 inch and permeability under one darcy for the systems studied. The disk was cut to the above dimensions from a Berea sandstone rod, which was epoxy coated to prevent fluid loss from the disk sides during the flow process.

Cutting of the Berea disk was accomplished with a diamond saw blade using 3.3 percent brine as the cutting fluid. The disk was briefly sonicated to remove sandstone fines from the disk faces, followed by vacuum drying. Subsequently, the disk was placed into a stainless steel holder equipped to measure the pressure drop across the disk by means of pressure transducers. The permeability was determined by flowing the 3.3 percent brine solvent, measuring the pressure drop, flow rate, and using Darcy's Law: KAdP rlL where Q = flow rate, cc/sec, K = permeability, darcies, A = disk area, cm2, n = fluid viscosity, cP, L = disk length, cm, and AP = pressure drop, atmospheres. Polymer solution was then injected using various flow rates.The extent of mechanical degradation was monitored by measuring the effluent viscosity by the Contraves Low Shear Rheometer at a shear rate of 11.0 s-1, which corresponds to a flux (flow) of 1 ft/day through the reservoir.

The SRAM Examples, 64 and 65, as shown in Table XVII, are similar in composition to Examples 42 and 56 shown in Table X, respectively. Examples 42 and 43 were composite blends of multiple syntheses. As shown, the mechanical stability of the SRAM polymers, Examples 64 and 65, was superior to a commercial partially hydrolyzed polyacrylamide system, HPAM. Examples 64 and 65, containing 20 and 30 mole percent AMP ffi with 0.75 mole percent hydrophobe, respectively, maintained 50 percent of their original viscosity up to a flux of about 1000 ft/day. In contrast, the HPAM system lost 50 percent of original viscosity at a flux of only 50 ft/day. This 20-fold increase in flux for 50 percent viscosity retention for the SRAM systems represents a significant improvement in the mechanical stability compared to hydrolyzed polyacrylamide.Thus, less polymer concentration would be needed to provide a fixed degree of mobility control in the reservoir during enhanced oil recovery operations.

Example 70 Resistance Factor In conjunction with monitoring the mechanical stability of the SRAM polymer systems, Examples 64 and .65, the polymer resistance factor was determined. The polymer resistance factor is the ratio of the brine and polymer solution mobilities (Mp) calculated by: R = Mw/Mp (Kw/rlw)(Kp/PIP) where the subscripts w and p refer to water (or brine) and polymer solution, respectively. In an oil-bearing formation, oil typically has a higher viscosity than the water phase, therefore, to improve the recovery of oil, the mobility ratio needs to be increased. This is accomplished by increasing the driving fluid (polymer solution) viscosity, which would increase the polymer resistance factor. As shown in Table XVIII, the resistance factor for the SRAM polymer systems, Examples 64 and 65, showed a pseudoplastic behavior with increasing flux between about 2 to 1000 ft/day.

By comparison an HPAM system had a significantly lower resistance factor at low flux and reached a plateau at about 10 ft/day flux, where a pseudoplastic response begins. At the typical flux of 1 ft/day in a reservoir, the higher resistance factor for the SRAM polymer indicates an improvement in thickening efficiency which could translate into a more economically attractive recovery process.

Table IX POLY (N-OCTYLACRYLAMIDE-ACRYLAMIDE-AMPSR SDS Initiator Example Hydrophobe AMP z Conc. Level Yield No. Mole% Mole% Wt% [M]/[I]0.5 a % 1 0.0 20 2.0 49 13.8 92 2 0.0 20 3.0 49 14.4 96 3 1.0 20 2.0 49 10.7 71 4 0.75 20 2.0 49 14.0 93 5 1.0 30 2.0 49 14.0 93 6 0.50 20 2.0 49 11.0 73 7 1.0 10 2.0 49 13.1 87 8 1.0 20 3.0 49 11.6 77 9 1.0 20 3.0 49 12.9 86 10 1.0 20 3.0 49 7.3 49 11 1.0 10 3.0 49 10.9 73 12 1.0 30 3.0 49 5.0 33 13 0.75 20 3.0 49 7.2 48 14 1.25 20 3.0 49 7.2 48 15 1.0 20 3.0 100 5.7 38 16 1.0 20 3.0 49 12.2 81 17 1.0 10 3.0 49 13.1 87 18 1.0 30 3.0 49 11.3 75 TABLE X POLY (N-OCTYLACRYLAMIDE-ACRYLAMIDE-AMPSR TERPOLYMERS Redox Initiators, Example Hydrophobe AMPSR Mx105 Yield No. Mole z Mole % K2S20 NaSv04 19 1.25 40 4.6 4.6 16.7 67 20 0.75 20 7.9 7.49 15.3 64 21 0.0 20 7.4 7.6 16.4 68 22 0.75 40 8.6 2.3 20.6 82 23 0.75 40 4.3 4.6 20.5 82 24 0.75 30 5.5 5.9 14.8 61 25 0.75 30 5.5 5.9 21.4 68 26 0.75 20 3.7 4.0 31.9 64 27 0.75 30 5.5 5.9 12.3 51 28 1.25 30 5.5 5.9 14.9 63 29 1.0 40 4.3 4.6 14.0 58 30 1.25 40 4.3 4.6 15.1 62 31 1.5 30 5.4 5.9 16.7 68 32 0.0 30 2.8 3.0 31.8 65 33 0.0 40 2.2 2.3 38.8 74 34 0.75 30 5.5 5.9 46.4 94 35 1.0 40 4.3 4.6 46.2 93 36 1.0 40 4.3 4.6 46.1 94 37 1.0 40 4.3 4.6 44.4 91 38 0.75 40 4.3 4.6 43.2 88 39 0.75 40 4.3 4.6 44.4 90 40 0.75 40 4.3 4.6 42.1 87 41. 0.75 40 2.1 2.3 47.0 92 TABLE IX SOLUTION PROPERTIES OF REDOX POLYMERIZED AMPSR TERPOLYMERS Viscosity, cP, at Intrinsic Example Hydrophobe AMPSR Viscosity Huggins' No. Mole % Mole % 1.3 sec-1 11 sec-1 dl/g Coefficient 20 0.75 20 73 26 10.0 1 .5 27 1.0 30 52 25 10.4 1 .3 30 1.25 40 289 45 9.2 1 ~4 21 0.0 20 4.8 4.8 13 0-4 TABLE XII EFFECT OF MONOMER CONCENTRATION ON TERPOLYMER SOLUTION VISCOMETRICS Polymer Composition: N-CgAM = 0.75 mole%, AMP w = 30.0 mole% Viscosity, cP at 1,500 ppm in Example Monomer 3.3% Brine No. Concentration 1.3 sec-1 11 sec-1 24 0.38 7.8 6.7 25 0.50 17 13 34 0.75 84 25 TABLE XIII EFFECT OF AMPSR ON TERPOLYMER SOLUTION VISCOSITY Viscosity, cP at Example AMPSR 1,500 ppm in 3.3% Brine No.Mole% 1.3 sec-1 11 sec-1 20 20 73 25 24 30 8 7 23 40 6 5 TABLE XIV EFFECT OF HYDROPHOBIC MONOMER CONCENTRATION Vicosity, cP at Exp. N-C8AM AMPSR 1,500 ppm in 3.3% Brine No. Mole% Mole% 1.3 sec~l 11 sec-1 32 0 30 5 5 24 0.75 30 8 7 27 1.0 30 378 54 28 1.25 30 79 25 31 1.5 30 13 8 33 0 40 4 4 22 0.75 40 6 5 29 1 40 7 6 19 1.25 40 279 44 TABLE XV EFFECT OF SURFACTANT CONCENTRATION ON TERPOLYMER SOLUTION VISCOSITY Hydrophobic Monomer = C8AM, AMPSR = 40 Mole% Viscosity, cP, at Example C8AM SDS 1,500 ppm in 3.3t Brine No. Mole% Wt .% 1.3 sec-1 11 sec-1 39 0.75 1.5 74 22 38 0.75 2 70 23 40 0.75 2.5 10 9 41 0.75 3 8 6 35 1 3 114 24 37 1 4.5 8 6 36 1 6 5 5 TABLE XVI Hydrolytic Stability of SRAM Polymers C8AM AMPSR Time Hydrolysis, mol% A.A.

Example No. mol% mol% days 80 C 93 C PAM* 0 0 20 -- 24 40 -- 45 60 -- 62 100 -- 78 20 0.75 20 20 8 15 40 15 26 60 19 31 100 28 47 19 1.25 40 20 8 12 40 13 20 60 15 27 100 16 36 * polyacrylamide TABLE XVII MECHANICAL STABILITY PROPERTIES OF SRAM POLYMERS Example AMPSR Hydrophobe Flux Viscosity, cP No. Mole % Mole % Ft/Day 11.0 Sec-1 42 20 0.75 1.8 34.

4.4 30.

9.2 30.

17.4 34.

26.6 37.

36.7 37.

46.8 39.

93.6 37.

192.8 35.

403.9 31.

605.9 25.

798.7 21.

1102.0 43 30 0.75 1.8 20.

4.6 21.

9.1 18.

27.9 21.

36.5 21.

45.7 21.

91.4 22.

182.7 23.

411.0 22.

630.5 17.

850.0 16.

1124.0 15.

Commercial 1.9 12.

HPAM 4.9 15.

10.0 14.

29.9 11.

51.0 8.

77.0 7.

149.0 5.

384.0 4.

683.0 3.

982.0 3.

TABLE XVII I RESISTANCE FACTOR OF SRAM POLYMER SYSTEMS Example AMPSR Hydrophobe Flux Resistance No. Mole% Mole% Ft/Day Factor 42 20 -0.75 1.8 359.

4.4 208.

9.2 134.

17.4 64.

26.6 57.

36.7 52.

46.8 60.

93.6 48.

192.8 34.

403.9 28.

605.9 30.

798.7 28.

1102.0 25.

43 30 0.75 1.8 376.

4.6 187.

9.1 151.

27.9 92.

36.5 84.

45.7 78.

91.4 57.

182.7 49.

411.0 44.

630.5 44.

850.0 42.

1124.0 40.

Commercial 1.9 70.

HPAM 3.1 124.

4.9 213.

10.0 265.

29.9 210.

51.0 155.

77.0 120.

149.0 79.

384.0 45.

683.0 32.

982.0 27.

Claims (9)

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) 100 to 5,000 ppm of a water soluble polymer having the formula:

wherein R1 is a C6 to C22 straight chained or branched alkyl or alkylcycloalkyl 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 A is selected from the groups consisting of SO3-M+, phenyl SO3-M+, and CONHC(CH3)2 CH2SO3-M+, wherein n+ is an alkali metal or ammonium cation, wherein x is 5 to 98 mole percent, y is 2 to 95 mole percent, z is 0.1 to 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 2 wherein R1 is an octyl group.

4. A process for recovering 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; (b) 0.1 to 5.0 weight percent of a surfactant; and (c) 100 to about 5,000 ppm of a water soluble polymer having the formula:

wherein R1 is a C6 to C22 straight chained or branched alkyl or alkylcycloalkyl 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 A is selected from the group consisting of SO3-M+, phenyl-SO3-M+ and CONHC(CH3)2SO3-M+, wherein M+ is an alkali metal or ammonium cation, wherein xis 5 to 98 mole percent, y is about 2 to 95 mole percent, z is 0.1 to 10.0 mole percent.

5. 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) 100 to 5,000 ppm of a water soluble polymer having the formula:

wherein R1 is a C6 to C22 straight chained or branched alkyl or alkylcycloalkyl 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 10 to 90 mole percent, y is 2 to 40 mole percent, z is 0.1 to 10.0 mole percent, and w is 1 to 80 mole %.

6. A process according to claim 5 wherein M+ is a sodium cation.

7. A process according to claim 5 or 6 wherein R1 is an octyl group.

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

wherein R1 is a C6 to C22 straight chained or branched alkyl or alkylcycloalkyl 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 10 to 90 mole percent, y is 2 to 40 mole percent, z is 0.1 to 10.0 mole percent, and w is 1 to 80 mole %.

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