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SPE 113845 Selection and Screening of Polymers for Enhanced-Oil Recovery David B. Levitt, SPE, and Gary A. Pope, SPE, The University of Texas at Austin Copyright 2008, Society of Petroleum Engineers This paper was prepared for presentation at the 2008 SPE/DOE Improved Oil Recovery Symposium held in Tulsa, Oklahoma, U.S.A., 19–23 April 2008. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright. Abstract A number of commercially available polymers have been tested for enhanced oil recovery based upon viscosity, filterability, and surfactant compatibility, and chemical and thermal stability testing has been carried out with some of these as well. Several high molecular weight polymers exhibited high viscosities at salinities up to 170,000 ppm NaCl and with greater than 17,000 ppm CaCl 2 present. Polyacrylamide polymers hydrolyze at high temperatures and beyond a certain point are subject to precipitation by calcium. If calcium concentrations can be kept below about 200 ppm, the use of polyacrylamide polymers is feasible up to reservoir temperatures of at least 100 °C. For higher concentrations of calcium, copolymers including AMPS moieties should be considered. Calcium tolerance can be improved with sodium metaborate or by using copolymers of acrylamide and sodium 2-acrylamido-2-methylpropane sulfonate (AMPS). The stability problems at elevated temperatures in the presence of iron can be mitigated by the use of chemicals such as sodium dithionite and sodium carbonate. The polymers tested did not lose viscosity after 220 days of aging at 100 °C with dithionite present. Introduction Following Muller’s (1981) terminology, we will use “chemical degradation” when referring to the hydrolysis of polymer functional groups and “thermal degradation” when describing the free radical induced breakdown of the acrylic backbone, resulting in molecular weight reduction. Chemical degradation leads to a higher degree of hydrolysis and can only be prevented by the inclusion of more chemically stable monomers, but does not necessarily limit application, and in fact, often results in higher viscosity. Thermal degradation results in a reduction of molecular weight and a loss in viscosity, but this degradation can be prevented in most situations. Chemical Degradation; Hydrolysis and Precipitation of Polymers. Solutions of non-hydrolyzed polyacrylamide (PAM) are nonionic, and hence the viscosity is essentially insensitive to salinity. At elevated temperature and/or pH, the amide moiety undergoes hydrolysis, resulting in a acrylate moiety and the evolution of ammonium ion, as illustrated in Figure 1. The anionic charges of the acrylate moieties results in intramolecular repulsions that increase the hydrodynamic radius of the polymer molecules and hence the solution viscosity. Because of this benefit, commercial polyacrylamide for EOR is usually either post-hydrolyzed by addition of alkali or produced as a copolymer of acrylamide (AM) and acrylic acid or its salt (AA). In either case, the molar fraction of the acrylate moiety is referred to as the degree of hydrolysis (τ), and is typically between 0.15 and 0.40 for commercial hydrolyzed polyacrylamide (HPAM) polymers used for enhanced oil recovery. During its residence in the reservoir at elevated temperature and/or pH, τ of polyacrylamide polymers increases. At high salinity, the acrylate moieties on HPAM are strongly associated with cations, and the viscosity approaches that of non-hydrolyzed polyacrylamide. Multivalent cations have a much stronger effect than monovalent cations. If τ exceeds approximately 0.33 (Zatouin and Potie, 1983), then precipitation is possible if excessive amounts of multivalent cations are present. The critical amount of calcium necessary to precipitate hydrolyzed polyacrylamide decreases with temperature, τ, and decreasing monovalent cation concentration. At a high degree of hydrolysis, this has been described as a site fixation phenomenon, and occurs at close to the stochiometric equivalence point between acrylate moieties and cations (around 200 ppm of calcium for a 1000 ppm polymer solution). At lower τ, the precipitation phenomenon is due to poor solvation (theta- type precipitation) (Ikagami, 1962). The kinetics of hydrolysis are a strong function of pH, and can be understood as a combination of acid and basic hydrolysis mechanisms and neighbor effects, as well as the buffering effect of evolved ammonium, as explained by Kheradmand (1987). In small molecules, amide hydrolysis proceeds through an acid or basic mechanism. The rate is lowest at neutral pH and increases linearly as pH rises or falls. In a macromolecule, the neighboring units can catalyze or retard

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Page 1: [Society of Petroleum Engineers SPE Symposium on Improved Oil Recovery - Tulsa, Oklahoma, USA (2008-04-20)] SPE Symposium on Improved Oil Recovery - Selection and Screening of Polymers

SPE 113845

Selection and Screening of Polymers for Enhanced-Oil Recovery David B. Levitt, SPE, and Gary A. Pope, SPE, The University of Texas at Austin

Copyright 2008, Society of Petroleum Engineers This paper was prepared for presentation at the 2008 SPE/DOE Improved Oil Recovery Symposium held in Tulsa, Oklahoma, U.S.A., 19–23 April 2008. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract A number of commercially available polymers have been tested for enhanced oil recovery based upon viscosity, filterability, and surfactant compatibility, and chemical and thermal stability testing has been carried out with some of these as well. Several high molecular weight polymers exhibited high viscosities at salinities up to 170,000 ppm NaCl and with greater than 17,000 ppm CaCl2 present. Polyacrylamide polymers hydrolyze at high temperatures and beyond a certain point are subject to precipitation by calcium. If calcium concentrations can be kept below about 200 ppm, the use of polyacrylamide polymers is feasible up to reservoir temperatures of at least 100 °C. For higher concentrations of calcium, copolymers including AMPS moieties should be considered. Calcium tolerance can be improved with sodium metaborate or by using copolymers of acrylamide and sodium 2-acrylamido-2-methylpropane sulfonate (AMPS). The stability problems at elevated temperatures in the presence of iron can be mitigated by the use of chemicals such as sodium dithionite and sodium carbonate. The polymers tested did not lose viscosity after 220 days of aging at 100 °C with dithionite present.

Introduction Following Muller’s (1981) terminology, we will use “chemical degradation” when referring to the hydrolysis of polymer functional groups and “thermal degradation” when describing the free radical induced breakdown of the acrylic backbone, resulting in molecular weight reduction. Chemical degradation leads to a higher degree of hydrolysis and can only be prevented by the inclusion of more chemically stable monomers, but does not necessarily limit application, and in fact, often results in higher viscosity. Thermal degradation results in a reduction of molecular weight and a loss in viscosity, but this degradation can be prevented in most situations.

Chemical Degradation; Hydrolysis and Precipitation of Polymers. Solutions of non-hydrolyzed polyacrylamide (PAM) are nonionic, and hence the viscosity is essentially insensitive to salinity. At elevated temperature and/or pH, the amide moiety undergoes hydrolysis, resulting in a acrylate moiety and the evolution of ammonium ion, as illustrated in Figure 1. The anionic charges of the acrylate moieties results in intramolecular repulsions that increase the hydrodynamic radius of the polymer molecules and hence the solution viscosity. Because of this benefit, commercial polyacrylamide for EOR is usually either post-hydrolyzed by addition of alkali or produced as a copolymer of acrylamide (AM) and acrylic acid or its salt (AA). In either case, the molar fraction of the acrylate moiety is referred to as the degree of hydrolysis (τ), and is typically between 0.15 and 0.40 for commercial hydrolyzed polyacrylamide (HPAM) polymers used for enhanced oil recovery. During its residence in the reservoir at elevated temperature and/or pH, τ of polyacrylamide polymers increases.

At high salinity, the acrylate moieties on HPAM are strongly associated with cations, and the viscosity approaches that of non-hydrolyzed polyacrylamide. Multivalent cations have a much stronger effect than monovalent cations. If τ exceeds approximately 0.33 (Zatouin and Potie, 1983), then precipitation is possible if excessive amounts of multivalent cations are present. The critical amount of calcium necessary to precipitate hydrolyzed polyacrylamide decreases with temperature, τ, and decreasing monovalent cation concentration. At a high degree of hydrolysis, this has been described as a site fixation phenomenon, and occurs at close to the stochiometric equivalence point between acrylate moieties and cations (around 200 ppm of calcium for a 1000 ppm polymer solution). At lower τ, the precipitation phenomenon is due to poor solvation (theta-type precipitation) (Ikagami, 1962).

The kinetics of hydrolysis are a strong function of pH, and can be understood as a combination of acid and basic hydrolysis mechanisms and neighbor effects, as well as the buffering effect of evolved ammonium, as explained by Kheradmand (1987). In small molecules, amide hydrolysis proceeds through an acid or basic mechanism. The rate is lowest at neutral pH and increases linearly as pH rises or falls. In a macromolecule, the neighboring units can catalyze or retard

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hydrolysis. At low pH protonated neighboring acrylate moieties catalyze further hydrolysis, while at high pH ionized acrylate moieties repel hydroxide molecules resulting in autoretard kinetics, which makes it difficult to achieve τ > 2/3 (Kulicke and Horl, 1985). Due to the evolution of ammonium (pKa= 9.7) the pH is driven towards around 8.2 as hydrolysis proceeds, complicating the interpretation of results in unbuffered solutions.

At near neutral pH, hydrolysis proceeds slowly but will result in complete hydrolysis of amide moieties after several months at temperatures of 100 °C and higher. At high pH, hydrolysis is more rapid and occurs within the first weeks, but may be limited due to the autoretard kinetics mentioned above.

Several researchers (Moradi et al., 1987b; Doe et al., 1987; Taylor and El Din, 1985; Martin et al. 1983) have examined the effects of including co-monomers to improve chemical stability or viscosity in the presence of salt. This may be achieved either by reducing the extent of hydrolysis or by finding other ways to increase the viscosity to negate the reduction of hydrodynamic radius due to shielding by cations. These include:

• Substituting some of the acrylamide moieties within the molecule with other nonionic monomers more resistant to chemical alteration, and whose presence may also stabilize neighboring acrylamide moieties. Examples are N-vinyl pyrrolidone (NVP) and diacetone acrylamide (DAAM).

• Substituting some of the acrylate moieties with another anionic monomer more resistant to cation shielding and/or precipitation itself, and which may also stabilize neighboring acrylamide moieties. An example is sodium 2-acrylamid-2-methylpropane sulfonate (AMPS).

• Sterically hindering the polymer chain so that the hydrodynamic radius does not fully collapse to a random coil configuration at high salinity. These have been called "comb" polymers.

• Adding a small amount of hydrophobe so that intermolecular associations increase solution viscosity, but still allow for flow through porous media.

These mechanisms are not mutually exclusive. The difficulty in using some of these improved polymers is that their flow properties and long term chemical stability

have not been tested to nearly the extent of HPAM. Available data on copolymers of PAM including AMPS and NVP (Moradi et al., 1987b; Parker and Lezzi 1993) indicate that both moieties are more chemically stable than acrylamide. Also a lower ultimate degree of hydrolysis may result after extended aging when AMPS or NVP is substituted for some of the AM moieties in HPAM (e.g. poly(AM-co-AMPS)), indicating neighbor effects are significant (Figure 2). For the case of AMPS, it has yet to be determined what properties the aged polymer will have, particularly in terms of calcium tolerance. As poly(AM-co-AMPS) ages at high temperature, the some of AM moieties will hydrolyze, leaving AA. Although sulfonate groups are typically regarded as more resistant to precipitation, it is unclear how the resulting terpolymer of AMPS, AA, and AM will behave in the presence of divalent cations. Additionally, Parker and Lezzi (1993) report that the AMPS moiety is susceptible to hydrolysis at temperatures above roughly 100 °C. They report a sudden and rapid hydrolysis of AMPS moieties after 50 days at 120 °C and after only 4 days at 150 °C. Using the rule of thumb that rate constants double every 10 °C, it could be expected that at 100 °C hydrolysis of AMPS moieties may occur after approximately 200 days, however this was not reported by Moradi et al. (1987b). In this paper, we use the symbol σ to represent the mol fraction of AMPS moieties on a polymer, analogous to the use of τ to represent mol fraction of AA moiety.

Thermal Degradation; Breakdown of the Acrylic Backbone. Viscosity loss during aging experiments is typically attributed to a reduction of molecular weight as a result of free-radical induced scission of the acrylic backbone (Grollman, 1982; Wellington, 1983). Sorbie (1991) presents an excellent review on thermal as well as chemical degradation.

Certain radicals are capable of abstracting hydrogen from acrylic polymers, the most favorable location being from the carbon on the acrylic backbone which attaches to the pendant group (Grollman, 1982). This results in a carbon-centered radical on the polymer backbone and can result in the cleavage of the backbone through the steps outlined by Grollman (1982) and others.

Previous research (Muller, 1981a and 1981b; Kheradmand, 1987; Ramsden and McKay, 1986) as well as our current work suggests that depolymerization reactions can occur when any two of the following three are present: (1) oxygen, (2) iron, and (3) residual initiators from polymerization. Antioxidants or easily oxidizable sacrificial species are typically added to prevent this. Notable exceptions to these depolymerization conditions are:

• When iron is not present, the degradation is only observed at elevated temperatures (Muller, 1981b). • At high pH (>pH 8) and if residual initiators are removed through purification, Ramsden and McKay (1986)

demonstrated that degradation was not observed, despite the visual evidence of rapid conversion of Fe(II) to Fe(III).

• Degradation occurs at room temperature when sulfite or dithionite is added to polymer solutions containing oxygen. However if only the residual 8 ppm or so of oxygen in solution is allowed to react, the degradation is limited. This assumes that no other source of oxygen (e.g., headspace) is present.

The types of radicals that may be present in dilute polymer solutions in the lab and the oil reservoir are carbon-centered (residual initiators), sulfur-centered (sulfite, dithionite), or oxygen centered (from decomposition of peroxide initiators or oxygen-mediated oxidation of iron (II)). While only oxygen-centered radicals are believed to be efficient hydrogen abstractors, both carbon-centered and sulfur-centered radicals react with oxygen rapidly (near diffusion controlled limits)

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producing oxygen-centered radicals (Fossey et al., 1995). Autoxidation of polymers and/or the presence of alkyl peroxide groups on the polymers, as theorized by some researchers (Wellington, 1983), is unlikely because of the reported stability of purified commercial polymers with oxygen at elevated temperatures (Muller, 1981b; Kheradmand, 1987). While the iron encountered in the region far from the wellbore is assumed to be largely in the reduced state, Kheradmand et al. (1988) have demonstrated that degradation occurs when either ferrous or ferric iron and residual impurities are present, even in the absence of oxygen.

Radical-induced oxidation is often attributed to oxygen-centered radicals, particularly the powerful hydroxyl radical. Hydroxyl radicals are formed by the reduction of a peroxide by iron as first observed by Fenton (1894). Peroxides may be present as residual initiators (Kheradmand, 1987), formed by the autoxidation of iron (Ramsden and McKay, 1986), or formed from peroxyl radicals generated by the reaction with carbon- or sulfur-centered radicals as discussed below. Hydroxyl radicals are the strongest known oxidants, and will abstract hydrogen almost indiscriminately from the nearest organic molecule. As such, true antioxidants are ineffective, however easily oxidized compounds such as alcohols or glycols may be used as sacrificial substrates (Anbar et al., 1966; Wellington, 1983; Shupe 1981). However, peroxide-decomposing antioxidants may be effective at preventing the formation of hydroxyl radicals.

Carbon-centered radicals are formed by the thermal decomposition of residual initiators at elevated temperature, or they may be present on a polymer backbone following the abstraction of hydrogen. These radicals are relatively weak, and are only able to abstract allyl hydrogen, hence their efficiency for radical polymerization of vinyl-backbone polymers such as HPAM. Though unable to abstract hydrogen from a saturated carbon backbone, these radicals will react with oxygen to form alkyl peroxyl radicals. The resulting radicals are described by different authors as "slightly reactive"(Fossey et al., 1995) and capable of abstracting hydrogen from a polymer backbone (Moad and Solomon, 2006), at least at high temperatures (Alfassi, 1997). In this case, the depolymerization reaction proceeds until the oxygen is consumed. Carbon-centered as well as peroxyl radicals may be trapped by certain antioxidants, as discussed below.

The dithionite ion rapidly decomposes to bisulfate and thiosulfate. The sulfur dioxide radical is a transient reaction product. Commercial samples of sodium dithionite frequently contain greater amounts of these radicals that decrease to an equilibrium amount of around 0.01 mole % in aqueous solution. The sulfur dioxide radical is also a reaction intermediate in the oxidation of sulfite to sulfate (Janzen, 1972). Sulfur dioxide radicals are strong reductants and will not abstract hydrogen directly, however they will rapidly react with oxygen to form superoxide radicals (Alfassi, 1999). While superoxide is not considered a strongly oxidizing radical, it is known to abstract hydrogen (Affanas'ev, 1989), and this may explain the loss of viscosity observed when scavengers are added to solutions with dissolved oxygen or when further oxygen is allowed to enter polymer solutions containing scavengers. In the absence of further oxygen, this depolymerization reaction will cease.

Antioxidants, Reducing Agents, and Sacrificial Substrates. Alcohols and other low molecular weight substrates with easily abstractable hydrogen can play a sacrificial role and prevent oxidation of the polymer backbone, as previously noted by Shupe (1981), Wellington (1983), Ryles (1983), and Yang and Treiber (1985). Primary alcohols are most easily oxidized. Anbar (1966) showed that the apparent rate constant for oxidation of various solvents by the hydroxyl radical increases with carbon length and decreases with additional OH groups. Shupe (1981) found better stabilization of viscosity using IBA than with IPA, which is in agreement with the difference in rate constants found by Anbar. Glycols and surfactants, particularly those of higher molecular weight, should also act as sacrificial agents that may be oxidized in lieu of the polymer backbone. It should be noted that rate constants for reactions between the hydroxyl radical and most of these compounds are near diffusion controlled limits, so the differences in rate may be less important than the ratio of substrate to polymer. Table 1 summarizes some of the factors that should be taken into account when choosing a cosolvent for use in a chemical flood.

Chloride, Bicarbonate, and Carbonate. Chloride, bicarbonate, and carbonate anions can all act as antioxidants, as

observed by Wellington (1983), Liao et al. (2000), and Rice and Wilkes (1994). Rice and Wilkes report an order of magnitude increase in the OH radical scavenging rate of CO3

2- over HCO3-.

Effect of pH. Transition metals such as iron play a catalytic role in many free radical reactions. Fenton (1894) first

observed that the combination of hydrogen peroxide with iron resulted in the breakdown of organic molecules, as generalized by the following:

2 2Fe H O Fe OH OH++ +++ −+ → + + •

At high pH, the oxidation of iron is extremely rapid and the solubility of iron is very low, particularly in the presence of carbonate (Hem, 1961). Shupe (1981) reported a significant reduction in polymer degradation in the presence of iron at pH=10. Ramsden and McKay (1986) noted that despite the rapid and visible change in oxidation state of iron, no change in viscosity was observed at pH=8 and above with a purified polymer. Others have noted that peroxides may break down at high pH through non-radical as well as radical pathways (Beylerian and Asaturyan, 2004). Any of these reasons or a combination of these reasons may explain the improved stability of polymers to radical degradation observed in this work and others. Figure 3 summarizes the importance of pH for polymer stability.

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Antioxidants. Antioxidants are capable of donating a proton or electron and trapping the radical species by forming a stable radical. Hindered phenol compounds are common antioxidants which form resonance stabilized radicals. The five-membered ring of NVP may trap radicals the same way. Strong radicals such as hydroxyl radicals react almost indiscriminately, and are therefore not effectively combated by such antioxidants. Peroxide decomposing or "secondary" antioxidants, such as phosphite and thiol compounds, prevent the formation of radical species.

Reducing Agents. Salts of dithionite, sulfite or bisulfite can be used to remove oxygen from polymer solutions. Sodium dithionite is a powerful reducing agent and exists in equilibrium with the SO2 radical, as mentioned above. Each

dithionite ion is capable of forming two radicals, which can each reduce Fe(III) to Fe(II). Dithionite is capable of reducing water and is thus quite unstable in solution (Carvalho and Schwedt, 2001) particularly at high temperature (Rinker et al., 1965), however the SO2 radical appears to persist sufficiently to propagate low Eh fronts in situ (Amonette et al., 1994; Istok, 1999). With transition metals such as iron kept in the reduced state, Fenton-type reactions are prevented.

Sodium sulfite is another reducing agent that can be used to scavenger oxygen, but is not powerful enough to reduce iron. Sodium bisulfate and ammonium bisulfate are closely related compounds which are often preferred in the field due to their higher solubility and lower molecular weight. The pKa of the disassociation of bisulfite to sulfite is 6.4 and it has been shown that sulfite is the species which reacts with oxygen, hence the rate of oxygen scavenging increases with pH, at least up to pH 10 (Miron, 1981). The difficulty in using sulfite or bisulfate is that the scavenging of oxygen can be quite slow unless catalyzed by a transition metal, which may itself cause degradation. There are reports that the use of sulfite may cause greater degradation to polymer than dithionite (Dunlop, 1973), however this may be due to the use of transition metals as a catalyst. If oxygen can be mostly or completely scavenged using sulfite, then dithionite could be added with minimal degradation.

Experimental Methods Polymers. Polymer samples were provided by SNF Floerger and Hengju and used as provided except as noted. Table 2 describes the commercial polymer samples tested.

Polymer Hydration. Proper hydration of powdered polymers can repeatedly be obtained in the lab by mixing a low salinity brine solution on a stir plate at 400-600 rpm using a cross shaped magnetic stir bar and slowly sprinkling the powder onto the shoulder of the vortex. A solid addition burette capable of slowly adding powdered polymer was designed and has made the process quicker and easier. As the polymer hydrates, the solution becomes more viscous and the stir bar must often be slowed to 100-200 rpm. After 1-2 hours the solution begins to appear homogenous. The solution is allowed to stir slowly for a minimum of 16 hours before use.

Polymer hydrated in the field was sprayed with water as it was slowly fed into a stirring tank. The tank was baffled to ensure polymer was allowed a minimum residence time of 1-2 hours.

Polymer powders were stored in a vacuum desiccator. Moisture content was routinely measured and found to be around 10%. The test weights given in the results section do not correct for this moisture.

Filtration. Filtration tests have been used here to rapidly gauge the potential for new polymers to be propagated through porous media in the same way that they have been used for years to ensure proper hydration of polymers. Approximately 250 mL of a 1500 ppm solution of polymer in 1000 ppm NaCl is placed in a 90 mm filter press bell with a 1.2 micron Millipore cellulose filter (part # RAWP09025) and filtered under 15 psi Argon. The time is recorded when 60, 80, 180, and 200 mL of cumulative fluid have been filtered. The filtration ratio is calculated as

FR = (t200 ml – t180 ml)/(t80 ml – t60ml)

Following the test, the filter is inspected qualitatively to see if any polymer has been filtered out due to fish eyes or other problems.

Polymer and Surfactant Compatibility. Surfactants and polymers used for surfactant-polymer flooding or alkaline-surfactant-polymer (ASP) flooding must be compatible in both the aqueous solution injected as a slug and in the microemulsions that form when the slug mixes with the oil. The first requirement is simply that the aqueous solution remains clear and stable when the polymer is added to the surfactant solution. Co-solvents are sometimes needed to meet this requirement. Dilute surfactant typically has very little effect on the viscosity of the HPAM polymer solutions when the mixtures are clear and stable. However, this is not true in general, so another compatibility condition is that the surfactant does not have a large effect on the polymer solution. In some cases, a polymer will cause a three phase microemulsion to form a separate, polymer-rich aqueous phase, so this is another type of incompatibility.

Viscosity Measurement. Viscosity is measured using a Contraves LS-30 couette viscometer. Viscosities are recorded over a range of shear rates from 0.02 to 129 s-1. Shear rates of 1 to 20 s-1 are expected in most of the reservoir except very close to wells, so this is typically the shear rate range of focus. Viscosities reported in the results section were measured at 23 °C and correspond to a shear rate of 11 s-1 unless otherwise stated.

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SPE 113845 5

Chemical Stability and Degree of Hydrolysis. Chemical stability refers to the hydrolysis of the amine from the acrylamide moiety and/or β,β dimethyl taurine (Audibert and Argillier, 1995) from the AMPS moiety. In either case this results in the formation of an additional acrylate (AA) moiety in the hydrolyzed polymer molecule. This was observed qualitatively by the increase in viscosity observed at most salinities as well as measured quantitatively by peak integration of 13C NMR spectra.

Initial degree of hydrolysis was determined by dissolving 4 wt% polymer in 10 wt% D2O with 3% NaCl. Dilute polymer samples were heated in 15 mL glass ampules that were flame sealed. Early experiments with off-the-shelf

ampules resulted in thermal fractures when ampules were sealed under vacuum. These fractured samples then were potted in epoxy to prevent air leakage of oxygen. Additional samples were blanketed with argon and then quickly sealed while open to atmosphere. In all of these initial samples, 1 ppm of resazurin was included in order to visually detect leaks. Resazurin is initially blue in solution. It is reduced irreversibly to a pink color and then reversibly to clear, after which it serves as a redox indicator, changing from clear to pink as the redox potential increases from about -100 mV to 0 mV using a Ag/AgCl combination electrode (Eh = -300 mV to -100 mV). Some samples turned pink when left overnight before heating or when placed in the oven, indicating a leak, and these samples were discarded. After heating, some samples did not turn pink when opened for measurement. This corresponded with low viscosity, indicating that extensive degradation occurred to the polymer as well as the resazurin. These samples were discarded.

Thicker, custom ampules and a manifold where then fabricated by the glass shop in the UT Department of Chemistry and Biochemistry so that the headspace of the ampules could be reliably cycled with purified argon and vacuum and sealed under vacuum. Experiments showed that cycling three times with argon and vacuum does not change the amount of dissolved oxygen in solution.

Samples of polymer heated as described above were analyzed viscometrically and 10% D2O was added to the samples prior to concentration to around 4 wt% using an ultrafiltration cell with a 100,000 MW filter.

NMR work was performed by UT Analytical Services Laboratory in the Department of Chemistry and Biochemistry using a Varion Inova 500. A delay time of 2 s was determined to be necessary from preliminary experiments, and approximately 1800 cycles were acquired (1 hour acquisition time). In some cases where insufficient amounts of polymer were recovered, acquisition time was increased to 16 or 64 hours. Peaks were assigned to the carbonyl carbon of acrylamide (180 ppm), acrylate (183 ppm), and AMPS (176 ppm) moieties (Lezzi and Parker, 1993). The mol % of each moiety was calculated as the fraction of the integration of the peak corresponding to the carbonyl carbon of that moiety divided by the sum of the integration of all carbonyl peaks.

Calcium Tolerance. Calcium tolerance of 8 million Dalton HPAM with degrees of hydrolysis varying from 0 to 100 were measured by preparing samples of 1500 ppm polymer and various amounts of sodium with incrementally more CaCl2 until cloudiness was observed. This was reported as a function of Na+ concentration with Na+ added as either NaCl or sodium metaborate (Na2B2O4·8H2O). Sodium metaborate supplies Na+ and also can sequester calcium. In some cases samples were placed in a 55 °C or 85 °C oven and observed quickly to determine the change in calcium tolerance with temperature. A sample of poly(AM-co-AMPS) copolymer (AN-125) was hydrolyzed at room temperature by the addition of NaOH and then neutralized with HCl. The sample was concentrated in an ultrafiltration cell and analyzed by NMR as described above. The salt was then removed by repeated dilution and concentration in the ultrafiltration cell, after which calcium tolerance was measured as above.

Thermal Stability. Stability of the acrylic backbone to radical degradation, as discussed above, can be observed viscometrically. At room temperature, any change in viscosity can be attributed to change in molecular weight distribution of the polymer. At elevated temperature, hydrolysis may also cause a change in solution viscosity and so observation of degradation is more complicated. Performing experiments in 3% NaCl or above will limit the change in viscosity due to hydrolysis. Some experiments were also performed using poly(ammonium acrylate) (PAA), a salt of poly(acrylic acid) which is similar to what is obtained if polyacrylamide is totally hydrolyzed. Because no further hydrolysis can occur viscometric interpretations are more readily made. In some cases iron was added as FeCl2, or FeCl3. When FeCl2 was used the solution was first bubbled with hydrogen gas to ensure iron was in the reduced state. Sodium dithionite and sodium sulfite were also added to some experiments. When either of these scavengers was used, care was taken to maintain an argon blanket over the polymer solution to ensure further oxygen did not enter the solution after the chemicals were added. All of these chemicals were obtained from Fisher Scientific.

Core Flooding. Core flooding was performed with some polymers, using methods described by Levitt (2006).

Results Preliminary Screening of Polymers. Filtration and surfactant compatibility data obtained with polymers are summarized in Table 3. Aside from hydrophobically modified associative polymers, all polymers were filtered through 1.2 micron filters with filtration ratios below 1.5. Higher MW polymers and poly(AM-co-AMPS) (AN-125) required longer hydration times before a low filtration ratio was obtained. It is unknown how the time required for proper hydration will scale up when field hydration equipment is used, or for what permeability range this filter test is appropriate. In the case of Flopaam 3330S good hydration, obtained after 1-2 days in lab conditions, was achieved after only 90 minutes using field hydration equipment.

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Associative polymers present filtration and surfactant compatibility issues and therefore require additional work before ready for application as mobility control agents. Levitt (2006), Jackson (2006) and Flaaten (2007) performed core floods in both outcrop and reservoir cores using Flopaam 3330S, 3630S and AN-125 with excellent transport (no plugging problems). We have observed excellent polymer behavior in core floods when the filtration ratio is less than 1.2. However, this may stricter than necessary in some cases and will certainly depend on the pore size and structure of the rock.

Viscosity of 1500 ppm solutions of several polymers as a function of NaCl concentration is summarized in Figure 4. For all except for Kypaam 5, the viscosity is nearly constant for salinities above 3% NaCl. This data is not presented for the purposes of comparing these products, as the viscosity is merely a reflection of the molecular weight, but to illustrate that high molecular weight HPAM polymers have sufficient viscosity for use in enhanced oil recovery even if the brine has a very high salinity. The same is true in the presence of calcium containing brines; Figure 5 presents data comparing viscosity of a HPAM in hard and soft brine. Even in the presence of 17,000 ppm CaCl2 viscosity is high.

Surfactant compatibility was also an issue for hydrophobically modified associative polymers. A large drop in viscosity was observed when surfactant was added to these polymer solutions. This is likely due to shielding of intermolecular associations of the polymer hydrophobes by the surfactant hydrophobe.

Flopaam 3330S, Flopaam 3630S, and AN-125 were compatible with the particular surfactants tested by Levitt (2006), Jackson (2006) and Flaaten (2007). Hengfloc 63020, Hengfloc 63026, and Kypaam 5 were not tested for surfactant compatibility.

The viscosities of samples of 8 million molecular weight HPAM with different degrees of hydrolysis are presented in Figure 6. These data show that the viscosity is nearly independent of the degree of hydrolysis at high salinities. However, as discussed elsewhere in this paper, the calcium tolerance is a strong function of the degree of hydrolysis.

Calcium Tolerance of Partially Hydrolyzed Polyacrylamides. Calcium tolerance of various acrylamide-co-sodium acrylate copolymers as a function of sodium concentration at room temperature is presented in Figure 7. At low salinity PAA, which is comparable to fully hydrolyzed polyacrylamide, is capable of tolerating about 200 ppm of Ca++ before precipitation is observed at 23 °C. Although precipitation of PAA with Ca++ occurred at close to stochiometric equivalence point with around 1500 ppm polymer, the precise proportionality to polymer concentration expected if this is purely a site fixation phenomenon was not observed. As Na++ concentration increases or τ decreases, calcium tolerance is quickly increased to at least 400 ppm. At 85 °C, the calcium tolerance is lowered by about 50 %. The addition of 1wt% IBA does not increase calcium tolerance of PAA.

In some cases, calcium tolerance is higher when sodium metaborate is used rather than sodium chloride, indicating sequestration of Ca++. However, when greater than about 1000 ppm Na+ (about 0.6 wt % sodium metaborate) was added, no improvement was noted. This amount corresponds closely to the apparent solubility of calcium metaborate (Nikolaev and Chelischeva, 1940). This is illustrated in Figure 8 for a copolymer of τ = 0.7.

Chemical Stability of HPAM and Poly(AM-co-AMPS). Samples of HPAM and poly(AM-co-AMPS) (AN-125)heated for 220 days at 85 °C were determined to be composed of approximately 68 mol% and 63 mol% anionic groups (τ and τ+ σ, respectively). The amount of AMPS present remained unchanged in the aged poly(AM-co-AMPS). The viscosity data for these experiments are presented in Figure 9. Because the initial pH was set to about 8.5 by the addition of 1000 ppm NaHCO3, the hydrolysis occurs very rapidly and then stops within about 2 weeks. By comparison, data presented by Moradi (1987a) exhibit much slower hydrolysis kinetics, consistent with the fact that the solutions were unbuffered and hence probably between pH 6 and 7. The ultimate degree of hydrolysis obtained agrees very well with that obtained by Moradi for PAM, as illustrated in Figure 2. It is interesting to note that, although the a lower proportion of the amide groups were hydrolyzed on the poly(AM-co-AMPS) sample, the total anionicity resulting was the same. Figure 10 compares the results of this paper and Moradi (1987a, 1987b) presented as total anionicity (τ+ σ).

In order to determine if the presence of some AMPS moieties instead of only AA moieties yields improved tolerance to divalent cations, a sample of poly(AM-co-AMPS) (AN-125) was hydrolyzed under basic conditions as described above. Total anionicity (τ+ σ) of the post hydrolyzed poly(AM-co-AMPS) sample was determined to be 67% by NMR. Calcium tolerance of this terpolymer is presented in Figure 11 along with a HPAM of similar total anionicity. Although precipitation can still occur, calcium tolerance is greatly increased by the presence of AMPS moieties. The transition to cloudiness as calcium is added is more gradual and thus slightly difficult to determine with these polymers. In order to more quantitatively observe the precipitation phenomena, the viscosity of both clear and cloudy solutions were measured before and after the point at which precipitation was observed. As illustrated in Figure 12, the viscosity decline experienced at the point where cloudiness is observed is similar with HPAM and hydrolyzed poly(AM-co-AMPS) polymer. It has been reported (Zaitoun and Potie, 1983) that observed turbidity is closely related to plugging behavior. Transport studies are necessary to determine if the same holds true for hydrolyzed poly(AM-co-AMPS), but based upon the above results it is the belief of the authors that the findings of Zaitoun and Potie (1983) would be true for these polymers as well.

As discussed above, calcium tolerance of HPAM polymers decreases with increasing temperature. Upon heating from 23 °C to 55 °C, calcium tolerance of hydrolyzed poly(AM-co-AMPS) samples decreased approximately 25%. Upon heating to 85 °C, calcium tolerance is reduced to approximately 50% of the amount observed at 23 °C. At 85 °C and with 20,000 ppm

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SPE 113845 7

NaCl present, the calcium tolerance of extensively hydrolyzed poly(AM-co-AMPS) (hydrolyzed AN-125) is around 800 ppm. This is about a four fold increase in the calcium tolerance of HPAM after aging at the same temperature.

Thermal Stability of HPAM and Poly(AM-co-AMPS). Figure 13 presents viscosity data collected with the same polymers at 100 °C, as well as a with SAV301, a terpolymer including thermally stable moieties such as those discussed above. As indicated by viscosity data in Figures 9 and 13, no further degradation was observed after a small initial drop in viscosity when sodium dithionite was added to polymer solutions containing dissolved oxygen. An initial increase in viscosity due to amide hydrolysis is followed by no further change in viscosity. For Flopaam 3630S at 100 °C, the viscosity first increased due to hydrolysis and then decreased, but is still at its original viscosity after 200 days. This may be due to the complete hydrolysis of this sample. As indicated in Figure 6, complete hydrolysis results in a quicker decline to the viscosity of the unexpanded coil for a given molecular weight, and this is consistent with the return to the original viscosity. These data indicate that in the presence of sodium dithionite these polymer solutions maintain their original viscosity for long periods of time up to at least 100 °C.

Stability of Acrylic Backbone. Adding either ferrous or ferric iron as iron chloride to polymer solutions had markedly different effects depending on the presence of Na2CO3. Viscosity of PAA in NaCl or Na2CO3 solutions after either oxidized or reduced iron is added is presented in Figure 14.

Viscosity reduction at high temperature due to the presence of residual impurities and oxygen was also strongly effected by the presence of Na2CO3. In the presence of residual impurities and with 8 ppm of oxygen originally in solution, polymer solutions aged at 100 °C containing sodium carbonate experienced much less degradation than similar solutions containing sodium metaborate or sodium chloride, even when the sodium chloride solution was adjusted to above pH 9 using NaOH, as illustrated in Figure 15.

The addition of sodium sulfite or sodium dithionite to polymer solutions containing dissolved oxygen results in a decrease in viscosity, as discussed above. Figure 16 compares the viscosity loss caused by each chemical as a function of initial dissolved oxygen present. Because viscosity is a complex function of salinity, pH, polymer molecular weight and concentration, these data are intended to provide an indication of the relative amounts and overall severity of degradation, and will not apply quantitatively if conditions are substantially changed. Degradation experienced when 400ppm sodium dithionite was added to polymer solutions from a 80,000 ppm stock solution containing 6-8 ppm oxygen ranged from 10% to 25%

An attempt was made to determine the optimal strategy for formulating polymer solutions with sodium dithionite. Less than 2% viscosity loss was experienced when uncatalyzed sodium sulfite was added to a brine solution prior to polymer hydration. After 20 hours of hydration under an argon atmosphere, sodium dithionite was added to the solution. The scavenging of oxygen when sodium sulfite was added prior to polymer was quicker and more extensive and resulted in higher viscosity than that observed when it was added after the polymer.

Additional experiments in Na2CO3 revealed that the viscosity loss encountered when dithionite is added to polymer solutions is also lower in the presence of Na2CO3 (8.5%) than in the presence of NaCl alone (14.7%).

Solutions of polymer to which only sodium sulfite was added experienced significant degradation upon the addition of iron(III) chloride.

Conclusions We have tested a number of commercially available polymers using filtration and viscosity testing as a preliminary screening step for enhanced oil recovery applications, followed by surfactant compatibility, and chemical and thermal stability testing. All of the commercially available polymers tested aside from a hydrophobically modified HPAM passed filtration testing through a 1.2 micron filter. Several high molecular weight polymers exhibited high viscosities at salinities up to 170,000 ppm NaCl and with greater than 17,000 ppm CaCl2 present.

Polyacrylamide polymers hydrolyze at high temperatures and beyond a certain point are subject to precipitation by calcium. If calcium concentrations can be kept below about 200 ppm, the use of polyacrylamide polymers is feasible up to reservoir temperatures of at least 100 °C. For higher concentrations of calcium, copolymers including AMPS moieties should be considered.

After extended aging at 85 °C, the calcium tolerance of poly(AM-co-AMPS) (AN-125) is about four times as high as a comparable HPAM polymer. Although amide hydrolysis is limited by inclusion of AMPS moieties, total anionicity (AMPS plus AA) resulting from aging at 85 °C for 200 days was approximately the same. Precipitation of hydrolyzed poly(AM-co-AMPS) with divalent cations is still possible, although the amount of calcium tolerated is much greater for a 20-30% AMPS copolymer. Thus poly(AM-co-AMPS) may be desirable in situations where 200 ppm or more of divalent ions will be present and moderate or extensive hydrolysis is expected. However, screening studies should first be performed to determine the amount of AM and AMPS moieties that will be hydrolyzed during aging for a given temperature, pH and initial composition, and tolerance to divalent cations should be determined for this resulting copolymer.

Calcium tolerance of hydrolyzed polyacrylamide polymers can be increased by adding sodium metaborate (Na2B2O4·8H2O), but if more than about 0.6 wt% is included the benefit becomes negligible.

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8 SPE 113845

Sodium carbonate improves the thermal stability of polymer solutions, resulting in less viscosity loss from radical attacks. The mechanism of this improved stability has not yet been fully elucidated. Polymer solutions with sodium dithionite added are thermally stable at temperatures up to at least 100 °C over long periods of time provided no further oxygen is added to the solution. An initial viscosity loss of 10-25% is observed and this is proportional to the amount of dissolved oxygen in solution. This can be minimized by adding sodium carbonate or by using uncatalyzed sodium sulfite to scavenge some of the oxygen prior to polymer addition.

Acknowledgements The authors wish to thank the industrial affiliates of the chemical enhanced oil recovery project at the University of Texas for funding of this research. We would also like to thank Larry Britton, Stephane Jouenne, Michel Duc, Jacques Kieffer, Nicolas Gaillard, Ludwig Gil, and R. Ravikiran for very helpful dialog. We are grateful for samples provided by Hengju, SNF Floerger, and Stepan Chemical Co. We are equally thankful for NMR services provided by Dr. Ben Shoulders and Steve Sorey in the UT Chemistry and Biology Analytical Service Laboratory as well as Mike Ronalter in the glass shop of the same department. We would like to thank the support staff of the Center for Petroleum and Geosystems Engineering and in particular Joanna Castillo. Finally, we thank Adam Jackson, Seun Magbagbeola, Mike Weatherl, Ryan Taylor, Josh Whitney, Ghazal Dashti, and Will Slaughter for work performed in the laboratory in conjunction with these experiments.

References Afanas'ev, I.B. 1989. Superoxide Ion: Chemistry and Biological Implications Volume I Boca Raton: CRC Press, Inc. Alfassi, Z. ed. 1997. Peroxyl Radicals West Sussex, England: John Wiley & Sons Ltd. Alfassi, Z. ed. 1999. S-centered Radicals West Sussex, England: John Wiley & Sons Ltd. Amonette, J.E., Szecsody, J.E., Schaef, H.T., Templeton, J.C., Gorby, Y.A., and Fruchter, J.S. 1994. Abiotic Reduction of Aquifer

Materials by Dithionite: A Promising In-situ Remediation Technology. Presented at the Thirty-Third Symposium on Health & the Environmental In Situ Remediation: Scientific Base for Current & Future Technologies, Richland, WA 7-11 November, Contract No. DE-AC06-76RLO 1830, US DOE.

Anbar, M., Meyerstein, D., and Neta, P. 1966. Reactivity of Aliphatic Compounds towards Hydroxyl Radicals. J. Chemical Society B: Physical Organic 742-747.

Audibert, A. and Lecourtier, J. 1992. Compatibility of Hydrosoluble Polymers With Corrodible Materials. SPEPE 7 (2): 193-198. SPE 21026.

Beylerian, N.M. and Asaturyan, M.Z. 2004. On the Mechanism of Hydrogen Peroxide Decomposition in Alkaline Medium. Oxidation Communications 27 (2): 263-273.

Carvalho, L.M. and Schwedt, G. 2001. Polarographic determination of dithionite and its decomposition products: kinetic aspects, stabilizers, and analytical application. Analytica Chimica Acta 436: 293-300.

Doe, P.H, Moradi-Araghi, A., Shaw, J.E., and Stahl, G.A. 1987. Development and Evaluation of EOR Polymers Suitable for Hostile Environments-Part 1: Copolymers of Vinylpyrrolidone and Acrylamide. SPERE 461-467.

Dunlop, A.K. 1973. Corrosion Inhibition in Secondary Recovery. In Corrosion Inhibitors, ed. C.C. Nathan, 76-88. Houston: NACE. Fenton, H.J.H. 1894. Oxidation of tartaric acid in presence of iron. J. Chem. Soc. 65: 899-910. Flataan, A. 2007. Experimental Study of Microemulsion Characterization and Optimization in Enhanced Oil Recovery: A Design

Approach for Reservoirs with High Salinity and Hardness. Master’s Thesis, University of Texas, Austin, Texas. Fossey, J., Lefort, D. and Sorba J. 1995. Free Radicals in Organic Chemistry Paris: Masson. Grollmann, U. and Schnabel, W. 1982. Free Radical-Induced Oxidative Degradation of Polyacrylamide in Aqueous Solution. Polymer

Degradation and Stability 4: 203-212. Hem, J.D. 1961. Stability Field Diagrams as Aids in Iron Chemistry Studies. J. AWWA 53 (2): 211-232. Ikegami, A. and Imai, N. 1962. Precipitation of Polyelectrolytes by Salts. J. Polymer Science 56: 133-152. Istok, J.D., Amonette, J.E., Cole, C.R., et. al. 1999. In Situ Manipulation by Dithionite Injection: Intermediate-Scale Laboratory

Experiments. Ground Water 37 (6): 884-889. Jackson, A. 2006. Experimental Study of the Benefits of Sodium Carbonate on Surfactants for Enhanced Oil Recovery. Master’s Thesis,

University of Texas, Austin, Texas. Janzen, E.G. 1972. Electron Spin Resonance Study of the SO2

.- Formation in the Thermal Decomposition of Sodium Dithionite, Sodium and Potassium Metabisulfite, and Sodium Hydrogen Sulfite. J. Physical Chemistry 76 (2): 157-162.

Kheradmand, H. 1987. Contribution A L'Etude De La Degradation Et La Stabilisation De Polyacrylamides En Solution Aqueuse. These Pour Obtenir Le Grade De Docteur D'Etat Es Sciences Physiques, Universite Louis Pasteur De Strasbourg, Strasbourg, France.

Kheradmand, H., Francois, J., and Plazanet, V. 1988. Degradation of Acrylamide-Sodium Acrylate Copolymer in Aqueous Solution. J. Applied Polymer Science 36: 1583-1600.

Kulicke, W.M. and Horl, H.H. 1985. Preparation and characterization of a series of poly(acrylamide-co-acrylates), with a copolymer composition between 0-96.3 mol-% acrylate units with the same degree and distribution of polymerization. Colloid & Polymer Science 263: 530-540.

Levitt, D. B. 2006. Experimental Evaluation of High Performance EOR Surfactantsfor a Dolomite Oil Reservoir. Master’s Thesis, University of Texas, Austin, Texas.

Liao, C.H., Kang, S.F., and Wu, F.A. 2001. Hydroxyl radical scavenging role of chloride and bicarbonate ions in the H2O2/UV process. Chemosphere 44: 1193-1200.

Martin, F.D., Hatch, M.J., Shepitka, J.S., and Ward, J.S. 1983. Improved Water-Soluble Polymers for Enhanced Recovery of Oil. Paper SPE 11786 presented at the International Symposium on Oilfield and Geothermal Chemistry, Denver, CO 1-3 June.

Miron, R.L. 1981. Removal of Aqueous Oxygen by Chemical Means in Oil Production Operations. Materials Performance 45-50.

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Moad, G. and Solomon D.H. 2006. The Chemistry of Radical Polymerization second fully revised edition Oxford, England: Elsevier. Moradi-Araghi, A. and Doe, P.H. 1987. Hydrolysis and Precipitation of Polyacrylamides in Hard Brines at Elevated Temperatures. SPERE

189-198. Moradi-Araghi, A., Cleveland, D.H., Jones, W.W., and Westerman, I.J. 1987. Development and Evaluation of EOR Polymers Suitable for

Hostile Environments: II-Copolymers of Acrylamide and Sodium AMPS. Paper SPE 16273 presented at the International Symposium on Oilfield Chemistry, San Antonio, TX 4-6 February.

Muller, G. 1981a. Thermal Stability of High-molecular-weight Polyacrylamide Aqueous Solutions. Polymer Bulletin 5: 31-37. Muller, G. 1981b. Thermal Stability of Polyacrylamide Solutions: Effect of Residual Impurities in the Molecular-weight-degradation

Process upon Heating. Polymer Bulletin 5: 39-45. Nikolaev, A.V. and Chelischeva, A.G. 1940. The 25° isotherm of the systems: CaO-B2O3-H2O and MgO-B2O3-H2O. Doklady Akademii

Nauk SSSR, Seriya A 28: 127-130. Parker, W.O. and Lezzi, A. 1993. Hydrolysis of sodium-2-acrylamido-2-methylpropanesulfonate copolymers at elevated temperature in

aqueous solution via 13C n.m.r. spectroscopy. Polymer 34 (23): 4913-4918. Ramsden, D.K. and McKay, K. 1986. The Degradation of Polyacrylamide in Aqueous Solutions Induced by Chemically Generated

Hydroxyl Radicals: Part II - Autoxidation of Fe2+. Polymer Degradation and Stability 15: 15-31. Rice, R.G. and Wilkes, J.F. 1994. Ozone chemistry applied to cooling tower water treatment. Chemical Oxidation 2: 78-111. Rinker, R.G., Lynn, S., Mason, D.M., and Corcoran, W.H. 1965. Kinetics and Mechanisms of the Thermal Decomposition of Sodium

Dithionite in Aqueous Solution. I&EC Fundamentals 4 (3): 282-288. Ryles, R.J. 1983. Elevated Temperature Testing of Mobility Control Reagents. 12008 Paper SPE 12008 presented at the SPE Annual

Technical Conference and Exhibition, San Francisco, CA 5-8 October. Shupe, R.D. 1981. Chemical Stability of Polyacrylamide Polymers. JPT 1513-1529. Sorbie, K. S. 1991. Polymer-Improved Oil Recovery Boca Raton, FL: CRC Press, Inc. Taylor, K. S. and Nasr-El-Din, H. A. 1995. Water-Soluble Hydrophobically Associating Polymers for Improved Oil Recovery: A Literature

Review. Paper SPE 29008 presented at the International Symposium on Oilfield Chemistry, San Antonio, TX 14-17 February. Wellington, S.L. 1983. Biopolymer Solution Viscosity Stabilization-Polymer Degradation and Antioxidant Use. SPEJ 901-912. Yang, S.H. and Treiber, L.E. 1985. Chemical Stability of Polyacrylamide Under Simulated Field Conditions. Paper SPE 14232 presented at

the SPE Annual Technical Conference and Exhibition, Las Vegas, NV 22-25 September. Zaitoun, A. and Potie, B. 1983. Limiting Conditions for the Use of Hydrolyzed Polyacrylamides in Brines Containing Divalent Ions. Paper

SPE 11785 presented at the International Symposium on Oilfield and Geothermal Chemistry, Denver, CO 1-3 June.

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Table 1—Suitability of Different Solvents for EOR

Solvent Flash point (°C)

K ●OH (X 10-8 LM-1s-1(Anbar 1966))

Cost ($/lb)

IPA 12 12.5 0.50 IBA 28 21 0.65 EGBE 65 - 0.65 DGBE 110 - 0.90

Table 2—Commercial Polymers Tested

Polymer Vendor MW X 10-6 Description Flopaam 3630S SNF Floerger 20 HPAM Flopaam 3330S SNF Floerger 8 HPAM AN-125 SNF Floerger 8 Poly(AM-co-AMPS), 20-30% AMPS SuperPusher SAV301 SNF Floerger 5 Confidential; PAM modified for enhanced thermal stability SuperPusher B192 SNF Floerger Unknown Hydrophobically modified PAM Hengfloc 63020 Hengju 20 Post hydrolyzed PAM Hengfloc 63026 Hengju 26 Post hydrolyzed PAM Kypam 5 Hengju Unknown Confidential; "comb" polymer Note: Molecular weights are approximate, as stated by suppliers, and have not been verified.

Flopaam 3330S and 3630S contain a confidential low-molecular weight antioxidant. An additional series of copolymers were supplied by SNF Floerger with degrees of hydrolysis ranging from 0 to 100 for testing their calcium and salinity tolerance.

Table 3—Preliminary Screening Results of Commercial Polymers

Polymer Mixing Time Filter Ratio Surfactant Compatibility

2 days (Lab) <1.2 Flopaam 3330S 90 min (Field) <1.1

Pass

Flopaam 3630S 4-5 days 1.2-1.4 Pass AN-125 4-5 days 1.2-1.4 Pass SuperPusher B192 N.A. Plugs Filter Causes viscosity loss SuperPusher SAV301 2 days <1.2 Untested KKYPAM 5 4-5 days 1.49 Untested Hengfloc 63026 11 days 1.31 Untested Hengfloc 63020 5 days 1.26 Untested

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At elevated temperatures and/or

high pH...

Hydrolysis of amide proceeds;

HPAM-> PAA

Creating less calcium tolerant polymers

+ Ca++

1τ →

H2C CH

C

NH2

O

H2C CH

C

O–

O

x y

H2O NH3

H+

H2C CH

C

O–

O

yH+

H2C CH

C

O–

O

xH+

Fig. 1—Hydrolysis of HPAM lowers calcium tolerance.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 20 40 60 80 100 120 140 160Temperature, °C

Deg

ree

of H

ydro

lysi

s

PAM (Moradi 1987a, 100Days)PAM-co-AMPS; 65/35 mol%(Moradi 1987b, 100 Days)HPAM (Authors, 220 Days)

PAM-co-AMPS (Authors,220 Days)

Fig. 2—Degree of hydrolysis after extended aging.

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Reservoir

Improved stability with iron

Protonation of carboxylate

Na2CO3 NaOH

NaBO2

7 8 9 10 1165 127 8 9 10 1165 12

Increasing rate, extent of hydrolysis Basic hydrolysis

M++(OH)2precipitation

pH

Relevant pKa’s:

RCOOH/RCOO- HCO3-/CO3

2-NH4+/NH3

Fig. 3—pH is a critical factor affecting polymer stability.

0

10

20

30

40

50

60

70

80

0 60,000 120,000 180,000NaCl Concentration, ppm

Vis

cosi

ty, c

P

20M Dalton HPAM (Flopaam 3630S)8M Dalton HPAM (Flopaam 3330S)20M Dalton HPAM (Hengfloc 63020)26M Dalton HPAM (Hengfloc 63026)'Comb' Polymer (Kypaam 5)

Fig. 4—Salinity tolerance of some commercially available polymers.

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SPE 113845 13

0

5

10

15

20

25

30

35

40

45

50

0 60,000 120,000 180,000Total Dissolved Solids, ppm

Vis

cosi

ty, c

P

NaCl only

9:1 NaCl/CaCl2

Fig. 5—Effect of brine hardness on salinity tolerance of 20M Dalton HPAM (Flopaam 3630S).

1

10

100

0 60,000 120,000 180,000NaCl Concentration

Vis

cosi

ty, c

P

30% hydrolyzed HPAM (Flopaam 3330S)

0% Hydrolyzed (PAM; FA920SH)

70% hydrolyzed HPAM

100% hydrolyzed (PAA)

Fig. 6—Relationship between degree of hydrolysis and salinity tolerance for 8M Dalton HPAM.

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0

200

400

600

800

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000Na+ Concentration, ppm

Ca+

+ C

once

ntra

tion,

ppm

τ = 0.66τ = 1τ = 0.45

Fig. 7—Effect of degree of hydrolysis on calcium tolerance.

0

200

400

600

800

1000

1200

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000Na+ Concentration, ppm

Ca+

+ C

once

ntra

tion,

ppm

NaClNa2B2O4

Fig. 8—Effect of sodium metaborate on calcium tolerance of hydrolyzed PAM.

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SPE 113845 15

0

5

10

15

20

25

30

0 50 100 150 200 250Time, days

Vis

cosi

ty, c

P

20M Dalton HPAM (Flopaam 3630S)

8M Dalton PAM-co-AMPS (AN-125)

τ + σ = 0.63

τ = 0.68

1500 ppm polymer, 30,000 ppm NaCl

Fig. 9—Stability of commerical polymers at 85 °C with sodium dithionite.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 20 40 60 80 100 120 140 160Temperature, °C

Deg

ree

of A

nion

icity

( σ+ τ

)

PAM (Moradi '87(a))

PAM-co-AMPS, 65/35 mol%(Moradi 1987b, 100 Days)HPAM (Authors, 220 Days)

PAM-co-AMPS (Authors, 220Days)

Fig. 10—Total anionicity after extended aging.

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16 SPE 113845

0

500

1000

1500

2000

2500

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000Na+ Concentration, ppm

Ca+

+ C

once

ntra

tion,

ppm

HPAM

Hydrolyzed PAM-co-AMPS

τ + σ = 0.67

τ = 0.66

Fig. 11—Effect of the presence of 20-30% AMPS moieties on the calcium tolerance of extensively hydrolyzed polymers, 23 °C.

0

2

4

6

8

10

12

0 1,000 2,000 3,000 4,000 5,000 6,000Calcium Concentration, ppm

Vis

cosi

ty, c

P

Extensively Hydrolyzed PAM-co-AMPS(Hydrolyzed AN-125, )Extensively Hydrolyzed HPAM

1500 ppm polymer, 20,000 ppm NaCl, 23 °C

Cloudy

τ + σ = 0.67

(τ = 0.66)

Fig. 12—Viscometric comparison of precipitation phenomena of extensively hydrolyzed PAM-co-AMPS and HPAM.

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0

5

10

15

20

25

0 50 100 150 200 250Time, days

Vis

cosi

ty, c

P20M Dalton HPAM (Flopaam 3630S)8M Dalton PAM-co-AMPS(AN-125)5M Dalton Modified PAM* (SAV 301)

τ = 11500 ppm polymer, 30,000 ppm NaCl

1500 ppm polymer, 30,000 ppm NaCl

2000 ppm polymer, 30,000 ppm NaCl

*Composition Confidential

Fig. 13—Stability of polymers at 100 °C with dithionite.

0

5

10

15

20

25

30

35

0 10 20 30 40 50Time, days

Vis

cosi

ty, c

P

NaCl Fe++Na2CO3 Fe++NaCl Fe+++Na2CO3 Fe+++

1500 ppm PAA, 23 °C

Fig. 14—Thermal stability acrylic backbone to radical attack in the presence of 8ppm O2, 20 ppm Fe(II) or Fe(III) and 1% NaCl or Na2CO3.

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18 SPE 113845

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30Time, days

Vis

cosi

ty, c

P

0.1M Na2CO3, pH-10

0.1M Na2B2O4, pH-9.9

0.1M NaCl, pH-6.5

0.1M NaOH/0.1M NaCl, pH-9.1

1500 ppm PAA

Fig. 15—Thermal stability acrylic backbone to radical attack in the presence of various salts at 100 °C with 8ppm O2 present.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6 7Original Dissolved Oxygen, ppm

% V

isco

sity

Ret

aine

d

1500 ppm PAM (FA920SH), 23 °C

Fig 16—Effect of original amount of dissolved oxygen on PAM degradation upon addition of 400 ppm sodium dithionite.