Chemical Stability Limits of Water-Soluble Polymers Used in Oil Recovery Processes

Abstract

Summary. This work describes long-term thermal stability limits of water-soluble polymers under anaerobic conditions. Polymers investigated included polyacrylamide, xanthan, scleroglucan, cellulose sulfate, and a heteropolysaccharide of unknown structure. The primary mechanism of polyacrylamide degradation was found to be amide group hydrolysis. polyacrylamide degradation was found to be amide group hydrolysis. Interaction between hydrolyzed polyacrylamide and divalent metal ions present in solution caused significant losses in solution viscosity, and present in solution caused significant losses in solution viscosity, and phase separation ultimately occurred in extreme conditions of high degrees phase separation ultimately occurred in extreme conditions of high degrees of hydrolysis or high concentrations of divalent ions. The rate of hydrolysis was found to depend mostly on temperature. At 50 degrees C [122 degrees F], the rate was quite slow and polyacrylamide solutions were stable for many months, even in the presence of high concentrations of divalent ions. At 60 to 70 degrees C [140 to 158 degrees F], the rate of hydrolysis was moderate and the rate of viscosity loss depended on the precise temperature and divalent ion concentration. At 90 degrees C [194 precise temperature and divalent ion concentration. At 90 degrees C [194 degrees F], hydrolysis was rapid and polyacrylamide solutions were stable to precipitation only when the divalent ion concentration was less than about 200 ppm. When the divalent ion concentration was zero, solution viscosity increased because of a further expansion of the polyelectrolyte coil. The stability of xanthan was determined primarily by temperature and was independent of divalent ions. Although performance varies from xanthan to xanthan, the useful limit was generally found to be less than 70 degrees C [less than 158 degrees F]. Viscosity retention was also found to be extremely shear-rate dependent. Other naturally occurring polymers exhibited variable performance. In alkaline brines, polyacrylamides were stable up to 90 degrees C [194 degrees F] for long periods of time, whereas xanthan was degraded at greater than 50 degrees C [ >122 degrees F]. Introduction Many reservoirs have extremely hostile environments that are well outside the limits of many water-soluble polymer applications; therefore, polymer degradation is an important factor in determining the efficiency of chemical EOR processes. Despite the plethora of articles on the the subject, the stability limits of most water-soluble polymers used in EOR remain questionable and unresolved. Although reservoir O2, concentration is the subject of some controversy, it is widely believed to be essentially zero because the environment is reducing in nature. Most work has therefore tried to emulate the anaerobic state, with various degrees of success. In many cases, poor control Of O2 concentration has limited a systeniatic study of degradation mechanisms. Indeed, it has been shown previously that proper control of O, is essential in laboratory previously that proper control of O, is essential in laboratory evaluation of thermal stability, and proper procedures have also been described. In this paper, we present a study of the effects of tem-perature, salinity, divalent ion, and pH on the anaerobic thermal stability of some water-soluble polymers used in EOR applications. In addition to the commercial EOR polymers, polyacrylamide, and xanthan, several other structurally dissimilar water-soluble polymers-scleroglucan, cellulose sulfate, and a heteropoly-saccharide-were polymers-scleroglucan, cellulose sulfate, and a heteropoly-saccharide-were investigated. Experimental Procedures Table 1 describes all polymers used in this study. Commercial Poly-acrylamides A, B, and C were 30, 30, and 26 mol% acrylate, Poly-acrylamides A, B, and C were 30, 30, and 26 mol% acrylate, respectively. Polyacrylamides A and B are copolymers of acrylamide and acrylate salt, whereas Polyacrylamide C was prepared by partial hydrolysis. Molecular weights were estimated by intrinsic partial hydrolysis. Molecular weights were estimated by intrinsic viscosity measurements to be 7 × 10, 14 × 10, and 7 X 10, respectively, for Polyacrylamides A, B, and C. Polymer solutions were prepared by procedures recommended by the appropriate manufacturer. Solution viscosities were determined at 25 degrees C [77 degrees F] with either a Brookfield LVT viscometer fitted with UL adaptor or a Haake CVIOO/RVIOO fitted with a Mooney-Ewart 45 measuring device. (Brookfield readings of 60, 30, 12, 6, and 3 rev/min correspond to shear rates of 73.5, 36.8, 14.7, 7.35, and 3.7 seconds, respectively.) The carboxyl content of hydrolyzed polyacrylamides was determined from the ratio of intensities of absorptions in the carbonyl region of the infrared (IR) spectrum. Polymer solutions were dialyzed before analysis to remove inorganic salts. Thermal stability tests were conducted in an anaerobic environment (about 1 ppb), as described previously. Polymer solutions were aged at the required temperature in sealed glass ampules to ensure exclusion of O2 throughout the experiments. Results and Discussion Thermal Stability of Polyacrylamides in Oilfield Brines Containing Ca2+, Ions. In the absence of oxidative degradation, it has been well established that the backbone chain of vinyl polymers, such as polyacrylamide, is quite thermally stable to temperatures as high as 120 degrees C [248 degrees F]. Indeed, at 90 degrees C [194 degrees F], polyacrylamide was found to be stable for at least 20 months under the controlled conditions shown in Table 2. At elevated temperatures, however, the pendant amide groups tend to hydrolyze, thereby increasing the total carboxylate content of the polymer. This results in significant changes in solution properties, rheology, and phase behavior; changes that are strongly dependent on the nature and concentration of inorganic salts present in solution. Also, it has been widely reported that hydrolyzed polyacrylamides interact strongly with divalent metal cations such as Ca2+ and Mg2+ . This phenomenon is commonly associated with reduction in molecular phenomenon is commonly associated with reduction in molecular dimensions (solution viscosity), or under extreme conditions, with phase separation-i.e., the formation of gels or precipitates. Fig. phase separation-i.e., the formation of gels or precipitates. Fig. 1 shows examples of polyacrylamide precipitation. Table 2 further demonstrates the deleterious effect of Ca2+ on polyacrylamide stability at elevated temperature. At 90 degrees C [194 polyacrylamide stability at elevated temperature. At 90 degrees C [194 degrees F], increasing concentrations of Ca2+ (greater than 500 ppm) caused precipita-tion within a matter of weeks. At lower concentrations ( less precipita-tion within a matter of weeks. At lower concentrations ( less than 200 ppm), a deterioration in solution viscosity was observed over many weeks of aging, even though no physical separation of phases occurred. The phase behavior of polyelectrolyte/metalion complexes depends on critical solution temperature, charge density (degree of hydrolysis), the nature of divalent ion, divalent ion concentration, coelectrolyte, and polymer concentration. SPERE P. 23