Stability of Partially Hydrolyzed Polyacrylamides at Elevated Temperatures in the Absence of Divalent Cations

At elevated temperatures in aqueous solution, partially hydrolyzed polyacrylamides (HPAM) experience hydrolysis of amide side groups. However, in the absence of dissolved oxygen and divalent cations, the polymer backbone can remain stable so that HPAM solutions were projected to maintain at least half their original viscosity for over 8 years at 100°C and about 2 years at 120°C. Within our experimental error, HPAM stability was the same with/without oil (decane). An acrylamideAMPS copolymer (with 25% AMPS) showed similar stability to that for HPAM. Stability results were similar in brines with 0.3% NaCl, 3% NaCl, or 0.2% NaCl + 0.1% NaHCO3. At temperatures of 160°C and above, the polymers were more stable in brine with 2% NaCl + 1% NaHCO3 than in the other brines. Even though no chemical oxygen scavengers or antioxidants were used in our study, we observed the highest level of thermal stability reported to date for these polymers. Our results provide considerable hope for the use of HPAM polymers in enhanced oil recovery at temperatures up to 120°C if contact with dissolved oxygen and divalent cations can be minimized. Calculations performed considering oxygen reaction with oil and pyrite revealed that dissolved oxygen will be removed quickly from injected waters and will not propagate very far into porous reservoir rock. These findings have two positive implications with respect to polymer floods in high-temperature reservoirs. First, dissolved oxygen that entered the reservoir prior to polymer injection will have been consumed and will not aggravate polymer degradation. Second, if an oxygen leak (in the surface facilities or piping) develops during the course of polymer injection, that oxygen will not compromise the stability of the polymer that was injected before the leak developed or the polymer that is injected after the leak is fixed. Of course, the polymer that is injected while the leak is active will be susceptible to oxidative degradation. Maintaining dissolved oxygen at undetectable levels is necessary to maximize polymer stability. This can readily be accomplished without the use of chemical oxygen scavengers or antioxidants. Introduction In chemical flooding applications for enhanced oil recovery, polymers are needed to provide effective sweep efficiency and mobility control. Depending on injection rates, formation permeability, and well spacing, the polymers must be stable for many years at reservoir conditions. Two chemical species are known to critically impact stability for partially hydrolyzed polyacrylamides (HPAM): divalent cations and oxygen. Effect of Divalent Cations. HPAM polymers are known to be unstable at elevated temperatures if divalent cations are present (Davison and Mentzer 1982, Zaitoun and Potie 1983, Moradi-Araghi and Doe 1987, Ryles 1988). For temperatures above 60oC, acrylamide groups within the HPAM polymer experience hydrolysis to form acrylate groups. If significant concentrations of divalent cations (especially Ca) are present, HPAM polymers can precipitate if the fraction of acrylate groups (i.e., the degree of hydrolysis) in the polymer becomes too high. These facts limit the utility of HPAM polymers for many potential EOR applications in warmer reservoirs. Moradi-Araghi and Doe (1987) indicated hardness limits in brines for various temperatures: 2,000 mg/L for 75°C, 500 mg/L for 88°C, and 270 mg/L for 96°C. For brines containing less than 20 mg/L divalent cations, they suggest that polymer hydrolysis and precipitation will not be a problem for temperatures of 204°C or greater. A few reservoirs exist that have low-cation resident formation brines that still allow the use of HPAM polymers, even though the reservoir temperature is relatively high (Tielong et al. 1998, Santoso et al. 2003). This precipitation problem can be overcome by copolymerizing acrylamide with monomer groups (such as AMPS or vinylpyrrolidone) that resist hydrolysis (Doe et al. 1987, Moradi-Araghi et al. 1987). These polymers have significantly improved resistance to precipitation; however, they are noticeably more expensive and less efficient viscosifiers than HPAM. Maitin (1992) and Sohn et al. (1990) proposed a concept that could considerably widen the applicability of HPAM polymers. They described polymer floods in the German Oerrel and Hankensbuettel fields that had resident brine salinities around 17% total dissolved solids (TDS). Because HPAM is not an efficient viscosifier in saline brines, polymer solutions were prepared and injected in fresh water. Conventional wisdom at the time argued that this process would not be effective because the saline formation water would mix with the low-salinity polymer solution, substantially decrease its viscosity, and compromise sweep. However, Maitin demonstrated that if the mobility of the injected polymer formulation was low enough, the fresh water polymer bank could maintain its integrity during displacement of oil in a reservoir with saline brine. In concept, this idea could be extended to application of HPAM solutions in hot reservoirs with saline, high-hardness brines. If the mobility of a low-hardness HPAM solution is sufficiently low, the polymer bank will displace oil and brine ahead of it with minimum mixing. Even though the HPAM in the polymer bank may experience hydrolysis with time, it will remain an effective viscosifier because there are insufficient divalent cations present to precipitate the polymer. Depending on circumstances, ion exchange from clays could allow dissolution of significant concentrations of divalent cations (Pope et al. 1978, Lake 1989). To avoid HPAM precipitation, the release of divalent cations from clays must be understood and controlled. Understanding divalent cation release requires characterization of the type, quantity, and current divalent-cation loading of clays present and the influence of polymer adsorption on the clays. Controlling the release of divalent cations may be accomplished by maintaining a fixed ratio of monovalent to divalent cations in the injection water (Lake 1989), which would require injection of low salinity water to keep the divalent cation concentration low (e.g., below 20 ppm). Other concepts that have been considered include (1) preconditioning the clays using a preflush and (2) polymer adsorption onto clays to slow cation release. What is the upper temperature limit for HPAM use in chemical EOR? •Above 60°C, acrylamide groups hydrolyze to form acrylate groups. • If the degree of hydrolysis is too high and too much Ca2+ or Mg2+ is present, HPAM polymers precipitate. Temperature, °C: 75 88 96 204 Max Ca2+ + Mg2+, mg/L: 2000 500 270 20 (from Moradi-Araghi and Doe, 1987) Effect of Dissolved Oxygen. The presence of dissolved oxygen by itself may not be detrimental to the stability of HPAM polymers (Knight 1973, Muller 1981). However, HPAM polymers can experience severe degradation by free radical attack if oxygen combines with metals (especially ferrous iron), residual initiators (remaining from polymerization), or other free radical generating chemicals (Knight 1973, Shupe 1981, Muller 1981, Yang and Treiber 1985). Fortunately, most reservoirs have a reducing environment, and produced waters typically contain no detectable dissolved oxygen. With good management of surface facilities (inert gas blanketing, minimizing leaks, and gas stripping where necessary), recycled produced water can be used to prepare EOR solutions that are oxygen free. If necessary, chemical oxygen scavengers and antioxidants can be used (Shupe 1981, Wellington 1983, Yang and Treiber 1985, Levitt and Pope 2008). However, use of these chemicals (1) are generally less cost-effective than gas stripping for oxygen removal, (2) can accelerate polymer degradation if oxygen is reintroduced after addition of the oxygen scavenger, and (3) can accentuate problems with microbial growth. Past studies have examined HPAM stability in “oxygen free” solutions to 105°C. Shupe (1981) reported a 13% loss of viscosity at 105oC (from 38 to 33 cp) over ~250 days for 2,000 mg/L HPAM (Dow Pusher 500TM) in brine with 3,841 mg/L TDS salinity, including 10 mg/L divalent cations. Ryles (1988) reported a 12% loss of viscosity at 90oC (from 11.7 to 10.3 cp) over ~580 days for 1,000 mg/L HPAM in brine with 1% NaCl and 0.4% sodium orthosilicate. Approaches to mitigating HPAM precipitation: 1. A few hot reservoirs exist with low hardness waters. 2. Hydrolysis-resistant monomers (AMPS, NVP) can be incorporated into PAM polymers (Moradi-Araghi, Doe 1987). They tend to be expensive and less effective as viscosifiers. 3. Fresh water HPAM solutions can provide efficient sweep with minimum mixing with saline brines if polymer mobility is sufficiently low (Maitin 1992). Requires that cation exchange be understood and controlled.