Effect of Dissolved Iron and Oxygen on HPAM Stability

This report describes an experimental study of the stability of an HPAM polymer and an HPAMATBS terpolymer in the presence of varying initial levels of dissolved oxygen (0 to 8000 parts per billion, ppb), Fe2+ (0 to 220 parts per million, ppm), and Fe3+ (0 to 172 ppm). A special method was developed to attain and confirm dissolved oxygen levels. Stability studies were performed at 23°C and 90°C. For Fe2+ concentrations between 0 and 30 ppm, viscosity losses were insignificant after one week so long as the initial dissolved oxygen concentration was 200 ppb or less. Above this level, significant viscosity losses were seen, especially if iron was present. If the temperature is high, a greater need arises to strive for very low dissolved oxygen content. For samples stored for one week at 90°C with only 10-ppb initial dissolved oxygen, contact with steel caused HPAM-AMPS solution viscosity losses greater than 30%. In contrast at 23°C, contact with steel caused no significant degradation so long as the dissolved O2 concentration was 1000 ppb or less. Several different methods are discussed to control oxidative degradation of polymers during field applications. We advocate physical means of excluding oxygen (e.g., stopping leaks, better design of fluid transfer, gas-blanketing, gas-stripping) over chemical means. Addition of Fe3+ to polymer solutions caused immediate crosslinking. Since crosslinked polymers were never observed during our studies with Fe2+, we conclude that free Fe3+ was not generated in sufficient quantities to form a visible gel. Introduction During polymer, surfactant/polymer, or alkaline/surfactant/polymer floods, the injected polymer solution must maintain viscosity for a substantial portion of the transit through the reservoir. When both Fe2+ and oxygen are present in aqueous HPAM solutions, redox couples or cycles can substantially degrade polymers (Pye 1967, Shupe 1981, Grollmann and Schnabel 1981, Ramsden and McKay 1986, Levitt et al. 2011a). In the absence of dissolved oxygen and oxidizing agents, HPAM can be quite stable in the presence of ferrous iron [Fe2+] (Shupe 1981, Yang and Treiber 1985, Seright et al. 2010). HPAM can also be reasonably stable in the presence of dissolved oxygen in the absence of Fe2+ and free-radical generating impurities, if certain conditions [oxidation-reduction potential (Eh), pH, brine composition, temperature] are met (Knight 1973, Muller 1981, Levitt et al. 2011a,b). Ramsden and McKay (1986) and Levitt et al. (2011a) observed that pH and Eh are key factors in HPAM/PAM stability in the presence of Fe2+. Polymer degradation is highest at acidic pH values and can be negligible under alkaline conditions. Levitt et al. point out that the pH dependence of polymer stability is closely tied to the pH dependence of iron solubility. Iron solubility can be especially low at higher pH values if carbonate/bicarbonate is present—leading to enhanced polymer stability. Seright et al. (2010) noted that even if ambient levels of dissolved oxygen are present (3-8 parts