SPE 169030 Effect of Dissolved Iron and Oxygen on Stability of HPAM Polymers

This paper describes an experimental study of the stability of an HPAM polymer and an HPAM-ATBS terpolymer in the presence of varying initial levels of dissolved oxygen (0 to 8000 parts per billion, ppb), Fe (0 to 220 parts per million, ppm), and Fe (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 Fe 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 Fe to polymer solutions caused immediate crosslinking. Since crosslinked polymers were never observed during our studies with Fe, we conclude that free Fe 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 Fe 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 [Fe] (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 Fe 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 Fe. 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 per million, ppm), the highly reducing conditions and iron minerals (e.g., pyrite, siderite) in an oil reservoir will usually consume that oxygen within hours or days, even at low temperature. In field applications of some low-to-moderate-temperature chemical floods, no attempt was made to exclude oxygen. In other cases, oxygen removal or exclusion was attempted, but atmospheric leakage occurred so that low levels of dissolved oxygen were present (e.g., < 50 parts per billion, ppb). Large differences of opinion exist on how to treat these situations, including (1) removing all iron (Levitt et al. 2011a), (2) removing all dissolved oxygen (Seright et al. 2010), (3) addition of free-radical scavengers or adjustment of Eh and/or pH (Wellington 1983, Gaillard et al. 2010), and (4) no action (Wang et al. 2008). Several studies characterized HPAM degradation in solutions with both Fe and ambient levels of dissolved oxygen (Grollmann and Schnabel 1982, Ramsden and McKay 1986). A number of investigations have examined HPAM stability with Fe present but without dissolved oxygen (Shupe 1981, Yang and Treiber 1985, Ryles 1988, Seright et al. 2010, Levitt et al. 2011a). Gaillard et al. (2010) examined polymer stability in the presence of 50-ppb and 500-ppb initial dissolved oxygen. 2 R.S. Seright and Ingun Skjevrak SPE 169030 Otherwise, we are not aware of any studies of HPAM stability in the presence of measured, low levels of dissolved oxygen with Fe present. Consequently, this paper describes an experimental study of the stability of an HPAM polymer and an HPAM-ATBS terpolymer in the presence of varying initial levels of dissolved oxygen (0 to 8000 ppb), Fe (0 to 220 parts per million, ppm), and Fe (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. Our hope was that insights from this study would help resolve which of the four approaches mentioned in the previous paragraph is most appropriate. Experimental Two polymers were used in this work, called HPAM and HPAM-ATBS. The HPAM polymer was SNF Flopaam 3630STM. This acrylamide-acrylate polymer had an average molecular weight of 18 million g/mol and 30% anionicity. The HPAMATBS polymer was SNF Flopaam 5115SHTM. This acrylamide-acrylate-ATBS terpolymer had an average molecular weight of 15 million g/mol, 15% ATBS (2-acrylamido-tertbutylsulfonic acid), and had a total anionicity of 25%. Two brines were used. Table 1 lists their compositions. They will be call “11%-TDS brine” and “2.85%-TDS brine”. The 11%-TDS brine describes the formation water associated with a particular polymer flooding field candidate. This brine was used in most experiments. The 2.85%-TDS brine describes alternative polymer make-up water for the target field polymer flood and was used in a few experiments. Brine was filtered through a 0.45-μm Millipore filter before de-oxygenation and polymer addition. Table 1—Brine compositions (ppm). Salt 11%-TDS brine 2.85%-TDS brine