When both Fe(II) and oxygen are present in aqueous HPAM solutions, redox couples can substantially degrade polymers. In the absence of dissolved oxygen and oxidizing agents, HPAM can be quite stable in the presence of Fe(II). HPAM can also be reasonably stable in the presence of dissolved oxygen in the absence of Fe(II) and free-radical generating impurities, if certain conditions [oxidation-reduction potential (ORP), pH, brine composition, temperature] are met. In field applications of chemical flooding, a gray area exists where significant iron (either solid or dissolved) may be present, along with low levels of dissolved oxygen (e.g., <50 parts per billion, ppb). Large differences of opinion exist on how to treat this situation, including (1) removing all iron, (2) removing all dissolved oxygen, (3) addition of free-radical scavengers or adjustment of ORP and/or pH, and (4) no action. This paper describes an experimental study of stability of an HPAM polymer and an HPAM-AMPS terpolymer with varying initial levels of dissolved oxygen (0, 10, 20, 50, 200, 500, 2000, and 8000 ppb), Fe(II) (0, 1, 5, 10, 20, 30, 50, 100, and 500 parts per million, ppm), and Fe(III) (0, 1, 5, 10, 50, 100, and 500 ppm). A special method was developed to attain and confirm dissolved oxygen levels. Stability studies were performed at 23°C and 90°C. With ≤2000 ppb initial O2, the dissolved oxygen dropped to 0 ppb within one week. Our findings reveal that once the available dissolved oxygen is depleted, no further oxidative degradation of polymer occurs. The results were insensitive to Fe(II) concentration for initial O2 ≤2000 ppb. For initial O2 of 8000 ppb, polymer degradation increased significantly with increased Fe(II). Solution viscosity losses were insignificant for O2 concentrations of 500 ppb or less. In the studies with Fe(III), the iron crosslinked the polymer to form a visible gel. However, in the studies with Fe(II), we saw no evidence of gel formation, suggesting when Fe(II) reacted with O2, any Fe(III) that formed was converted to solid iron oxide before any polymer crosslinking could occur. The significance of these observations for field applications will be discussed.
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