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), 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.