Effect of Dissolved Iron and Oxygen on Stability of Hydrolyzed Polyacrylamide Polymers

R.S. Seright, Ingun Skjevrak – 2015

Abstract

This paper describes an experimental study of the stability of HPAM and an HPAM-ATBS terpolymer in the presence of varying dissolved oxygen (0–8,000 ppb), Fe²⁺ (0–220 ppm), and Fe³⁺ (0–172 ppm). At 23°C with Fe²⁺ concentrations between 0 and 30 ppm, viscosity losses were insignificant after one week when dissolved oxygen was 200 ppb or less. Above this level, significant viscosity losses occurred, especially with iron present. At 90°C with only 10 ppb dissolved oxygen, contact with steel caused HPAM-ATBS viscosity losses greater than 30%. At 23°C, contact with steel caused no significant degradation up to 1,000 ppb dissolved oxygen. Fe³⁺ addition caused immediate crosslinking. Physical oxygen exclusion methods are advocated over chemical methods.

Key Takeaways

  • At 23°C with ferrous iron (Fe²⁺) up to 30 ppm, HPAM stability is acceptable as long as dissolved oxygen is kept at or below 200 ppb — above this threshold, significant viscosity loss occurs, especially if iron is present.
  • At 90°C, the oxygen tolerance is dramatically reduced: even at just 10 ppb dissolved oxygen, contact with steel surfaces caused more than 30% viscosity loss in HPAM-ATBS after one week — demonstrating that high-temperature applications require near-zero dissolved oxygen, not just ‘low’ oxygen.
  • Ferric iron (Fe³⁺) caused immediate crosslinking of HPAM — not gradual degradation — and free Fe³⁺ was not generated in sufficient quantities from Fe²⁺ to form visible gels under the conditions tested, confirming Fe²⁺ and Fe³⁺ have distinct degradation mechanisms.
  • At 23°C, contact with steel caused no significant HPAM degradation at dissolved oxygen concentrations up to 1,000 ppb — at low temperature, mild oxygen exposure alone (without sufficient iron catalyst) does not trigger rapid HPAM backbone attack.
  • Physical oxygen exclusion methods (stopping leaks, better fluid transfer design, gas blanketing, gas stripping) are recommended over chemical oxygen scavengers — chemical methods are less reliable and only address oxygen after it has already entered the system.
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