Stability of Partially Hydrolyzed Polyacrylamides at Elevated Temperatures in the Absence of Divalent Cations

R.S. Seright, A.R. Campbell, P.S. Mozley, Peihui Han – 2010

Abstract

HPAM solutions at elevated temperatures experience progressive hydrolysis of amide side groups. However, in the absence of dissolved oxygen and divalent cations, the polymer backbone can remain stable — HPAM solutions were projected to maintain at least half their original viscosity for more than 8 years at 100°C and approximately 2 years at 120°C, without chemical oxygen scavengers or antioxidants. HPAM stability was the same with and without oil (decane). An acrylamide-AMPS copolymer with 25% AMPS showed similar stability to HPAM. Results were similar across brines with 0.3% NaCl, 3% NaCl, or 0.2% NaCl + 0.1% NaHCO₃. Above 160°C, polymers were more stable in 2% NaCl + 1% NaHCO₃. Calculations showed dissolved oxygen entering the reservoir before polymer injection will be consumed quickly and will not propagate far — providing two practical protections: pre-injection oxygen depletion and fast recovery from surface oxygen leaks.

Key Takeaways

  • In the absence of dissolved oxygen and divalent cations, HPAM backbone stability is remarkably high — projected to maintain at least half original viscosity for more than 8 years at 100°C and approximately 2 years at 120°C without any chemical stabilizers.
  • Divalent cations (Ca²⁺, Mg²⁺) are the critical co-threat with oxygen: HPAM hydrolysis at high temperature produces acrylate groups that precipitate in the presence of divalent cations, requiring minimization of both divalent ions and dissolved oxygen for high-temperature applications.
  • HPAM and a 25% AMPS acrylamide copolymer showed similar thermal stability in the absence of oxygen and divalents — AMPS incorporation alone does not dramatically extend the stability window without also eliminating divalent cations.
  • Dissolved oxygen injected into the reservoir before polymer injection is consumed by reaction with oil and pyrite quickly and will not propagate far — providing important protection: residual oxygen from pre-polymer injection is self-cleaning before the polymer arrives.
  • Physical means of oxygen exclusion (gas blanketing, leak prevention, gas stripping) provide HPAM stability at high temperature without chemical oxygen scavengers — the highest thermal stability results to date at time of publication were achieved without any chemical stabilizers.
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