Stability and Behavior in Carbonate Cores for New Enhanced-Oil-Recovery Polymers at Elevated Temperatures in Hard Saline Brines

Randall S. Seright, Kathryn E. Wavrik, Guoyin Zhang, Abdulkareem M. AlSofi – 2021

Abstract

This study identifies viable polymers for polymer flooding of high-temperature carbonate reservoirs with hard, saline brines. New ATBS polymers with high degree of polymerization, scleroglucan, NVP-based polymers, and hydrophobic associative polymers were examined for stability over a 2-year period under oxygen-free conditions at temperatures up to 180°C in brines with TDS from 0.69% to 24.4%, including divalent cations from 0.034% to 2.16%. Arrhenius analysis — a novel feature of this study — was used to project viscosity half-lives without waiting decades for conventional stability results. A set of ATBS polymers showed viscosity half-lives over 5 years at 120°C and over 50 years at 99°C — a major advance for extending polymer flooding to higher temperatures. Five promising polymers were evaluated in anaerobic corefloods at 99°C using carbonate cores, examining retention, rheology, mechanical degradation, and residual resistance factor.

Key Takeaways

  • A new class of high-polymerization ATBS acrylamide copolymers achieves a viscosity half-life of over 5 years at 120°C and over 50 years at 99°C — a major advance that extends viable polymer flooding to higher-temperature reservoirs without oxygen scavengers or chelating agents.
  • Arrhenius analysis — running accelerated stability tests at elevated temperatures and projecting long-term behavior — allows reliable viscosity half-life estimates within 2 years rather than waiting decades, enabling rapid polymer selection for high-temperature applications.
  • Scleroglucan shows excellent stability in high-TDS, high-hardness brines at elevated temperatures where HPAM would precipitate due to acrylate-divalent cation interactions — a viable alternative for carbonate reservoirs with saline, hard formation water.
  • NVP-based polymers and hydrophobic associative polymers were also screened — results define their stability windows and identify which reservoir conditions fall within vs. outside their viable operating ranges.
  • Anaerobic corefloods at 99°C in carbonate cores confirmed that the most stable candidate polymers maintained acceptable retention, rheology, mechanical degradation resistance, and residual resistance factor under representative high-temperature carbonate reservoir conditions.
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