Polymer flooding in high temperature, high salinity conditions: Selection of polymer type and polymer chemistry, thermal stability

After 50 years of research and numerous commercial field applications around the world, polymer flooding is considered as a proven EOR technique. However, until recently, its application was limited to reservoirs with temperature not exceeding 100 degrees C. The development of polymer withstanding harsh conditions such as those encountered in Middle East carbonate reservoirs (temperature higher than 100 degrees C and salinity up to 280 g/L), would unlock a lot of reserves. In this paper, thermal stability, limits and application range of biopolymers and acrylamide-based polymers are reviewed. It appears that a new class of acrylamide-based copolymers containing high level of 2-acrylamido-2-methyl propane sulfonate (AMPS) fulfills the main criteria for massive field injection: industrial availability, moderate cost, thermal stability (we present aging tests for which 80% of the viscosity is maintained after 3 years at 120 degrees C in a hard brine) and ability to propagate in a wide range of permeabilities by tailoring the molecular weight. A viscosity model we recently developed is applied for comparing the viscosifying power of hydrolyzed polyacrylamides (HPAM) with a AMPS-rich polymer (SAV10). The latter presents a better salinity tolerance, especially when divalent cations are present and a better viscosifying power in temperature. The model is successfully applied when viscosity is predicted at high temperature from a viscosity measurement at ambient temperature. After reviewing the effect of CO2 and H2S on viscosity and degradation, it is shown that SAV10 viscosity is not sensitive to pH and should remain constant whatever the CO2 content. This insensitivity to pH is taken as an advantage: a method is proposed for minimizing oxidative degradation on solutions sampled manually on the field.