Long Term Stability of Acrylamide Based Polymers during Chemically Assisted CO2 WAG EOR
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CO2 flooding often results in poor sweep efficiency due to the high mobility ratio caused by its low viscosity. To mitigate this problem alternate injection of water and CO2 slugs, known as the water-alternating-gas process (WAG) is widely applied. Recently, numerical simulation and core flood experiments indicate that the use of chemicals in the water slug may improve mobility control during WAG, thus increasing the ultimate recovery and reducing the requirements for newly purchased or recycled CO2. Therefore, stability studies of common polymers used for EOR applications in CO2 saturated environments becomes necessary to address the technical and economic feasibility of this process. In this paper, we report the results of two commonly used EOR polymers, a co-polymer of acrylamide and acrylate and a co-polymer of acrylamide and ATBS (2-acrylamido-terbutylsulfonic acid). To establish a base line for comparison, parallel experiments were conducted in two different oxygen free environments: one with CO2 and the other with nitrogen. Samples were hydrated and aged at reservoir temperature over 328 days. To isolate the effect of CO2 on the polymer, the water used for their hydration was stripped out of dissolved oxygen and the divalent cation concentration was decreased to reduce thermal and chemical degradation. Polymer samples were distributed in several bottles and removed at different times, and their apparent viscosity was measured as a function of shear rate and fitted to a power law model. The ability of the polymer solutions to retain their original apparent viscosity over time was used to quantify polymer degradation. The results of this work show that CO2 impacts polymer stability, causing further degradation in both polymers tested when compared with the solutions aged with N2. The co-polymer of acrylamide and ATBS exhibited higher resistance to CO2 degradation as it was able increase its viscosity to 104 % of its initial apparent viscosity after 328 days, compared with the co-polymer of acrylamide and acrylate which only retained 54 % of its initial apparent viscosity after 215 days. Chemical degradation, hydrolysis and pH that affected the behavior of apparent viscosity over time also are addressed. We conclude that commercial polymers can be used during chemically assisted CO2 WAG when low divalent cation water is used at a reservoir temperature of 122 F.
author list (cited authors)
Tovar, F. D., Barrufet, M. A., & Schechter, D. S.