Long term stability of ATBS type polymers for enhanced oil recovery

Abstract In offshore enhanced oil recovery (EOR) applications polymer residence time within the reservoir could be many years. Long term stability (preservation of solution viscosity) within the anaerobic reservoir is a fundamental property of the polymer chemical and consequently highly important for choosing a proper product. For high temperature and/or high salinity reservoirs co-polymers of acrylamide (AM) and acrylamide tertiary butyl sulfonic acid (ATBS) have become common in the industry. The present work aimed at developing generic stability diagrams for such polymers dissolved in seawater (SW). Herein a two-step approach was attempted where stability versus ATBS content was measured at elevated temperature. Thereafter these data were extrapolated to lower temperature. To arrive at a sound extrapolation knowledge of hydrolysis kinetics for the two species, AM and ATBS, was needed. Aqueous solutions of AM-ATBS and AA-ATBS co-polymers were therefore stored at elevated temperature (82–140 °C) and progressive changes in microstructure followed by use of 13 C NMR spectroscopy. A steady increase in polymer stability with ATBS content was observed up to around 70 mol%. From 70 to 80 mol% the robustness sharply improved. A striking observation was that stability loss, defined as For the brines tested herein hydrolysis rate was virtually uninfluenced by hardness/salinity, while viscosity was largely impacted. Hence it was concluded that different viscosity loss in various brines stems from carboxylate complexation equilibria and not altered hydrolysis rates. Temperature extrapolation was found to fit a simple Arrhenius approach and it has been suggested that the function should be sound up to around 70 mol% ATBS. With this methodology, the current dataset enabled the construction of a stability diagram for ATBS polymers (0–70 mol% ATBS) dissolved in seawater.