| Popis: |
When EOR by polymer flooding is applied in the offshore environment, polymer can be expected to face long reservoir retention times of up to several years. The stability and viscosity retention of the polymer needs to be evaluated throughout this time. Copolymers of acrylamide (AM) and acrylamide tertiary butyl sulfonic acid (ATBS) are a potential polymer choice for relatively high temperature and salinity reservoirs. In this study, it was evaluated how the long term stability of AM-ATBS copolymers can be predicted by accelerated aging tests at elevated temperature. The main cause of viscosity loss in AM-ATBS copolymer solutions is the hydrolysis reaction turning AM and ATBS groups into acrylic acid (AA). This hydrolysis was studied in strictly anaerobic conditions at 70-120°C. Besides temperature, the impact of ATBS content in the copolymer (15-55 mol%), pH, salinity and divalent ion concentration were studied. In addition, some tests were done with copolymers of ATBS and AA to elucidate the effect of neighboring monomer on ATBS hydrolysis. The reaction progress was followed by 13C NMR and viscosity measurements. Over a range of conditions, it was observed that AM-ATBS copolymers aged at different temperatures but at otherwise equal conditions, had similar AA/AM/ATBS ratios at equivalent degrees of hydrolysis. Additionally, the shapes of viscosity retention vs. time curves were corresponding in the studied temperature interval after being normalized into the same time scale. It is proposed that AM hydrolysis is the rate determining step, resulting in a similar hydrolysis pathway over a range of temperatures. AM hydrolysis reaction rates were fitted into an Arrhenius function to estimate acceleration at elevated temperatures. The data generated in this study helps to predict how the results from accelerated tests (e.g. at 100-120°C) correlate to stabilities at lower temperature (e.g. actual reservoir temperature at 70-80°C). Specifically, a practical overview of expected time of stability (viscosity retention >80%) has been generated for AM-ATBS copolymers in seawater. |
