Hydrolysis kinetics of ATBS polymers at elevated temperature, via 13C NMR spectroscopy, as basis for accelerated aging tests

Abstract

In offshore enhanced oil recovery application polymer residence time within the reservoir could be many years. Accelerated laboratory methods, for chemical stability testing, are therefore desired to perform efficient product type screening. For high temperature and/or salinity reservoirs co-polymers of acrylamide (AM) and acrylamide tertiary butyl sulfonic acid (ATBS) have become common in the industry. Simple high temperature storage was, however, not believed to be a viable accelerated test approach for these products. The reason being different hydrolysis rates for AM and ATBS, to acrylic acid (AA), as function of temperature. Hence the microstructure of a polymer aged at for example 120 °C should be significantly different from one stored at 80 °C.To develop an accelerated testing methodology, improved understanding of hydrolysis kinetics is necessary. Aqueous solutions of AM-ATBS and AA-ATBS co-polymers were in the present work therefore stored at elevated temperature (70–140 °C) and progressive changes in microstructure followed by use of 13C NMR spectroscopy. Short chained polymers (Mw ≈ 250 kg/mol) at high concentration (≈11%) were applied to reduce NMR scanning time.In AM-ATBS co-polymers initial AM reaction rate followed a first order kinetic trend. After a significant hydrolysis (>20–30%) the kinetics of both AM and ATBS clearly deviated from first order and showed a distinct pH dependence. This was attributed to interaction with appearing AA neighbouring groups.AA-ATBS co-polymers yielded higher hydrolysis rates than in AM-ATBS polymers, thus proving the intramolecular catalytic effect of AA on ATBS hydrolysis. A marked pH influence indicated the non-dissociated acid to be more efficient in catalysing ATBS removal than the carboxylate.Polymer microstructure remained similar as function of hydrolysis degree in the whole studied temperature interval. This was herein proposed to stem from AM hydrolysis being the rate determining step. From a practical point of view this observation is intriguing as it indicates that simple high temperature storage could in fact be a viable route to accelerated laboratory screening of polymer stability.A noticeable rate difference between pH 6 and pH 7 suggested that historically performed long term aging tests, in un-buffered solutions, may have yielded too optimistic predictions. Hence, a recommendation following the present work would be to critically evaluate whether to include pH buffering in future polymer stability tests.