An approach to evaluate polyacrylamide-type polymers' long-term stability under high temperature and high salinity environment

Abstract

Water-soluble polymers have been widely used in chemical flooding either independently or as part of surfactant-polymer (SP) and alkaline-surfactant-polymer (ASP) processes. The polymer viscosifies the injected water thereby reducing the displacing fluid's mobility and increasing sweep efficiency. The key to efficient sweep is attaining sustainable mobility control (i.e. maintenance of sufficient viscosity during propagation in the reservoir). Long-term stability is therefore a crucial parameter in the screening of appropriate polymers for application, especially in high temperature and high salinity reservoirs. In general, the evaluation of a polymer solution's long-term stability is a very time-consuming process. There is a need to develop fast and reliable means to assess the feasibility of polymers from a long-term stability standpoint. Different from conventional methods posed in the literature, this paper presents a new facile approach to evaluate the polymers in powder form and identify their molecular decomposition. The approach is correlated and confirmed against conventional long-term stability results obtained from the evaluation of polymer solutions.Thermogravimetric analysis (TGA) was used in this work to study the decomposition of polymers and their individual constituents. The derivative of the TGA curve with respect to temperature, is known as the derivative thermal gravimetric (DTG) thermogram, which can clearly identify differences in decomposition rates of screened polymers. Furthermore, conventional long-term stability tests were performed on polymer solutions prepared in synthetic seawater with a salinity of 57,670 ppm. The solutions were aged at a temperature of 95 °C under anaerobic conditions and monitored by rheological measurements for viscosity loss, total organic carbon (TOC) analyses for material loss, and gel permeation chromatography (GPC) for molecular weight loss.The thermal stability of 12 commercial water soluble polymers was tested in this work. The long-term stability results are consistent with the TGA results. The two polymers showing good thermogravimetric stability exhibited significant viscosity retention in conventional long-term stability tests. TOC and GPC results further supported these TGA results. The developed method provides a fast approach to screen polymer candidates for high temperature and high salinity reservoirs.