A New Facile Approach to Estimate EOR Polymers Thermal Stability at Harsh Reservoir Conditions

Abstract

Abstract Water-soluble polymers have been widely used in chemical enhanced oil recovery (EOR) either independently or part of surfactant-polymer (SP) and alkaline-surfactant-polymer (ASP) processes. The polymer viscosifies the injected water thereby reducing displacing fluid mobility and sweep efficiency. Key to efficient sweep is attaining a sustainable mobility control (i.e. maintenance of sufficient viscosity during the propagation in the reservoir). Therefore, long-term stability is a crucial parameter in screening of appropriate polymers for EOR application, especially in high temperature and high salinity reservoirs. Generally, the evaluation of polymer solution's long-term stability is time-consuming process. Accordingly, there is a need to develop fast and reliable means to assess the feasibility of polymers from a long-term stability standpoint. Different from the methods 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 on polymer solutions. Thermogravimetric analysis (TGA) was used in this work to study the decomposition of polymers and their individual constituents. The derivative of TGA curve with respect to temperature is known as the DTG, 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 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 thermal stability exhibited significant viscosity retention in conventional long-term stability tests. TOC and GPC results further supported the TGA results. The developed and demonstrated method provides a fast approach to screen polymer candidates for high temperature and high salinity reservoirs.