Development of Surfactant-Polymer SP Processes for High Temperature and High Salinity Carbonate Reservoirs

Abstract

Oil recovery during waterflooding of carbonate reservoirs is often low due to their oil-wetness and heterogeneity. Surfactant-Polymer (SP) flooding can improve the oil recovery from these reservoirs through ultra-low interfacial tension (IFT), mobility control and wettability alteration. However, there are several challenges associated with this process in high salinity and high temperature carbonate reservoirs related to thermal stability of polymers at elevated temperatures, compatibility of surfactants with high concentration of divalent cations present in formation brines, and geochemical interactions with carbonate minerals. This paper addresses the following challenges: surfactant interaction with formation brine containing high concentration of divalent cations and thermal stability and transport of polymers in carbonate rocks at a high temperature (80 C). Surfactant phase behavior experiments were performed to identify promising surfactant candidates which showed ultralow IFT with crude oil and aqueous stability at high temperature in high salinity and high hardness brines. A systematic study was performed to understand the effect of surfactant hydrophobe length on phase behavior, oil recovery, and surfactant retention in coreflood experiments. Novel surfactants with very short hydrophobes and cosolvent-like properties were also included to further optimize the phase behavior. Surfactants of larger hydrophobe length, containing similar number of EO and PO groups, gave higher solubilization ratio (and lower IFT) and lower optimum salinity. Specialty synthetic polymers with good thermal stability and salinity tolerance (TDS > 90,000 ppm) were investigated for their transport in single-phase corefloods. Results showed successful transport of polymer, without degradation in-situ, and improvement in mobility control. SP core floods were conducted using selected formulations in Indiana limestone cores. Coreflood experiments showed small increases in oil recovery over waterflood after the injection of the chemical formulation. Succesful polymer transport was observed in SP corefloods at high temperature.