Synergistic effects of surfactants on depressurization and augmented injection in high salinity low-permeability reservoirs: Formula development and mechanism study

Abstract To solve the application problem of surfactant in depressurization and augmented injection in high salinity low-permeability reservoirs, through various laboratory experiments, we studied the synergistic effects of surfactants, and a new surfactant formula composed of an anionic surfactant and a non-ionic surfactant for depressurization and augmented injection in high salinity, low permeability reservoirs was acquired. The proposed formula remained stable after 3 months at 85 ℃ and produced ultra-low interfacial tension in high salinity conditions (TDS = 794–65,010 mg/L). With total surfactant concentration of 0.40%, the proposed formula effectively decreased the water-injection pressure and unlocked the remaining oil trapped in small pores (0.05–0.5 µm). With average adsorption loss of 8.25%, the novel formula was capable of reducing the water-injection pressure by 24.77% and recover additional 28.86% oil. Moreover, higher salinity is conducive to a lower interfacial tension, therefore, at higher salinity conditions, lower surfactant concentration may be required and the corresponding adsorption loss, would therefore be lower. According to our study, the developed formula lowered injection pressure and improved oil recovery mainly through interfacial tension reduction, emulsification, and wettability alteration. The microscopic functional mechanisms of surfactant mixtures include; (1) adhesion work reduction, boundary fluid activation and free flow area increase; (2) Promote the formation of oil belt and facilitate the displacement process; (3) Increase the oil-water two-phase mobility ratio, reduce the flow resistance and increase sweep efficiency. The test results of the given formula showed that the salinity in water is a key factor affecting the efficiency of depressurization and augmented injection by surfactants, and the concentration as well as the stability of surfactant also showed different rules under different TDS. The synergistic effect between surfactants made the system perform better at high salinity. The findings of this study not only bring more enlightenment on the principle of depressurization and augmented injection when surfactants are used in petroleum development process, but also provide a basis for the establishment of high- performance surfactant depressurization and injection system.