Revisiting the effect of oil type and pressure on optimum salinity of EOR surfactant formulation using phase behavior evaluation

Salinity screening and brine/surfactant/oil phase behavior are normally carried out at ambient pressure using dead oil samples diluted with hydrocarbon solvents to match the oil viscosity at subsurface conditions. However, diluting the crude oil changes its microemulsion behavior. The reported optimum salinity to achieve ultralow interfacial tension changes when gas is present in the oil. The crude oil composition in the same reservoir can vary from one location to another due to pressure changes. Hence, it is mandatory to evaluate the influence of both gas-oil ratio and pressure on the optimum salinity of surfactant formulations. This paper presents an extensive literature review encompassing relevant research works that have been done over the last 50 years on the impact of pressure and live-oil on microemulsion phase behavior. The findings have been summarized in a structured manner and main similarities and contradictions between different papers have been presented. Overall, this review combines an extensive literature review, a comprehensive summary of findings, and a critical analysis to assess the effect of live-oil and pressure on microemulsion phase behavior. The derived conclusions from this review demonstrate that the utilization of live-oil in phase behavior generally contributes to the decrease of the optimum salinity, while the microemulsion phase behavior shifts toward type I with increasing pressure. However, the contradictory results are also discussed and the suggestions to improve the understanding of the combined effect of live-oil and pressure are also presented. The new approach will contribute to a better comprehension of this phenomenon, therefore, improving this area in chemical enhanced oil recovery.