Modeling of Pressure and Solution Gas on Microemulsion Phase Behavior

The effect of pressure and solution gas in crude can significantly change the optimal salinity of a surfactant flood (SP). The shift in the optimal salinity must currently be measured using high pressure experimental equipment, which is difficult to perform. This paper presents a novel approach to predict optimum salinity for live crudes at high pressure based solely on optimal salinity and solubilization ratio measurements made at atmospheric pressure. We use the well established relationship that the logarithm of the optimum salinity is a linear function of the equivalent alkane carbon number (EACN) at atmospheric pressure. The procedure corrects that data for the effect of pressure and methane content in the oil. We compare our predicted results to all high-pressure data available in the literature. We show that the linear trend of the logarithm of optimum salinity with EACN can be accurately corrected to higher pressures based solely on alkane density changes at atmospheric pressure. A similar correction can also be made for the optimum solubilization ratio. The optimal salinity for live crude is determined from the calculated EACN for the live oil, and the calculated optimum salinity versus EACN relationship at the pressure of interest. The results very accurately predict and explain all measured data in the literature, including that of Puerto and Reed (1983). We further incorporate the method in UTCHEM, where the optimum can vary spatially based on calculated methane content (using a cubic EOS) and pressure.