Chapter 8 - Optimum Phase Type and Optimum Salinity Profile in Surfactant Flooding

Abstract

A microemulsion can exist in three types of systems depending on salinity—type II(−), which occurs below a certain salinity Csel; type II(+), which occurs above a certain salinity Cseu, or type III, if the salinity is between Csel and Cseu. In a type III system, the interfacial tension (IFT) of microemulsion/brine is lower than that in a type II(+) system, and the IFT of microemulsion/oil is lower than that in a type II(−) system. Optimum salinity is defined as the middle of Csel and Cseu, when the two IFTs are equal. The literature about optimum phase types, the relationship between optimum salinity and surfactant concentration, and optimum salinity gradients is reviewed. A fine core-scale model is used to study the optimum phase type and optimum salinity profile in surfactant flooding. Furthermore, the chapter discusses the effects of kr curves, optimum phase type, and phase viscosity. The effect of negative salinity gradient is further discussed under conditions where different relationships between optimum salinity and surfactant concentrations occur. Because the highest oil recovery factor depends on the type of microemulsion, it must be ensured that the surfactant slug is in the phase type that leads to the highest oil recovery factor. A concept of optimum salinity profile (OSP) is proposed.