Existence of a Secondary Optimal Salinity in Microemulsion Systems

Abstract

ABSTRACT The optimal salinity has become the most important criterion for structuring surfactants to maximize solubilization of oil and water in microemulsion systems. Laboratory corefloods have demonstrated that surfactant systems show maximum oil recovery at this optimal salinity which also corresponds to a minimum interfacial tension. However, many oil reservoirs exhibit high salinities over 200,000 ppm and temperatures above 90°C. Since most surfactants produce low optimal salinities, the interfacial tension under such severe conditions is expected to be rather high. In this study, we have found the existence of a secondary optimal salinity which occurs, at a salinity higher than the primary optimal salinity (previously known as the optimal salinity). The presence of a secondary optimal salinity has been tested for ethoxylated sulfonates of varying ethoxylation (EO) number with an alkene and also with crude oil. For example, for EO=8 while the primary optimal salinity with dodecane at 90°C takes place at 8% salinity, the secondary optimal salinity occurs at 14% salinity. Increasing the EO number shifts both the primary and secondary optimal salinities indicating that the secondary optimal salinity is not an artifact. Moreover, the data obtained so far reveal that the secondary optimal salinity yields even lower interfacial tension. The mechanism governing this phenomenon is elucidated on the basis of partition coefficients, phase inversion temperature and infrared data. These results in addition to their theoretical interest are expected to open new avenues in the design of surfactant formulations for high-salinity and high-temperature oil reservoirs.