EOR Perspective of microemulsions: A review

Since conventional oil recovery techniques only produce a limited fraction of the original oil in place (OOIP), the need for Enhanced Oil Recovery (EOR) methods has become imperative. Chemical enhanced oil recovery (cEOR) techniques have been widely studied for decades. Surfactants are frequently employed in cEOR processes. Micelles are formed when the amount of surfactant exceeds the critical micelle concentration (CMC) by aggregation of monomers; this can potentially result in the formation of microemulsions when the surfactant solution comes in contact with reservoir fluids. These microemulsions are homogenous and thermodynamically stable. This review focuses on factors affecting the formation of microemulsions such as pressure, temperature, salinity, nature of cosurfactants, cosolvent type and concentration, water-oil-ratio, pH, and oil properties. This review also covers the characterization of microemulsions, such as via: phase behavior and rheological properties, visual inspections, polarized light microscopy studies, their interfacial, surface tension, and zeta potential measurement plus investigations of the particle size distributions. Finally, a comprehensive review of surfactant systems that have been used to generate microemulsions at different reservoir conditions is presented. This review reveals that anionic surfactants are capable of generating a Winsor III type microemulsion – which is optimal in EOR processes. On the other hand, cationic surfactants usually do not exhibit Winsor III type behavior, and nonionic surfactants are normally only applicable as cosurfactants in EOR contexts.