Mechanism studies on the application of the mixed cationic/anionic surfactant systems to enhance oil recovery

Mixed cationic/anionic surfactant systems have showed great potential in various fields. Ionic liquids with extraordinary physical and chemical properties are regarded as novel alternatives to traditional surfactants. In this report, we evaluated the performances of the mixed surfactant systems comprising a cationic surface-active ionic liquid 1-dodecyl-3-methylimidazolium chloride (M12) and an anionic surfactant sodium dodecyl sulfate (SDS) in terms of surface/interfacial tension reduction, emulsifying ability, wettability alteration and core flooding tests. Molecular dynamic simulation was employed to explore the surface/interfacial activity of the mixed surfactant systems. The interfacial tension (IFT) between the mixed surfactant solutions and crude oil could be dramatically decreased to the ultralow level (<10−2 mN/m) at high-salinity and high-temperature conditions, indicating their potential application in the harsh reservoir. Bottle tests demonstrated the mixed surfactants could effectively stabilize the crude oil-in-water emulsions. The mechanism of wettability alteration was investigated via the measurements of contact angles, FTIR and zeta potential. The electrostatic attraction in the mixed systems might cause the fabrications of the pseudo two-tailed co-surfactants and their intensive arrangements at the surface/interface, which should be responsible for their remarkable performances. Moreover, core flooding tests were conducted to confirm the high efficiency of mixed systems in the improvement of crude oil recovery.