Microemulsions Prepared by the Synergistic Effect of Ionic Liquids and Zwitterionic Surfactants for Enhanced Oil Recovery

Microemulsion flooding can significantly enhance the oil recovery in low-permeability reservoirs. A novel ionic microemulsion is developed by the synergistic effect of ionic liquids [C16mim] [Br] and zwitterionic surfactants CHSB. The system achieves ultralow oil–water interfacial tension (8.3 × 10–3 mN/m) under optimal salinity. The microemulsion exhibits stable viscosity (4.0 ∼ 6.5 mPa·s) with varying salinities, shear rates, and durations. Core flooding experiments demonstrate superior recovery efficiency (30.1%) compared to surfactant-only solutions, with optimal injection parameters of 0.2 cm3/min and 0.5 PV. The alteration of core wettability from lipophilic to hydrophilic promotes residual oil stripping. The mechanism of enhanced oil recovery is reduced adhesion work, increased capillary number, and controlled mobility ratio. This work provides a practical strategy of efficient exploitation for challenging reservoirs.