Experimental study of in situ-formed middle-phase microemulsion flooding for enhanced oil recovery after polymer flooding

• An in situ-formed middle-phase microemulsion flooding system (IFMF) was developed. • The structure of extended surfactant was optimized to match the molecular size of Daqing crude oil. • Live oil phase behavior tests were conducted to verify the effect of the optimum IFMF formulation. • In situ-formed middle-phase microemulsion flooding could significantly enhance oil recovery after polymer flooding, and the produced fluid analysis confirmed in-situ formation of bicontinuous middle-phase microemulsions under reservoir conditions. In situ-formed microemulsion flooding (IFMF) presents a promising method for enhanced oil recovery (EOR) after polymer flooding by forming nanoscale middle-phase microemulsions with ultralow interfacial tension (IFT) and superior solubilization capabilities. In this work, extended ethylene oxide (EO)/propylene oxide (PO) carboxylate (EPEC) and alkylbenzene sulfonate (ABS) were used as primary and secondary surfactants without adjuvants (alcohols). Phase behavior tests were conducted to optimize the molecular structure of EPEC, the proportion of compound surfactants and the type of added salt to determine the optimum IFMF formulation. The comprehensive performance of the formulation was tested through instrumental analysis and physical simulation experiments. The results demonstrated that the combination of the identified EPEC (C24 Guerbet, 10-EO/25-PO) and ABS was the optimal surfactant blend, achieving ultralow IFT (10 −3 mN/m) and favorable phase behavior characteristics. The addition of petroleum sulfonate further reduced optimum salinity from 1.1 wt% to 0.4 wt% and shortened equilibrium time. At low surfactant concentrations (0.3–1.2 wt%), the optimum salinity of the IFMF formulation decreased with increasing temperature (42–50 °C) and decreasing surfactant concentrations. Laboratory displacement experiments demonstrated remarkable effectiveness, with a 45 % incremental oil recovery in Berea sandstone while maintaining extremely low surfactant retention (0.09–0.14 mg/g rock). Microstructural analysis verified the in situ formation of middle-phase microemulsions under reservoir conditions. These findings establish IFMF as a promising and cost-effective technology for recovering highly dispersed residual oil in low-salinity reservoirs of the Daqing Oilfield.