Abstract: To solve the problems of low residual oil saturation and high dispersion after polymer flooding,based on surfactant structure-property relationships,phase-behavior experiments,instrumental tests and physical simulation were conducted.An oil-displacement system capable of forming a middle-phase microemulsion in situ was developed,and formulation parameters optimization research was conducted,including the hydrophobic-chain lengths of the extended surfactant,the type of hydrophilic group,the numbers of propylene oxide(PO)/ethylene oxide(EO)units,and the blending ratio of mixed surfactants.Results show that the primary surfactant is a double-tailed C₂₄ extended carboxylate surfactant(APEC)containing 10 EO units and 25 PO units,which provides both favorable hydrophobic performance and cost effectiveness.The most suitable auxiliary surfactant is an alkylbenzene sulfonate surfactant(ABS).When combined with APEC,it generates ultralow interfacial tension at extremely low salinity,exhibits Winsor-type phase behavior,and provides a middle-phase microemulsion salinity window of 0.20%;the optimal mass ratio of APEC to ABS is 4∶6.The system can form a middle-phase microemulsion in situ under reservoir conditions.After waterflooding,in situ microemulsion flooding can improve recovery by 45.9%,which is 8.7 percentage points higher than weak-alkali alkaline-surfactant-polymer(ASP)flooding.This study provides an important basis for developing new post-polymer-flooding EOR technologies in Daqing Oilfield.

Key words: microemulsion flooding, phase behavior experiment, extended surfactant, middle-phase microemulsion, enhanced oil recovery(EOR)