An alkali-free surfactant system based on petroleum sulfonate and extended surfactant for enhanced oil recovery

Abstract To address the scaling and corrosion problems associated with long-term weak-alkali ASP flooding in Daqing Oilfield, an alkali-free oil-displacement system based on petroleum sulfonate (WPS) and an extended surfactant (C12P9E3S) was developed. The EACN of the target crude oil was used to guide hydrophobe selection, while the extended surfactant and subsequent formulation were further selected through experimental evaluation of interfacial tension and emulsification performance. The optimal system, with a WPS/C12P9E3S mass ratio of 2:1, total surfactant concentration of 0.3 wt%, and NaCl concentration of 1.6 wt%, achieved ultralow interfacial tension at the 10 –3 mN/m level and exhibited improved emulsion stability compared with the conventional weak-alkali ASP system. Core-flooding experiments showed that the alkali-free system achieved an average incremental oil recovery of 36.73% beyond water flooding, which was 5.81 percentage points higher than that of the weak-alkali ASP system. In addition, the system demonstrated more stable displacement behavior and a longer effective flooding stage. The improved oil recovery performance is associated with enhanced emulsion stability and more favorable dispersed oil-phase characteristics during displacement in porous media. The results indicate that the developed alkali-free system can effectively reduce alkali-related operational risks while maintaining favorable oil-displacement efficiency, providing insights into the development of alkali-free chemical flooding systems for mature oil reservoirs.