In this study, a comparative study of alkaline flooding and alkaline/surfactant flooding was conducted for Zhuangxi heavy oil with viscosity of 325 mPa·s at 55 °C. The results of core flooding tests show that, the tertiary oil recovery of alkaline-only flooding with the alkaline concentration in the range of 0.4–1.0 wt% can reach 22% – 31% OOIP. The coexistence of the surfactant and alkali can reduce the IFT between the heavy oil and aqueous phase to an ultralow level. However, the tertiary oil recovery of alkaline/surfactant flooding are lower than those of alkaline-only flooding. Further flood study via glass-etching micromodel tests demonstrates that injected alkaline-only solution can penetrate into the oil phase and creates some discontinuous water droplet inside the oil. The subsequent formation of droplet flow tends to reduce the mobility of the water phase and damp viscous fingering, leading to the improvement of sweep efficiency. While for alkaline/surfactant flooding, because of the addition of the surfactant, these systems exhibit better capacity in emulsification and IFT reduction, the viscous oil can be break up into fine droplets and entrained along with the flowing aqueous phase. Therefore, viscous fingering phenomena are occurred during the displacement, resulting in its relatively lower oil recovery. Comparing the results of alkaline flooding and alkaline/surfactant flooding, improving the sweep efficiency by the droplet flow in alkaline flooding is more effective than lowering IFT by alkaline/surfactant flooding for heavy oil recovery.

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