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ASP-foam (ASPF) is a system prepared by injecting natural gas into the conventional alkali-surfactant-polymer (ASP) system. Foam can be formed in the porous media by the interaction of gas and surfactant in the ASP system. With the ASPF system, oil recovery is improved as the interfacial tension (IFT) is reduced to a relatively low level, and the swept volume is enlarged. In this paper, four surfactants were evaluated and characterized by IFT between ASP system and oil and the foaming performance. Alkyl benzene sulfonate (ORS-41) was chosen as the surfactant to best reduce IFT between displacement fluids and oil and improve the foaming performance. The mechanisms of ASPF flooding were studied in this paper, the results show that the ASPF flooding not only enlarges the swept volume but also enhances the displacement efficiency. The effects of reservoir heterogeneity, the gas-liquid ratio of ASPF system, and the concentrations of polymer and surfactant on the displacement efficiency were studied. A field trial of ASPF flooding has also been conducted. Both the laboratory results and the field trial results show that the ASPF flooding can significantly increase the oil recovery, with a 30% increase in the proportion of the original oil in place recovered compared with water flooding.