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Surfactants are usually added to the fracturing fluids to enhance the imbibition between fractures and matrix, which is an important measure to improve oil recovery. The popular perspective believes that altering water-wet conditions and keeping a relatively high IFT value are the keys to achieving the imbibition process. Recently, both indoor experiments and oilfield practice have shown that the surfactant systems with ultra-low IFT can also achieve imbibition and effectively recover oil from the matrix. For the problem that the imbibition mechanism of ultra-low IFT system is still unclear, the wettability alteration system, ultra-low IFT system, and emulsification system are constructed, the characteristics of imbibition systems and imbibition recovery are clarified, the oil-water transport patterns and dominant mechanisms of wettability alteration system and ultra-low IFT system with excellent emulsification are revealed by using mathematical models. The results show that the applicability range of the wettability alteration system and ultra-low IFT system with excellent emulsification is different. The wettability alteration system has better imbibition effects under 0.1 mD, but the ultra-low IFT system with excellent emulsification performs better under 0.01 mD and 0.001 mD. The imbibition process of the wettability alteration system is a layer flow pattern dominated by capillary force, the rate at which the wetted phase fills the pores decreases rapidly with the decrease of permeability, and the imbibition recovery decreases significantly from about 45% to 18%. The micro-nano emulsions can be formed by the ultra-low IFT system with excellent emulsification, which has a special emulsification and diffusion mechanism. In the early stage, oil is mainly recovered by emulsification and diffusion, in the late stage, the layer flow pattern of IFT reduction and wettability alteration gradually works, and the imbibition recovery decreases from about 38% to 23%. The emulsification and diffusion mechanism of the ultra-low IFT system with excellent emulsification is suitable for the imbibition of tighter reservoirs, and has a broad application in fracturing and stimulation of tight reservoirs. [ABSTRACT FROM AUTHOR]