Imbibition is a crucial approach for improving oil recovery in tight reservoirs. A shut-in strategy is usually adopted to enhance oil recovery by imbibition after fracturing，and chemical agents such as surfactant and nanomaterials are used to improve imbibition efficiency. However，the enhanced oil recovery（EOR）mechanisms of different types of imbibition agents need be distinguished and explained. In this study，four types of imbibition agent systems were employed to conduct imbibition experiments，including silica nanofluid and pure water with high interfacial tension（IFT）（?30 mN/m），pure surfactant solution with medium IFT（1—10 mN/m）and surfactants compound system with ultra-low IFT（＜10^-3 mN/m）. The results showed that，as for the silica nanofluid and pure surfactant solution，the wettability alteration was the main EOR mechanism to improve the imbibition recovery in oil-wet reservoirs. Furthermore，the pure surfactant system could reduce the IFT and increased the deformability of oil droplets when the droplets passed through the pore throat. Therefore，the Jamin effect was alleviated，and the possibility of the oil droplets being snapped off and retained in the pore space is reduced，thereby improving the utilization degree of unrecovered oil. Nevertheless，the recovery results indicated that IFT reduction had a limited impact on the recovery enhancement，and that wettability improvement was more significant for improving recovery than IFT reduction. However，in the strong oil-wet pores，when the improvement of wettability was not effective and capillary force could not work ，the surfactant compound system exhibited the best imbibition performance. The ultra-low IFT between oil and water favored the spontaneous emulsification of oil，and changed the imbibition pattern from capillary force-driven imbibition to gravity-driven imbibition，and the emulsions formed spontaneously were drained out from the pore under the effect of gravity. The gravity-driven imbibition， which relies on the ultra-low IFT to promote the spontaneous emulsification of crude oil，provides a novel approach for enhancing imbibition recovery.