The current studies regarding the effect of interfacial tension (IFT) reduction on removing the water blockage of tight sandstones are significant, but the migration characteristics of trapped water in the stimulation process have not been researched. These issues lead to the stimulation mechanism of IFT reduction after hydraulic fracturing is unclear. In this work, a new coreflood platform was designed to simulate the water invasion, shut-in, and flowback process, and how the IFT affects the water blockage was further studied from pore levels. The oil production rates before and after shut-in were measured, which were used to detect the regained permeability of tight sandstones. The T2 spectrum signals, 1D frequency, and magnetic resonance imaging (MRI) based on the nuclear magnetic resonance (NMR) experiments were used to explore the migration characteristics of trapped water under different conditions. The results indicate that the core damage induced by water invasion is severe. The regained permeability is decreased to less than 25% after shut-in. IFT reduction is an effective way to improve the regained permeability, but the emulsification effect of fracturing fluid needs to be avoided, which will reduce the permeability of tight sandstones by the Jiamin effect. The NMR signals of the 1D profile show the water saturation of cores gradually decreases from the fracture face to the exit end, which demonstrates that the water blockage occurs mainly in the area near the fracture face. The T2 spectrum signals show that the residual water saturation of mesopores and macropores after flowback can be reduced by decreasing the IFT values, but the reduction of residual water saturation in micropores is insignificant. This result demonstrates that the core damage caused by water blockage may mainly come from mesopores and micropores. Our study reveals a deeper mechanism of removing water blockage during the IFT reduction process, which can guide the application of surfactants in the oil field.

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