Experimental Study on the Effects of IFT Reduction and Shut-In on Water Blockage after Hydraulic Fracturing in Tight Sandstone Reservoirs Based on the NMR Method

Fracturing fluid is often trapped in the matrix near the fracture face after hydraulic fracturing, hindering the flow of oil. Reducing the interfacial tension (IFT) and shut-in are usually employed to remove the water block. However, the field reports and the experimental or simulation studies show that the results of the above measures on the water blockage and oil production are mixed. And the migration characteristics of trapped water are not well understood. These issues lead to the stimulation mechanism of shut-in and IFT reduction after hydraulic fracturing is unclear. In this work, the effects of shut-in and IFT reduction on the water blockage were studied. The imbibition experiments were performed to determine the shut-in period. Coreflood experiments were conducted to measure the oil production rates and detect the regained permeability ratio. Nuclear magnetic resonance (NMR) experiments were conducted to investigate the migration characteristics of trapped water. The results indicate that the regained permeability ratio is reduced to less than 35% after water invasion, and shut-in in the early stage can further decrease the permeability due to the refill of water from fractures to the matrix. Reducing the IFT is a beneficial approach to enhance regained permeability ratio; however, emulsification should be avoided since it may lead to reduced permeability via the Jiamin effect. NMR results suggest that the residual water saturation of mesopores and macropores after flowback can be reduced by decreasing the IFT, but it is insignificant. Additionally, the residual water saturation of micropores and mesopores is much larger than that of macropores, the core damage may mainly come from micropores and mesopores. This work further explains the mechanism of removing water blockage from the pore level, which can guide the stimulation process of oil fields.