Optimum surfactant criteria for controlling invasion-induced water blockage in tight water-wet cores

Flowback after hydraulic fracturing shows a poor recovery of the fracturing fluid. The unrecovered fluid is believed to invade the matrix and creates a formation damage called water blockage, which affects the optimum oil production from the reservoir. Surfactant is often added into the fracturing fluid, because it can improve the recovery of the fracturing fluid, and hence minimum water blockage. This objective is achieved by the surfactant's interfacial tension (IFT) reduction and wettability alteration capabilities. This study investigates the optimum surfactant criteria, in terms of the IFT and wettability, to minimize the water blockage. From the perspective of regained oil relative permeability, our history-matched core-scale model reveals that the optimum criteria depend on the matrix capillarity. In conventional reservoirs, the optimum criteria are to select surfactants which offer ultralow IFT (even as low as 0.001 mN/m) and maintain the water-wetness of the rock. However, in tight reservoirs, the optimum criteria are surfactants which strongly alter the rock wettability to oil-wet, while the IFT requirement is more flexible (i.e., from 15 down to 0.001 mN/m). The contradicting optimum criteria can be explained by the relative dominance between relative permeability and capillary pressure. This study emphasizes that the optimum surfactant criteria for conventional reservoirs are not necessarily the same as those for tight reservoirs.