Water blocking mechanism of low-permeability reservoirs and screening principle of flowback surfactants

After hydraulic fracturing in low-permeability reservoirs, fracturing fluid entering the rock matrix will cause water blocking damage, thereby reducing the matrix permeability. The low-permeability rock is characteristic of small pore throat, large capillary force, and shallow producing depth of crude oil, which can aggravate the water blocking damage in rock matrix near the fracture face due to the capillary end effect. To determine the damage degree and duration of this type of water blocking, core flow experiments under real-time monitoring by CT scanning is conducted in this study. Results indicate that this form of water blocking delays the recovery of matrix permeability during the flowback, and its duration increases linearly with the decrease of the core physical parameter (K/)1/2 and exponentially increases with the increasing water invasion depth. Results also indicate that shut-in treatments has little effect on mitigating such water blocking in low-permeability reservoirs, and it is necessary to add flowback surfactants to the fracturing fluid to enhance the flowback of fracturing fluid and increase production. By comparing three surfactant systems that can achieve the oil-water interfacial tension increased by five orders of magnitude (10-4~23 mN/m), the paper clarifies the significant effects of flowback surfactants in water-wet low-permeability reservoirs, and that of wettability-alteration surfactants in oil-wet low-permeability reservoirs. © 2020, Editorial Office of ACTA PETROLEI SINICA. All right reserved.