Experimental investigation on the effect of slickwater on methane adsorption/desorption/diffusion and pore structure of shale

After the slickwater fracturing method is adopted, a portion of the slickwater remains in the reservoir, impacting shale gas production. To address this limitation, shale samples from the Longmaxi Formation were soaked and washed with varying concentration of polyacrylamide in slickwater under different pressures to simulate the flowback process following slickwater fracturing. The effects of slickwater on methane adsorption, desorption, and diffusion in shale were examined through isothermal adsorption experiments, while the evolution of shale pore structure was assessed using low-temperature nitrogen adsorption experiments. Results indicate that slickwater adheres to the surfaces of microcracks and macropores, creating new small pores and increasing the specific surface area and pore volume of macropores with sizes exceeding 5 nm. Slickwater may also block pore throats, causing some open or semi-open pores to become closed, thereby reducing the accessible specific surface area and pore volume and hindering the adsorption, desorption, and diffusion of methane. At a pressure of 9 MPa, treatment with 0.7% slickwater reduces the Langmuir volume to 40% of that of the original shale, while the diffusion coefficient decreases to 47% of its original value. Furthermore, as soaking pressure or concentration increases, the hysteresis of methane desorption in shale initially decreases before subsequently increasing. The findings of this research provide theoretical guidance for the further enhancement of shale gas development.