Experimental study on the methane desorption-diffusion behavior of Longmaxi shale exposure to supercritical CO2

Desorption-diffusion behaviors of CH4 in shale after supercritical CO2 (ScCO2) injection are closely related to CO2 enhanced shale gas recovery (CO2-ESGR). A self-developed gas desorption test system was used to measure the desorption kinetics of CH4 in shale collected from Sichuan Basin at different ScCO2 exposure pressures (8, 12, 16 MPa) and temperatures (40, 60, 80 degrees C), and the low-pressure N-2 adsorption was performed to evaluate the variations of pore structure of shale. Results indicate that the desorption kinetics curves of CH4 in shale exhibit typical Langmuir characteristics under different desorption pressures, and the kinetics data were fitted by unipore and bidisperse diffusion models, manifesting that the diffusion of CH4 in the shale samples are dominated by macroporous channel (>95%). With increasing desorption pressure, the desorption capacity of CH4 in shale (n(de)) increased, while the diffusion coefficient of CH4 in shale (D) decreased. Additionally, the n(de) and D increased after ScCO2 exposure, and exhibited a slight exposure pressure and temperature dependence, which is mainly caused by the decrease of specific surface area and the widening of microstructure in shale, implying that the ScCO2-shale interaction can promote the desorption-diffusion of CH4 in shale. This study provides a theoretical reference for CO2-ESGR.