A review of microscopic seepage mechanism for shale gas extracted by supercritical CO2 flooding

Abstract Shale gas exploitation with supercritical CO2 is a promising technology that not only improve gas recovery but also achieve geological storage of CO2. This paper presents the research status of shale gas seepage mechanism under supercritical CO2 conditions from several aspects including the adsorption and desorption of gas, the competitive adsorption of CO2 and CH4, the multi-scale spatial gas mass transfer process and the shale gas seepage mechanism under multi-field coupling. Adsorption potential theory provides a method for predicting the real shale reservoir adsorption capacity under supercritical conditions. The desorption model needs to be considered simultaneously due to the existence of hysteresis. The adsorption selectivity of CO2/CH4 is always greater than 1, and there is an optimal selectivity coefficient to optimize the in-situ process conditions. For the competitive adsorption mechanism, the reason why CO2 adsorption capacity of shale is stronger than that of CH4 is explained in terms of kinetic diameter, critical temperature, molecular polarity and diffusion rate. The interaction between CO2 and CH4 will make the shale gas seepage mechanism more complex, so further research is needed on the mass transfer process involving multi-component gas. The changes in reservoir physical properties after supercritical CO2 fracturing, the mathematical model of multi-field fully coupled gas microscopic seepage and the universal stress-sensitive model are the focus of future research.