Occurrence characteristics and enrichment regularities of CH4 and CO2 in clay reservoirs: Implication for in-situ CO2 sequestration with shale gas recovery

• In-situ occurrence characteristics of CH 4 and CO 2 in clay reservoirs were explored. • The adsorbed and free CH 4 and CO 2 were distinguished and quantified individually. • Enrichment regions of pure CH 4 and CO 2 in the clay reservoirs were obtained. • Preferential adsorption manners of CH 4 and CO 2 on clay surfaces were identified. • Favorable geological depth and CO 2 pressure were predicted for CS-EGR technique. Understanding the in-situ occurrence states and enrichment characteristics of CH 4 and CO 2 in shale reservoirs is a prerequisite for optimizing the approach of CO 2 sequestration with enhanced gas recovery (CS-EGR) technology. The divergent structural characteristics of montmorillonite (high surface area/swelling capacity) and illite (stable framework) critically govern gas adsorption and transport dynamics in shale formations. In this work, composite clay reservoirs comprised of montmorillonite and illite were modeled to investigate the adsorption behaviors of CH 4 and CO 2 at actual geological depth by molecular simulations. Density functional theory calculations demonstrate the physisorption manners of CH 4 and CO 2 on heterogeneous clay surfaces. Al- and Mg-doped sites show preferential adsorption to gas molecules, particularly to CO 2 . Based on grand canonical Monte Carlo and molecular dynamics simulations, the density distributions, adsorption capacities and absorption heats of CH 4 and CO 2 were analyzed to characterize their in-situ occurrence states and enrichment regularities as well as the efficiency of CO 2 displacing CH 4 . The enrichment region of adsorbed CH 4 is predicted to be beyond 3200 m, while the optimal depth for CO 2 storage is around 800 m, accounting for the high CH 4 displacement efficiency. A CO 2 /CH 4 pressure ratio of 1/4 was found to be economically favorable for efficient displacement of adsorbed CH 4 . These findings have clarified the adsorption mechanisms of CH 4 and CO 2 in clay reservoirs, which are essential for the reliable evaluation and economic exploitation of shale gas and the sequestration of CO 2 in clay reservoirs.