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

Abstract

Understanding the in-situ occurrence states and enrichment characteristics of CH₄ and CO₂ in shale reservoirs is a prerequisite for optimizing the approach of CO₂ 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 CH₄ and CO₂ at actual geological depth by molecular simulations. Density functional theory calculations demonstrate the physisorption manners of CH₄ and CO₂ on heterogeneous clay surfaces. Al- and Mg-doped sites show preferential adsorption to gas molecules, particularly to CO₂. Based on grand canonical Monte Carlo and molecular dynamics simulations, the density distributions, adsorption capacities and absorption heats of CH₄ and CO₂ were analyzed to characterize their in-situ occurrence states and enrichment regularities as well as the efficiency of CO₂ displacing CH₄. The enrichment region of adsorbed CH₄ is predicted to be beyond 3200 m, while the optimal depth for CO₂ storage is around 800 m, accounting for the high CH₄ displacement efficiency. A CO₂/CH₄ pressure ratio of 1/4 was found to be economically favorable for efficient displacement of adsorbed CH₄. These findings have clarified the adsorption mechanisms of CH₄ and CO₂ in clay reservoirs, which are essential for the reliable evaluation and economic exploitation of shale gas and the sequestration of CO₂ in clay reservoirs.