Mechanistic study of the effect of clay hydration swelling on CH4/CO2 competitive adsorption

The study on the hydration swelling mechanism of different types of shale clay minerals and the influence of water content on the adsorption characteristics of CH4/CO2 is of great significance for CO2 storage in shale reservoirs and enhanced shale gas recovery. Molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) were employed to establish different types of shale clay minerals (sodium montmorillonite, calcium montmorillonite, and illite). Hydration swelling simulations were performed at water contents ranging from 0.0 wt% to 20.7 wt%. Additionally, CH4 adsorption and CH4/CO2 competitive adsorption simulations were conducted at varying water densities. The effects of clay mineral type and water content on hydration swelling degree, gas adsorption capacity, and CO2/CH4 adsorption selectivity were analyzed. The simulation results indicate that variations in surface charge enrichment and interlayer cationic ionic radius account for the differences in hydration degree. Under the same water content, the hydration degree of the three clay minerals follows the order: sodium montmorillonite > illite > calcium montmorillonite. The gas adsorption capacity of clay minerals decreases with the increase of water content, and the greater the water content, the smaller the reduction range. With the increase of water content, the adsorption selectivity of CO2/CH4 first decreased and then increased, and the increasing range of different clay minerals was sodium montmorillonite > illite > Calcium montmorillonite. The research results of this paper have a certain reference value for the reasonable development of shale gas and the study of CO2 storage in shale.