To elucidate the microscopic mechanisms of competitive adsorption between CO2 and CH4 under deep coal seam geological conditions, and to clarify the intrinsic advantages of CO2 adsorption sequestration and CH4 displacement from a molecular-scale perspective, this study conducts a systematic investigation using molecular simulation methods. A molecular structure model of coal was constructed and optimized based on elemental analysis and petrographic data of a representative deep coal sample. Combined grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were employed to systematically analyze the adsorption capacity, density distribution, adsorption heat, adsorption potential energy, and adsorption selectivity of equimolar CO2/CH4 mixtures under geologically constrained conditions of high temperature (308-348 K), high pressure (0-20 MPa), and experimentally measured moisture contents (0-3.58%). The results demonstrate that CO2 consistently exhibits a significantly stronger competitive adsorption advantage over CH4 within the coal matrix, with higher adsorption capacity, adsorption heat, and adsorption potential energy and an adsorption selectivity coefficient always greater than 1. Increasing the temperature generally weakens the adsorption capacity of both gases, with CO2 showing greater sensitivity to temperature variations. At intermediate temperature (328 K) and moderate-to-high pressure, the CO2 adsorption density exhibits a nonmonotonic trend, reaching a peak, whereas the CH4 adsorption density decreases monotonically with increasing temperature. Increasing overall moisture content suppresses adsorption behavior; however, at low moisture content (approximately 1.22%), CO2 adsorption capacity and density are enhanced under high-pressure conditions, creating a favorable microscopic environment for CO2 retention. With further increases in the moisture content, the saturated adsorption capacity and density of both gases decrease markedly, with a more pronounced decline observed for CO2. This study reveals, at the molecular scale, the regulatory mechanisms of temperature, pressure, and moisture content on CO2-CH4 competitive adsorption behavior and clarifies the critical role of coal matrix adsorption in geological CO2 sequestration. The findings provide a theoretical basis for evaluating the potential of CO2-enhanced coalbed methane recovery (CO2-ECBM) and adsorption-based sequestration in deep coal seams.
Liu et al. (Thu,) studied this question.