Insights into influence of CH4 on CO2/N2 injection into bituminous coal matrix with biaxial compression strain in molecular terms

Abstract

Under the influence of strong mining disturbances in coal mines, the coal gradually deforms, which affects the extraction of coalbed methane (CBM). In this study, the microscopic mechanism of CH4 affecting CO2/N2 injection into a bituminous coal matrix with biaxial compression strain was investigated by analyzing its microporous structure, adsorption isotherm, CH4 content effect, strain effect, adsorption density, intermolecular adsorption potential energy and adsorption energy using grand canonical Monte Carlo simulation, molecular dynamic simulation and density functional theory. The results show that with increasing (negative) compression strain (0.00 → −0.20), the distribution range of the gas adsorption potential energy field decreases, with a deviation order of CO2 > CH4 > N2. Also the Henry constant of CO2 increases while those of CH4 and N2 stabilize. The orders of influence of CH4 content and strain on adsorption are N2 > CO2 > CO2/N2 and CO2 > CO2/N2 > N2, respectively. Additionally, the potential energy field distribution ranges of CO2 and N2 injections decrease with increasing CH4 content. More importantly, CH4 is adsorbed in the Up orientation, while CO2 and N2 are adsorbed parallel to the aromatic groups on the pore surface of coal matrix. Also, pre-adsorbed CH4 increases the negative adsorption energy of CO2 and N2 by 45.37–50.26%, thereby effectively inhibiting their injections. This research lays the foundation for improving CBM recovery efficiency in the deformation zone of CBM reservoirs with realistic exploitation.