Coal Swelling Induced by CO2/CH4 Exchange Sorption under Laboratory-Simulated In Situ Stress Conditions

Abstract

CO2 injection into methane-bearing coal causes competitive CO2/CH4 sorption and coal-matrix swelling, which may reduce permeability and limit the efficiency of CO2-enhanced coalbed methane recovery. This study simultaneously quantified the CO2/CH4 exchange sorption balance and the accompanying volumetric deformation of a hard coal briquette under constant confining pressures of 1.5, 5, 10, 20, and 30 MPa. Before CO2 injection, the sample was saturated with CH4 at pressures of 0.15, 0.40, and 0.80 MPa. Increasing confining pressure had only a limited effect on the equilibrium CH4 sorption capacity but substantially reduced sorption-induced swelling and slowed the CO2/CH4 exchange process. The final exchange sorption balance remained similar throughout the investigated stress range: the sorption of approximately 1 mmol/g of CO2 was accompanied by the displacement of nearly all initially sorbed CH4, amounting to approximately 0.5 mmol/g. In contrast, the volumetric response was strongly stress-dependent. At a confining pressure of 1.5 MPa, the total sample swelling increased from approximately 0.69% after CH4 saturation to 1.58% after CO2/CH4 exchange, whereas at 30 MPa it increased from approximately 0.30% to 0.94%. The slope of the swelling–exchange sorption relationship systematically decreased with increasing effective stress. A first-order linear extrapolation indicated that complete suppression of exchange sorption-induced swelling would require an effective stress of the order of 100 MPa, with a conservative estimate of approximately 90 MPa for the tested material. The results demonstrate that laboratory-simulated in situ stress conditions affect the deformation and kinetics of CO2/CH4 exchange more strongly than its final sorption balance and provide a quantitative basis for coupled sorption–deformation models of CO2-ECBM processes.