Hydromechanical responses of coal powders by CO₂ adsorption

Enhanced coal bed methane recovery is a promising method to produce methane gas while storing carbon dioxide (CO₂) in coal bed seams. Although the adsorption of the injected carbon dioxide on coals is known to cause swelling and softening of coals, its roles in the worsening of transport and mechanical characteristics have not been fully quantified. Thus, this study examined the effect of carbon dioxide adsorption on the mechanical characteristics (stiffness and strength), permeability and P-wave velocity of coals by using a modified triaxial strength apparatus. Anthracite coal powder samples from Kyung-Dong, Korea, were used in this study. It was confirmed that carbon dioxide adsorption reduced the permeability due to the carbon dioxide-induced swelling and increased the P-wave velocity, possibly owing to additional consolidation of softened coals. Upon completion of carbon dioxide adsorption, a series of multistage triaxial shear strength tests were performed under a consolidated-drained condition. Clear reductions in the mechanical strength and stiffness of the coal samples were observed, confirming the carbon dioxide-induced softening effect on coals. The results suggest that carbon dioxide adsorption heavily affects the mechanical and hydraulic properties of coals, in particular, confirming the carbon dioxide-induced swelling and softening effects on coals.