Variation of mechanical properties of bituminous coal under CO₂ and H₂O saturation

CO2 injections into coal seams can boost the recovery of coalbed methane (CBM), while simultaneously sequestering a greenhouse gas. However, alteration of the mechanical attributes of coal is observed during CO2 injection. Most studies to date have focused on the influence of single-fluid saturation on coal, and few investigations have been dedicated to various and coupled-fluid saturations on the geomechanical attributes of coal. This work therefore examines the effect of CO2, water and water + CO2 saturation on bituminous coal. It was found that the average uniaxial strength of the untreated coal samples is 46.07 MPa and it reduces after CO2 and water saturation, this reduction increases with increasing CO2 saturation pressure, especially for supercritical CO2, as the much higher adsorption affinity of supercritical CO2 results in greater structural alteration. Further 5.29% and 9.69% strength reductions were found for water+6 MPa CO2 and water+8 MPa CO2 (supercritical) saturated samples, from 26.33 MPa to 19.00 MPa–23.90 MPa and 14.53 MPa, respectively, compared with the corresponding single CO2 saturations, because of the enhanced structural alteration and mineral dissolution observed in SEM images. The coal structure is relaxed with increased Young's modulus after exposure to CO2 or water. More progressive strain development was found for the water + CO2 treated sample, which showed a more ductile collapse compared with the shear-dominated failure of the untreated sample. The weakening effect was further evidenced by acoustic emission (AE) results, according to which the AE energies decreased considerably after CO2 or water saturation, and this reduction intensified for the water + CO2 saturated sample with increase of crack initiation stress and reduction of stable crack propagation. The findings highlight that while single CO2 or water saturation weakens coal strength, the coupled influence of water + CO2 saturation, which is the most likely scenario in actual field operations, induces greater strength alteration and threatens the overall stability of the system.