Mechanical properties of coal-rock combined bodies under the action of ScCO2: A study of damage and failure mechanism

•Studied mechanical damage of different coal-rock combinations with ScCO2 soak.•Analysed failure patterns of the coal-rock combination after ScCO2 soak.•Analysed coal-rock combination's failure mechanism based on a mechanical model.•Based on the study, some advice for geological storage of CO2 is provided. Injecting CO2 into deep unrecoverable coal seams can help achieve carbon sequestration targets and improve the coalbed methane output rate. However, CO2 reaches a supercritical in situ storage state that impacts the coal storage structure. Based on the independently developed ScCO2 soak system, RFPA3D numerical simulation, and theoretical calculations, the mechanical damage characteristics and failure mechanism of RCR samples with three types of coal seam thicknesses and three types of top and bottom plate lithologies under the action of ScCO2 are investigated. The study reveals that (1) under the action of ScCO2, with the increase in the coal thickness, the deterioration degree of compressive strength and modulus of elasticity of the RCR combined body gradually increases and decreases, respectively, (2) ScCO2 will promote the plastic failure of the coal mass and exacerbate the tendency of the RCR combined body to change from tensile splitting failure to shear plastic failure, and the degree of plastic failure of the RCR combined body has a positive correlation with the coal thickness and the rock-coal strength ratio, and (3) ScCO2 accelerates the failure of the RCR combined body, and the coal mass enters the plastic failure stage earlier, while the rock mass ends the elastic strain energy accumulation and unloads and releases energy earlier, reducing the accumulation and release of energy in the combined body. The research results can provide a theoretical reference for the geological storage of CO2 deep inaccessible coal seams.