Effects of sub-/super-critical CO2 on the fracture-related mechanical characteristics of bituminous coal

Injecting carbon dioxide CO 2 into a coal seam is an important way to improve coalbed methane recovery and to store geological carbon. The fracture mechanical characteristics of bituminous coal determine the propagation and evolution of cracks, which directly affect CO 2 storage in coal seams and the efficiency of resource recovery. This study applied CO 2 adsorption and three-point bending fracture experiments using bituminous coal samples in a gaseous state (4 MPa), subcritical state (6 MPa), and supercritical state (8 and 12 MPa) to investigate the influence of CO 2 state and anisotropy on the fracture-related mechanical response of bituminous coal. The results show that the change in mechanical properties caused by CO 2 adsorption is CO 2 state-dependent. The supercritical CO 2 adsorption at 8 MPa causes the largest decrease in the mode-I fracture toughness ( K IC ), which is 63.6% lower than the toughness before CO 2 adsorption. The instability characteristics of bituminous coal show the transformation trend of “sudden-gradual-sudden fracture”. With or without CO 2 adsorption, the order of the K IC associated with three types of bituminous coal specimens is crack-divider type > crack-arrester type > crack-short transverse type. Phenomenologically, the fracture toughness of bituminous coal is positively correlated with its specific surface area and total pore volume; the toughness is negatively correlated with its average pore size.