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

Abstract

Injecting carbon dioxide CO₂ 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 CO₂ storage in coal seams and the efficiency of resource recovery. This study applied CO₂ 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 CO₂ state and anisotropy on the fracture-related mechanical response of bituminous coal. The results show that the change in mechanical properties caused by CO₂ adsorption is CO₂ state-dependent. The supercritical CO₂ 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 CO₂ adsorption. The instability characteristics of bituminous coal show the transformation trend of “sudden-gradual-sudden fracture”. With or without CO₂ 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.

Graphical abstract

Keywords

energy development / CO₂ geological storage / rock mechanics / bituminous coal

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