Review of the Impact of Cross-Scale Coal Damage on Gas Migration Mechanism under Supercritical CO₂ Conditions

Supercritical CO₂ (ScCO₂) plays a crucial role in enhancing coalbed methane recovery and CO₂ geological sequestration. Currently, predicting gas flow remains a major scientific challenge due to difficulties in coupling multiple physical fields. The reasons for discrepancies between observations from laboratory and field studies and model predictions remain unclear. This paper reviews nearly 20 projects funded by the National Natural Science Foundation of China and several related publications on coal-gas interactions under ScCO₂. Recent advances are systematically summarized, focusing on multiscale coal damage characteristics and multiphysics gas transport. The fracture propagation and pore structure evolution of coal are analyzed at macro-, meso-, micro-, and molecular scales. Models that assess the effects of cleat structure and mechanical coal damage on permeability under constant-volume conditions are reviewed. The importance of developing a coupled damage-stress-flow-thermal model to elucidate gas transport mechanisms under free-expansion conditions is highlighted. Moreover, the main factors affecting CH₄ displacement by ScCO₂including coal adsorption properties, CO₂ concentration, and mechanical coal deformationare examined. The effects of these factors on physical field distributions and displacement mechanisms are also analyzed. Finally, future research priorities for CO₂-enhanced (CO₂-ECBM) recovery in experimental, simulation, and field applications are proposed.