Mechanical properties and fracture mechanism of supercritical CO₂ infiltrated particle coal based on time effect

Enhanced coalbed methane (ECBM) recovery technology through CO₂ sequestration in deep unminable coal seams, denoted as CO₂-ECBM, is one of the pivotal strategies for achieving the dual-carbon targets. However, the long-term safety of CO₂ sequestration within coal strata remains a pressing issue that necessitates resolution. The crux of this challenge lies in the investigation of the evolution and failure mechanisms of coal mechanical properties at various scales under the influence of supercritical CO₂ (ScCO₂), with a particular scarcity of research on the mechanical characteristics and fracture mechanisms of particulate coal. This study focuses on anthracite from the Guojiahe coal mine in Shaanxi Province, China, and conducts a study on the mechanical properties of particulate coal under varying durations of ScCO₂ saturation. By employing uniaxial compression tests on particulate coal and integrating scanning electron microscopy (SEM) experiments, we explore the time effects of ScCO₂ saturation on the mechanical properties and the evolution of fracture mechanisms over different periods. The findings reveal significant disparities in the mechanical properties of particulate coal contingent upon the duration of ScCO₂ saturation, with a demarcation at 24 hours. This threshold distinguishes the short-term physical rapid deterioration effects, characterized by rapid impacts on the elastic deformation stage due to micro-cracks induced by ScCO₂ adsorption expansion, from the long-term chemical slow deterioration effects, dominated by changes in coal molecular structure, organic matter extraction, and mineral dissolution, which exert a slower and less potent influence on coal mechanical properties. The expansion, organic extraction, and mineral dissolution induced by ScCO₂ lead to the release of elastic strain energy and a reduction in surface energy of particulate coal, thereby accelerating energy release during crack propagation. By constructing a model for the evolution of particulate coal fracture mechanisms, we further elucidate the impact of ScCO₂ time effects on these mechanisms, with short-term physical rapid deterioration effects primarily influenced by an increase in micro-crack density and long-term chemical slow deterioration effects characterized by an increase in stress concentration points and internal porosity, as assumed by the model.