Experimental study on fracture behavior of deep coal rock under supercritical CO₂ exposure

The Carboniferous Benxi Formation in the Ordos Basin holds substantial potential for deep coalbed methane (CBM) development. However, hydraulic fracturing operations commonly induce damage to coal reservoirs, resulting in low gas recovery rates. In this study, systematic experiments were conducted on deep coal rock samples from the Benxi Formation in the Ordos Basin, including supercritical (SC)-CO₂ soaking, three-point bending tests, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) analyses. As indicated by the research results, SC-CO₂ exposure exerts a time-dependent, nonlinear influence on the mechanical properties of coal, characterized by an initial short-term enhancement followed by long-term deterioration. Peak load, fracture toughness, and elastic modulus reached relatively high values after 5 days of soaking. With prolonged exposure, however, the pore structure progressively deteriorated due to dissolution, leading to a 25.62% reduction in fracture toughness and a 33.29% reduction in elastic modulus after 20 days. Meanwhile, SC-CO₂ significantly enhances pore connectivity and alters fluid occurrence patterns. Loosely structured coal rocks with well-developed porosity exhibits greater sensitivity, with the total T₂ spectrum area increasing by 40.2%. In terms of failure modes, the fractures of untreated coal rocks were predominantly of brittle vertical penetration with relatively regular propagation paths. After SC-CO₂ soaking, fracture propagation is increasingly controlled by the interplay between bedding development and dissolution-induced weakening. This led to more complex fracture geometries, as evidenced by an increase in fracture fractal dimensions. The most pronounced fluctuations occur in samples with bedding perpendicular to the loading direction, with a maximum fractal dimension increase of 12.8%. SEM observations indicate that SC-CO₂ dissolution induces multiscale damage to the microstructure of coal rocks. At the micrometer scale, calcite dissolution results in localized pore networks. At the hundred-micrometer scale, micro-cracks initiate at dissolution zone boundaries and propagate into through-going fractures. At the millimeter scale, accumulated damage drives micro-crack coalescence along mechanically weakened pathways, ultimately forming complex fracture networks.