Fracture evolution and mechanical response of coal with damage induced by oil-gas seepage

In coal-oil-gas co-storage zones, long-term oil and gas seepage significantly alters the intrinsic properties of coal, thereby posing substantial challenges to the safe extraction of coal resources. However, existing research on the evolutionary mechanisms of coal properties under coal-oil-gas co-storage conditions remains insufficient. To address this knowledge gap, this study systematically investigated coal specimens collected from a specific coal seam in the Shuangma Coal Mine, employing an integrated technical approach that combines high-resolution CT scanning, uniaxial compression testing, and synchronous acoustic emission monitoring, coupled with fractal theory. The aim was to quantitatively analyze the effects of oil and gas seepage-induced damage on the fracture evolution and mechanical behaviors of coal. The integrated test results and analytical findings are as follows: Fracture density, cumulative fracture length, and box-counting fractal dimension exhibit a monotonic decreasing trend with increasing distance from legacy oil wells, which confirms a negative correlation between seepage-induced damage degree and well proximity; Coal specimens adjacent to oil wells show significant mechanical degradation, characterized by lower peak strength, elastic modulus, and peak strain, and are dominated by shear failure modes. In contrast, specimens distant from wells gradually transition to mixed shear-tensile failure modes; The AE characteristic analysis during the uniaxial compression process reveals four distinct deformation stages: compaction, elastic deformation, rapid crack propagation, and post-peak softening. Notably, high-amplitude AE events with high RA values (rise angle) and low AF values (average frequency) can serve as effective precursory indicators for imminent coal instability; Oil and gas seepage-induced deterioration of coal mechanical properties in oil- and gas-enriched zones facilitates the formation of migration pathways for oil and gas hazards. Simultaneously, the stress-relief effect arising from seepage-induced damage in coal reduces the occurrence probability of various mining-induced dynamic disasters. These research findings provide a theoretical basis for hazard prevention in coal-oil-gas coexisting reservoirs.