Rapid desorption-induced multiscale structural alterations in gas-bearing coals from Sydney Basin: Implications for coal and gas outburst

Coal and gas outbursts are among the most severe hazards in deep underground mining, driven in part by the rapid desorption of gas from coal seams. This study investigates the multi-scale structural alterations induced by the rapid desorption of CH 4 and CO 2 in deep-buried coal samples from the Sydney Basin. A novel methodology integrating high-resolution 3D micro-computed tomography (μ-CT), scanning electron microscopy (SEM), low-pressure gas adsorption (LPGA-N 2 and LPGA-CO 2 ), and helium-based void volume measurements for full-scale quantification was employed. Results reveal that rapid gas desorption facilitates the expansion of existing fractures and the formation of new ones, with fracture volume increasing proportionally with equilibrium pressure. SEM reveals widening and propagation of surface microfractures consistent with μ-CT observations. CH 4 desorption caused greater structural damage than CO 2 under equivalent gas content, due to its higher equilibrium pressure and gas expansion energy. Microscale analysis revealed a slight reduction in micropore volume (-2.49%) and a significant increase in mesopores (9.67%), enhancing gas diffusion and potentially intensifying outburst severity. Structural damage was most significant in areas with pre-existing fractures. These findings establish a clear linkage between gas pressure, desorption dynamics, and coal structural alteration, providing a scientific basis for refining gas threshold limits and improving outburst-risk management in deep coal seams, thereby supporting safer and more sustainable mining practices.