Anisotropy evolution and gas transport characteristics of coal structure under tectonic stress: Insights into multiscale microstructural controls on methane desorption and diffusion

The pore architecture within coal media critically influences gas emission behavior. This study used fluid injection and photoelectric radiation methods to systematically analyze the microporous structures of primary and tectonic coals across nanometer to micrometer scales. It reveals how tectonic stress-induced anisotropic evolution of coal pore structures affects gas desorption and diffusion mechanisms. The results show that tectonic activity significantly accelerates pore formation in macropore and mesopore regimes. In macropores, surfaces become smoother and migration pathways simplify, while mesopores feature rougher surfaces that provide more gas adsorption sites and simpler spatial structures that offer favorable migration channels—collectively promoting efficient free gas transport. At the micrometer scale, tectonic coal's primary fracture network is disrupted, forming a more extensive network of micrometer-scale fractures. This leads to a significant increase in pore throats and cavities, with a reduction in long-sized throats and development of short-sized ones, effectively shortening gas migration paths. In primary coal, throats show a preferred orientation (270°–330°), reflecting strong anisotropy. In contrast, tectonic coal exhibits reduced tortuosity, with throat orientation angles distributed across multiple directions, weakened anisotropy, and enhanced pore connectivity. Tectonic deformation enhances gas transport capacity through synergistic optimization across scales: creating more efficient pathways at the nanoscale (via modifications to surface and spatial configurations) and significantly improving connectivity at the microscale (through reorganization of fracture networks). This synergistic effect weakens the overall structural anisotropy of the coal matrix, leading to markedly higher methane desorption and diffusion rates. The comparative study of coal microstructure characteristics provides critical theoretical insights for understanding gas emission in tectonic coal, offering scientific guidance for coal mine gas disaster prevention and coalbed methane exploitation.