An innovative coal permeability model based on elastoplastic mechanics: Development and verification

With the continuous mining of shallow coal resources, deep mining has increasingly become the norm. However, the migration mechanism of coalbed methane (CBM) in coal seams becomes exceptionally complex due to the combined influence of multiple factors in deep mining, posing considerable challenges to coal and gas co-mining. Therefore, studying the coal's mechanical behavior and seepage evolution mechanisms during deep mining is necessary. This study established a coal permeability model based on elastoplastic mechanics, considering the impacts of coal matrix destruction on the average fracture aperture. It assumed that the fracture aperture follows an exponential distribution and further introduced plastic strain to characterize the damage process in coal. The proposed permeability model was validated using the indoor experimental data. Subsequently, the control mechanisms of force-heat coordination effects on coal permeability were discussed, and the sensitivity of model parameters was analyzed. The results demonstrated that the established permeability model effectively described the evolution of coal permeability under the combined impacts of temperature and effective stress. Moreover, the fracture number ratio (eta) and the influence coefficient of plastic strain increment on the average fracture aperture (beta) not only connected the dilation of microfractures and plastic deformation in coal but also effectively reflected the relationship between permeability and plastic deformation during the failure process of coal. The results presented in this paper contributed to understanding the evolution of permeability during coal and gas co-mining, which should be of great significance for reducing coal and gas outburst hazards.