A novel elastoplastic damage–permeability coupling model for coal–gas co-mining: Development and validation

Violent gas explosions in coal mines, in addition to coal and gas outbursts, exhibit extremely destructive potential and pose severe threats to coal mining safety. The emergence of coal and gas co-mining technology not only enhances mining safety but also promotes low-carbon energy transition through the utilization of coalbed methane (CBM). However, increasing mining depths have led to plastic deformation areas inevitably developing ahead of the working face and around gas drainage boreholes, significantly impeding the safe and efficient implementation of coal and gas co-mining. Therefore, it is imperative to further investigate coal mechanical response characteristics, including the change mechanisms of gas seepage during extraction. In this study, based on generalized plasticity theory, the plastic strain was calculated using a non-associated flow rule. A damage variable was introduced by considering the degradation of mechanical properties in coal during plastic deformation. Based on the equivalent fractured coal theory, the established damage variable was incorporated into a simplified permeability equation, ultimately yielding a coupled elastoplastic damage–permeability model. The model's accuracy was validated using published experimental data. Furthermore, the evolution mechanisms of plastic strain and damage variable under varying conditions were systematically analyzed. The variation patterns of elastic modulus reduction ratios (Rm) under combined action of temperature and gas pressure effects during CBM extraction were investigated, followed by a comprehensive sensitivity analysis of the model parameters. The theoretical findings of this study provide critical insights into coal mechanical behavior and permeability evolution, thereby providing a theoretical basis for coal–gas co-mining.