Coupling between Damage Evolution and Permeability Model with the Adsorption Effect for Coal under Gas Extraction and Coal Mining Conditions

During coalbed methane (CBM) extraction and coal mining, the stress state, adsorption/desorption, and seepage behavior in coal will change significantly, which is the principal cause of the outburst accidents. Therefore, it was considered particularly important to investigate permeability behavior under damage conditions. In this paper, to simulate gas extraction and coal mining conditions, two triaxial seepage experiment schemes were designed: the first relating to the coal seepage experiment of gas pressure reduction under constant external stress (GPR seepage experiment) and the other relating to the coal seepage experiment of gas pressure constant under different confining pressures (GPC seepage experiment). Based on the principle of strain equivalence, the total damage variable was obtained, which revealed the combined result between adsorption stress and external stress. The internal expansion coefficient was introduced into the permeability model, which was developed from the poroelastic deformation theory. Moreover, when the effects of damage evolution on coal permeability were considered, a coal damage-permeability model was proposed. The results revealed that, during the pore pressure reduction process, the coal permeability increased slowly at first and then sharply. When pore pressure was constant, permeability decreased with an increase in confining pressure. Whether it was in the stage of reducing pore pressure or in the whole stress-strain stage, the proposed permeability model had a good correspondence with the experimental results. Finally, the cohesion force expression was modified to take into account the mechanical induced effect and adsorption induced effect on the gas. The effect of adsorbed gas on the coal reservoir stress state (strength criterion and Mohr circle) and in the in situ state was further discussed. The proposed theoretical findings could be applied to CBM extraction and coal mining problems to describe the coal damage evolution and gas seepage behavior.