Evolution of Permeability and Gas Seepage in Deep Coal Under 3D Stress

More and more gas extraction in deep coal seam is conducted under extreme geological conditions. The coal permeability determines the efficiency of gas extraction. To research the evolution of permeability and gas seepage in deep coal seam under three-dimensional (3D) stress condition, a permeability model (DCTDS model) was established by introducing the ratio of internal swelling and the elastic modulus reduction ratio under 3D stress. The influence of adsorption-desorption and effective stress on fracture aperture was considered in the DCTDS model. Subsequently, the DCTDS model was validated. Sensitivity analysis of parameters was conducted based on the laboratory test data under various boundary conditions. The results demonstrate that the change of coal permeability is effectively presented by the DCTDS model. The coal permeability decreased more with increasing pressure when the porosity and Biot's coefficient were smaller in the dominant range of adsorption swelling. The coal permeability increased more when the porosity, Poisson's ratio, and ratio of internal swelling were smaller in the dominant range of effective stress. Finally, based on the constructed DCTDS model, gas extraction in deep coal seams was simulated. The extraction effect was improved with smaller borehole spacing, and elastic modulus, Poisson's ratio of coal. The extraction effect became more notable with larger borehole aperture, Biot's coefficient, and initial permeability. The gas extraction efficiency decreased gradually with extension of extraction time.