Numerical study on fracture evolution and gas migration in the surrounding rock of large diameter drilling in crushed-soft coal seam

In view of the low efficiency of gas extraction practice in a crushed-soft outburst coal seam, the method to reduce the gas content in coal seam and prevent the gas outburst disaster by large diameter drillings and gas extraction was implemented. On the basis of the governing equations of gas adsorption and migration, coal deformation, and damage, the coupling model of stress damage-seepage in a crushed-soft gas bearing coal seam was established, which reflected the coupling response of gas and coal seam. COMSOL Multiphysics software was used to simulate the damage evolution and gas migration in surrounding rock of large diameter drilling in crushed-soft coal seam. The change of gas content in the damaged area near the drilling and the extraction standards of different drilling spacing, and optimize the spacing of large diameter borehole were analyzed. The research results show that a large range of damage and failure areas present in the coal mass around the large diameter drilling, and its permeability increases greatly. Gas flow from the coal mass in the damaged area to the extraction drilling was significantly enhenced. Efficiency of gas extraction in crushed-soft coal seam was therefore increased. The large diameter drillings with a length of 6 m and aperture of 300 mm are determined for a efficient gas pre-extraction. The gas content in coal seam is less than 6.2 m³/t, which meets the standard conditions of extraction and reduces the construction cost.