Gas flow in hydraulic slotting-disturbed coal seam considering stress relief induced damage

Gas extraction in destressed coal seams is one of the effective measures to improve degasification efficiency and reduce gas disasters in deep coal seams. In order to reveal the influence of damage zone induced by stress relief on gas extraction of slotted boreholes, a multi-field model incorporating stress, damage, gas diffusion and gas flow in heterogeneous coal was established. By using this model, the stress relief induced coal damage around slotted boreholes was simulated with the example of Guhanshan Coal Mine. Besides, the effects of homogeneity coefficient, Langmuir volumetric strain constant and overburden stress on coal seam damage and gas extraction were analyzed. The following results were drawn: The rationality and engineering applicability of the model were verified by comparing the numerical simulated results with the field test data in Guhanshan Coal Mine. The stress relief induced coal damage zone around slotted boreholes rises gradually with the increase of the homogeneity coefficient and overburden stress, and the coal seam permeability rises gradually with the increase of Langmuir volumetric strain constant and the decrease of overburden stress. The reasonable borehole distance is determined to be 8 m which is obtained based on the variations of total gas production and gas pressure with different borehole distances. Therefore, reasonable borehole distances under varying conditions can be determined by integrating different geological conditions of coalmines with the proposed simulation method. The simulation results can provide theoretical guidance for the layout of stress relief boreholes and the improvement of gas extraction efficiency in the future.