Determination of Effective Radius under the Condition of Nonuniform Gas Occurrence and Borehole Plastic Failure

In order to solve the problem of the blindness of borehole layout when the nonuniform occurrence of gas and the neglect of plastic failure around the borehole will underestimate the gas production, a gas-solid coupling model is established for considering plastic failure. The finite element software COMSOL Multiphysics is used to investigate the evolution of the effective radius during the nonuniform occurrence of gas, and the relationship between the effective radius and the borehole spacing is determined. The results show that the permeability around the borehole increases significantly when plastic failure is considered and the rising area is butterfly shaped, with a maximum increase of 94 times. Not only does the peak value of the gas drainage flow increase but the time of its occurrence is also delayed. The maximum gas drainage flow rate increases by 40.7%, and the cumulative drainage volume increases by 37%. The relationship between the effective radius and reasonable borehole spacing in nonuniform gas occurrence is obtained. This research can provide a reference for the selection of borehole spacing under the condition of nonuniform distribution of coal seam gas.