Study on optimization of gas drainage parameters with borehole drilled along seam based on coupling of multi physical field

In order to reveal the gas drainage seepage mechanism of the borehole drilled along the mining seam under the coupling effect of the multi physical field,a fluid-solid coupling model with a consideration of the Klinkenberg effect,the effective stress and the desorption shrinkage influence was established. The model was applied to optimize the drainage parameters of the borehole drilled along the seam. The results showed that the drainage time would be very effected to the effective drainage radius. When the drainage time was a day,the effective drainage radius would be 0.38 m. When a drainage time of the borehole was 10 days,the effective drainage radius would be increased by 1.55 times only. When the drainage time was 180 days,the effective drainage radius would be increased by 8.26 times. When in the high gas pressure area,the porosity and permeability of the coal around the borehole in the seam were in a decreasing tendency. With an influence degree of the borehole gas drainage decreased,the desorption and shrinkage effect of the coal matrix would be steadily improved,the declining tendency of the porosity and permeability would steadily be slow and the simulation results and the theoretical analysis would be well consistent. With the diameter of the gas drainage borehole increased,the gas pressure reduced scope in the seam around the borehole would be steadily increased and the effective drainage radius and the borehole diameter could meet the power function relation. Under the prerequisite to ensure the gas drainage effect of the borehole,the most rational borehole diameter determined in No. 29031 coal mining face of a mine was 94 mm.