Study on mechanism of permeability improvement for gas drainage by cross-seam cavitation borehole

As coal mining gradually extends to the deep level,the pressure and content of coal seam gas increase,and the permeability of coal seam is generally low,which increases the difficulty of gas extraction.The technology of pressure relief by cavity drilling is an effective method to improve the permeability of coal seam and the efficiency of gas extraction. Based on the engineering background of No.15 coal seam cavity drilling in the auxiliary transportation roadway of north-wing of Sijiazhuang Coal Mine,and according to the elastic mechanics softening model and diffusion-seepage coal seam gas flow theoretical model,the authors established the evolution equation of coal seam permeability and the gas flow equation of cross-seam cavitation borehole. Using COMSOL Multiphysics software,the models were calculated,and the permeability increasing mechanism and distribution law after cross-seam cavitation borehole were analyzed. The gas extraction amount,gas pressure distribution and permeability were obtained. Combining the simulation results with field conditions,the optimal cavity radius was 0.6 m,and the optimal hole spacing was 6.0～7.0 m. It provided a guidance for the borehole radii and spacing in field construction. Finally,the conventional drilling and cavity drilling were carried out in No.15 coal seam of Sijiazhuang Coal Mine,and the gas extraction effect was compared. Results showed that the gas drainage rate,gas concentration and purity were increased by 2.7,2.0 and 5.7 times compared with the conventional drilling after the implementation of hydraulic cavity drilling technology. The period of gas drainage and the amount of drilling construction were reduced about 2 times,which improved the economic benefits of gas drainage technology. Thus,the simulation results are reliable and the gas drainage design is feasible,which can guide the field construction.