Coal seam permeability improvement and cbm production enhancement by enlarged borehole: Mechanism and application

The permeability is a key factor to determine the efficiency of coalbed methane (CBM) production. The borehole enlargement technology using hydraulic and mechanical measures to cut coal is an effective method to increase the coal seam permeability and improve the efficiency of gas drainage. Reasonable design of the layout of bore-holes is the prerequisite for efficient and economical gas drainage. In this paper, based on the strain-softening model, the stress and permeability model of the coal seam around the enlarged borehole was built, and based on the dual-medium model, the gas migration model in the coal seam was established. Then the borehole enlargement gas drainage engineering of E9/10 coal seam in Pingdingshan No. 8 coal mine was simulated by using COMSOL Multiphysics software. The distribution of stress and permeability in the coal seam around a borehole was analyzed, and the reasonable borehole radius of 0.25 m and reasonable borehole spacing of 6 m were deter-mined. Finally, in Pingdingshan No. 8 coal mine, field application was carried out in E9/10 coal seam-21070 work-ing face from the high-level gas drainage roadway. The results show that the actual average coal slag discharge rate is 77.82%, which achieved borehole enlargement. The natural gas flow rate from an enlarged borehole is 2.3–7.3 times that of a normal borehole, and the influence range of enlarged boreholes is more than 6 m. The average gas drainage concentration of a group of enlarged boreholes is about 42%, and the average gas drainage amount is about 0.53 m3/min. After two months of gas extraction, the outburst risk in this area was eliminated, which provides a guarantee for safe coal mining. © 2021, Tech Science Press. All rights reserved.