Ground development technology and engineering application of CBM in coal mine goafs： A case study of Jincheng mining area in Qinshui Basin

There are still rich CBM resources in goafs after coal mining, and the resource evaluation, development and utilization have dual significance of environmental protection and resources. In recent years, ground drilling has been actively explored in Jincheng, Xishan and Yangquan mining areas in Shanxi Province to extract CBM in the goafs. However, in the process of ground development of CBM, due to water accumulation in the goafs, confined water gushing from overlying strata and other reasons, there is no gas production or low gas extraction from ground drilling. Taking Yuecheng Coal Mine in Jincheng, Shanxi as an example, this paper studies the occurrence characteristics of coal-bed methane in goafs under different coal mining technologies, the influence of goaf water accumulation and rock permeability in goaf fracture zones on the ground development of coal-bed methane in the goafs, and puts forward the layout principle and extraction technology of goaf wells. To solve the problem that rock powder enters the fracture zones blocking the migration channel of coal-bed methane in goafs during the three-open air drilling, the application of hydraulic punching method is explored to improve the rock permeability in goaf fracture zones. The research shows that the well layout principle of the coal mine goafs is as follows: (1) The goaf wells should be arranged outside the goaf water accumulation area; (2) For the spatial form of goafs formed by room-and-pillar mining, the goaf wells should avoid the protective coal pillar and finally complete drilling into the goaf space. In view of the spatial form of goaf formed by long arm caving method, the optimal drilling area of goaf well is the “O” circle boundary connection line and the stope boundary near the terminal line. An integrated drainage and gas extraction system for goaf wells in coal mines was developed. The drainage system realized the simultaneous drainage of accumulated water at the bottom of goaf wells and the extraction of CBM, and solved the problem of CBM production decline caused by the pressure water inflow of the overlying strata in goafs. The CBM extraction amount of goaf wells increased by 33.3% after the optimization of the drainage system. The hydraulic punching is applied to solve the problem of permeability decline in fractured zones of the goaf caused by drilling cuttings. The engineering test results show that the maximum increase rate of daily average extraction of goaf wells before and after hydraulic punching transformation is 11.30%. The suggestion of using foam under-balanced drilling technology to solve the problem of drilling rock powder invading into fractured zones of the goaf is put forward.