Study on key technology for surface extraction of coalbed methane in coal mine goaf from Sihe Wells Area,Jincheng

The coalbed methane (CBM) of coal mine goaf has become one of the important resources with the continuous deepening of CBM exploration. Based on the analysis of goaf distribution characteristics in Sihe Wells Area of Jincheng Mining Area,the resource conditions of CBM in goaf,the key technology of surface extraction are systematically studied through geological analysis, CBM composition，concentration experiment in goaf,and resource model calculation. The occurrence law of CBM in goaf is revealed, the calculation model and method of CBM resources under different occurrence states are given, and the key technology of surface extraction of CBM in goaf is explored. It is shown that the CBM in coal mine goaf comes from free gas and adsorbed gas in coal pillar, residual coal seam, adjacent undeveloped coal seam,and surrounding rock. According to the calculation model of adsorbed gas and free gas, the total resource of CBM in Sihe Wells Area is 213.016×108m3, among them, the free gas resource is 0.102 × 108m3, and the adsorbed gas resource is 212.914×108m3. The coal mining method and goaf airtightness have an important impact on the source and enrichment degree of CBM in goaf. According to the characteristics of fracture development in the upper part of goaf, the wellbore structure of CBM extraction wells in goaf is optimized from the second opening to the third opening. The upper aquifer of the fracture zone is sealed by cementing in the second opening, and the slotted casing protection is installed in the third opening, which effectively solves the problems of the drilling stability and the pumping effect caused by water gushing from the upper aquifer of the goaf. On this basis, the drilling technology of down-the-hole hammer (DTH hammer) + compressed air (nitrogen) has been developed. Nitrogen is used to replace air as circulating medium and a set of drilling technology for safely exposing gas bearing fracture zones is formed, which provides an effective way for safe extraction of coalbed methane in goaf