Research progress on surface drainage of coalbed methane in abandoned mine gobs of Jincheng Mining Area

There are abundant coalbed methane (CBM) resources in abandoned coal mines, and the extraction of CBM from abandoned coal mines has become one of the important CBM resources in coal mining areas.Based on the practice of CBM extraction in abandoned mines in Jincheng mining area of Shanxi Province, this paper systematically introduced the occurrence characteristics of CBM resources in abandoned mine gob and its surface drainage technology, and analyzed the research status and existing problems of CBM surface drainage in abandoned mine gob. The current key scientific problems and research contents of surface drainage of CBM in abandoned mine gob in China have been put forward.The research shows that the caving-fracture zone formed by coal mining provides a reservoir space for the occurrence of CBM in the gob. The CBM in the abandoned mine goaf mainly contains adsorbed gas and free gas which comes from the coal pillars, residual coal seams, adjacent unmined coal seams and surrounding rocks. Based on the volumetric method, the calculation methods of adsorbed gas and free gas resources are used to calculate and evaluate the CBM resources of abandoned coal mine. In order to effectively solve the influence of water inrush from the upper aquifer in the gob on the stability of the drilling well and the drainage effect, the wellbore structure of the CBM extraction well in the gob is optimized from secondspud to third spud structure, and compressed air down-the-hole (DTH) hammerdrilling technology is preferentially used, and the nitrogen is used instead of air as the circulating medium to safely expose the well technology of the gas-bearing fracture zone.According to the variation characteristics of pressure and daily gas production in the extraction process of CBM in gob of abandoned mine in Jincheng mining area, the CBM extraction in gob is divided into four stages: free ejection, rapid gas decline, stable gas production and quantitative gas production, and a “four-stage” extraction control method suitable for coalbed methane extraction in abandoned mine gobs is proposed. Finally, it points out the two key scientific issues currently facing “the occurrence law of coalbed methane in abandoned mines and its control mechanism and the evaluation of coalbed methane extraction conditions in abandoned mines and the optimization of extraction parameters” and ”the occurrence law of coalbed methane in abandoned mines and its research content such as control mechanism, CBM resource evaluation models and methods in abandoned mines, and CBM extraction technology in abandoned mine gobs.”