Study on gas extraction from parabolic high level drilling during the initial mining period of fully mechanized caving face

The gas extraction effect during the initial pressure on the working face by using conventional high-level directional long drilling is poor. In order to solve the above problem, taking the project background of 12309 fully mechanized caving face of Wangjialing Mine of China Coal Huajin Group Co., Ltd. as an example, the paper analyzes the overburden rock structure and fracture development law of the coal seam roof in the fully mechanized caving face during the initial mining period by physical similar simulation and numerical calculation. During the initial mining period, the height of fractures is low and the number of fractures is small in the coal seam roof. With the advancement of working face, the roof fractures gradually develop, the height and range of fractures increase with the increase of advancement distance. The maximum height of the fractures is about 28 m, and the pressure step distance of the working face is about 50 m. During the initial mining period, the gas concentration and emission volume of the working face are relatively large. Along the working face tendency, the gas concentration gradually increases from 20 to 150 supports. Along the working face direction, the gas concentration gradually increases from 300 mm of the coal wall to the rear scraper conveyor. The gas emission volume of the goaf mining area accounts for more than 50% of the gas emission volume of the working face, and the gas emission volume as a whole is on an upward trend with obvious phase characteristics. According to the actual situation of the site and the gas emission characteristics, overburden rock structure and fracture evolution law of the fully mechanized caving face during the initial mining period, the trajectory of the conventional high-level directional drilling during the initial mining period is optimized, and the parabolic high-level directional drilling gas extraction method during the initial mining period is proposed. The final hole of the borehole is designed in the coal seam, which is directly connected with the goaf mining area and used for low level gas extraction in the mining area during the initial mining period of the working face. It solves the problem of high level of conventional high-level long borehole during the initial mining period. The field application results show that compared with the conventional high-level directional drilling, the use of parabolic high-level directional drilling can extract the low level gas in the goaf mining area effectively before the initial pressure of the basic roof of the working face. The average pure volume of gas extraction increases by about 37%, and the maximum gas volume fraction in the upper corner and return air flow is less than 0.80%. The result has achieved the expected effect of gas extraction.