Well location layout of surface drilling for gas extraction in an abandoned gob

The gob is formed by large-scale voids left after the completion of underground coal mining. A large amount of desorbed gas still remains in the gob. Surface drilling is used to extract gas from abandoned coal mines. The location of surface drilling in abandoned gobs is the key to ensure efficient gas extraction. In this paper, an abandoned gob is divided into the start mining area, the middle area, and the end mining area based on the result of physical similarity simulation experiment. The distribution characteristics of methane concentration in the abandoned gob under the surface drilling extraction are calculated by using the numerical simulation software Ansys. The gas extraction effect in the start mining area, the middle area, and the end mining area is compared. Results show that the methane control range by extraction pipe presents different characteristics along the X-, Y-, and Z-axes of the abandoned gob. When Z = 55 m, the methane control range by extraction pipe reaches the maximum. When Y = 40 m and 60 m, the methane control range by extraction pipe reaches the maximum. In the start and end mining areas, the methane control range by extraction pipe reaches the maximum when X = 70 m and 30 m, respectively. In the middle area, the methane control range by extraction pipe reaches the maximum when X = 100 m and 150 m. The gas extraction effect is the best in the end mining area, followed by the start mining area and finally the middle area. The preferred location of surface drilling is the end mining area of the abandoned gob. The research results provide theoretical basis for gas extraction in abandoned coal mines.