Drainage Test of Surface Drilling Crossing Multi-gobs and Characteristics Analysis of Well Breakdown

Coalbed methane can be extracted by surface drilling efficiently. According to the situation that there are multiple gobs above the mining seam in Wuhushan Coal Mine, pipe-follow drilling technology using eccentric bit combined with pipe shoe was applied to solve the problems effectively of drill sticking or burying when borehole passed through gob. Based on the pipe-follow drilling technology, surface drilling operation was successfully completed. During the longwall face mining, the surface drilling extracted gas for 9 days, and the accumulated drainage amount of pure gas was 2.31×10⁴ m³. Due to the poor drainage effect, drainage was forced to stop. The forced characteristics of casing under the action of mining disturbance were researched by using ABAQUS finite element software. The results show that the stress of drilling casing in the position of overlying gob was greater, and this was the key position for the drilling breaking.