Delayed Effect Analysis of Gas Extraction Boreholes Instability in Coal Seam

Aiming at the delay phenomenon of gas extract borehole structural instability in rick jointed coal seam, this paper analyzed gas flow field and jointed cracks field around gas extraction boreholes. The results showed that the drilling structural instability is the combined effect of gas pressure and joint cracks, the change of gas pressure would affect normal stress of jointed cracks surface, increasing the possibility of jointed cracks development and failure, and the performance was borehole instability in the macroscopic. The internal cause of the instability delay of gas extraction boreholes is the existence of the jointed cracks in coal seam, and external cause is gas pressure changes in jointed seam. With practical examples, this paper illustrated that the instability delay phenomenon of gas extraction drilling could cause gas extraction concentration and flow decimated and other drilling failure problems.