Analysis on borehole instability under coupled multiple stress and dynamic sealing technology in soft coal seam

The engineering problems such as borehole shrinkage, deformation, wall collapse and plugging holes are prone to result in drainage problems such as low hole-making rate, poor sealing, short service time and big resistance of drainage during drilling and gas extraction. Aiming at the above-mentioned problems, based on the tectonic evolution of soft coal seams, the paper analyzed the effects of gravity stress, tectonic stress, mining-induced stress and gas stress on the stability of gas drainage boreholes in soft coal seam, the paper obtained the instability mechanism of borehole under the coupling effect of multiple stress. And according to the instability law of gas drainage borehole in soft coal seam, it proposed the the integrated protection and sealing technology of gas drainage borehole in soft coal seam, that is to take protecting hole as the basis and self-adaptive dynamic sealing as critical. Engineering tests show that the sealing system made the gas extraction concentration increase to above 90% in single hole and the gas concentration of the single hole increase by 2~3 times, the extraction concentration was stable.