Failure analysis of the methane drainage borehole in the weak coal seam

Methane pre-drainage by borehole is one of the main approaches to control methane in Chinese coal mines. The factors, which cause drainage failure in soft coal seam, including the borehole collapse, gas outburst, water intrusion, geological instability and the mechanical fault of drilling equipment in weak coal seam, were studied in this paper. The result indicated that borehole collapse in soft coal seam contributed most to the drainage failure. A geomechanical model depicting the changes of collapse pressure with time was established and the period of borehole instability was obtained through the model. The study illustrated an approximately linear growth of the maximum collapse pressure with the increase of original gas pressure in coal seam. High maximum collapse pressure leaded to a long risky period of borehole instability. The collapse pressure increased with the decrease of the cohesion, resulting in a longer risky cycle of borehole instability. During the borehole collapse risky period, a good reinforcement of gas drainage borehole in soft seam and the installation of liners could shorten the risky period of borehole collapse.