A fully multifield coupling model of gas extraction and air leakage for in-seam borehole

Air leakage around the in-seam borehole leads to a poor quality of gas extracted, thus gas outburst danger is difficult to eliminate but also may cause other disasters, such as coal spontaneous combustion and gas explosion. Moreover, the low concentration gas discharged into the atmosphere always possesses an intense greenhouse effect. To investigate multiphase and multifield coupling problem of gas extraction, a dual medium coupling model was developed in this study to describe the gas–air mixture flow by considering the influences of matrix shrinkage and effective stress on coal permeability. And the coupling model was calculated by the 3D numerical simulation and presented an ideal matching result with the field data. Then, the key factors affecting the gas extraction quality were also investigated with the model. It was found that the gas concentration elevated with the increase in the sealing length and borehole diameter, and reduced with the augmentation of extraction negative pressure and coal permeability. Therefore, we developed an improved sealing technology, which can fully seal the various fractures around the borehole for improving gas extraction. The field test indicated that the gas concentration by this improved sealing technology was 1.91 times that of the traditional sealing method, and still maintained a relatively high extraction level in the subsequent extraction stage with a superior application effect. The results of this work can help in optimizing the borehole sealing technology for improving gas extraction quality, which is of great significance to CBM exploitation and utilization, reduce greenhouse gas emission and ensure safe mining.