Enhancement of gas drainage efficiency in a special thick coal seam through hydraulic flushing

Coal seam gas is a kind of precious energy, but it also will be a terrible disaster in coal mining if not well controlled. Underground coal seam gas drainage is one of the most effective methods for lowering the risk of gas disaster. However, it is difficult to extract gas in many coal seams with low permeability, especially in tectonic coal seams, which were severely crushed or even pulverized by tectonism. The key to improving the gas drainage efficiency is stress relief and permeability increase. There are many conventional technologies to improve the gas drainage efficiency of a single coal seam, but they are inoperative on the special thick coal seam with a soft mylonitic tectonic coal sub-layer at the lower part. In this paper, to enhance gas drainage in this coal seam, we presented a hydraulic flushing gas drainage technology. Numerical simulation was conducted to analyze the stress evolution, plastic damage, permeability changes and gas drainage of the coal after hydraulic flushing. Further, abundant field experiments were carried out to investigate and verify the mechanism of stress relief and permeability increase. The results show that, after adopting the hydraulic flushing gas drainage technology, more tectonic coal in the thick coal seam was flushed out and the big hole in the tectonic coal sub-layer enlarged the range of stress relief and permeability increase, and the gas drainage efficiency increased significantly.