Numerical Evaluation on Stress and Permeability Evolution of Overlying Coal Seams for Gas Drainage and Gas Disaster Elimination in Protective Layer Mining

Coalbed methane development is an important way to effectively utilize clean energy and reduce carbon dioxide and other gas emissions. However, due to the low permeability of coal seams, the development of coalbed methane in China has encountered great difficulties. Protective layer mining can efficiently minimize in situ stress and increase coal seam permeability, thus improving the gas drainage and eliminating the gas disaster. This study analyzed the characteristics of gas migration and the change in permeability during in situ stress releasing process through experiments and numerical simulation to study the gas drainage effect of a coal mine in an unloaded coal seam under specific geological conditions. First, a stress–strain-seepage coupled test of coal is carried out, and the evolution features of coal permeability during stress loading and unloading processes are investigated. Second, experimental results are used to develop a mathematical model of coal seepage under mining unloading conditions. Finally, a numerical model is developed based on the real geological conditions of the Wulan coal mine to explore the characteristics of in situ stress release and permeability evolution, as well as the gas drainage process in an unloaded coal seam. Combining protective layer mining and gas drainage technology, the risk of gas outburst is significantly eliminated.