Disaster-causing mechanisms of gas migration under loading and unloading conditions

The aim of studying the mechanisms of coal and gas outbursts is to master the occurrence and development process of outbursts and to determine the reasons and conditions for outburst occurrence. Gas permeability is an important parameter for characterizing the difficulty of gas migration in a coal seam and an important factor in studying coal and gas outbursts. By utilizing a THM-2 type thermo-fluid-solid coupling test system of gas-bearing coal developed at Chongqing University, China, gas-bearing coal was experimentally investigated under loading and unloading paths. The results demonstrate that permeability is significantly affected by stress paths and gradually decreases with an increase in stress. The permeability of coal samples under unloading increased with the number of loading and unloading cycles and the irreversible strain was negatively correlated with permeability. In situ stress mainly affects the difficulty of migration and the flow direction of gas in the coal seam by controlling the pore and fracture systems of the coal mass. The point of maximum bearing pressure is the boundary point of the gas migration direction. In the coal mining process, mining activities break the in-situ stress state and induce stress concentration. As the mining face advances, the abutment pressure curve shifts to the deep part of the coal seam as does the permeability. Gas is prone to accumulate in the area between the maximum abutment pressure and the in situ stress zone, which increases the gas concentration, thus forming a zone with increased outburst risks. Highlights The direction of gas transportation in the coal seam was determined, and a model was established. The gas migration in the coal seam is divided into different areas. It was found that the maximum supporting pressure point is the boundary point of the direction of gas migration, which is the main factor that makes gas migration difficult.