The effect of gas migration on the deformation and permeability of coal under the condition of true triaxial stress

The dynamic characteristics of gas flow are often not considered in a true triaxial stress environment, making the simulation test hard to meet the requirement for coal and gas outburst in reality. To solve this issue, we investigated the deformation and gas flow characteristics of coal-like briquette under true triaxial stress conditions using self-developed triaxial servo-controlled seepage equipment for fluid-solid coupling of coal containing methane. Analysis of the experimental data revealed the relationship of coal body's stress to strain as well as the relationship of their permeability. The results showed that in the conventional triaxial and true triaxial compression seepage tests, the relationship of stress-strain-permeability varies at different stages. The stress-strain curves in the whole stress-strain process of coal show a trend of decrease after increase, while the permeability-strain curves show a trend of increase after decrease. There is a "stress-sensitive zone" in the influence of the intermediate principal stress on the minimum permeability of the coal. The gas pressure promotes the "fluctuation" strain of the coal body, resulting in an instantaneous increase in the strain rate. The research provides the experimental basis for studying the permeability of coal body in the true triaxial stress environment.