Experimental and theoretical study on the dynamic effective stress of loaded gassy coal during gas release

In the process of mining coalbed methane (CBM), an unsteady state often arises due to the rapid extraction, release and pressure relief of CBM. In this case, the effective stress of coal changes dynamically, affecting the stability of the gassy coal seam. In this paper, gas release tests of gassy coal under conventional triaxial compression were performed, and the dynamic effective stress (DES) during gas release was obtained indirectly based on a constitutive equation and deformation of coal. The results show that the maximum increases in DES caused by the release of free gas and adsorbed gas under the stress of 1.1 MPa were 0.811 and 5.418 MPa, respectively, which seriously affected the stress state of the coal. During the gas release, the free gas pressure and the adsorbed gas volume were the parameters that directly affected the DES and showed a positive linear relationship with the DES with an intercept of zero. The DES of the coal sample increased exponentially with time, which was determined by the contents of free and adsorbed gas. Based on the experimental results and theoretical analysis, an effective stress model was obtained for loaded gassy coal during gas release. The results of verification indicated accuracy greater than 99%.