Experimental study on permeability and mechanical properties of coal under different pore pressure and confining pressure

With the continuous increase of coal mining depth, the response of coal mechanics and the mechanism of gas migration have become extremely complicated. In order to explore the  coal damage evolution and gas seepage mechanism under the integrated operation of first extraction and subsequent mining in engineering, theK₂ coal seam briquette sample of Chongqing Songzao Coal Mine was used as the research object. Using the triaxial servo seepage device of thermal-fluid-solid coupling of gas-bearing coal, the reduced pore pressure seepage test and the triaxial compression-seepage test were successively carried out on the same specimen. According to the elasto-plasticity theory, a statistical damage constitutive model that characterized the whole stress-strain relationship of coal was derived, and the permeability model of coal under consideration of damage was further constructed. The results of the research shown that, in the reduced pore pressure seepage test, the permeability of coal under constant external stress shown a trend of first rising gently and then rising sharply with the decrease of pore pressure. In this process, the change of coal permeability was affected by the competition between effective stress and gas desorption. In the process of the triaxial compression-seepage test, the characteristics of coal deformation stages under different confining stresses were basically similar. As the confining stress increased, the coal mechanics properties were strengthened. The coal permeability curve changed as a negative exponential function with the increasing axial strain . The damage variable curves and plastic strain curves shown a trend of first rising slowly and then rising sharply with the increase of axial strain, the damage evolution process was corresponded to the whole stress-strain curve of each stage of coal deformation and failure. The rationality of the constructed damage constitutive model and permeability model were verified by comparison with test data, which shown that the model can more accurately reflect the characteristics of coal deformation stages and the law of gas seepage.