Study on dual pore permeability model of coal under the conditions of pore pressure rise and fall

Pore pressure is the key factor to control the permeability of coal and rock. In order to explore the response mechanism of coal and rock permeability in the process of pore pressure increasing and decreasing, the 3-axis seepage test device for gas bearing coal was used to carry out seepage tests on the increase and decrease of pore pressure under different average stress conditions. Based on the characteristics of dual pore structure medium of coal and rock, considering the mechanical effect of coal, slippage effect, adsorption expansion, and thickness change of adsorption layer in the process of pressure boost, a dual pore permeability model including matrix and fracture was constructed. By introducing a modified function L(p), the change of coal and rock permeability during depressurization was further quantified, and the rationality of the new permeability model was verified by using test data. The research results show that：①When the average stress is constant, the matrix permeability decreases sharply at first and then slowly with the increase of pore pressure, and the change law of fracture permeability is close to that of the total permeability of coal and rock. ②When the average stress is constant, the total permeability of coal and rock in the process of pore pressure increasing and decreasing shows a “V” shape. But for the same pore pressure, the total permeability in the process of pressure drop is lower than that in the process of pressure rise. ③The model is verified by seepage test data, and it is found that the new dual pore permeability model is consistent with the test results. ④With the change of pore pressure, the sensitivity coefficient c in the correction function L(p) affects the slope of the permeability curve，and the sensitivity coefficient d affects the overall height of the permeability curve. ④The sensitivity coefficient c in the modified function L(p) affects the slope of the permeability curve, and the sensitivity coefficient d affects the overall height of the permeability curve.