Study on permeability characteristics of coal rock under the interaction of coal matrix and fracture

In order to study the influence of temperature and gas pressure on the adsorption and seepage characteristics of coal and rock in the process of the mining of coalbed methane, the isothermal adsorption test device and the triaxial seepage test device for gas-bearing coal were used to carry out the adsorption isotherm test and the triaxial seepage test with variable gas pressure under different temperature conditions respectively. Considering the capillary fractal characteristics under stress, the stress sensitivity model of fracture volume is established. On this basis, a coal permeability model considering the expansion and deformation of the coal matrix, temperature and gas pressure changes is established. The results show that: ① At the same temperature, as the gas pressure increases, the gas adsorption capacity of coal and rock gradually increases, but the adsorption rate shows the opposite trend. Under the same gas pressure, the gas adsorption capacity shows a downward trend as the temperature increases. When the effective stress is constant, the amount of coal adsorption deformation increases with the increase of gas pressure, and decreases with the increase of temperature. ② Under the action of external stress, the capillary side inside the coal shrinks and expands radically. The fracture compression coefficient calculated by the new fracture volume stress sensitivity model is in the same order of magnitude as the laboratory value, and it shows a downward trend with the increase of effective stress. ③ The new permeability model can better reflect the evolution of permeability under different temperatures and gas pressures. At the same temperature as the gas pressure increases, the permeability of coal first drops sharply and then tends to be gentle. The influence of matrix expansion and deformation around pores and cracks on permeability gradually decreases.