Study on the evolution mechanism of coal permeability during gas production and coal mining

In order to simulate the operation process of gas mining first and then coal mining on site, the gas-containing coal thermal-fluid-solid coupling triaxial servo seepage device was used to conduct coal seepage tests under reduced pore pressure and full stress strain-seepage tests. By deriving the expression of the change in the width of coal cracks when the temperature rises, a coal permeability model considering temperature-stress coupling was constructed, and the evolution mechanism of coal gas seepage under the action of the temperature and the stress was discussed. The newly-built coal permeability model includes four influencing factors such as effective stress, adsorption/desorption, thermal expansion and thermal cracking, and uses damage variables to characterize the matrix expansion effect(thermal cracking) produced during the crack expansion process. The results show that, when the external stress is constant, the permeability first decreases slightly and then increases rapidly with decreasing the pore pressure at different temperatures, and when the pore pressure is constant, the permeability as a whole decreases first and then increases as the temperature increases. During the full stress strain-seepage test of coal, the permeability first decreases and then increases with increasing the axial stress, and both the elastic model and the peak intensity have a negative correlation with the temperature. The calculated values by the new permeability model are basically the same as the measured values, indicating that the model can better characterize the evolution of the permeability with the pore pressure and the effective stress. Based on the definition of the internal expansion stress, the contribution of the internal expansion deformation to the permeability during the action process of temperature and stress was explored. It is also shown that, when the temperature is constant, the permeability decreases with increasing the internal expansion factor, that the width and the damage of coal fractures are related and that the permeability decreases with increasing the temperature mutation coefficient. © 2022, Science Press. All right reserved.