Effect of Pore Pressure on Seepage Characteristics of Coal and Rock at Different Temperatures

The ground temperature is higher and the pore pressure gradually decreases in the process of gas extraction in deep coal seams, however there are few studies on the coupling effect of temperature and pore pressure on the permeability of coal. Seepage experiments on the raw coal from Guizhou mining area are carried out by the self-developed triaxial seepage equipment with an adjustable outlet pressure to study the influence of pore pressure and temperature on the permeability. A permeability matching model with temperature effect is also developed in this study. An exponential relationship between the permeability and pore pressure is found and presented. The results show that the permeability decreases with increasing pore pressure, and it decreases as the differential pressure increases. Also, the permeability of coal seam decreases with increased temperature, and the decreasing rate and magnitude of permeability are different under different temperatures. Therefore, the differential pressure should be as small as possible to reduce the error and help develop the permeability model with different boundary conditions in the physical simulation experiments of coalbed methane (CBM) extraction. As the temperature increases, the mutation coefficient of temperature increases. While, the mutation coefficient of temperature decreases as pore pressure increases. It has been found these two characteristics that the mutation coefficient of temperature is not a constant in the whole stage and the realistic model matching with a variable cleat compressibility coefficient can reflect the development process of CBM.