Effect of Temperature on Permeability and Mechanical Characteristics of Lignite

Underground in situ pyrolysis and gasification is an important method to enable clean utilization of lignite in China. In this study, using the high-temperature triaxial permeability test equipment for different ranges of temperature and pore pressure, the permeability and mechanical characteristics of lignite from the Pingzhuang Mine Area in Chifeng have been examined. The results show that, at constant confining pressure, the elastic modulus of lignite decreases with increasing temperature. For temperature up to approximately 75°C, the elastic modulus is close to the modulus under the uniaxial state. As the temperature increases, the stress-strain curves during loading and unloading are different. The differences between the curves during loading and unloading are greater at higher temperature due to the greater residual deformation. In addition, in different temperature ranges (i.e., 150–650°C), the triaxial creep curves of lignite are different. In particular, at 300–450°C, the triaxial creep curve of lignite alternates between the accelerated creep and the steady creep. Moreover, the permeability change rule in the lignite is complex, and it is governed by the temperature and pore pressure. Hence, for different temperature range and pore pressure, the variations in the permeability are different. In fact, as the temperature increases, the permeability of lignite fluctuates.