Influence of High Pressure and Temperature on the Mechanical Behavior and Permeability of a Fractured Coal

Understanding mechanical behavior and permeability of coal at ambient and high temperature is key in optimizing high-temperature in-situ processes such as underground coal gasification. The main objectives of this study were to characterize thermal deformation, stress-strain behavior, and gas permeability of coal samples acquired from the Genesee coal mine in Central Alberta, Canada under various temperatures and confining stresses. These measurements were conducted in a high-pressure high-temperature triaxial apparatus. Initial thermal expansion of the coal was followed by contraction in both axial and lateral directions at about 140 degrees C. This temperature corresponds to occurrence of pyrolysis in the coal. All specimens showed brittle behavior during shear while forming complex shear planes. The specimens exhibited compressional volumetric strain responses at all temperatures. Deformation localization initiated at various stage during shearing. Specimens sheared at 200 degrees C showed higher peak stresses and larger axial strains compared to those tested at room temperature (24 degrees C). Fluctuations of permeability were observed with confining stress and temperature. Permeability dropped at 80 degrees C due to thermal expansion of coal and closure of initial fractures; however, it increased at 140 and 200 degrees C due to a combined response of thermal expansion and pyrolysis. Small axial strain during shear was observed to reduce permeability.