Multi-scale x-ray computed tomography analysis of coal microstructure and permeability changes as a function of effective stress

Gas permeability (k) and porosity (φ) are the most important parameters in CBM/ECBM and CCS in deep unmineable coal seams. k and φ depend on the coal microstructure, and k and φ significantly change with varying effective stress. However, how the coal microstructure is related to such permeability and porosity changes is only poorly understood. We thus imaged sub-bituminous coal samples at two resolutions (medium - 33.7 μm and high - 3.43 μm voxel size) in 3D with an x-ray micro-computed tomography as a function of applied effective stress; and investigated how cleat morphology, k and φ are influenced by the changes in effective stress and how these parameters are interrelated. In the images, three phases were identified: microcleats (void), a mineral phase (carbonate) and the coal matrix. When effective stress increased, the cleats became narrow and closed or disconnected. This resulted in a dramatic permeability drop with increasing effective stress, while porosity decreased only linearly.