Characterizing the shape, size, and distribution heterogeneity of pore-fractures in high rank coal based on X-ray CT image analysis and mercury intrusion porosimetry

The shape, size, and surface characters of pore-fractures in coal are the basis for understanding the occurrence and migration of coalbed methane (CBM), which requires quantitative characterization. In this study, X-ray CT image analysis was used to quantitatively characterize the size distribution and volume contribution of pore-fractures in coal at three cohesive-scales (nanometer-scale, micrometer-scale, and macro-scale). A shape factor was introduced to divide the pore-fracture shapes into five types: spherical, tubular, slot, and slit pores and flat fractures, which mainly develop in diameters of 0.2-0.8 mu m, 0.8-10 mu m, 10-30 mu m, 30-40 mu m, and 50-240 mu m, respectively. Compared to mercury intrusion porosimetry (MIP), it was found that the pore diameters measured by X-ray CT image analysis were relatively larger, the pore volume contribution increased then decreased with increasing pore diameter, and pores with diameters of 10-30 mu m were the largest. When the pore diameter is 0.2-1 mu m and 30-40 mu m, the specific surface area by X-ray CT image analysis was 20.38% and 51.53% higher than that from MIP. When the pore diameter is 1-10 mu m, the specific surface area measured by the two methods were similar. When the pore diameter is 10-30 mu m, the specific surface area by MIP was 43.02% higher than that from X-ray CT image analysis. X-ray CT image analysis is more suitable for testing the morphologies than MIP. Additionally, these shape models of pore-fractures in this work can provide some understanding for the permeability calculations.