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

Abstract

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 μm, 0.8–10 μm, 10–30 μm, 30–40 μm, and 50–240 μm, 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 μm were the largest. When the pore diameter is 0.2–1 μm and 30–40 μm, 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 μm, the specific surface area measured by the two methods were similar. When the pore diameter is 10–30 μm, 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.