Morphological Characterization of Pore Connectivity in Coal: Insights from Quantitative Analysis of Constricted Pore Network Accessibility

The connectivity and accessibility of the coal pore network structure determine the mechanism of CBM storage and migration, which is of crucial significance for efficient utilization as well as for mine safety and disaster prevention. This study employed the multitechniques of mercury intrusion, physisorption, and low-field NMR to comprehensively characterize the morphological pore network of original and tectonic coals with metamorphism. Additionally, different from the limitations of qualitative analyses of pore connectivity, this study proposes a quantitative characterization method for a constricted pore network based on the accessibility response index λ, thus supplementing and expanding the evaluation methods for coal pore structure connectivity. The research findings indicate that compared to the original coal, tectonic coal exhibits stronger adsorption capacity and instant desorption ability. Moreover, the development of meso- and macropores in tectonic coal is more pronounced, indicating that meso- and macropores may be more sensitive to the pore-enhancing behavior of tectonics. After centrifugation, the T2 relaxation curve of low-field NMR could reveal significant residual water, illustrating the presence of constricted pore configurations composed of interconnected and semiopen pores restricted by throats in coal, which may cause a rapid decline in pore connectivity under capillary and barrier effects. Similarly, the hysteresis loops of mercury injection-ejection curves and the physisorption isotherm may be closely related to such a constricted configuration. After tectonism, the tortuosity and the pore-to-throat ratio of middle-high-ranked coal decreased from 2.211 and 1.121 to 2.186 and 0.829, respectively. Meanwhile, the λ values of low-ranked tectonic coal and middle-high-ranked tectonic coal increased by 22.0% and 16.8%, respectively. These changes reveal that tectonism has modified the constricted pore configurations in coal, thereby enhancing the internal gas migration pathways and ultimately strengthening the connectivity and accessibility of the pore network. The above-mentioned results may provide significant guidance for uncovering the abnormal enrichment of coal mine gas and the mechanism of gas disaster.