Integration of fluid-invasive, scattering, and imaging methods in resolving pore structures in coal and shale

Abstract

In this study, coal and shale samples were collected from the gas-rich Barakar Formations and investigated using various analytical and imaging methods, to quantify their pore attributes. The results indicate that coal contains an abundance of nanopores that occur in clusters, along with evidence of microfractures in its structure, as observed through scanning electron microscopy (SEM). The accessible micropore surface area (SA) of coal samples is around 2.5 times higher than that of shale samples, while the total mesopore SA in coal is around half that of shales. However, the average pore width of coal samples is approximately 0.82 times that of shale samples. These findings suggest that a higher percentage of organic carbon in coal contributes to an abundance of organic pores, which results in greater porosity in coal samples when compared to shale. The total SA determined by gas adsorption for the entire spectrum of pore sizes in coal is around two times that of shale. Interestingly, despite the difference in the pore SA and the pore volume, the pore surface roughness in the studied coals is almost equal to or slightly higher than that of shales. The study observations show that the total organic carbon and mineral composition in coal and shale play little influence on the degree of pore connectivity. The degree of pore connectivity for the coal samples varies from 0.4–0.93, whereas for shale samples it ranges from 0.50–0.82. This study provides analytical insights into the pore structure of coal and shale collected from the same reservoir by considering factors such as depth, mineralogical content, and surface roughness. During CO₂ injection, coal and shale reservoirs may experience swelling induced stress changes, potentially impacting their mechanical stability. Thus, this study provides insight into estimating the gas-storage capacities of both coal and shale reservoirs and aims to optimise the gas adsorption and maintain structural integrity. This approach ensures the long-term feasibility of implementing Enhanced Coalbed Methane (ECBM) recovery and shale gas recovery in other gas basins.