Influencing mechanisms of SC-CO₂ extraction on the microstructures of high-rank tectonic coals

This study aims to reveal the influencing patterns of supercritical CO₂ (SC-CO₂) on the microstructures of tectonic coals during the injection of CO₂ into deep coal seams. The study conducted SC-CO₂ extraction experiments on two types of high-rank coal samples with different damage degrees, namely the lean coal samples from the Daping coal mine in Shanxi Province and the anthracite samples from the Zhenxing coal mine in Guizhou Province. Accordingly, this study compared the parameters of pore structures and aromatic carbon stacking structures in the coal samples before and after the extraction experiments. Finally, this study explored the influencing mechanisms of SC-CO₂ on the microstructures of high-rank coals with different damage degrees. The results show that: (1) With an increase in the damage degree, the two types of high-rank coal samples showed increased pore volume and specific surface area and enhanced pore connectivity. However, compared with primary structural coal samples, the tectonic coal samples showed higher fractal dimension and more complex pore structure when the pore size>4 nm. (2) Under the influence of SC-CO₂ fluid, the two types of coal samples showed significantly increased average pore size, decreased fractal dimension of pores, and simpler pore structure. However, the coal samples with different damage degrees underwent complex changes in pore volume, specific surface area, and pore connectivity. Specifically, the tectonic coal samples from the Daping coal mine showed significantly decreased specific surface area and pore volume of micropores and mesopores, which significantly decreased the total specific surface area and the total pore volume and impaired the pore connectivity. By contrast, the tectonic coal samples from the Zhenxing coal mine showed significantly increased specific surface area and pore volume of mesopores, which increased the total specific surface area and the total pore volume. (3) Under the influence of SC-CO₂ extraction, the high-rank coals with different damage degrees showed increased interlayer spacing (d₀₀₂) of carbon layers. However, their ductility (L_a), stacking height (L_c), and the number of aromatic layers (N_c) showed an upward trend. Moreover, the amplitude of these changes tended to be increasingly significant with an increase in the damage degree, making the parameters of the carbon layers of the coal samples with different damage degrees tend to be consistent. The following conclusions can be reached from this study: (1) The dissolution of coals by the products of the reactions between the SC-CO₂ fluid and coals causes a pore expansion effect, which increases the average pore size of high-rank coals. However, different amounts of the products and the pore plugging effect caused by product retention are two major reasons for the differential changes in the pores of the two types of high-rank coals with different damage degrees. (2) The SC-CO₂ fluid and the tectonic damage produce opposite effects on the structural parameters of aromatic carbon layers of the two types of high-rank coals, leading to the looseness and compaction of the aromatic carbon layers, respectively.