Effect of supercritical CO₂ on heterogeneity and connectivity of coal micro-pore structure

Supercritical CO₂ fracturing technology can not only improve coalbed methane recovery but also realize in-situ CO₂ sequestration. Supercritical CO₂ soaking modification experiments were carried out on coal samples collected from Huanghou Coal Mine in Yangquan. Combined with low-temperature N₂ and low-pressure CO₂ adsorption tests, the differences in the microporous and mesoporous structure of coal samples before and after modification were characterized. Based on the multifractal theory, the changes in pore size distribution heterogeneity and connectivity were quantitatively analyzed. The results show that supercritical CO₂ treatment promotes pore development of coal samples: the number of mesopores increases significantly, while the number of micropores decreases relatively. After treatment, the N₂ adsorption capacity of coal samples increases, the adsorption-desorption hysteresis weakens, the pore inlet and outlet become wider, the retention effect decreases, and the pore connectivity is enhanced. The heterogeneity of micropore size distribution decreases with significantly improved connectivity, whereas the heterogeneity of mesopore size distribution increases with reduced connectivity. Compared with raw coal, coal samples treated by supercritical CO₂ show overall enhanced pore connectivity, which is conducive to improving coal reservoir permeability. The pore surface becomes rougher, the pore size distribution is more uneven, and the complexity of the pore structure is significantly improved. The metamorphic degree and maceral composition of coal jointly affect the heterogeneity and connectivity of the pore structure. High-rank coal has a complex pore microstructure, and its connectivity is significantly improved after CO₂ treatment. For low-rank coal, the permeability enhancement effect lags under the action of CO₂ extraction and swelling. Supercritical CO₂ modification endows the coal with higher pore heterogeneity over a wider pore size range and forms a more connected pore-fracture network.