Mechanistic effects of supercritical CO₂-H₂O-coal geochemical reaction on pore-fracture networks in deep high-sulfur coal seams

Deep high-sulfur coal seams exhibit good structural integrity and favorable sealing properties, demonstrating significant potential as geological reservoirs for CO₂ sequestration. However, there has been a lack of focused research on CO₂ sequestration in deep high-sulfur coal. Therefore, focusing on the No.17 (conventional coal) and No.21 (high-sulfur coal) from Sanhejian Mine, through ScCO₂-H₂O-coal geochemical reaction, X-ray diffraction and pore structure characterization tests, to study variation in mineral composition, pore structure and fracture network, and investigate the pore-fracture evolution mechanisms in deep high-sulfur coal seams induced by the ScCO₂-H₂O-coal reaction and explore the feasibility of CO₂ sequestration in deep high-sulfur coal seams. The results indicate that: During the process of the ScCO₂-H₂O-coal reaction, the dissolution of carbonate minerals and the extraction effect of ScCO₂ improved the pore and fracture structure, manifested as positive effects of pore expansion and increase. While coal matrix swelling, kaolinite hydration, secondary gypsum precipitation, and CO₂ mineralization products cause the pore throat constriction and reduced connectivity, manifested as negative effects of compressing and blocking the pore. However, the influence of pore pressure changes on pore and fracture has both positive and negative dual effects. Overall, the positive effect plays a dominant role. The differences of the degree of dissolution of carbonate minerals after ScCO₂-H₂O-coal reaction leads to the difference in the property of pore and fracture between high-sulfur coal and conventional coal seams, while the influence of the variation in content of pyrite is limited. The research suggests that the ScCO₂-H₂O-coal reaction improves the property of pore and fracture of high-sulfur coal seams, increases the CO₂ adsorption sites and diffusion paths in the pores of high-sulfur coal, which is conducive to the injection, migration, diffusion and adsorption of CO₂. In the initial phase of the reaction, the improvement effect is significant, which is conductive to CO₂ sequestration in high-sulfur coal seams. This study confirmed the feasibility of CO₂ sequestration in deep high-sulfur coal at the microscopic level.