Evolution law of coal pore structure under SCCO₂-H₂O-coal coupled interaction

For CO₂ geological sequestration in deep coal seams, focusing on the evolution of coal pore structure under the long-term coupling of CO₂-H₂O-coal, lean coal from the 7th member of the Upper Triassic Xujiahe Formation in Shidonggou Coal Mine was taken as the research object. Scanning electron microscopy, low-temperature nitrogen adsorption, and mercury intrusion porosimetry were adopted to reveal the dynamic evolution mechanism of coal pore structure under the SCCO₂-H₂O-coal coupled interaction. The results show that the coal surface structure evolves in stages at short term(15 d), medium term(30 d), and long term(60 d) under SCCO₂-H₂O interaction. In the short term, small-molecule extraction and mineral dissolution dominate, expanding the pore-fracture network. Fracture filling dominales in the medium term. In the long term, the carbon content decreases by 15%, mineral debris migrates with fluids, and mineral transformation and precipitation take place, resulting in a smoother coal surface and sealed pores. Regarding micropore and mespore structure, short-term SCCO₂ extraction increases the micropore volume proportion from 7% to 9% with a 61.6% increase in the cumulative specific surface area. In the medium term, mineral precipitation reduces the micropore volume by 43.2% with mesopores accounting for 51%. In the long term, micropores nearly disappear, the specific surface area is reduced compared to that of raw coal, and the CO₂ adsorption capacity first increases and then decreases. For macropore structure, the short-term macropore proportion is 55% and the porosity is 47.6%, leading to low seepage resistance. A mesopore network develops and its proportion rises in the medium term. In the long term under high temperature and high pressure, macropores increase but most are blocked by precipitates, reducing the effective seepage space and weakening CO₂ migration ability. The findings provide a reference for CO₂ geological sequestration in medium-rank bituminous coal reservoirs.