Effects of CO2 intrusion on pore structure characteristics of mineral-bearing coal: Implication for CO2 injection pressure

CO₂ intrusion has a crucial effect on the pore structure of mineral-bearing coal. In this study, we selected long flame coal, lean coal, and anthracite after CO₂ adsorption at different pressures and tested the coal samples using X-ray diffraction, mercury intrusion porosimetry, and N₂ (77 K) adsorption methods. The tests were conducted to determine the variations in mineral content, pore structure, and fractal characteristics. The results showed that supercritical CO₂ had a greater ability to dissolve minerals in coal than that of subcritical CO₂. Although the total pore volume and BET specific surface area gradually increased with the increase in CO₂ intrusion pressure in coal, the transformation of different pores and partial new pores caused by the dissolution of minerals and the adsorption swelling of coal matrix caused the micro-macropores in the three coal samples to exhibit different trends. The pore surface roughness and pore structure complexity of seepage pore in the long-flame coal after CO₂ adsorption increased while those of the lean coal and anthracite decreased. Meanwhile, CO₂ intrusion caused the surface of the adsorption pore in coal to become smooth, and the pore structure was more regular, except for the lean coal. A conceptual model of the mineral-bearing coal was developed to describe the relationship between the mineral composition and pore structure induced by CO₂ intrusion. These findings help to understand the transformation effect of CO₂ on coal seams. Thus, a higher CO₂ injection pressure should be used to obtain a larger injection volume and shorter injection time during CO₂ storage implementation.