The effect of subcritical and supercritical CO2 on the pore structure of bituminous coals

The influence of CO2 on the pore structure of coals has a significance for CO2-ECBM recovery and CGS. In this study, three coal samples from two typical regions in China were treated with subcritical and supercritical CO2 at 40 °C. The porosity, total pore volume (TPV), pore size distribution (PSD) and fractal dimension of coal samples after CO2 treatment at different pressures were characterized. Changes in the mineral composition and surface chemical structure of coal samples treated with ScCO2 at 8 MPa were also tested. The results indicate that the porosity and TPV of coals gradually increase at 7–10 MPa, while decrease at 10–16 MPa. The overall trend is an inverted U shape reaching a peak at 10 MPa. The PSD of samples is increased after CO2 treatment, especially that of adsorption pores. ScCO2 more obviously improves the PSD than SubCO2. In addition, the fractal dimension of samples does not change significantly with increasing pressure. The increase in porosity and TPV of coals after CO2 treatment is mainly due to the acid dissolution of minerals and the larger pores in newborn minerals, which is manifested by the significant decrease in the proportion of kaolinite and quartz. The effect of ScCO2 on the surface chemical structure of coals is mainly reflected in the shedding of aliphatic hydrocarbons and the reduction in the degree of condensation (DOC) of aromatic rings. These findings will provide certain guidance for pressure selection when CO2 is injected into bituminous coal seams for CO2-ECBM recovery and CGS.