Supercritical CO2 effect on the physicochemical structure of coal and its adsorption capacity: Implications for CO2 geological sequestration

The temperature and pressure conditions of coal seams at different burial depths influence the CO₂ storage potential. In this study, Fourier transform infrared spectroscopy, low-temperature nitrogen adsorption, low-field nuclear magnetic resonance, and isothermal CO₂ adsorption experiments were employed to investigate the changes in functional groups, pore structures, and carbon dioxide adsorption capacity of four coal samples with varying burial depths after supercritical carbon dioxide (ScCO₂) treatment under simulated in-situ temperature and pressure conditions (8 MPa to 20 MPa, 35 °C to 65 °C). Results show that ScCO₂ treatment reduces the aromatic carbon content, aromatization, condensation degree of aromatic rings, aliphatic structure abundance, and the ratio of CO/CC in the coal samples. Additionally, ScCO₂ treatment induced increases in the pore structure of coal, promoting the transformation from micropores to mesopores. Following ScCO₂ treatment, the mesoporous volume of coal samples increased within the range of 2.22–41.38 %, whereas the specific surface area decreased between 9.40 % and 59.57 %. Moreover, the CO₂ adsorption volume exhibited an increase ranging from 3.81 cm³ /g to 6.92 cm³ /g. A positive correlation is observed between the enhancement in CO₂ adsorption capacity and both the increase in mesoporous volume as well as the decrease in CO/CC. Notably, the maximum adsorption capacity was attained at a depth of 1000 m, making it the optimal target for CO₂ injection. These findings provide theoretical guidance for selecting suitable burial depths for CO₂ storage in coal seams and elucidating the interaction mechanism between ScCO₂ and coal.