Influence of sub-supercritical CO2 on pore structure and fractal characteristics of anthracite: An experimental study

The geological storage of CO2 in coal seams has become one of the most effective means to alleviate the greenhouse effect, and the choice of CO2 injection and storage pressure is very critical. In this study, the effects of sub-supercritical CO2 intrusion on coal pore structures were investigated by nuclear magnetic resonance (NMR) and low-pressure N2 adsorption method (LP-N2GA), and the causes were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy. The results showed that total porosity increased by 1.25%, 4.53%, and 5.82%, respectively, and micro-minipore volume increased by 10.1%, 42.2%, and 42.7%, respectively, after anthracite was treated with 4, 8, and 12 MPa CO2. The reasons for the change of coal pore structure caused by CO2 intrusion mainly include mineral dissolution, extraction of functional groups, destruction of aromatic layers in the microcrystalline structure, and rearrangement of macromolecular structure. Additionally, supercritical CO2 possesses more of these capabilities than subcritical CO2, resulting in greater changes in coal pore structure. These findings guide us that the injection pressure of CO2 should be increased as much as possible within the scope of cost and technology and the storage pressure of CO2 near the critical pressure point should be selected.