Potential effect of carbon dioxide injection on the functional groups of medium volatile bituminous coals analysed using in-situ diffuse reflectance Fourier-transform infrared spectroscopy

CO2 sequestration with enhanced coalbed methane recovery (CO2-ECBM) is a promising technology for miti-gating CO2 emissions. To implement CO2-ECBM, complex interactions must be elucidated between CO2 and coal during CO2 sequestration, but the mechanism through which coal and CO2 interact remains controversial mainly because coal contains a complex macromolecular structure. Therefore, we used in-situ high-pressure diffuse reflectance Fourier-transform infrared spectroscopy to investigate the potential impact of the CO2 injection on the functional groups of medium volatile bituminous (MVB) coals. The results of this study showed that CO2 physisorption could be found near the MVB coal macromolecular C-H bonds, and the interactions between the MVB coal macromolecular C-H bonds and CO2 kept strengthening with increasing CO2 injection pressure. Injecting CO2 into the MVB coal disrupted the weak interactions between aromatic layers, and supercritical CO2 (ScCO2) could disrupt the physical association between small molecular compounds and the coal macromolec-ular structure, which extracted hydrocarbon and oxygen-containing compounds, and thus reduced the content of oxygen-containing functional groups and -CH2- in the MVB coal. The coal macromolecular C--O bonds were not conducive to methane adsorption on the coal surfaces, injecting high-pressure CO2 (P = 7 MPa, 8 MPa) into the MVB coal matrix irreversibly damaged the coal macromolecular C--O bonds, which enhanced the CH4 adsorption capacity and was not conducive to CH4 desorption. The results of this study showed that injecting ScCO2 into the MVB coal could change the coal macromolecular structure and reduce the content of oxygen -containing functional groups and-CH2-, thus affecting the properties of MVB coal reservoir, such as porosity, permeability, and mechanical characteristics, which could advance knowledge of the theoretical usefulness for CO2-ECBM recovery and sequestration in the MVB coal reservoirs.