Influence of supercritical CO2 on pore structure and functional groups of coal: Implications for CO2 sequestration

To better understand the effects of CO2 sequestration and long-term storage, it is worth studying the interactions between supercritical CO2 (SC-CO2) and coal, and its influence on coal properties or, more specifically, the changes in coal pore structure and functional groups caused by SC-CO2. In this study, three different metamorphic grades of coal were sampled and exposed to SC-CO2 (∼40 °C and 10 MPa) for 120 h through a geochemical reactor, simulating CO2 storage in deep coal seams. The functional groups and pore structure of different coal ranks before and after SC-CO2 treatment were measured by Fourier Transform Infrared Spectroscopy (FTIR), Mercury Intrusion Porosimetry (MIP) and physical adsorption method. The results show that the absorption peak intensity of -OH groups, with intramolecular association and C-H stretching vibrations, clearly changed for anthracite compared to others. Compounds with weakly polar functional groups, such as hydrocarbons, epoxy and lipid compounds (ether or ester), decreased significantly, whereas strongly polar functional groups exhibited only a slight change. Pore structure and distribution of each pore phase showed the diversity present in different coal ranks. The development of seepage-flow pores (mesopore and macropore) was promoted by SC-CO2. For high rank and medium rank coals, the degree of pore development was significantly altered by SC-CO2, while pore development in low rank coal was largely unaltered. The results of this study contribute to the understanding of coal structure evolution and its effects on coal reservoir during long-term geological sequestration.