Mechanism of supercritical CO2 on the chemical structure and composition of high-rank coals with different damage degrees

The interaction between carbon dioxide (CO2) and coal plays a vital role in CO2 geological storage enhanced coalbed methane recovery (CO2-ECBM). To investigate the mechanism of supercritical carbon dioxide (SC-CO2) extraction in technically deformed coals under deep coal seam conditions, the chemical composition and structure of technically deformed coals before and after SC-CO2 treatment were analyzed by Fourier transform infrared spectroscopy (FTIR), mass spectrometry (GC–MS) and X-ray diffraction (XRD). The results show that, after SC-CO2 extraction, the absorption peak intensities of weakly polar and non-polar compounds in coal are weakened. With the increase in tectonic deformation degree, the dissolution effect of aromatic compounds increases, while aliphatic compounds show a complementary evolution trend. The dissolution effect of SC-CO2 on carbonate minerals is the most significant, with the highest dissolution amount of 100 %, followed by clay minerals, with the dissolution amount between 1.3 % and 30.5 %, while oxide minerals are relatively stable and hardly react. With the increasing degree of tectonic deformation, the solubility of various minerals tends to increase. In summary, SC-CO2 has a dissolution effect on the organic functional groups and minerals of technically deformed coal, and the effect tends to be significant with the increase of coal tectonic deformation degrees. In view of the important role of mineral and organic composition on the porosity and adsorption of coal, when CO2 is injected into deep coal seams, the response characteristics of porosity, permeability and adsorption caused by the change in mineral and organic composition should also be considered.