Experiment study on the chemical structure changes of different rank coals under action of supercritical carbon dioxide and water

ScCO₂-H₂O-coal geochemical reaction can exert a great influence on the physical and chemical structure of coal, playing a critical role in the CO₂ injection and enhancing CH₄ recovery. To investigate the effects of ScCO₂-H₂O on chemical structure in coal, three coal samples of different metamorphic grades were exposed to supercritical CO₂ and deionized water under 1000m depth temperature and pressure conditions for 240h using a high pressure reactor(ScCO₂/H₂O simulation system and coal/rock geochemical reaction device). X-ray diffraction and Raman spectroscopy were used to provide change characteristics of crystal structure and carbon order degree of coal before and after the ScCO₂-H₂O treatment. The results show that the ScCO₂-H₂O reaction destroys the crystal structural integrity and carbon order degree and changes the macromolecular structure of coal, the effect mainly depends on the coal rank, i.e., for low rank coals, the degree of parallel orientation increases and makes its structure more compact, the lamellae spacing decreases; while for medium-high rank coals, the disordered structure and lamellae spacing of the coal microcrystalline structure increases. This study result has laid a micro-scale foundation for the adsorption/desorption, pore and permeability characterization of coal reservoir before and after ScCO₂-H₂O action in the implementation of the CO₂-ECBM project.