Microstructural alterations of coal induced by interaction with sequestered supercritical carbon dioxide

A total of four samples with different coal ranks were exposed to supercritical CO2 (SC-CO2) fluids for one week at 313.15 K and 12 MPa. FTIR and X-ray diffraction were combined to explore the changes in functional groups, mineral compositions and aromatic microcrystallite structures of coal samples before and after SC-CO2 interaction. The results indicate that a great number of aliphatic hydrocarbons in coal are extracted and the content is reduced by 47.83-83.08 %, whereas the oxygen-containing groups are slightly changed within 8 %. The content of carbonate minerals is decreased by 29.05-46.11 % after SC-CO2 saturation, while quartz content is enhanced. The changes in coal crystalline structures are attributed to the mineral dissolution and hydrocarbon extraction during SC-CO2 saturation. Numerous voids are generated among aromatic layers after mineral dissolution, which directly enlarges the layer spacing of aromatic sheets. The extraction of aliphatic hydrocarbons can not only further increase layer spacing, but also loosen the coal macromolecular structure. Consequently, the directional alignment of aromatic microcrystallites is reduced within coal, leading to the decline of stacking height and aromatic layers. These findings are of significance for assessing both the recovery of CO2-enhanced coalbed methane and the trapping of CO2 in coal reservoirs.