Effect of Supercritical Carbon Dioxide (ScCO2) on the Microstructure of Bituminous Coal with Different Moisture Contents in the Process of ScCO2 Enhanced Coalbed Methane and CO2 Geological Sequestration

Water plays an important role in carbon dioxide (CO2) enhanced coalbed methane exploitation and CO2 geological sequestration at deep geological conditions. We performed mercury intrusion porosimetry, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction analysis on coal samples with different moisture contents after supercritical carbon dioxide (ScCO2) treatment to study the effect of different moisture contents on deep coal seam treatment by ScCO2. Using these experimental techniques, the effects of ScCO2 treatment on the microstructure of coal samples with different moisture contents were obtained. The results showed that the combination of ScCO2 and water in coal samples can cause mineral dissolution, increase the damage degree of coal structural defects, reduce the number of aromatic structures and oxygen-containing functional groups, and then lead to the expansion of the pore and fracture volume, especially the micropore volume. Moreover, with increasing the moisture content, the micropore volume of the coal samples under ScCO2 interaction with the presence of water exhibited an increasing trend. The number of oxygen-containing functional groups in the coal samples decreased. The peak position difference (G – D1) decreased first and then plateaued, and when the moisture content was in the range of 5.85–7.19%, the damage degree reached the maximum. The effect of water and ScCO2 on the dissolution of clay minerals in the coal samples was greater than that on the carbonate minerals.