Changes in the microstructure of low-rank coal after supercritical CO2 and water treatment

With a focus on different CO2 pressures and H2O, the influences of the ScCO2–H2O coupling effect on the microstructures of low-rank coal samples were compared and analyzed, offering further analysis of the CO2 sequestration capacity in coal seams rendered unworkable owing to the effect of water. By using nuclear magnetic resonance (NMR) and X-ray powder diffraction (XRD), the changes in porosity, pore size, pore size distribution (PSD), fractal dimension, and minerals in coal samples of the two states were compared and analyzed. XRD analysis revealed that a large number of carbonate rocks (calcite) and aluminosilicate minerals (clay minerals) were found in coal. ScCO2 presented the optimal dissolution effect in the water-saturated samples. NMR analysis showed that compared with a single CO2 fluid, the pore structures of the coal samples varied more remarkably under the coupling effect of CO2 and H2O. Moreover, under the supercritical state of CO2, the effect on the coal sample was the greatest. With the increase in pressure during the CO2 treatment, the porosity of the coal samples increased, and the proportion of macropores grew, indicating the transformation of small pores into large pores. The roughness of adsorption pores increased, whereas the complexity and heterogeneity of seepage pores in the coal samples after ScCO2 treatment declined. Generally, the ScCO2–H2O coupling effect on coal samples caused further changes in the pore structures, promoting mineral dissolution in the coal samples.