Investigation of microstructural damage evolution in high-rank coal under supercritical CO2 pulsating action

To characterize the microcomponents and pore structure of high-rank coals under supercritical CO2 (SC-CO2) pulsation, the effects of SC-CO2 pulsation on the mineral composition, organic functional groups, pore structure and pore fractal characteristics of high-rank coals were investigated on the basis of a combination of X-ray diffraction (XRD), Fourier transform infrared (FTIR), Mercury intrusion porosimetry, low-temperature liquid nitrogen, and low-field nuclear magnetic resonance experiments. The results showed that after SC-CO2 pulsation, the mineral compositions and organic functional group contents of the coals changed to different degrees, among which the contents of calcite, ilmenite and illite decreased, the contents of ferro-dolomite and kaolinite increased, and the characteristic peaks of oxygen-containing, benzene-ring-five-substituted H, and aliphatic functional groups decreased. The pore and fractal structure of the coals after SC-CO2 pulsation significantly altered, the total pore volume of the coal increased by 88%, the percentage of seepage pore volume increased by 13 times, and the porosity increased by 8.272%. The fractal dimensions of the coal samples after SC-CO2 pulsation all showed a decreasing trend, and the pore structure complexity and surface roughness of the coal samples decreased. This will be highly important for increasing the permeability of coal, sequestering CO2, and improving the effect of coalbed methane extraction by using SC-CO2.