Effect of supercritical CO2 transient high-pressure fracturing on bituminous coal microstructure

Carbon dioxide phase change fracturing is an effective permeability enhancement technology, and its influence mechanism on the coal microstructure remains to be further investigated. To explore the impact of CO2 fracturing on the coal chemical structure, the self-developed transient high-pressure fracturing of CO2 test platform was independently developed. Based on X-ray photoelectron spectrometer (XPS), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), the elemental morphology, functional group and crystal structure of the two bituminous coal samples before and after fracturing were analyzed. The results showed that the fracturing process resulted in the disruption of hydrogen bonds within coal, leading to a reduction in the cross-linking density of the coal structure. Moreover, it facilitated the breaking of fatty alkyl side chains and oxygen-containing functional groups, causing the detachment of side chains and shortening of the fatty chains. The interlayer spacing of aromatic layers (d200) of fractured coal decreased, and the average transverse dimension (La), the average stack height (Lc) and the aromaticity (fa) increased. With the improvement of aromaticity, the arrangement between aromatic layers was looser. The volume and basic unit structure of coal crystal nucleus became larger, and the flattening degree increased. The research results contribute to the understanding of the change of coal microstructure under supercritical CO2 transient high-pressure fracturing and provide theoretical support for exploring the methane adsorption capacity of fractured coal.