Evolution mechanism of coal chemical structure after supercritical CO2 transient fracturing

Abstract

This study investigates the chemical structural evolution of coal during supercritical CO2 transient fracturing using a self-developed experimental platform. Coal samples with different metamorphic ranks were subjected to transient fracturing under different pressures. Characterization techniques, including X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy, analyzed the fractured coal samples. Initially, supercritical CO2 dissolved minerals and non-aromatic structures, rearranging organic compounds. As fracturing intensified, alkanes and substituents rapidly released, causing large aromatic rings to break into smaller ones. The breakage of small aromatic rings created pores and increased aromatic structure dimensions. Moreover, the interaction between fractured small aromatic rings and oxygen atoms in the C=O group facilitated the formation of new C–C bonds, promoting the connection of aromatic rings to form larger structures. In the early stages of fracturing, the molecular structure of coal relaxed, and subsequently, as the fracturing intensified, the hydrogen bonds initially broken could reconnect to aromatic structures to form OH-π interactions, enhancing intermolecular attraction. This promoted molecules to be more inclined to form stable, dense structures. Understanding this evolution mechanism aids in analyzing the transformation of fractured coal pores, thereby enhancing our understanding of the mechanism for improving coal seam permeability using this technology.