Insights into supercritical CO2 mediated multi-scale pore-fracture evolution combined experiments and parameter correlation analysis

To elucidate the pore-fracture structure evolution of low-, medium-, and high-rank coals under supercritical CO 2 (SC-CO 2 ) treatment, samples of varying coal ranks were subjected to SC-CO 2 soaking at 10 MPa and 40 °C for 1–3 d. Multi-scale pore and fracture properties were characterized via low-temperature nitrogen adsorption, nuclear magnetic resonance, scanning electron microscopy, with Pearson correlation analysis employed to quantify relationship between parameters. The study found that SC-CO 2 modifies the pore structure by promoting the transformation of micropores into mesopores, optimizing connectivity, and increasing surface roughness, with coal rank playing a decisive regulatory role. Due to their loose structure and high content of soluble components, low- and medium-rank coals exhibit significant parameter fluctuations and morphological reshaping under the synergistic effects of “dissolution-extraction-expansion.” In contrast, high-rank coals, due to their high aromatic content and dense structure, exhibit limited modification. Correlation analysis confirms that the response of low-, medium-, and high-rank coals to treatment duration follows a sequence of strong correlation, fluctuating behavior, and weak correlation, respectively. This study not only reveals the patterns of pore evolution but also provides a critical theoretical basis for optimizing injection parameters in CO 2 -enhanced coalbed methane extraction and geological sequestration.