Molecular-Structural Evolution Mechanisms in Coal Induced by Supercritical CO ₂ : Insights from Depth-Dependent Treatment

A clear understanding of how supercritical carbon dioxide (ScCO2) interacts with coal’s chemical structure across different burial depths─particularly its influence on extracting small molecules and reconstructing the macromolecular carbon skeletal structures─is currently lacking, hindering the assessment of CO2 sequestration potential in deep coal seams. This study simulated ScCO2 treatment at conditions equivalent to 800–2000 m depth. The evolutionary mechanisms of coal’s chemical structure were elucidated using a combination of carbon-13 nuclear magnetic resonance (13C NMR), gas chromatography–mass spectrometry (GC–MS), and CO2 adsorption isotherms. Results demonstrate that ScCO2 preferentially cleaves long-chain aliphatic structures and promotes the release of short-chain hydrocarbons, with a more pronounced effect on saturated components than aromatic systems. ScCO2 exposure consistently increased the relative content of total aromatic carbon (fa) while reducing aliphatic carbon (fal), indicating aromatic condensation and aliphatic dissolution (e.g., fa rose from 68.87% to 69.28% at 800 m; 88.64% to 88.90% at 1500 m; 88.37% to 88.75% at 2000 m). Depth-dependent extraction rates revealed three interaction regimes: extraction-dominated at 800 m (−9.20%), transitional at 1000 m (−3.52%), and swelling-dominated at 1500 m (+7.59%). At 2000 m, high aromatization limited extractability (+0.32%). These structural and compositional changes are corroborated by systematic variations in CO2 adsorption behavior, reflecting mechanisms of coal extraction and swelling. The study provides mechanistic insight into ScCO2-driven molecular restructuring in coal under depth-dependent physicochemical conditions.