Impact of Supercritical Carbon Dioxide on Pore Structure and Gas Transport in Bituminous Coal: An Integrated Experiment and Simulation

The injection of CO₂ into coal reservoirs occurs in its supercritical state (ScCO₂), which significantly alters the pore structure and chemical composition of coal, thereby influencing the adsorption and diffusion behavior of methane (CH₄). Understanding these changes is crucial for optimizing CH₄ extraction and improving CO₂ sequestration efficiency. This study aims to investigate the effects of ScCO₂ on the pore structure, chemical bonds, and CH₄ diffusion mechanisms in bituminous coal to provide insights into coal reservoir stimulation and CO₂ storage. By utilizing high-pressure CO₂ injection adsorption, low-pressure CO₂ gas adsorption (LP-CO₂-GA), Fourier-transform infrared spectroscopy (FTIR), and reactive force field molecular dynamics (ReaxFF-MD) simulations, this study examines the multi-scale changes in coal at the nano- and molecular levels. The following results were found: Pore Structure Evolution: After ScCO₂ treatment, micropore volume increased by 19.1%, and specific surface area increased by 11.2%, while mesopore volume and specific surface area increased by 14.4% and 5.7%, respectively. Chemical Composition Changes: The content of aromatic structures, oxygen-containing functional groups, and hydroxyl groups decreased, while aliphatic structures increased. Specific molecular changes included an increase in (CH₂)_n, 2H, 1H, and secondary alcohol (-C-OH) and phenol (-C-O) groups, while Car-Car and Car-H bonds decreased. Mechanisms of Pore Volume Changes: The pore structure evolves through three distinct phases: Swelling Phase: Breakage of low-energy bonds generates new micropores. Aromatic structure expansion reduces intramolecular spacing but increases intermolecular spacing, causing a decrease in micropore volume and an increase in mesopore volume. Early Dissolution Phase: Continued bond breakage increases micropore volume, while released aliphatic and aromatic structures partially occupy these pores, converting some mesopores into micropores. Later Dissolution Phase: Minimal chemical bond alterations occur, but weakened π-π interactions and van der Waals forces between aromatic layers result in further mesopore volume expansion. Impact on CH₄ Diffusion: Changes in pore volume directly affect CH₄ migration. In the early stages of ScCO₂ interaction, pore shrinkage reduces the mean square displacement (MSD) and self-diffusion coefficient of CH₄. However, as the reaction progresses, pore expansion enhances CH₄ diffusion, ultimately improving gas extraction efficiency. This study provides a fundamental understanding of how ScCO₂ modifies coal structure and CH₄ transport properties, offering theoretical guidance for enhanced CH₄ recovery and CO₂ sequestration strategies.