Abstract
The injection of CO(2) into coal reservoirs occurs in its supercritical state (ScCO(2)), which significantly alters the pore structure and chemical composition of coal, thereby influencing the adsorption and diffusion behavior of methane (CH(4)). Understanding these changes is crucial for optimizing CH(4) extraction and improving CO(2) sequestration efficiency. This study aims to investigate the effects of ScCO(2) on the pore structure, chemical bonds, and CH(4) diffusion mechanisms in bituminous coal to provide insights into coal reservoir stimulation and CO(2) storage. By utilizing high-pressure CO(2) injection adsorption, low-pressure CO(2) gas adsorption (LP-CO(2)-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 ScCO(2) 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 (CH(2))(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 CH(4) Diffusion: Changes in pore volume directly affect CH(4) migration. In the early stages of ScCO(2) interaction, pore shrinkage reduces the mean square displacement (MSD) and self-diffusion coefficient of CH(4). However, as the reaction progresses, pore expansion enhances CH(4) diffusion, ultimately improving gas extraction efficiency. This study provides a fundamental understanding of how ScCO(2) modifies coal structure and CH(4) transport properties, offering theoretical guidance for enhanced CH(4) recovery and CO(2) sequestration strategies.