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

Abstract

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.

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