Impact of Supercritical Carbon Dioxide on Pore Structure and Gas Transport in Bituminous Coal: An Integrated Experiment and Simulation
The injection of CO<sub>2</sub> into coal reservoirs occurs in its supercritical state (ScCO<sub>2</sub>), which significantly alters the pore structure and chemical composition of coal, thereby influencing the adsorption and diffusion behavior of methane (CH<sub>4</...
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| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2025-03-01
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| Series: | Molecules |
| Subjects: | |
| Online Access: | https://www.mdpi.com/1420-3049/30/6/1200 |
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| Summary: | The injection of CO<sub>2</sub> into coal reservoirs occurs in its supercritical state (ScCO<sub>2</sub>), which significantly alters the pore structure and chemical composition of coal, thereby influencing the adsorption and diffusion behavior of methane (CH<sub>4</sub>). Understanding these changes is crucial for optimizing CH<sub>4</sub> extraction and improving CO<sub>2</sub> sequestration efficiency. This study aims to investigate the effects of ScCO<sub>2</sub> on the pore structure, chemical bonds, and CH<sub>4</sub> diffusion mechanisms in bituminous coal to provide insights into coal reservoir stimulation and CO<sub>2</sub> storage. By utilizing high-pressure CO<sub>2</sub> injection adsorption, low-pressure CO<sub>2</sub> gas adsorption (LP-CO<sub>2</sub>-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<sub>2</sub> 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<sub>2</sub>)<sub>n</sub>, 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<sub>4</sub> Diffusion: Changes in pore volume directly affect CH<sub>4</sub> migration. In the early stages of ScCO<sub>2</sub> interaction, pore shrinkage reduces the mean square displacement (MSD) and self-diffusion coefficient of CH<sub>4</sub>. However, as the reaction progresses, pore expansion enhances CH<sub>4</sub> diffusion, ultimately improving gas extraction efficiency. This study provides a fundamental understanding of how ScCO<sub>2</sub> modifies coal structure and CH<sub>4</sub> transport properties, offering theoretical guidance for enhanced CH<sub>4</sub> recovery and CO<sub>2</sub> sequestration strategies. |
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| ISSN: | 1420-3049 |