Abstract
Evaluating the coupling of moisture content with supercritical CO2 (ScCO2) in coal is important for CO2 geological sequestration and enhanced coalbed methane recovery. Changes of minerals and microstructure in bituminous coal after ScCO2-water treatment were explored employing X-ray powder diffraction and micro-computed tomography (CT), and the seepage behavior evolution was further investigated by performing computational fluid dynamic analysis after 3D CT reconstruction. The results show that carbonate minerals dissolved remarkably after ScCO2-water treatment, but a reversible chemical reaction occurred in calcite minerals. The induced mineral dissolution, pore-fracture formation and expansion changed the pore-fracture structure in coal significantly. As a result, the amount and diameter of pores and throats obviously increased as the total volume and surface area of the pore-fracture increased to be nearly twice of the original coal. Additionally, the pore-fracture connectivity improved from 44.7 to 67.6% with a coordination number greater than 3 after ScCO2-water treatment. Pores of the equivalent radius of 75 μm were also found to contribute most to the permeability, rather than pores of the largest equivalent radius, as reported previously, indicating both the size and volume proportion of pores should be considered in permeability evaluation. Numerical modeling reveals that pore pressure decays faster along flow pathways after ScCO2-water treatment due to pore-fracture volume enhancement. The ScCO2-water treatment not only increased seepage channels in coal but also intensified the preferential flow. Along with the pore-fracture volume enhancement, the permeability heterogeneity in coal decreased after the ScCO2-water treatment, but the permeability enhancement along different directions varied.
Article highlights
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A reconstructed coal model was developed to analyze the pore-fracture variations.
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Largest permeability contribution did not belong to the pores with the maximum equivalent radius.
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ScCO2-water-coal interactions reduced the permeability heterogeneity and intensified the preferential flow.
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This study was financially supported by the National Natural Science Foundation of China (Grant No. 51674047 & Grant No. 51911530152).
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Appendix A. Minerals analysis by XRD
Appendix A. Minerals analysis by XRD
Figure
Variation of mineral composition in coal samples subjected to different pressure conditions
18 Shows the variation of mineral compositions in coal samples induced by CO2-water-coal interaction. The coal samples employed for XRD testing were collected from the same location in the same block and contained the same mineral composition.
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Luo, P., Zhang, Z., Geng, X. et al. Evaluation of ScCO2-water performance on bituminous coal: insights from experiments and 3D CT image reconstruction. Geomech. Geophys. Geo-energ. Geo-resour. 8, 118 (2022). https://doi.org/10.1007/s40948-022-00420-3
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DOI: https://doi.org/10.1007/s40948-022-00420-3