Multifractal Characterization of Pore Heterogeneity and Water Distribution in Medium- and High-Rank Coals via Nuclear Magnetic Resonance
Comprehensive assessment of pore structure and multiphase water distribution is critical to the flow and transport process in coalbed methane (CBM) reservoirs. In this study, nuclear magnetic resonance (NMR) and multifractal analysis were integrated to quantify the multiscale heterogeneity of nine m...
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2025-04-01
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| author | Huan Liu Shasha Zhang Yu Qiao Danfeng Xie Long Chang |
| author_facet | Huan Liu Shasha Zhang Yu Qiao Danfeng Xie Long Chang |
| author_sort | Huan Liu |
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| description | Comprehensive assessment of pore structure and multiphase water distribution is critical to the flow and transport process in coalbed methane (CBM) reservoirs. In this study, nuclear magnetic resonance (NMR) and multifractal analysis were integrated to quantify the multiscale heterogeneity of nine medium- and high-rank coals under water-saturated and dry conditions. By applying the box-counting method to transverse relaxation time (<i>T</i><sub>2</sub>) spectra, multifractal parameters were derived to characterize pore heterogeneity and residual water distribution. The influencing factors of pore heterogeneity were also discussed. The results show that pore structures in high-rank coals (HCs) exhibit a broader multifractal spectrum and stronger rightward spectrum than those of medium-rank coals, reflecting micropore-dominated heterogeneity and the complexity induced by aromatization in HCs. The vitrinite content enhances micropore development, increasing the heterogeneity and complexity of pore structure and residual water distribution. Inertinite content shows opposite trends compared to vitrinite content for the effect on pore structure and water distribution. Volatile yield reflects coal metamorphism and thermal maturity, which inversely correlates with pore heterogeneity and complexity. Residual water mainly distributes to adsorption pores and pore throats, shortening <i>T</i><sub>2</sub> relaxation (bound water effect) and reducing spectral asymmetry. The equivalence of the multifractal dimension and singularity spectrum validates their joint utility in characterizing pore structure. Minerals enhance pore connectivity but suppress complexity, while moisture and ash contents show negligible impacts. These findings provide a theoretical reference for CBM exploration, especially in optimizing fluid transportation and CBM production strategies and identifying CBM sweet spots. |
| format | Article |
| id | doaj-art-3da75b1b1e84432794aa9429e1910472 |
| institution | DOAJ |
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| language | English |
| publishDate | 2025-04-01 |
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| spelling | doaj-art-3da75b1b1e84432794aa9429e19104722025-08-20T03:14:39ZengMDPI AGFractal and Fractional2504-31102025-04-019529010.3390/fractalfract9050290Multifractal Characterization of Pore Heterogeneity and Water Distribution in Medium- and High-Rank Coals via Nuclear Magnetic ResonanceHuan Liu0Shasha Zhang1Yu Qiao2Danfeng Xie3Long Chang4The Prevention and Control Genter for the Geological Disaster of Henan Geological Bureau, Zhengzhou 450003, ChinaState Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University), Jiaozuo 454003, ChinaSchool of Resources and Enviroment, Henan Polytechnic University, Jiaozuo 454003, ChinaThe Prevention and Control Genter for the Geological Disaster of Henan Geological Bureau, Zhengzhou 450003, ChinaThe Prevention and Control Genter for the Geological Disaster of Henan Geological Bureau, Zhengzhou 450003, ChinaComprehensive assessment of pore structure and multiphase water distribution is critical to the flow and transport process in coalbed methane (CBM) reservoirs. In this study, nuclear magnetic resonance (NMR) and multifractal analysis were integrated to quantify the multiscale heterogeneity of nine medium- and high-rank coals under water-saturated and dry conditions. By applying the box-counting method to transverse relaxation time (<i>T</i><sub>2</sub>) spectra, multifractal parameters were derived to characterize pore heterogeneity and residual water distribution. The influencing factors of pore heterogeneity were also discussed. The results show that pore structures in high-rank coals (HCs) exhibit a broader multifractal spectrum and stronger rightward spectrum than those of medium-rank coals, reflecting micropore-dominated heterogeneity and the complexity induced by aromatization in HCs. The vitrinite content enhances micropore development, increasing the heterogeneity and complexity of pore structure and residual water distribution. Inertinite content shows opposite trends compared to vitrinite content for the effect on pore structure and water distribution. Volatile yield reflects coal metamorphism and thermal maturity, which inversely correlates with pore heterogeneity and complexity. Residual water mainly distributes to adsorption pores and pore throats, shortening <i>T</i><sub>2</sub> relaxation (bound water effect) and reducing spectral asymmetry. The equivalence of the multifractal dimension and singularity spectrum validates their joint utility in characterizing pore structure. Minerals enhance pore connectivity but suppress complexity, while moisture and ash contents show negligible impacts. These findings provide a theoretical reference for CBM exploration, especially in optimizing fluid transportation and CBM production strategies and identifying CBM sweet spots.https://www.mdpi.com/2504-3110/9/5/290pore structurewater distributionnuclear magnetic resonancemultifractal characterizationcoalbed methane |
| spellingShingle | Huan Liu Shasha Zhang Yu Qiao Danfeng Xie Long Chang Multifractal Characterization of Pore Heterogeneity and Water Distribution in Medium- and High-Rank Coals via Nuclear Magnetic Resonance Fractal and Fractional pore structure water distribution nuclear magnetic resonance multifractal characterization coalbed methane |
| title | Multifractal Characterization of Pore Heterogeneity and Water Distribution in Medium- and High-Rank Coals via Nuclear Magnetic Resonance |
| title_full | Multifractal Characterization of Pore Heterogeneity and Water Distribution in Medium- and High-Rank Coals via Nuclear Magnetic Resonance |
| title_fullStr | Multifractal Characterization of Pore Heterogeneity and Water Distribution in Medium- and High-Rank Coals via Nuclear Magnetic Resonance |
| title_full_unstemmed | Multifractal Characterization of Pore Heterogeneity and Water Distribution in Medium- and High-Rank Coals via Nuclear Magnetic Resonance |
| title_short | Multifractal Characterization of Pore Heterogeneity and Water Distribution in Medium- and High-Rank Coals via Nuclear Magnetic Resonance |
| title_sort | multifractal characterization of pore heterogeneity and water distribution in medium and high rank coals via nuclear magnetic resonance |
| topic | pore structure water distribution nuclear magnetic resonance multifractal characterization coalbed methane |
| url | https://www.mdpi.com/2504-3110/9/5/290 |
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