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<p>Multiple episodes of metamorphism and tectonic activity induce significant alterations in the physical and chemical composition of coal microstructure, leading to increased heterogeneity and anisotropy. The complexity of coal microstructure directly impacts coalbed methane occurrence, development, and transport properties. This paper proposes a microstructure complexity coefficient, i.e., a comprehensive, quantitative evaluation method on pore network heterogeneity and gas migration efficiency, characterized by spatial distribution, roughness, pore shape, pore connectivity, porosity, tortuosity, and so on. We employed fractal porous media theory to hypothesize and analyze original coals characterized by cleat development features and tectonic coals exhibiting particle compression characteristics. Additionally, micro-CT was utilized for nondestructive 3D visualization of coal morphological microstructure, associated with further analyses on image-based topological microstructure parameters to innovatively propose a normalized and integrated evaluation coefficient of microstructure complexity. The 3D reconstructions and equivalent pore network models demonstrated that original coal have uniformly dispersed sheet-like pore-connected clusters, whereas tectonic coals have more sporadically pore fragments, consistent with the assumption of fractal geometrical structure. Tectonic coal, compared to original coal, has higher quantity of pore-throat equivalent lengths below 100&#xa0;μm compared, implying that tectonism may progressively reduce longer pore-throat into shorter ones; opposite trends could be found in the pore-throat equivalent diameters with metamorphism. By constructing pore network models with topological structure parameters into fractal geometric parameters, the micro-CT-based complexity coefficient of original coal microstructure exhibited the highest index of 1.89, followed by tectonic coal samples’ 1.67, 1.61, and 1.60; meanwhile, the complexity coefficient showed positive correlation with metamorphism. Isothermal desorption experiments further confirmed the relevance between microstructure complexity and gas transport capacity, which in turn verified the reliability relation with equivalent diameter and length distribution of pore-throats. Based on the analyses, a summative schematic demonstration was developed to elucidate the fractal complexity mechanism governing original and tectonic coals, providing critical insights for quantifying gas transport efficiency under the influence of inherent coupling mechanism of microstructural multiparameter. The outcome of this study may provide a theoretical foundation for quantitative evaluation on gas disaster prediction and risk of tectonic coal reservoir.</p>

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Micro-CT Image-Based Characterization of Microstructure Complexity in Coal: From the Perspective of Fractal Geometrical Theory

  • 期刊论文

  • 发表日期：

    2025年6月11日

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  • Kaizhong Zhang,
  • Xiangqian Xing,
  • Liang Wang,
  • Bo Yang,
  • Jiayang Li,
  • Haoyu Zhang

摘要

Multiple episodes of metamorphism and tectonic activity induce significant alterations in the physical and chemical composition of coal microstructure, leading to increased heterogeneity and anisotropy. The complexity of coal microstructure directly impacts coalbed methane occurrence, development, and transport properties. This paper proposes a microstructure complexity coefficient, i.e., a comprehensive, quantitative evaluation method on pore network heterogeneity and gas migration efficiency, characterized by spatial distribution, roughness, pore shape, pore connectivity, porosity, tortuosity, and so on. We employed fractal porous media theory to hypothesize and analyze original coals characterized by cleat development features and tectonic coals exhibiting particle compression characteristics. Additionally, micro-CT was utilized for nondestructive 3D visualization of coal morphological microstructure, associated with further analyses on image-based topological microstructure parameters to innovatively propose a normalized and integrated evaluation coefficient of microstructure complexity. The 3D reconstructions and equivalent pore network models demonstrated that original coal have uniformly dispersed sheet-like pore-connected clusters, whereas tectonic coals have more sporadically pore fragments, consistent with the assumption of fractal geometrical structure. Tectonic coal, compared to original coal, has higher quantity of pore-throat equivalent lengths below 100 μm compared, implying that tectonism may progressively reduce longer pore-throat into shorter ones; opposite trends could be found in the pore-throat equivalent diameters with metamorphism. By constructing pore network models with topological structure parameters into fractal geometric parameters, the micro-CT-based complexity coefficient of original coal microstructure exhibited the highest index of 1.89, followed by tectonic coal samples’ 1.67, 1.61, and 1.60; meanwhile, the complexity coefficient showed positive correlation with metamorphism. Isothermal desorption experiments further confirmed the relevance between microstructure complexity and gas transport capacity, which in turn verified the reliability relation with equivalent diameter and length distribution of pore-throats. Based on the analyses, a summative schematic demonstration was developed to elucidate the fractal complexity mechanism governing original and tectonic coals, providing critical insights for quantifying gas transport efficiency under the influence of inherent coupling mechanism of microstructural multiparameter. The outcome of this study may provide a theoretical foundation for quantitative evaluation on gas disaster prediction and risk of tectonic coal reservoir.