Abstract To enhance the gas extraction efficiency of deep low-permeability coal seams, this study systematically uncovered the dynamic fracture evolution and permeability enhancement mechanisms of coal and rock under impact. This was achieved using a self-developed electromagnetic split Hopkinson pressure bar (SHPB) system, combined with three-dimensional (3D) micro-CT and scanning electron microscopy (SEM). Based on CT data, a 3D fractal tortuosity model was constructed. This model integrates the fracture number fractal dimension, tortuosity fractal dimension, and porosity to quantitatively characterize the 3D complexity of the damage structure. Additionally, the classical permeability model was extended to three dimensions, initially forming a permeability prediction framework based on CT image reconstruction. Results show that after impact, the fractal dimension decreases, with a more significant decline at higher energy absorption levels. This indicates that energy dissipation promotes micro-crack coalescence and penetration, reducing fracture filling density. Meanwhile, the tortuosity also decreases. These two effects jointly reveal the 3D physical mechanism of permeability enhancement. SEM observations confirmed for the first time that under moderate strain rates, the microscopic damage morphology of CO2 phase-change fracturing is highly similar to that of SHPB impact, providing direct experimental evidence for this dynamic simulation. This study establishes a quantitative link between microscopic fracture geometry and macroscopic seepage properties, laying a methodological foundation for efficient gas extraction and for evaluating fracture effects caused by similar engineering dynamic disturbances. Highlights <UnorderedList Mark="Bullet"> <ItemContent> A quantitative system integrating 3D fracture quantity fractal dimension and tortuosity fractaldimension has been built, enabling a leap from 2D to 3D in describing damage structure complexity. </ItemContent> <ItemContent> A self-developed electromagnetic-driven SHPB device, coupled with 3D CT and SEM, facilitates multiscale observation and analysis of the dynamic impact damage and structural evolution process. </ItemContent> <ItemContent> The classical permeability model has been innovatively expanded to 3D space, forming a preliminary theoretical framework for permeability prediction based on CT image reconstruction. </ItemContent> <ItemContent> Experimental verification has demonstrated a remarkable similarity in the micro-damage patterns between CO2 phase-transition fracturing and SHPB impact. </ItemContent> </UnorderedList>

3D Fractal Tortuosity Quantification of Impact-Induced Coal Fractures: A Permeability Enhancement Model for Gas Extraction

期刊论文
发表日期：

2026年4月9日

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Jiyun Zhang,
Zhihao Fang,
Xinsheng Zhang,
Yongqiang Yu,
Shuren Wang,
Yunxing Cao

摘要

Abstract

To enhance the gas extraction efficiency of deep low-permeability coal seams, this study systematically uncovered the dynamic fracture evolution and permeability enhancement mechanisms of coal and rock under impact. This was achieved using a self-developed electromagnetic split Hopkinson pressure bar (SHPB) system, combined with three-dimensional (3D) micro-CT and scanning electron microscopy (SEM). Based on CT data, a 3D fractal tortuosity model was constructed. This model integrates the fracture number fractal dimension, tortuosity fractal dimension, and porosity to quantitatively characterize the 3D complexity of the damage structure. Additionally, the classical permeability model was extended to three dimensions, initially forming a permeability prediction framework based on CT image reconstruction. Results show that after impact, the fractal dimension decreases, with a more significant decline at higher energy absorption levels. This indicates that energy dissipation promotes micro-crack coalescence and penetration, reducing fracture filling density. Meanwhile, the tortuosity also decreases. These two effects jointly reveal the 3D physical mechanism of permeability enhancement. SEM observations confirmed for the first time that under moderate strain rates, the microscopic damage morphology of CO₂ phase-change fracturing is highly similar to that of SHPB impact, providing direct experimental evidence for this dynamic simulation. This study establishes a quantitative link between microscopic fracture geometry and macroscopic seepage properties, laying a methodological foundation for efficient gas extraction and for evaluating fracture effects caused by similar engineering dynamic disturbances.

Highlights

A quantitative system integrating 3D fracture quantity fractal dimension and tortuosity fractaldimension has been built, enabling a leap from 2D to 3D in describing damage structure complexity.

A self-developed electromagnetic-driven SHPB device, coupled with 3D CT and SEM, facilitates multiscale observation and analysis of the dynamic impact damage and structural evolution process.

The classical permeability model has been innovatively expanded to 3D space, forming a preliminary theoretical framework for permeability prediction based on CT image reconstruction.

Experimental verification has demonstrated a remarkable similarity in the micro-damage patterns between CO₂ phase-transition fracturing and SHPB impact.