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Processes 2018, 6(8), 127; https://doi.org/10.3390/pr6080127

A Strain-Based Percolation Model and Triaxial Tests to Investigate the Evolution of Permeability and Critical Dilatancy Behavior of Coal

1
School of Mechanics and Civil Engineering, China University of Mining and Technology, Beijing 100083, China
2
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400030, China
3
Key Laboratory of Safety and High-efficiency Coal Mining, Anhui University of Science and Technology, Huainan 232001, China
*
Author to whom correspondence should be addressed.
Received: 5 July 2018 / Revised: 4 August 2018 / Accepted: 6 August 2018 / Published: 13 August 2018
(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)
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Abstract

Modeling the coupled evolution of strain and CH4 seepage under conventional triaxial compression is the key to understanding enhanced permeability in coal. An abrupt transition of gas-stress coupled behavior at the dilatancy boundary is studied by the strain-based percolation model. Based on orthogonal experiments of triaxial stress with CH4 seepage, a complete stress-strain relationship and the corresponding evolution of volumetric strain and permeability are obtained. At the dilatant boundary of volumetric strain, modeling of stress-dependent permeability is ineffective when considering the effective deviatoric stress influenced by confining pressure and pore pressure. The computed tomography (CT) analysis shows that coal can be a continuous medium of pore-based structure before the dilatant boundary, but a discontinuous medium of fracture-based structure. The multiscale pore structure geometry dominates the mechanical behavior transition and the sudden change in CH4 seepage. By the volume-covering method proposed, the linear relationship between the fractal dimension and porosity indicates that the multiscale network can be a fractal percolation structure. A percolation model of connectivity by the axial strain-permeability relationship is proposed to explain the transition behavior of volumetric strain and CH4 seepage. The volumetric strain on permeability is illustrated by axial strain controlling the trend of transition behavior and radical strain controlling the shift of behavior. A good correlation between the theoretical and experimental results shows that the strain-based percolation model is effective in describing the transition behavior of CH4 seepage in coal. View Full-Text
Keywords: enhanced permeability; deviatoric stress; mechanical behavior transition; CH4 seepage; volumetric strain; strain-based percolation model enhanced permeability; deviatoric stress; mechanical behavior transition; CH4 seepage; volumetric strain; strain-based percolation model
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Xue, D.; Zhou, J.; Liu, Y.; Zhang, S. A Strain-Based Percolation Model and Triaxial Tests to Investigate the Evolution of Permeability and Critical Dilatancy Behavior of Coal. Processes 2018, 6, 127.

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