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

Abstract

Modeling the coupled evolution of strain and CH 4 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 CH 4 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 CH 4 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 CH 4 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 CH 4 seepage in coal.Processes 2018, 6, 127 2 of 22 seams, to improve permeability for easy extraction, premining the protective coal is often carried out to relieve the stress [4,5]. At the working face, the hydraulic fracturing method is often employed to achieve local stress adjustment. The huge demand in CH 4 promotes a deep understanding of sudden changes in permeability, especially induced by the coupled effect of gas and stress. The key to all these problems is to accurately understand the transition of permeability. We need a new model that can illustrate the whole change in permeability, even sudden transitions.Under triaxial compression, the permeability of CH 4 mainly depends on the shearing-induced dilatancy of the coal. Modeling enhanced permeability with deviatoric stress is useful for understanding the coupled mechanism between the dilatant deformation and the seepage or percolation channel. Many outstanding achievements focus on the study of stress-dependent permeability without considering transition effect. Many models [6][7][8][9][10][11], as shown in Table 1, have been made based on the excavation damage zone (EDZ), which depends on continuous media to explain the coupled process [12]. To our knowledge, the stress-dependent model can effectively explain the coupled effect before the dilatant boundary, taking coal as a continuous porous mediu...

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