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.
Highlights
Enhanced coal permeability after failure under triaxial stress can be determined quantitatively by the stress-dependent model
We propose the sudden changes of mechanical behavior from pore-dominated structure to fracture-dominated structure as the phase transition according to percolation theory [19,20]
Fractal dimension is often used to measure the roughness of a fracture surface, and spatial roughness reflects the complexity of the distribution of shear planes
Summary
Enhanced coal permeability after failure under triaxial stress can be determined quantitatively by the stress-dependent model. There must be a state transition of the continuous-discontinuous behavior controlling the coal permeability by connected clusters of multiscale networks [10]. The multiscale network of pore-crack-fracture is mainly generated at the dilatant boundary, and the focus of study is on the multiscale behavior and sudden transition of permeability. Percolation theory [21,22,23,24] is very effective at explaining such phase transitions in discontinuous transition depends the cluster triaxial generation of multiscale as well disordered media. For coal under on conventional compression, thepore-crack-fracture, continuous-discontinuous as the structural phase transition behavior of the connected clusters. Percolation models have been transition depends on the cluster generation of multiscale pore-crack-fracture, as well as the structural used to analyze the fracture-induced transitions of various rocks [23,24,25,26]. Experiments of conventional triaxial compression for coupled stress-strain permeability
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
