Calculation of the Stress-dependent permeability in coal fracture networks by Lattice Boltzmann Method

摘要

The cleat or natural fracture system is a dominant factor controlling the permeability of coal seams. Since coal permeability is sensitive to stress, fundamental understanding of gas flow in coal fracture networks and permeability change with varying effective stress is necessary in the processes of CBM production. Considering the fracture aperture, roughness and density, coal fracture networks are reconstructed on the basis of the Voronoi tessellations. Based on the rock elastic theory, the stress-strain model of reconstructed coal fracture networks under stress condition is established. Then the lattice Boltzmann method (LBM) is applied to simulate fracture flows and to predict the associated permeability. For comparison purposes, gas permeability and porosity of coal samples with artificially generated fractures are measured under varying effective stress. Based on the experimental results and the Walsh model, fracture width and roughness are estimated. The predicted stress-dependent permeability in a single coal fracture by LBM simulation is in acceptable agreement with laboratory measurement. Furthermore, the stress-dependent permeability in fracture networks is investigated. The present study provides a promising approach to predict the associated permeability and transport characteristics in coal fracture networks.