Evolution of Coal Permeability under Constant Effective Stresses: Direct Measurements and Numerical Modeling

Coal permeability is normally measured under the assumed condition of local equilibrium, i.e., the fracture pressure is equalized with the matrix pressure. When the local equilibrium condition is not met, the coal is in a transient state. In this study, we define coal permeability at a transient state as nonequilibrium permeability. A gas expansion-based experimental technique is developed to directly measure both the overall strain and the fracture strain of coal while the matrix strain is calculated based on the volumetric relation. This technique was applied to the case of gas injection into a coal sample. Carbon dioxide was injected into a coal sample at a range of pressures from 0.925 to 5.876 MPa and the confining pressures from 3.925 to 8.876 MPa, while the effective stress is maintained as a constant of 3 MPa. After continuous injection of carbon dioxide into the sample for 10 h, the experimental permeability is measured by the transient method, the overall strain is measured by the strain analyzer, and the fracture strain is measured by the gas expansion method. These strain measurements are used to directly calculate the fracture permeability, the matrix permeability, and the resultant permeability, respectively. The calculated permeability and the resultant permeability match with the experimental observations reasonably well. All three permeability profiles are similar, but their magnitudes are slightly different. These permeability profiles are used to benchmark a numerical model that includes the fracture-matrix interaction. The benchmarked numerical model is used to create a permeability map for the coal under the constant effective stress condition. The map represents a complete characterization of coal permeability from initial to ultimate equilibrium. The results demonstrate that the transient effects are significant and cannot be ignored for coal during coal seam gas extraction.