Investigation of Anisotropic Deformation and Stress-Dependent Directional Permeability of Coalbed Methane Reservoirs

Abstract

Two types of experiments were conducted under different boundary conditions to characterize the anisotropic deformation and directional permeability of the coal sample. Gas sorption-induced strains in the three principal directions were studied to examine the anisotropic deformation. The results show that the coal sample can be treated as isotropic in those planes parallel to the bedding direction. However, the strong strain anisotropy of coal indicates that the gas sorption-induced matrix strain in the direction parallel to the bedding is smaller than that which is perpendicular to the bedding, under different hydrostatic pressures for unconstrained conditions. Permeability measurements were conducted under different effective stresses to determine the magnitude and orientation of directional permeability. It was found that cleat distribution plays a dominant role in the magnitude of the measured permeability. After the two principal permeabilities parallel to the bedding planes are calculated, Mohr’s circle of permeability can be then determined and used to estimate permeability in other directions of the bedding. The experimental results show that the principal permeability directions vary under different effective stresses, illustrating that coal cleat system would be reoriented during gas production. It was also found that cleat reorientation is attributed to three factors: stress contrast, the difference of Biot’s coefficient, and the difference in the sorption-induced stresses in the direction perpendicular to and parallel to the bedding planes. However, cleat reorientation had little influence on changes in the magnitude of coal permeability. Permeability anisotropy degree between the three principal directions varies dynamically under different effective stresses, and then possibly changes direction of gas flow. This study demonstrates that stress-dependent coal anisotropy should be considered for simulating gas flow behavior and predicting coalbed methane production.