Permeability Anisotropy in High Dip Angle Coal Seam: A Case Study of Southern Junggar Basin

Coal seam with high dip angle has relatively high permeability anisotropy, which has great impact on the production of coalbed methane. In this study, a laboratory method was established to evaluate the influence of dip angle on coal permeability with three steps. Firstly, five groups of coal columns with different dip angles were obtained by wire cutting technology from the southern Junggar Basin. Then, all samples were tested for initial (no confining pressure) porosity and permeability. Finally, some samples with different dip angles were selected for the permeability test under confining pressure. According to the test process and results, numerical simulations were established to explain how dip angles affect permeability under the experimental conditions. In the initial permeability test, as the dip angle increases, although the porosity was almost the same, the coal samples showed a decreasing trend in permeability. This trend is caused by the anisotropy of coal samples, which can be characterized by numerical simulation. Under confining pressure, the permeability decreases exponentially with increasing confining pressure. When the inclination angle gradually increased from 0° to 90°, the rate of permeability decrease had a tendency of first decrease and then increase. The reason for this phenomenon is that the strain, including the shear strain, can be changed anisotropically by the dip angle, which is proved by good agreement between test results and simulation results. The results are helpful to analyze the law of permeability change in coal seam with high dip angle.