The coal permeability is a key parameter for the gas seepage characteristics and the migration law, and the structural anisotropy of the coal seam makes the permeability obvious orientation dependent. In this paper the gas permeability of raw coal specimens is measured in directions of face cleats and butt cleats under different gas pressures by a coal and rock permeability testing system. According to the equivalent displacement principle, the calculation model for the permeability of anisotropic coal is established and the directional characteristics of gas seepage are numerically simulated. It is shown that the structural anisotropy of coal has a great influence on the gas permeability of coal, the permeability in the direction of face cleats is larger than that in the direction of butt cleats, and the difference of coal permeability in the two directions reaches one order of the magnitude, and the more degraded the coal is, the greater the difference is. With the increase of the coal rank, the permeability decreases both in the directions of raw coal face cleats and butt cleats. With the increase of the gas pressure, the permeability decreases exponentially and the directionality coefficient peak increases and the directionality of gas seepage in the coal seam enhances. And under the same gas pressure, the lower the coal rank is, the weaker the directionality of gas seepage will be. This study provides some insight for optimizing the design and the layout of the gas drainage borehole in the coal seam, improving the gas drainage effect and achieving optimal extraction.

permeability  /  coal seam  /  structural anisotropy  /  directionality coefficient