Anisotropy of gas permeability associated with cleat pattern in a coal seam of the Kushiro coalfield in Japan

An underground investigation has been performed in a subbituminous coal seam exhibiting a particular cleat pattern in the Kushiro coalfield, Japan. The coal cleat pattern shows some analogy to isolated straight joints, and is believed to have been formed during the late Tertiary period by a compressive tectonic stress, roughly in the east-west direction. Three cylindrical coal specimens representing the three orthogonal axes of the coal seam with respect to the bedding plane and its associated cleat were cored from a large block of coal. Gas permeabilities of the three coal specimens were measured under the same hydrostatic pressure conditions. Results clearly revealed anisotropy in permeability of the coal seam under relatively low confining pressures of less than about 12 MPa. The specimen cored parallel to both the bedding plane and cleat strike showed the highest permeability, even though the cleats were partly filled with calcite and clay minerals. The permeability in this direction was 2.5 times higher than perpendicular to the bedding plane, and 3 times higher than in the direction parallel to the bedding plane but perpendicular to the cleat strike. This suggests that the cleats play a greater role than bedding planes in controlling fluid flow in the coal seam. The permeability in the three orientations, however, converged to the same value at confining pressures above about 16 MPa. This may suggest that both cleats and bedding planes in a coal seam can close due to earth pressure if the coal seam is located below a certain depth. It further indicates that the traditional view that gas permeability is always greater parallel to the coal bedding than perpendicular to it should be reconsidered.