The Effect of Directional Net Stresses on the Directional Permeability of Coal

Many researchers have found that coal permeability is strongly inversely proportional to the net stress levels. However, most of the supporting lab data was gathered with hydrostatic/isotropic external stresses, quite unrepresentative of in situ conditions. It is well known that the in situ stress vectors are quite anisotropic, with the maximum principal stress (usually horizontal tectonic) being as much as 50 % greater than the minimum principal stress (whether horizontal or vertical). Also, most coal basins exhibit face cleat orientation parallel to the maximum principal stress. The inferred stress-shear strain relationship poses the question of how would the directional permeability of a coal seam be affected by changing directional external net stresses, when these are parallel, perpendicular or oblique to the face cleats or other directions? This paper addresses the question and summarises tests done on Permian age coal from China’s Sunan Basin and Australia’s Bowen Basin, utilising a True Triaxial Stress Coal Permeameter. This test facility delivers three independently controlle