Permeability: coal petrography

The permeability of coal seams can be influenced by geological structure variations. Coal seam permeability is affected by the changes in a geological structure of coal particularly in the vicinity of vault, dyke or fold. The level of coal permeability is influenced by the severity of the structural variations. Generally, favourable areas for coal bed methane drainage are likely to have a relatively simple geological structure to ensure the continuity of reservoirs. Gently folded areas in coal seams tend to have higher permeability than steeply folded and faulted areas. Cleat system in gently folded coal is likely to have the cleats open and continuous, thus facilitating increased permeability. However, in the structurally complex areas of dykes and faults, the cleat system tend to be severely damaged leading to low permeability,especially where severe structural compression has occurred.
The permeability among different coal litho-types varies even under the condition of similar coal rank. The coal petrological composition affects the overall permeability of the coal-bed through controlling the development of the pore and fissure system. The permeability of a vitrinite-rich coal reservoir is around 10 times higher than an inertinite rich one (Symth, 1993). Also, the inertinite-rich coal absorbed more methane than a middle-rank coal sample, but absorbs the same amount as a high rank coal sample.
Bartosiewicz and Hargraves (1984) examined various coal samples from Australian coal basins, and the results showed significant variations in permeability in different directions. Bedding plane permeability is significantly greater than the permeability normal to the bedding. However, Lingard, Phillips and Doig (1982) reported no significant difference between coal samples cut parallel and perpendicular to the bedding plane. Gash et al. (1993) tested the permeability of American coal samples and found that the permeability in the face cleat direction was greater.