Permeability: coal petrography
The permeability of coal seams can be influenced by geological structure
variations. Coal seam permeability is affected by the changes in geological
structure of coal particularly in the vicinity of a fault, 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 tends 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.
