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Permeability is a physical property of porous materials,
which determines the flow of fluid through the material by an applied pressure
gradient. It may be described as the “fluid conductivity” of
the porous material. Permeability is also used to describe the resistance
of strata to the passage of gas through it (Mordecai and Morris, 1974).
One of the principal physical parameters
governing gas emission from coal is cleat permeability (Jones et al., 1982).
To properly
plan the gas drainage system in the mine, information about gas pressure,
gas composition, location of the gas reservoirs and permeability of the
seam is required. Highly permeable coal offers good opportunity to recover
methane. The unit of measuring the permeability is the Darcy. The
Darcy unit represents the flow capacity required for 1 ml of fluid to flow
through 1cm2 for a distance of 1 cm when 1 atmosphere of pressure is applied.
The permeability values vary according to gas type and differential pressure
across the coal seam
Permeability of coal can be calculated using the following Darcy’s equation:
where:
k = Permeability, Darcy
dp/dx=
Pressure gradient, atm/cm
A = Cross sectional area, cm2
 =
Viscosity of the fluid, centipoise
Q = Volumetric flow, cm3/s
Australian coals are commonly of low permeability (less than 5 md) compared
with coal seams mined in other countries,and represents a significant problem
for the exploitation of the gas. Hayes (1982) reported that the Bulli seam
permeability is considerably less than 1 mD. Lingard, Phillips and Doig (1982)
reported permeability of Australian coals from Appin, West Cliff and Leichhardt
(now closed) collieries that varied from less than 0.1 mD to 100 mD.
The following factors influence the coal permeability (Enever and Hennig,
1997)
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