Permeability: Water
In virgin coal seams, water normally fills pore spaces,
cleats, and fractures and any gas present is dissolved within the seam
water or absorbed on the internal surface of the coal, while the reservoir
and its fluid components are in equilibrium (Van der Meer, 2004). Permeability
of a coal seam to gas is dependent on the concentrations of water (Thakur
and Davis, 1977). Usually free gas only comes out of the coal into the
cleat space when the water pressure drops below the sorption pressure (Gray,
2000). The permeability of coal to water is increased by decreasing the
pressure (Dabbous et al., 1974). Kissell and Edwards (1975) reported that
the relative permeability of a coal seam increases as the water in the
seam decreases, thus making more space available for the gas phase to flow.
It means that initially permeability will decrease with a drop in reservoir
pressure around the production hole, followed by an increase, as significant
desorption induced shrinkage occurs as water and gas are produced from
the seam, with the effective stress increases leading generally to a
reduction
in permeability. However many coal seams exhibit an increase in permeability
with production, because of seam de-stressing and coal shrinkage due to
gas desorption. Coal shrinkage reduces the lateral stress in the seam and
shifts the stress into the surrounding rocks. The opposing effects on effective
stress mean that the permeability of the seam may either increase or decrease
with the removal of gas and water from the seam.
Generally, permeability is reduced by an increase in moisture content (Bartosiewicz
and Hargraves, 1985). However, it should be pointed out that the gas permeability
of a coal mass is influenced by the degree to which the permeable volume of the
pore is filled with natural moisture. Natural moisture decreases the permeable
volume of pores by a factor of greater than two (Ayruni, 1981).
