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).