Coal reservoirs have a dual porosity, naturally fractured structure. As in other naturally fractured formations coal permeability is sensitive to the effective stress, decreasing exponentially as the effective stress increases. In contrast to other naturally fractured reservoirs, coal shrinks as gas desorbs. This shrinkage plays an important role in determining reservoir permeability behaviour as it leads to a geomechanical response that counteracts the effective stress increase due to the pore pressure drawdown during production. It has been shown that under the right conditions this shrinkage effect can lead to a rebound in the reservoir permeability.
There are a range of existing models for coal reservoir permeability behaviour that represent the coupled effects of pore pressure change and shrinkage, with the models by Shi and Durucan (2004, 2005) and Palmer and Mansoori (1996, 1998) finding widespread use. An important obstacle to the meaningful application of coal permeability models is characterisation of the various physical properties involved. There are also important questions about the behaviour of these properties with changes in pressure and effective stress. For example, many coal permeability models assume that the cleat compressibility and geomechanical properties are constant. This paper presents the results of a laboratory program of work to measure the coal properties that influence absolute reservoir permeability during gas production; these are matrix shrinkage, cleat compressibility, and the geomechanical properties (see Figure 1). These measurements were made on core samples from the Bowen Basin of Australia, an important area for coal seam methane production. The results are compared to the field based analyses of Mazumder et al. (2012).