Laboratory characterisation of coal matrix shrinkage, cleat compressibility and the geomechanical properties determining reservoir permeability

This paper presents the results of a laboratory program of work to measure the coal properties required to apply models for the behaviour of the absolute reservoir permeability during gas production. These measurements were made on core samples from the Bowen Basin of Australia, an important area for coal seam methane production, and involved applying an integrated testing methodology. During the testing the pore pressure was increased in a stepwise fashion with gas adsorption equilibration allowed at each pressure step. The gas content of the intact sample was estimated from the gas taken up during equilibration and the sample swelling in response to adsorption measured. After adsorption had equilibrated, the geomechanical properties were determined through axial loading and measurement of the deformation and the permeability measured with respect to confining pressure. These permeability measurements were then used to estimate the cleat compressibility by fitting the Seidle model to the observations. The results from five coal samples are presented. A method is presented for the calculation of the cleat porosity, a difficult property to determine experimentally as it represents the proportion of the porosity involved in Darcy flow. Thus, the presented method uses a property determined from flow measurements; the cleat compressibility. The measured properties are used in the Shi-Durucan model to predict permeability behaviour with pressure drawdown. The results are compared to the field based estimates from the analysis of Mazumder et al. (2012).