Pore Scale Characterisation of Coal: An Unconventional Challenge

Abstract

Abstract Coal seam gas is an unconventional resource for natural gas that is becoming popular due to its environmental benefit and abundance. This paper reviews recent developments on the pore-scale characterisation of coal from coal seam gas reserviors. The development of micro-computed tomography (micro-CT) imaging has enabled for the 3D characterization of the fracture system in coals. This provides detailed insights into understanding flow in these unconventional reservoirs. A novel image calibration method in which the skeleton of the fracture system is obtained from micro-CT imaging while the fracture apertures are measured from scanning electron microscopy (SEM) is described. We also show the application of micro-CT imaging for studying diffusion processes in ultralow permeability matrices and discuss the incorporation of the data into calculations of gas production from unconventional reservoirs. The extraction of statistical information from micro-CT images to reconstruct coal cleat system are also demonstrated. This technique allows for preserving the key attributes of the cleat system while the generated fracture network is not limited in terms of size nor resolution. The developments of microfluidic methods for understanding the complex displacement mechanisms in coal seams are also described. These low-cost experimental methods can provide unique information about the displacement mechanisms occurring during gas production from coal seam reservoirs. Variation of coal contact angle with pressure is analysed and results demonstrate important wettability processes that occur in coal seams. We describe numerical methods for prediction of petrophysical properties from micro-CT images of coal and discuss the associated limitations when dealing with coal samples. The paper concludes by addressing the challenges faced when characterising coal at the micro-scale and approaches for population of coal data into reservoir simulators for relaible prediction of reservoir behaviour during gas production as well as CO2 sequestration in coalbeds.