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Abstract
In the past few decades, unconventional gas resources and in particular coalbed methane have received a lot of attention to satisfy the increasing demands of energy requirements. The characterization of coal fracture networks is crucial for understanding flow in coal seams. In this study, X-ray micro-computed tomography (micro-CT) imaging technique is used to acquire high quality 3D images of coal at the core-scale. Coal in terms of permeability and porosity is characterized and a unique contrast agent technique using micro-CT is employed to study coal features. The technique allows for visualization of coal fractures that are not readily visible with conventional imaging methods, particularly sub-resolution fractures. This facilitates identifying and partitioning the resolved and sub-resolution fractures and consequently results in a more deterministic representation of the coal fracture system. Also, high-resolution scanning electron microscope (SEM) methods are used for segmentation and analysis of the digital images. A unique calibration technique that captures and adjusts the fracture apertures of coal is introduced. The calibration method provides images in which the fracture network is obtained from the micro-CT images while the aperture sizes are assigned based on the calibration curve obtained from high-resolution SEM data. Furthermore, the image-based porosity estimation is validated against experimental methods, both gravimetric and gas (Helium) porosity, and the correlations between mineral contents, porosity, lithotype and permeability in coal are investigated. The results indicate that there is no relationship between porosity and permeability while there is a direct relationship between coal lithotype and permeability, i.e. the bright coals offer more permeability than dull coals. It is shown that de-mineralization of coals has significant effect in improving permeability of coal. The applied calibration technique preserves a large amount of valuable information that is neglected when non-calibrated images are used. This produces a more precise permeability result, i.e. close to the experimental permeability. Overall, the results of this study provide profound insights into the parameters that control the gas flow in coals.