A 3d Feature Extraction-Based Method for Coal Cleat Characterization Using X-Ray Μct and its Application to a Bowen Basin Coal Specimen

Cleats are the dominant microscopic fracture network controlling the macroscopic mechanical and hydrological behaviour of coal. Improved understanding of the spatial characteristics of cleat networks is therefore important to both the coal mining and coal seam gas industries. Discrete fracture networks (DFN) are increasingly used in engineering analyses to spatially model fractures at various scales. The reliability of DFNs largely depends on the confidence in the input fracture statistics such as orientation, spacing, intensity, and aperture. Traditionally, estimates of these parameters have relied on surficial measurements from drill core, outcrop exposures, or downhole geophysical logs with limited regard for internal variability. Therefore, new methods for improved 3D characterisation of internal fracture characteristics are needed. In this paper, a new method is introduced for the 3D characterisation of coal cleat based on X-ray microscopic computed tomography (μCT) and feature extraction to identify and separate distinct cleat groups. The new method is applied to a 80 mm diameter by 100 mm length coal specimen acquired from an Anglo American Metallurgical Coal mine in the Bowen Basin, Queensland, Australia. Convenient methods for automated calculation of properties for the key spatial metrics of the separated cleat groups are also presented. It is demonstrated that the new method produces reliable cleat separation capable of defining individual cleats and preserving 3D topology after separation. Bedding-parallel cleats are also identified and separated, which has historically been challenging to delineate and rarely reported in the rock mechanics literature. A variety of cleat statistics are measured which not only quantitatively characterize the cleat system but also can be used for DFN modelling. Finally, variability and heterogeneity with respect to the core axis are investigated with a series of cross-sections through the 3D dataset. Significant heterogeneity is observed and suggests that the representative elementary volume of the cleat groups for engineering purposes may be a complex problem requiring careful consideration.