MEASUREMENT AT THE EDGE OF RESOLUTION: CHARACTERIZING FRACTURES IN 3D USING HIGH-RESOLUTION X-RAY COMPUTED TOMOGRAPHY

When open and connected fractures are present in the subsurface they dominate both fluid flow and solute transport. The transport properties of fracture networks are heavily dependent on the aperture and roughness of the individual fractures. Precise and thorough measurement of these properties is typically problematic, particularly in tandem. High-resolution X-ray computed tomography is able to image fractures in solid samples, presenting the possibility of making precise 3D measurements. This measurement task is challenging, however, in that apertures are inevitably small compared to their horizontal extent, leading to aspect ratios of up to several orders of magnitude. Fracture apertures are thus frequently imaged at close to the resolution limit of CT instruments, particularly when the research objective involves characterizing fracture length and width simultaneously, as in the case of flow pathways. A number of methods have been put forward to extract from CT data maps of fractures that precisely quantify their geometry, both from the academic literature and in commercial software such as Avizo Fire and VG Studio Max. We compare some of these methods on a series of natural fractures that have been imaged in both relatively homogeneous and heterogeneous rocks and have well-controlled apertures. In general, most methods are sensitive to user input, and the greatest potential for making reproducible measurements lies in choosing input parameters in an unbiased way. Maximizing accuracy, on the other hand, requires that the measurement method account for the physical system as completely as possible, including the heterogeneity of the material being imaged and the limited resolution of the imaging modality.