Within geologic reservoirs the flow of fluids through fractures is often orders of magnitude greater than through the surrounding, low-permeability rock. Because of the number and size of fractures in geological fields, reservoir-scale discrete-fracture simulators often model fluid motion through fractures as flow through narrow, parallel plates. In reality fractures within rock are narrow openings between two rough rock surfaces. In order to model the geometry of an actual fracture in rock, a ∼9 cm by 2.5 cm fracture within Berea sandstone was created and the aperture distribution was obtained with micro-Computed Tomography (CT) scans by Karpyn et al. . The original scans had a volume-pixel (voxel) resolution of 27 by 27 by 32 microns. This data was up-scaled to voxels with 120 microns to a side to facilitate data transfer and for practicality of use. Using three separate reconstruction techniques, six different fracture meshes were created from this up-scaled data set, each with slightly different final geometries. Flow through each of these fracture meshes was evaluated using the finite-volume simulator FLUENT. While certain features of the fracture meshes, such as the shape of the fracture aperture distributions and overall volume of the void, remained similar between the different geometric reconstructions, the flow in different models was observed to vary dramatically. Rough fracture walls induced more tortuous flow paths and a higher resistance to flow. Natural fractures do vary in-situ, due to sidewall dissolution and mineral precipitation, smoothing and coarsening fracture walls respectively. Thus for our study the range of fracture properties was actually beneficial, allowing us to describe the flow through a range of fracture types. A compromise between capturing the geometric details within a domain of interest and a tractable computational mesh must always be addressed when flow through a physical geometry is modeled. The fine level of detail that is currently available from micro-CT scanning equipment can compound this problem. This study evaluates several methods of obtaining rational CFD meshes from a complex physical geometry, and discusses the benefits and disadvantages of the different procedures as they pertain to flow through a natural fracture in rock.
Conversion of a Micro-CT Scanned Rock Fracture Into a Useful Model
Crandall, D, Bromhal, G, & Smith, DH. "Conversion of a Micro-CT Scanned Rock Fracture Into a Useful Model." Proceedings of the ASME 2009 Fluids Engineering Division Summer Meeting. Volume 1: Symposia, Parts A, B and C. Vail, Colorado, USA. August 2–6, 2009. pp. 2101-2108. ASME. https://doi.org/10.1115/FEDSM2009-78118
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