CHARACTERIZATION OF FRACTURE SURFACE ROUGHNESS AS A STEP TOWARD PREDICTING FLOW IN FRACTURES

Open and connected fractures, where present, control fluid flow and dominate solute transport. Flow through fractures has major implications for water resource management, underground waste repositories, contaminant remediation, and hydrocarbon exploitation. Complex fracture morphology makes it difficult to quantify and predict flow and transport accurately. The difficulty in usefully describing the complex morphology of a real fracture from a small 3-D volume or 2-D profile sample remains unresolved. Furthermore, even when complex fracture morphology is measured across three-dimensions, accurate prediction of discharge remains difficult. High resolution x-ray computed tomography (HXRCT) data collected for over 20 rock surfaces and fractures provide a useful dataset to study fracture morphology across scales of several orders of magnitude. Samples include fractured rock of varying geologic fabric; data were used to test the interdependence of fracture physical characteristics, such as grain-size and surface roughness. Results suggest that the influence of grain-size on surface roughness is not readily apparent. Flow tests of HXRCT-scanned fractures provide real discharge data to test numerical fracture flow models such as the local cubic law. The hydraulic aperture, the aperture value output when modeling the measured discharge as the cubic law, is found to be best estimated by the geometric mean aperture. Furthermore, the arithmetic mean aperture measured across a 2-D fracture profile (such as on an outcrop) significantly overestimates the hydraulic aperture. Scale-invariant descriptions of surface roughness can produce constrained estimates of aperture variability and possibly yield better predictions of fluid flow through fractures. Further research should address upscaling fracture morphology from hand samples to outcrops and characterizing entire fracture networks from samples of single fractures.