Characterization of Fracture Morphology for Predicting Flow Through 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.  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 lithology, including sandstone, volcanic tuffs and crystalline igneous and metamorphic rocks.  Scale-invariant descriptions of surface roughness can produce constrained estimates of aperture variability and possibly yield better predictions of fluid flow through fractures.  We compare a variety of local aperture measurements, including the apparent aperture, 2-D and 3-D “true” apertures, and the arithmetic, geometric, and harmonic mean apertures.  The mechanical aperture, the arithmetic mean of the apparent local aperture, is always the largest aperture.  The other aperture metrics vary in their ranking, but remain similar.  Results suggest that, in practice, it may not be necessary to differentiate between the apparent and true apertures.  Fracture aperture is the predominant control on permeability, and surface roughness controls fracture aperture.  A variety of surface roughness characterizations using statistical and fractal methods are compared.  Mated fracture surfaces are observed to have nearly identical characterizations of fracture surface roughness, suggesting that rock fractures can be sampled adequately by using only one surface, resulting in a significantly easier sampling requirement.    These results allow better constraints for predicting fracture permeability thereby providing a better understanding of subsurface fracture flow for applications to contaminant remediation and water and hydrocarbon management.  Further research must address upscaling fracture morphology from hand samples to outcrops and characterizing the morphology of entire fracture networks from samples of single fractures.