INVESTIGATING THE EFFECT OF GRAIN SIZE ON NATURAL FRACTURE SURFACE ROUGHNESS

Open and connected fractures, when present, dominate fluid flow and solute transport. The characterization of fracture morphology and its relationship to fluid flow is still poorly understood. It is assumed that the impact of roughness on fracture flow at field-scales can be characterized from samples of natural fractures on the order of 100cm2. Some research suggests fracture surface roughness often exists at the same scale as fracture aperture and that surface roughness influences discharge at length scales of hand-size samples. Using the high-resolution x-ray computed tomography (HRXCT) facility at the University of Texas at Austin, we measure surface roughness and aperture for fractures in granite and tuff at hand-sample size scale. Our past work investigated aperture scaling, roughness scaling, and fracture skin properties. Current work involves investigating the influence of varying grain-size on fracture roughness and flow for natural granite fractures. From statistical analyses of fracture surfaces, we investigate detection of variation in grain-size for granite fractures with different skin properties. Statistical analyses consist of 3-dimensional characterization of the entire surface and 2-dimensional characterization of surface profiles. Roughness is integrated into numerical flow simulations by calculating the local cubic law with varying aperture. Some past research suggests, for rock fractures created in the laboratory, roughness properties vary with rock type due to grain effects. By combining HRXCT measurements of surface roughness with flow simulations for natural fractures of varying grain-size, we present research investigating what relationship exists between flow through fractures and rock characteristics such as grain-size.