Introducing the Effects of Rough Surfaces into Predictive Modeling of Fluid Flow through Fractures

Fractures dominate fluid flow and transport of solutes when they are open and connected. The prediction of flow through fractured media has implications for petroleum reservoir exploitation, development of water resources, contamination and remediation assessment, and site evaluation for waste repositories. Assessing the impact of surface roughness on fluid flow and solute transport through fractured media from samples on the order of 100 cm² assumes the existence of a relationship between fracture morphology and discharge that is scale invariant or at least smoothly transformable. Although some studies assume that the length scale at which surface roughness significantly contributes to the discharge through a fracture falls within the size of a typical hand sample, there is a dearth of empirical data supporting an extension of the relationships found at small scales to larger samples. Furthermore, an appropriate metric to describe a fracture volume accurately must be chosen. The University of Texas HRXCT facility provided computed tomography representations of several fractures that allow analysis of surface roughness and aperture statistics at 0.25mm grid resolution, which also form the basis for transmissivity field inputs to numerical models. I compile data from physical flow tests and numerical modeling of two discrete consecutive natural fractures in Santana Tuff from Trans Pecos, TX. These data show: 1) a fracture aperture profile of any orientation has little utility for predicting fluid flow through that fracture; 2) that a representative elemental surface of 10cm² can describe surface roughness of natural fracture faces at least two orders of magnitude larger; 3) an arithmetic mean aperture considerably overestimates flow through natural rough fractures; and 4) that a roughness ratio calculated from a single surface may be used to modify a statistical distribution of apertures to enable more accurate modeling of fractured units.