ASPERITY-INDUCED EPISODIC PERCOLATION THROUGH UNSATURATED FRACTURED ROCK

Many models of liquid flow through unsaturated, fractured rock employ steady infiltration-boundary conditions that yield steady flow conditions in the simulated domain. However, pore-scale heterogeneities provide a great deal of uncertainty regarding the steady vs. episodic nature of the actual flow processes. In this study, heterogeneous fracture asperities were investigated as a possible means of inducing episodic percolation events, even under steady-infiltration boundary conditions. The existence of a "pinch-point" aperture in a larger "storage" aperture was shown to create a capillary barrier capable of producing episodic accumulation and drainage events within the fracture. An episodicity factor was derived that represents the fraction of time in which flow actually drains through the fracture. The drainage event, while generally short in duration, was large in magnitude relative to the infiltration boundary condition. Results from an application of this model to the potential repository at Yucca Mountain indicated that the distribution of site-derived fracture apertures can yield a large range of episodicity factors (spanning several orders of magnitude), which resulted in considerable variability in the magnitude and frequency of the simulated percolation events. These simulated pulses of percolation were then used in analytical solutions to determine the amount of water that could penetrate into a superheated region surrounding the simulated repository drifts.

This work was supported by the Yucca Mountain Site Characterization Office as part of the Civilian Radioactive Waste Management Program, which is managed by the U.S. Department of Energy, Yucca Mountain Site Characterization Project. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.