Investigator: John S. Selker, Oregon State University
Proposal Number: EAR 1446915
Fractured rock aquifers are the only source of water in many parts of the United States and much of the rest of the world. Understanding these waters, and the materials that water transports, is critically needed for providing and protecting safe water drinking water. In addition, the role of water flow fractured rocks is important in the formation of mineral deposits and in the development of deep geothermal systems. This research will also benefit the extraction of oil and gas from fractured reservoirs, including those created by fracking. Thus, this study is both theoretical and potentially very practical in nature. Methods that can identify fluid flow paths in fractures have been very limited. This research will conduct lab experiments that will use unique fluids that can sample the major flow paths more effectively. These are the Non-Newtonian fluids (e.g., guar gum and other food grade fluids). The novel methods proposed seek a practical means of gaining insight into flow in fractured rock aquifers and other fractured rock systems. In parallel to the laboratory work, a field experiment will be performed at the observatory site of Ploemeur, France, where a unique fracture-flow field site has been established for just this kind of experiment. In addition, two undergraduate students will participate in the research.
The identification of dominant flow paths, their connectivity, and their hydraulic properties in fractured rocks is critical for fluid flow and solute transport. Tracer testing can characterize such aquifer properties from laboratory to field scales. Classical tracer test interpretations allow defining a mean "effective hydraulic" aperture based on simplified transport models. This research will: 1) develop an innovative tracer approach using non-Newtonian ("shear-thinning") fluids to identify aperture distributions of preferential flow paths in natural fractured networks and 2) investigate flow behavior of these "smart" tracers, knowing their rheology, in order to characterize the hydraulic properties of fracture systems. By adjusting the viscosity, the research will be able to select for specific thresholds or aperture size that allow flow, while smaller apertures will be essentially "frozen" in a gel-filled condition. This will be tested in the laboratory and at the instrumented field site to demonstrate the utility in characterizing aquifer properties. The experimental observations of the movement of shear-thinning fluid in realistic rock apertures will be compared to theoretical models of fluid transport. Additionally, numerical modeling will explore the hypothesis identified by the experimental approach (e.g., stable (non-Newtonian chasing water) vs unstable (water chasing non-Newtonian) fluid displacement). Based on preliminary results, the field experimental approach will to test the validity of the proposed approach. Coupled time-lapse GPR measurement will be used during tracer tests in order to document the preferential paths taken by the tracer. The methodology will be tested in a unique, well-characterized fractured granitic formation in France where complete fracture geometry and hydraulic characteristic have already been defined based on multiple hydro-geophysical approaches. Finally, numerical models will investigate the sensitivity of first order parameters that control flow transport of non-Newtonian fluids in such context. Theoretical concepts related to fractured aquifer hydrogeology will be presented and addressed experimentally in the lab.
