PACKER AND TRACER TESTING FOR FRACTURE CHARACTERIZATION IN A CRYSTALLINE BEDROCK AQUIFER

Previous investigations at the Fractured Rock Research Site in Floyd County, VA have indicated the existence of a fracture zone associated with a fault plane aquifer that intersects multiple wells on the site. Preliminary studies at a pair of wells have indicated a rapid pressure response between wells during aquifer testing, suggesting that this may be a significant flow pathway in the fault-zone aquifer. However, little is known about the characteristics of this flow pathway, including aperture, porosity, and hydraulic conductivity.

This study uses a multi-faceted approach to characterize this fracture zone. Optical televiewer logs in wells of interest allow for the visual identification of fractures that intersect a borehole wall. Coupling these images with heat-pulse flowmeter measurements during both ambient and pumping conditions provides confirmation of their activity as conductive flow pathways and an estimation of the hydraulic head within these zones.

Isolating the conductive fractures with packers during aquifer testing allows for the evaluation of cross-borehole hydraulic conductivity. Aperture can also be estimated during aquifer testing by the cubic law. No universally appropriate method exists for analyzing drawdowns from pumping tests in fractured rocks due to the high variability of these systems, so several methods must be assessed to determine their suitability for this specific site. The methods considered in this study include a generalized radial flow model, a double porosity model, and an equivalent porous medium model.

Finally, tracer tests using both an inert saline tracer and heated water provide a means of identifying the cross-borehole travel time and effective porosity of a conductive fracture. Comparison of the breakthroughs for each of these tracing methods potentially offers information concerning retardation or diffusion rates into the host rock. By combining the data collected from multiple tools and analyses, a comprehensive description of the fracture zone and its characteristics is obtained. The evaluation of these methods for characterizing fractures is important for determining the most effective techniques of analyzing these features in both the Blue Ridge province and other localities of fractured crystalline rock.