ABSTRACT:

Groundwater seepage critical affects subsurface leakage and chemical transport. Fluid flow in fractured media is highly variable and difficult to characterize. Several characterization methods were applied at a fractured granite environmental site. We collected flow velocity measurements over a depth interval of about 100 feet using Point Velocity Probe (PVP) and Passive Flux Meter (PFM). The PVP is based on measuring the travel time of an electrically conductive tracer from the center to the circumference of the instrument. The PFM is based on measuring the loss of tracer mass leaching from sorbent within a period. In general, PVP and PFM were in good agreement on delineating the high-and low flow regions. In one borehole, which was also profiled by PVP, a downhole acoustic televiewer (ATV) survey revealed the densest set of fractures in this zone. The fracture orientation was also logged by the ATV survey. In boreholes nearby, we inserted FLUTeTM liners to obtain transmissivity profiles to a depth of 150 feet. The transmissivity profiles are in qualitative agreement with the flow profiles, revealing a high transmissivity zone in the same depth as a high-flow zone identified by the other methods.

1. Introduction

Fractures are important geological features since they provide preferential pathways for fluid flow through an otherwise relatively impermeable rock mass (Schmelling and Ross, 1989). Many aquifers, as well as petroleum and geothermal reservoirs, are formed in fractured rock (National Research Council, 1996). The factors controlling flow through fractures are fracture density, orientation, aperture, and rock matrix type. Characterization methods for fractures, such as geophysical imaging or geomechanical analysis, are easily collected, but rarely provide accurate information about the effective fracture conductivity, since the flow behavior can vary depending on the fracture properties. For these methods, ground-truthing are necessary. Therefore, measurements of the actual flow behavior are crucial for evaluating the effective fracture conductivity. In recent years, techniques for measuring groundwater flow velocity have been developed, such as tracer tests, borehole flowmeter, colloidal borescopes (Ferry et al. 1995), acoustic Doppler flowmeters, Passive Flow Meter (PFM, Hatfield et al., 2004), In-Well Point Velocity Probe (IWPVP, Osorno and Devlin, 2018), and FLUTe (FLUTe, 2012) liner technology. Although these methods have been successfully applied to measure flow through porous media, their applications to fractured flow have not be established. In this study, we applied the PFM, IWPVP, and FLUTe methods to measure groundwater velocity within the same depth interval at a fractured rock site. In addition, the Acoustic Borehole Televiewer (ATV) technology was applied to depict fracture location and aperture. This paper presents a comparison of the results.

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