Hydraulic Conductivity Calibration of Logging NMR in a Granite Aquifer, Laramie Range, Wyoming

In granite aquifers, fractures can provide both storage volume and conduits for groundwater. Characterization of fracture hydraulic conductivity (K) in such aquifers is important for predicting flow rate and calibrating models. Nuclear magnetic resonance (NMR) well logging is a method to quickly obtain near-borehole hydraulic conductivity (i.e., K-NMR) at high-vertical resolution. On the other hand, FLUTe flexible liner technology can produce a K profile at comparable resolution but requires a fluid driving force between borehole and formation. For three boreholes completed in a fractured granite, we jointly interpreted logging NMR data and FLUTe K estimates to calibrate an empirical equation for translating borehole NMR data to K estimates. For over 90% of the depth intervals investigated from these boreholes, the estimated K-NMR are within one order of magnitude of K-FLUTe. The empirical parameters obtained from calibrating the NMR data suggest that intermediate diffusion and/or slow diffusion during the NMR relaxation time may occur in the flowing fractures when hydraulic aperture are sufficiently large. For each borehole, intermediate diffusion dominates the relaxation time, therefore assuming fast diffusion in the interpretation of NMR data from fractured rock may lead to inaccurate K-NMR estimates. We also compare calibrations using inexpensive slug tests that suggest reliable K-NMR estimates for fractured rock may be achieved using limited calibration against borehole hydraulic measurements.