Hydraulic testing of low-permeability Silurian and Ordovician strata, Michigan Basin, southwestern Ontario

Abstract

• Hydraulic testing of Silurian and Ordovician sediments – eastern Michigan Basin. • Test equipment, methods and analysis specifically adapted for low-permeability rock. • Parameter perturbation analysis method allows quantification of uncertainty. • K values range from 4E−14 to 4E−8 m/s (Silurian) and 2E−16 to 2E−10 m/s (Ordovician). • Scaled Ordovician K values consistent with laboratory and natural analog estimates. Straddle-packer hydraulic testing was performed in 31 Silurian intervals and 66 Ordovician intervals in six deep boreholes at the Bruce nuclear site, located near Tiverton, Ontario, as part of site-characterization activities for a proposed deep geologic repository (DGR) for low- and intermediate-level radioactive waste. The straddle-packer assembly incorporated a hydraulic piston to initiate in situ pulse tests within low hydraulic conductivity (<1E−10 m/s) test intervals. Pressure transient data collected during the hydraulic tests were analyzed using the well-test simulator nSIGHTS to estimate the hydraulic properties specified as fitting parameters for the tested intervals, quantify parameter uncertainty, and define parameter correlations. Horizontal hydraulic conductivities of the Silurian test intervals range from approximately 4E−14 to 4E−8 m/s. The average horizontal hydraulic conductivities of the Ordovician intervals range from 2E−16 to 2E−10 m/s. The Lower Member of the Cobourg Formation, the proposed host formation of the DGR between 660 and 688 meters below ground surface, was found to have a horizontal hydraulic conductivity of 4E−15 to 3E−14 m/s. The formation pressures inferred from the hydraulic testing, confirmed by long-term monitoring, show that the Upper Ordovician and Middle Ordovician Trenton Group are significantly underpressured relative to a density-compensated hydrostatic condition and relative to the overlying Silurian strata and underlying Black River Group and Cambrian strata. These underpressures could not persist if hydraulic conductivities were not as low as those measured.