Downhole nuclear magnetic resonance logging in glaciomarine sediments near Ottawa, Ontario, Canada

Abstract

Downhole nuclear magnetic resonance technology was applied in four boreholes intersecting glaciomarine sediments of the Ottawa Valley, Ontario. The study evaluated the ability of slim-hole nuclear magnetic resonance tools to measure in situ volumetric water contents (porosities in saturated sediments) for geohazard and hydrogeological applications. The sediments are composed of clay-and silt-sized grains of glacially eroded rock flour derived from the Precambrian Shield containing trace amounts of magnetic minerals, and porosities ranging from 40 to 74 porosity units (PU, 1% porosity = 1 PU). Two nuclear magnetic resonance instruments were deployed with echo times of 0.5 and 1.0 ms, and diameters of investigation ranging from 14.0 to 30.5 cm. Quantitative nuclear magnetic resonance porosities in the sediments were typically within ±5 PU (95% within ±10 PU) of core calibration data sets in the silty clays where threshold bulk magnetic susceptibility values were <30 × 10 −4 SI. This was found to deviate, however, where the concentration and mineralogy of magnetic particles changed, interpreted to be shortening relaxation times which led to underestimation of true water contents. This effect is correlated with a change in depth from magnetite to superparamagnetic nanoparticles of greigite (low-temperature iron monosulphide) magnetism, interpreted to occur at the sulphate-methane interface. Clay mineralogy and pore water chemistry were also examined as contributing factors, but were not found to significantly shorten nuclear magnetic resonance relaxation responses. Very short T 2 times (<2 ms) are typical in these particular silty clays, requiring a tool with an echo spacing of <1.0 ms. Due to increased potential for sediment disturbance around a well caused by the geotechnical sensitivity of the sediments, a nuclear magnetic resonance instrument with multiple frequencies provided signal penetration out to various diameters around the tool, giving needed information about the size of the disturbed zone surrounding the casing.

Evaluation of Models for Estimating Hydraulic Conductivity in Glacial Aquifers withNMRLogging

“…When NMR logging data have been used to identify the presence of free and bound water in near‐surface materials, the same cutoff time of 33 ms established for sandstone cores is used, albeit with reservation (Walsh et al 2013; Tan et al 2015; Kirkland and Codd 2018; Spurlin et al 2019; Crow et al 2020; Davis et al 2020). Although more investigation is needed to determine the appropriate cutoff time for near‐surface applications, such determination was beyond the scope of this study.…”

Hydraulic Conductivity from Nuclear Magnetic Resonance Logs in Sediments with Elevated Magnetic Susceptibilities

“…The influence of magnetic minerals on NMR response must be considered in glacial sedimentary settings proximal to Precambrian Shield terrain, common to much of the southern fringe of the Laurentide Ice Sheet in southern Canada (Roy et al 2008; Legchenko et al 2010; Crow et al 2020a). In the mid‐1990s, the Geological Survey of Canada began using high‐sensitivity MS logging as an effective means of lithological interpretation in glacial sediments across Canada (McNeill et al 1996).…”