Investigating internal magnetic field gradients in aquifer sediments

Fay, Emily; Knight, Rosemary; Song, Yi‐Qiao

doi:10.1190/geo2014-0445.1

Cited by 17 publications

(12 citation statements)

References 35 publications

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“…We note that if the iron content were the same in the materials sampled by the laboratory and logging instruments, we would expect the impact of diffusion relaxation on the two measurements to be the same, resulting in no difference in the measured T 2ML . The magnitude of internal gradients is affected by the background magnetic field strength and the echo spacing, but, as shown in Fay et al (2015), for the combination of field strengths and echo spacing used in this study (2 MHz and 0.3 ms for the laboratory data;~0.3 MHz and 2 ms for the logging data), the internal field contribution to the T 2 measurements should be the same.…”

Section: Resultsmentioning

confidence: 87%

Successful Sampling Strategy Advances Laboratory Studies of NMR Logging in Unconsolidated Aquifers

Behroozmand

Knight

Müller‐Petke

et al. 2017

Geophysical Research Letters

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The nuclear magnetic resonance (NMR) technique has become popular in groundwater studies because it responds directly to the presence and mobility of water in a porous medium. There is a need to conduct laboratory experiments to aid in the development of NMR hydraulic conductivity models, as is typically done in the petroleum industry. However, the challenge has been obtaining high-quality laboratory samples from unconsolidated aquifers. At a study site in Denmark, we employed sonic drilling, which minimizes the disturbance of the surrounding material, and extracted twelve 7.6 cm diameter samples for laboratory measurements. We present a detailed comparison of the acquired laboratory and logging NMR data. The agreement observed between the laboratory and logging data suggests that the methodologies proposed in this study provide good conditions for studying NMR measurements of unconsolidated near-surface aquifers. Finally, we show how laboratory sample size and condition impact the NMR measurements.

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Section: Resultsmentioning

confidence: 87%

Successful Sampling Strategy Advances Laboratory Studies of NMR Logging in Unconsolidated Aquifers

Behroozmand

Knight

Müller‐Petke

et al. 2017

Geophysical Research Letters

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“…Internal gradient magnitudes were estimated using repeated CPMG measurements with variable echo spacing, using 14 t E steps from 200 μs to 1 ms. The average internal gradient magnitude G int was calculated from the slope of 1∕T 2ML versus t E2 following the method described in Fay et al (2015).…”

Section: Methodsmentioning

confidence: 99%

“…In geologic materials, magnetic susceptibility contrasts between the pore fluid and the solid matrix can lead to significant pore-scale background magnetic field inhomogeneity. Previous work has found that large internal gradients exist in many natural sediments due to the presence of ferromagnetic materials (Walbrecker et al, 2014) and paramagnetic materials (Sun and Dunn, 2002;Washburn et al, 2008;Fay et al 2015), both of which have magnetic susceptibility values very different from that of water. A list of magnetic susceptibility values of some common minerals is given in Table 1.…”

Section: Internal Gradientsmentioning

confidence: 99%

“…Internal gradients also vary with their proximity to the pore walls (Cho et al, 2009), with the highest localized gradients occurring near inflection points (such as sharp corners) on a pore surface (Brown and Fantazzini, 1993;Zhang et al, 2003;Grombacher et al, 2016). The background field strength controls the magnitude of internal gradients, such that measurements conducted at lower fields will be less impacted by internal gradients (Mitchell et al, 2010; see also Figure 11 in Fay et al 2015). This is relevant because different logging tools operate at different frequencies (e.g., 2 MHz, Prammer et al [2001] versus 2 kHz, Walsh et al, 2013).…”

Section: Internal Gradientsmentioning

confidence: 99%

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Investigating the effect of internal gradients on static gradient nuclear magnetic resonance diffusion measurements

2017

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Nuclear magnetic resonance (NMR) methods can be used to measure the diffusion coefficient D of fluids. In porous materials, diffusion of the pore fluid is restricted by pore boundaries, such that D may be smaller than the diffusion coefficient of the bulk fluid. This reduction in D provides information about the geometry of the pore space. Significant overestimates of D can, however, occur due to internal gradients caused by magnetic susceptibility contrasts between the pore fluid and the solid phase. We have investigated the way in which internal gradients can impact the measured diffusion coefficient and obscure the link to pore geometry in unconsolidated sediments. We focus on measurements of D obtained with a static gradient diffusion-editing sequence, which can be used with NMR logging tools to measure D in subsurface sediments. Laboratory measurements of D measured with a diffusion-editing D-T 2 sequence indicate significant impacts from internal gradients, including D values several orders of magnitude larger than D of bulk water. The log-mean D values were found to be highly correlated with estimated internal gradient magnitudes and indicate no clear relationship to pore size. Samples with heterogeneous magnetic susceptibility of the solid phase indicate D distributions with multiple peaks, reflecting the nonuniform distribution of internal gradients in the sediment. We found evidence of high internal gradients impacting a majority of our samples, resulting in increased D values that do not reflect pore size even in samples with low magnetic susceptibility.

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“…In sediments where internal magnetic gradients are large, however, diffusion relaxation can further shorten decay times. Internal magnetic gradients increase with magnetic susceptibility and are also large in the case of small disseminated magnetic grains (Dunn et al ., 2002; Keating and Knight, 2007a; Fay et al ., 2015), which is an important consideration in settings containing magnetic minerals.…”

Section: Nuclear Magnetic Resonance Logging Technologymentioning

confidence: 99%

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

Crow

Enkin

Percival

et al. 2020

Near Surface Geophysics

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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.

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Investigating internal magnetic field gradients in aquifer sediments

Cited by 17 publications

References 35 publications

Successful Sampling Strategy Advances Laboratory Studies of NMR Logging in Unconsolidated Aquifers

Successful Sampling Strategy Advances Laboratory Studies of NMR Logging in Unconsolidated Aquifers

Investigating the effect of internal gradients on static gradient nuclear magnetic resonance diffusion measurements

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

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