Borehole Nuclear Magnetic Resonance (bNMR)
Basic Concept
The borehole nuclear magnetic resonance (bNMR) method provides direct, depth-dependent measurements of critical subsurface hydrologic properties by exploiting some of the atomic properties of hydrogen. bNMR was developed primarily for hydrocarbon exploration but has recently become more widely available for use in environmental and hydrogeological studies that require slimhole tools. There are a number of bNMR tools designed to fit in standard PVC-cased or open boreholes that can collect either stationary or continuous measurements.
The bNMR method is based on the principle of nuclear magnetic resonance and allows for the detection and quantification of the water within subsurface materials. Analysis of bNMR data can provide information regarding the porosity of fluid-filled pores, moisture content in the vadose (i.e., unsaturated) zone, and pore-size distribution of porous materials. Additionally, hydraulic conductivity (K) estimates can be calculated by applying bNMR data and site-specific parameters to established empirical relations.
Theory
The borehole nuclear magnetic resonance (bNMR) method exploits the subatomic processes occurring within and the measurable responses produced by materials during nuclear magnetic resonance (NMR). Proton NMR occurs when the proton spins associated with hydrogen atoms within a magnetic field are subjected to a perturbation and results in and electromagnetic signal. Simplistically, a bNMR measurement involves three stages that are repeated multiple times: 1) establish equilibrium, 2) perturb, and 3) measure.
A standard bNMR tool contains internal magnets that are significantly stronger (i.e., about 50 times) than the magnetic field of Earth. Due to their intensity and proximity, the magnets induce a background magnetic field (B0) that dominates the materials surrounding the tool. As such, the protons within the hydrogen atoms in the bNMR measurement zone become magnetized in alignment with (i.e., equilibrate to) the background field.
After equilibration, the tool emits a radio frequency (RF) pulse, which is tuned at the Lamour frequency, that overpowers the background magnetic field. The RF pulse tips (i.e., perturbs) the magnetization normal to (i.e., 90 degrees from) the background (B0) into the transverse plane (B1). After the pulse, the protons process about and back into parallel with B0. This precession creates a measurable magnetic field that decays over time into the background field.
The measured electromagnetic signal over time is referred to as the transverse (T2) decay. T2 is a vector that represents the proton-magnetization orientations and decreases over time with proton precession. The three-stage process is repeated multiple times, during which a series of RF pulses is used to refocus the spin and rotate successive signals 180 degrees. The results are stacked and filtered to remove noise, and a multi-exponential curve is fit to the T2 decay data.
The initial magnetic-field strength, which is represented by the T2 value at time zero, is directly proportional to the total amount of signal-inducing water. The T2-decay rate is related to the pore-size distribution such that bound water in small pores have shorter relaxation times than mobile water in large pores. The multi-exponential curve is inverted, and total water content is separated into bound- and mobile water based on T2-decay times (Straley and others, 1997).
The pore-size distribution results are used to estimate hydraulic conductivity (K) using established empirical equations and site-specific parameters. The two relevant unit-dependent equations are the Schlumberger-Doll research (SDR) equation (Kenyon and others, 1988) and the sum of echoes (SOE) equation (Allen and others, 2000). The SDR equation uses the measured values of total porosity (ϕ) and the mean log T2 (MLT2), whereas the SOE equation uses the summed amplitudes of the signal echoes in the T2 decay.
Applications
Borehole nuclear magnetic (bNMR) tools can be used in both PVC cased and open boreholes, but logging in steel can be dangerous. It is important to select an appropriate bNMR-logging probe, as data quality and accuracy can decrease by unsuitable tool selection, poor well construction, or unknown borehole parameters. Additionally, like most electromagnetic geophysical methods, bNMR data are susceptible to erroneous noise. However, there exist ways to help mitigate these effects in both the data collection and processing phases.
Multiple bNMR tools exist, each of which has a specific vertical resolution and radii of investigation (or measurement zone). The measurement zone is at a distance from the center of the tool that depends on tool diameter and frequency and can be visualized as 2-mm thick cylindrical shells surrounding the probe. To ensure that data are collected in the natural, undisturbed formation, the measurement zone must lie beyond the zone disturbed by drilling.
The bNMR method has two main objectives: 1) collect in situ water-content measurements and 2) determine the mobile/immobile fractions of water and estimate hydraulic conductivity. bNMR data can be presented as point measurements or one-dimensional vertical profiles of total-, mobile-, and bound-water content and hydraulic-conductivity estimates. In saturated conditions, bNMR measurements represent total porosity, whereas, in unsaturated zones water content is shown as a volumetric percentage of measured material. Additionally, bNMR logs have aided:
- Characterizing water content in unconsolidated deposits
- Estimating hydraulic conductivity in unconsolidated deposits
- Vadose-zone studies
- Soil-moisture experiments
- Hydrocarbon prospecting in consolidated deposits
Examples/Case studies
Behroozmand, A.A., Keating, K., and Auken, E., 2015, A Review of the Principles and Applications of the NMR Technique for Near-Surface Characterization: Surveys in. Geophysics, v.36:1, p. 27–85, doi:10.1007/s10712-014-9304-0.
Abstract: This paper presents a comprehensive review of the recent advances in nuclear magnetic resonance (NMR) measurements for near-surface characterization using laboratory, borehole, and field technologies. During the last decade, NMR has become increasingly popular in near-surface geophysics due to substantial improvements in instrumentation, data processing, forward modeling, inversion, and measurement techniques. This paper starts with a description of the principal theory and applications of NMR. It presents a basic overview of near-surface NMR theory in terms of its physical background and discusses how NMR relaxation times are related to different relaxation processes occurring in porous media. As a next step, the recent and seminal near-surface NMR developments at each scale are discussed, and the limitations and challenges of the measurement are examined. To represent the growth of applications of near-surface NMR, case studies in a variety of different near-surface environments are reviewed and, as examples, two recent case studies are discussed in detail. Finally, this review demonstrates that there is a need for continued research in near-surface NMR and highlights necessary directions for future research. These recommendations include improving the signal-to-noise ratio, reducing the effective measurement dead time, and improving production rate of surface NMR (SNMR), reducing the minimum echo time of borehole NMR (BNMR) measurements, improving petrophysical NMR models of hydraulic conductivity and vadose zone parameters, and understanding the scale dependency of NMR properties.
Dlubac, K., Knight, R., Song, Y., Bachman, N., Grau, B., Cannia, J., and Williams, J., 2013,
Use of NMR logging to obtain estimates of hydraulic conductivity in the High Plains aquifer, Nebraska, USA: Water Resources Research, v.49:4, p. 1871-1886, doi:10.1002/wrcr.20151.
Abstract: Hydraulic conductivity (K) is one of the most important parameters of interest in groundwater applications because it quantifies the ease with which water can flow through an aquifer material. Hydraulic conductivity is typically measured by conducting aquifer tests or wellbore flow (WBF) logging. Of interest in our research is the use of proton nuclear magnetic resonance (NMR) logging to obtain information about water‐filled porosity and pore space geometry, the combination of which can be used to estimate K. In this study, we acquired a suite of advanced geophysical logs, aquifer tests, WBF logs, and sidewall cores at the field site in Lexington, Nebraska, which is underlain by the High Plains aquifer. We first used two empirical equations developed for petroleum applications to predict K from NMR logging data: the Schlumberger Doll Research equation (KSDR) and the Timur‐Coates equation (KT‐C), with the standard empirical constants determined for consolidated materials. We upscaled our NMR‐derived K estimates to the scale of the WBF‐logging K(KWBF‐logging) estimates for comparison. All the upscaled KT‐C estimates were within an order of magnitude of KWBF‐logging and all of the upscaled KSDR estimates were within 2 orders of magnitude of KWBF‐logging. We optimized the fit between the upscaled NMR‐derived K and KWBF‐logging estimates to determine a set of site‐specific empirical constants for the unconsolidated materials at our field site. We conclude that reliable estimates of K can be obtained from NMR logging data, thus providing an alternate method for obtaining estimates of K at high levels of vertical resolution.
Kirkland, C.M. and Codd, S.L., 2018, Low-Field Borehole NMR Applications in the Near-Surface
Environment: Vadose Zone Journal Abstract, v.17:1, doi:10.2136/vzj2017.01.0007.
Abstract: The inherent heterogeneity of the near subsurface (<200 m below the ground surface) presents challenges for agricultural water management, hydrogeologic characterization, and engineering, among other fields. Borehole nuclear magnetic resonance (NMR) has the potential not only to describe this heterogeneity in space nondestructively but also to monitor physical and chemical changes in the subsurface with time. Nuclear magnetic resonance is sensitive to parameters of interest like porosity and permeability, saturation, fluid viscosity, and formation mineralogy. Borehole NMR tools have been used to measure soil moisture in model soils, and recent advances in low-field borehole NMR instrumentation allow estimation of hydraulic properties of unconsolidated aquifers. We also demonstrate the potential for low-field borehole NMR tools to monitor field-relevant biogeochemical processes like biofilm accumulation and microbially induced calcite precipitation at laboratory and field scales. Finally, we address some remaining challenges and areas of future research, as well as other possible applications where borehole NMR could provide valuable complementary data.
Knight, R., Walsh, D.O., Butler Jr, J.J., Grunewald, E., Liu, G., Parsekian, A.D., Reboulet, E.C.,
Knobbe, S., and Barrows, M., 2016, NMR Logging to Estimate Hydraulic Conductivity in Unconsolidated Aquifers: Groundwater, v.54:1, p. 104-114. doi:10.1111/gwat.12324.
Abstract: Nuclear magnetic resonance (NMR) logging provides a new means of estimating the hydraulic conductivity (K) of unconsolidated aquifers. The estimation of K from the measured NMR parameters can be performed using the Schlumberger‐Doll Research (SDR) equation, which is based on the Kozeny–Carman equation and initially developed for obtaining permeability from NMR logging in petroleum reservoirs. The SDR equation includes empirically determined constants. Decades of research for petroleum applications have resulted in standard values for these constants that can provide accurate estimates of permeability in consolidated formations. The question we asked: Can standard values for the constants be defined for hydrogeologic applications that would yield accurate estimates of K in unconsolidated aquifers? Working at 10 locations at three field sites in Kansas and Washington, USA, we acquired NMR and K data using direct‐push methods over a 10‐ to 20‐m depth interval in the shallow subsurface. Analysis of pairs of NMR and K data revealed that we could dramatically improve K estimates by replacing the standard petroleum constants with new constants, optimal for estimating K in the unconsolidated materials at the field sites. Most significant was the finding that there was little change in the SDR constants between sites. This suggests that we can define a new set of constants that can be used to obtain high resolution, cost‐effective estimates of K from NMR logging in unconsolidated aquifers. This significant result has the potential to change dramatically the approach to determining K for hydrogeologic applications.
Walsh, D., Turner, P., Grunewald, E., Zhang, H., Butler Jr, J.J., Reboulet, E., Knobbe, S., Christy, T.,
Lane Jr, J.W., Johnson, C.D., Munday, T., and Fitzpatrick, A., 2013, A Small-Diameter NMR logging Tool for Groundwater Investigations: Groundwater, v. 51:6, p. 914-926, doi:10.1111/gwat.12024.
Abstract: A small‐diameter nuclear magnetic resonance (NMR) logging tool has been developed and field tested at various sites in the United States and Australia. A novel design approach has produced relatively inexpensive, small‐diameter probes that can be run in open or PVC‐cased boreholes as small as 2 inches in diameter. The complete system, including surface electronics and various downhole probes, has been successfully tested in small‐diameter monitoring wells in a range of hydrogeological settings. A variant of the probe that can be deployed by a direct‐push machine has also been developed and tested in the field. The new NMR logging tool provides reliable, direct, and high‐resolution information that is of importance for groundwater studies. Specifically, the technology provides direct measurement of total water content (total porosity in the saturated zone or moisture content in the unsaturated zone), and estimates of relative pore‐size distribution (bound vs. mobile water content) and hydraulic conductivity. The NMR measurements show good agreement with ancillary data from lithologic logs, geophysical logs, and hydrogeologic measurements, and provide valuable information for groundwater investigations.
References
Allen, D., Flaum, C., Ramakrishnan, T.S., Bedford, J., Castelijns, K., Fairhurst, D., Gubelin, G., Heaton, N., Minh, C.C., Norville, M.A., Seim, M.R., Pritchard, T., and Ramamoorthy, R., 2000, Trends in NMR Logging: Oilfield Review, v. 12, no. 3, p. 2-19.
Behroozmand, A.A., Keating, K., and Auken, E., 2015, A Review of the Principles and Applications of the NMR Technique for Near-Surface Characterization: Surveys in Geophysics, v.36:1, p. 27–85, doi:10.1007/s10712-014-9304-0.
Coates G.R., Xiao, L., and Prammer, M.G., 1999, NMR Logging Principles and Applications: Houston, Halliburton Energy Services, 251 p.
Dlubac, K., Knight, R., Song, Y., Bachman, N., Grau, B., Cannia, J., and Williams, J., 2013, Use of NMR logging to obtain estimates of hydraulic conductivity in the High Plains aquifer, Nebraska, USA: Water Resources Research, v.49:4, p. 1871-1886, doi:10.1002/wrcr.20151.
Kenyon, W.E., Day, P.I., Straley, C., and Willemsen, J.F., 1988, A Three-Part Study of NMR Longitudinal Relaxation Properties of Water-Saturated Sandstones: Society of Petroleum Engineers Formation Evaluation, v. 3, no. 3, 15 p., doi:10.2118/15643-PA.
Kirkland, C.M. and Codd, S.L., 2018, Low-Field Borehole NMR Applications in the Near Surface Environment: Vadose Zone Journal Abstract, v.17:1, doi:10.2136/vzj2017.01.0007.
Knight, R., Walsh, D.O., Butler Jr, J.J., Grunewald, E., Liu, G., Parsekian, A.D., Reboulet, E.C., Knobbe, S., and Barrows, M., 2016, NMR Logging to Estimate Hydraulic Conductivity in Unconsolidated Aquifers: Groundwater, v.54:1, p. 104-114, doi:10.1111/gwat.12324.
Straley, C., Rossini, D., Vinegar, H., Tutunjian, P., and Morris, C., 1997, Core Analysis By Low Field NMR: The Log Analyst, Society of Professional Well-Log Analysts, v.38, no.2, p. 84-94.
Walsh, D., Turner, P., Grunewald, E., Zhang, H., Butler Jr, J.J., Reboulet, E., Knobbe, S., Christy, T., Lane Jr, J.W., Johnson, C.D., Munday, T., and Fitzpatrick, A., 2013, A Small-Diameter NMR logging Tool for Groundwater Investigations: Groundwater, v. 51:6, p. 914-926, doi:10.1111/gwat.12024.