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Successful Sampling Strategy Advances Laboratory Studies of NMR Logging in Unconsolidated Aquifers: NMR Studies of Unconsolidated Aquifers
Geophysical Research Letters
Abstract
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.
... In water-filled porous media, the T2 distribution gives the pore size distribution within the media [37][38][39]. In this study, the T2 distribution of the sand modeled porous media (the contrast sample in Figure 5a) that are saturated with sterile LB nutrients consist of two types of pore size distribution: the fast component and the slow component, which indicates the distributions of the mesopore and the macropore within the sand filled sample. ...
... In water-filled porous media, the T 2 distribution gives the pore size distribution within the media [37][38][39]. In this study, the T 2 distribution of the sand modeled porous media (the contrast sample in Figure 5a) that are saturated with sterile LB nutrients consist of two types of pore size distribution: the fast component and the slow component, which indicates the distributions of the mesopore and the macropore within the sand filled sample. ...
To in situ and noninvasively monitor the biofilm development process by low-field nuclear magnetic resonance (NMR), experiments should be made to determine the mechanisms responsible for the T2 signals of biofilm growth. In this paper, biofilms were cultivated in both fluid media and saturated porous media. T2 relaxation for each sample was measured to investigate the contribution of the related processes to T2 relaxation signals. In addition, OD values of bacterial cell suspensions were measured to provide the relative number of bacterial cells. We also obtained SEM photos of the biofilms after vacuum freeze-drying the pure sand and the sand with biofilm formation to confirm the space within the biofilm matrix and identify the existence of biofilm formation. The T2 relaxation distribution is strongly dependent on the density of the bacterial cells suspended in the fluid and the stage of biofilm development. The peak time and the peak percentage can be used as indicators of the biofilm growth states.
... There is a growing interest to use the downhole NMR method for the investigation of water in vadose zones or aquifers [14][15][16][17][18] with the advantages of high vertical resolution and low data acquisition time. Moreover, the measurement can be performed in the existing pumping and observation wells cased with polyvinyl chloride pipes [16,19]. ...
The low-field nuclear magnetic resonance (NMR) technique is widely used as a noninvasive method to characterize the water content of subsurface porous media, such as aquifers and hydrocarbon reservoirs, but the quantitative correlation between the water saturation and the NMR relaxation signal has not been fully addressed. We conducted a laboratory study to measure the NMR signals of sandstone samples with different water saturations and to develop an empirical model for estimating the water saturation. The partially saturatinthe irreducible water saturationg states were derived by a high-speed centrifuge. The result shows that the water saturation is proportional to the geometric mean of the transverse relaxation time and can be fitted through a power function. Moreover, it has been found that the fitting parameters vary with the porosity and exhibit similar behaviors with the parameters of the classical Archie equation. The water saturation as well as its mobility state can be estimated with the NMR signals and porosity data. The proposed method has the potential to be applied to detect and quantify the water content in vadose zones, phreatic aquifers, permafrost regions, and gas hydrate reservoirs.
... The transverse relaxation time (T 2 ) of the NMR signal is the time for the nuclear spins to re-equilibrate to their initial state after being perturbed by an energizing pulse [30]. The area under the T 2 curve of a certain relaxation time is proportional to the number of pores with an equivalent pore or crack size, while the connectivity and the number of pores or crack in a damaged rock can be estimated from the amplitude and peak area of the T 2 curve [31]. ...
... The nuclear magnetic resonance (NMR) methods measure a response from water molecules that are stored in pores and provide information about water-filled porosity and pore size distributions . Surface NMR has been used in groundwater studies to map the water table and structural variations within the aquifer (Behroozmand et al., 2017a;Chalikakis et al., 2008;Costabel et al., 2017). Additionally, surface NMR can be used during MAR to detect and characterize water in the unsaturated zone, as shown in a study by Walsh et al. (2014) at a managed aquifer storage and recovery facility in Arizona. ...
Core Ideas
We introduce a new geophysical imaging method to assess managed aquifer recharge sites.
This method provides high‐resolution 3D imaging of the subsurface down to a minimum depth of 50 m.
We introduce Resistivity Distribution Plot as a new approach to assign a saturated–unsaturated boundary.
In many places around the world, much attention is focused on managed aquifer recharge (MAR) because of reduced groundwater levels due to droughts. To assess the suitability of a site for MAR, detailed three‐dimensional (3D) information about the subsurface materials and their hydraulic properties is needed. In areas where the groundwater level is at an intermediate depth (e.g., 20–40 m), such information is needed from the ground surface down to a minimum depth of ∼50 m. To achieve this goal, we used a new geophysical imaging system: a towed time‐domain electromagnetic system that is efficient for acquiring data at a significantly improved resolution and a scale needed for MAR. During a 2‐d period, we acquired ∼92 line‐kilometers of data in one almond [Prunus dulcis (Mill.) D.A. Webb] grove, one pistachio (Pistacia vera L.) grove, one open field, and two active recharge basins in the Tulare Irrigation District in the Central Valley of California. At each site, a detailed 3D resistivity model with a resolution down to the 10‐ by 10‐m scale is presented in terms of resistivity distribution plots, which are then used to assign a saturated–unsaturated boundary. In addition, we used a resistivity–lithology transform to interpret the resistivity models and create lithology maps at each site. We used this information to assess the suitability of each site for MAR.
Sc-CO2 fracturing would be a potential stimulation method for Hot Dry Rock. A series of Sc-CO2 fracturing experiments were performed on granite under different temperature and stress conditions. Quantitative and qualitative analysis of injection pressure curves and cracks were conducted to explain the Sc-CO2 fracturing mechanism under high temperature and high stress conditions. Under the same stress conditions, as the temperature increases, the breakdown pressure decreases. Concurrently, the volume and length of macro-cracks on the sample surface decrease, whereas the volume of micro-cracks within the sample increases. Under the same temperature conditions, as the stress increases, the breakdown pressure increases. However, this increasing trend is less noticeable at high temperatures. Compared with hydraulic fracturing, due to the lower density and viscosity of CO2, Sc-CO2 fracturing takes longer from injection to breakdown and has lower breakdown pressure. The effect of high temperature on fracturing mainly manifests in the generation of microscopic thermal cracks and a reduction in viscosity and density of Sc-CO2. Low viscosity and low density CO2 are more likely to penetrate into the thermal cracks of the sample, generating a diffuse micro-crack network, leading to an increase in pore pressure and a reduction in effective stress near the wellbore. Consequently, there is propagation of these micro-cracks, resulting in an increase in the volume of micro-cracks while the volume and length of macro-cracks decrease, ultimately leading to a decrease in breakdown pressure. High stress primarily influences the fracture process by reducing the opening width of microscopic thermal cracks. This reduction inhibits the diffusion of Sc-CO2 through these cracks, ultimately leads to an increase in breakdown pressure. The findings of this experimental study provide a theoretical basis for efficient fracturing and crack creation in hot dry rock reservoirs.
Hydraulic fracturing in the exploitation of hot dry rock (HDR) resources could significantly enhance the permeability and heat production of the reservoir. However, the fracturing mechanism of HDR at high temperatures is still not fully understood. In this study, hydraulic fracturing experiments at room temperature and 200 °C were performed respectively on granite under different true triaxial stress to analyze their different fracturing mechanisms. Optical microscope and nuclear magnetic resonance were applied to identify pore and crack characteristics of fractured samples from micro- to macro-scale. The test results show that hydraulic fracturing at 200 °C can significantly reduce the breakdown pressure and fracture initiation pressure under the same stress condition compared to hydraulic fracturing at room temperature. The wellbore pressurization stage at 200 °C deviates distinctly from linearity. The cloud fracture with multi-scale crack, rather than a dominant fracture at room temperature, was formed at 200 °C even under a horizontal stress difference of 20 MPa. Moreover, the nuclear magnetic resonance result shows an increase in fracturing volume caused by the increment of micro-scale crack in the fractured sample at 200 °C. The main reason for the above transition is that the pore pressure diffusion at 200 °C generates more micro-scale cracks.
This study assesses the performance and limitations of slim-hole borehole nuclear magnetic resonance (NMR) technology from a hydrogeologic perspective in fractured, porous rock. NMR logging was carried out in dolomitic and sandstone bedrock boreholes at two research test sites in Ontario, Canada, where aquifer and aquitard units provide a range of clay contents as well as a variety of primary and secondary porosity types (e.g. discrete fractures, reefal structures, vugs and karstic conduits). Results were compared to core measurements, geophysical logs, and hydrogeophysical testing. The vertical response curve of the instrument tested was found to produce 60% of the signal from within a 0.2m span surrounding the measuring point. The repeatability of the total porosity measurements in stationary mode is excellent where the porosity is greater than 0.15. Below that threshold, repeatability is scattered at ±0.05 porosity about the mean, with the variability primarily within the clay- and capillary-bound fractions. The NMR porosity estimates agreed with core measurements to within ±0.04 porosity in both the dolostone and sandstone, but the correlation deteriorates in finely bedded lithologies, and where fracturing is present. Much of the discrepancy is attributed to scaling in a finely layered geologic sequence, as the core samples are much smaller than the entire volume measured with NMR probes. Data collection with the probe in motion (continuous logging) added variability to the response when compared to stationary recordings. Although broadscale trends were comparable, the details and depth-specific insights of the bound fluid fractions varied with logging rates. Overall, NMR provides a robust measurement of the bulk matrix porosity and pore size distribution of lithologies intersected, both of which are critically important parameters in understanding hydrogeologic conditions and contaminant distributions in layered sedimentary rock systems.
Nuclear magnetic resonance (NMR) relaxation time (T 2) distributions are well known to be linked to the pore size distribution (PSD) and show promise as a method of estimating soil texture. As traditional laboratory methods used for soil texture estimates in soil science are generally time consuming, in this study, we explore an alternative approach based on NMR T 2‐distributions to estimate the soil texture of water‐saturated soil samples collected from three field sites. Using two T 2 cutoff times, T 2a and T 2b , the T 2‐distribution of a soil was partitioned into three regions, short‐, intermediate‐, and long‐relaxation times, each of which represents the fraction of clay, silt, and sand respectively. Two approaches for determining the cutoff times were used: the first used T 2 cutoff times was determined from the data from all sites and the second used site‐specific T 2 cutoff times. The NMR estimates of soil texture were compared to measurements of soil texture made using the sieve‐pipette method and laser diffraction particle size analysis (LDPSA).
The results show that there is no universal cutoff time for estimating the clay, silt, and sand fraction based on the NMR T 2‐distributions. The accuracy of NMR measurements to estimate the soil texture depends on the magnetic susceptibility of the measured material. For soils with low magnetic susceptibility (< 2×10‐4 SI) using site‐specific cutoff times, the NMR‐derived soil texture (RMSE = 9.43%) more closely matches the soil texture measured from the sieve‐pipette method than the soil texture determined using LDPSA (RMSE = 11.88%). However, the NMR estimate of soil texture breaks down for soils with high magnetic susceptibility (> 4×10‐4 SI). These results suggest that the NMR method can provide reasonable estimates of the soil texture for soils with low magnetic susceptibility.
The presence of materials with high magnetic susceptibility are known to have an impact on ¹H nuclear magnetic resonance (NMR) measurements and in laboratory data this often results in poor estimates of porosity from NMR data. To quantify and understand the cause of the poor NMR porosity estimations, in this laboratory study, we examine the effect of magnetic susceptibility, NMR measurement parameters, and NMR instrument design on NMR measurements. Data were collected with two instruments, each with a different Larmor frequency (2 MHz and 485 kHz), on water-saturated unconsolidated sediments with magnetic susceptibility values ranging from 3.6 × 10⁻⁶ to 7020 × 10⁻⁶ SI. The results show that for materials with low magnetic susceptibility (<378 × 10⁻⁶ SI), the gravimetric porosity is accurately predicted from the NMR measurements. For the samples with high magnetic susceptibility (>987 × 10⁻⁶ SI) the gravimetric porosity is poorly predicted from the 2 MHz NMR measurements made at all echo times (from 0.2 to 3.0 ms). In contrast, the gravimetric porosity is more accurately predicted at an echo time of 0.2 ms for measurements made using the 485 kHz instrument, although at larger echo times (>1.0 ms), the porosity estimate becomes poor. The 485 kHz NMR instrument has non-zero internal magnetic field gradients, similar to those found in borehole instruments, in contrast to the 2 MHz NMR instrument, which has a homogeneous applied magnetic field. We conclude that differences is the magnetic field strength and higher magnetic field inhomogeneities in the 485 kHz NMR instrument contribute to a reduction of the impact of inhomogeneities in the magnetic field caused by materials with high magnetic susceptibility, allowing for improved porosity estimation. These results indicate that NMR measurements collected at short echo times in low, inhomogeneous static fields, e.g., borehole instruments, may provide accurate estimates of porosity in water saturated sediments, even in the presence of magnetic minerals.
Core Ideas
NMR relaxation rates are a function of water content but do not exhibit hysteresis.
NMR sum of echoes does not exhibit hysteresis with imbibition and drainage.
The results are consistent across the synthetic sands and natural soils investigated.
Results suggest NMR can characterize the WRC but not distinguish drainage from imbibition.
In this laboratory study, nuclear magnetic resonance (NMR) relaxation data were collected on unconsolidated sediment to determine the NMR response, characterized by the mean‐log transverse relaxation rate, T 2ML ⁻¹ , and the sum of echoes, SOE, during drainage and imbibition. Measurements were made on four synthetic sands, with a range of grain sizes and Fe content, and two natural loamy sand soils. A porous ceramic plate apparatus was used to induce drainage and imbibition. Water content (θ) was plotted vs. matric potential (ψ) to give the water retention curve (WRC). The drainage and imbibition branches of the WRC were then compared with the corresponding branches of the θ– T 2 ML ⁻¹ and θ–SOE curves. We observed the expected linear trend between NMR signal magnitude and θ. The θ– T 2ML ⁻¹ or θ–SOE curves did not exhibit drainage–imbibition hysteresis, even though T 2ML ⁻¹ and SOE varied with θ and the WRC did exhibit hysteresis. Using a simple pore network model, we show that, for well‐connected networks, the surface‐area/volume ratio of the water‐occupied porosity is similar during drainage and imbibition, explaining the lack of hysteresis present in the measured θ– T 2ML ⁻¹ or θ–SOE curves. For materials with poorly connected pore networks, hysteresis may be observed. We conclude that, for the materials used in this study, it is not possible to distinguish drainage from imbibition using T 2ML ⁻¹ or SOE. However, because T 2ML ⁻¹ and SOE depend on θ, NMR data may be useful for characterizing a single branch or an average of the two branches of the WRC by relating these NMR parameters to ψ.
Buried tunnel valleys are common features in formerly glaciated areas, and where present, they are very important for the groundwater recharge and flow. Delineation of the structures and modelling of the infill is therefore very important in relation to groundwater mapping. Typically, borehole information is too sparse to enable a detailed delineation of the structures, whereas densely covering airborne electromagnetic data have proven to be very useful for this. In the last decades, the mapping approach has been studied carefully, but the 3D modelling of the valley structures has not been described to the same degree yet. In this study, we create a 3D geological model of an area that is characterised by a complex network of buried valleys mapped with a spatially dense airborne electromagnetic survey. Due to the comprehensive dataset, the modelling requires formulation of an advanced strategy. This contains a number of steps, where the AEM-derived resistivity data are initially interpreted based on the geological background knowledge to identify the buried valleys and build a conceptual geological model. Secondly, the age relationships between the valleys are established from the valley orientations and their internal cross-cut relationships. Thirdly, the deep erosional surfaces are modelled. Subsequently, the interpreted age relationships are utilised to trim the valley floor surfaces, such that younger valleys cut older. Finally, a voxel model is built and populated with lithofacies and stratigraphical units. The model is constructed as a combined layer-based and voxel model in order to map both the overall structures as well as the lithological variations within the 3D model domain. The final model contains 20 buried valleys that show a complex cross-cut setting that indicate the presence of at least eight valley generations. Most of the valley infills show lithological variations, and the final voxel model thus contains 42 different geological units.
The pore-size distribution (PSD) of geologic materials is an important rock parameter to understand the flow of water in the subsurface. PSDs can be obtained from sieving analyses, mercury porosimetry measurements, and imaging techniques, but none of these methods is available for in situ measurements. Nuclear magnetic resonance (NMR) measurements are controlled by rock parameters such as the surface-area to porevolume ratio. NMR is available for in situ measurements. State-of-the-art NMR relaxation time measurements need a calibration of the surface relaxivity ? to extract pore-size information. State-of-the-art NMR diffusion measurements avoid the calibration of ? but are limited to small pores. We developed an approach that estimates the average pore size without calibrating ? by means of incorporating higher order modes into the signal interpretation of NMR relaxation times. We conducted forward-modeling studies using an analytic solution for cylindrical tubes, 2D finite-element simulations to incorporate fractal pore spaces, and laboratory experiments on synthetic and natural samples. Our experimental data indicated that relaxation can occur outside the fast-diffusion regime not only for coarse-grained materials, but also for fine- to medium-grained unconsolidated sandy materials due to high surface relaxivities. We found that the rock-fluid interface's roughness had a significant impact on the diffusion regime and led to an apparent increase in ?, which may cause intermediate or slow diffusion. The methodology was limited to materials with a narrow PSD and uniform distribution of ? because we assumed multiexponential decay due to diffusion in single isolated pores.
Internal magnetic field gradients in porous materials, if sufficiently large, can be a source of error in nuclear magnetic resonance (NMR) measurements of the transverse relaxation time T2 and the diffusion coefficient D. Given that these measurements can provide information about the pore fluid and the pore geometry, it is important to determine the magnitude of internal gradients and assess their potential impact. We estimated the effective internal gradients in aquifer sediment samples using three methods. We used a 2D NMR method to map the distribution of internal gradients versus T2 and found gradients up to 1000 G/cm with peak gradient values in the range of 20 - 100 G/cm for most of the samples. The average effective gradient values, calculated from the slope of the mean log relaxation rate versus the squared echo time, typically fell above the peak gradient values in the 2D distributions, with a range from 12 to 230 G/cm. The maximum effective gradients, calculated from the magnetic susceptibility of the samples, were found to be the upper bounds for most of the gradient distributions. The mean gradient was found to increase with increasing magnetic susceptibility of the sample; however, pore size was also found to impact gradient magnitudes. Given that the distribution of internal gradient magnitudes is determined by the properties of the sediment and by the magnitude of the background field, our results have implications for the acquisition of logging and surface NMR data. We expect the internal gradients in many aquifer sediments to impact NMR logging measurements; this should be considered when selecting logging parameters and interpreting NMR logging data. In contrast, we expect internal gradients to have a negligible impact on surface NMR measurements because of the much smaller magnitude of the background magnetic field.
The accuracy of NMR-derived permeability estimates in sands and gravels are examined through simulations on numerical grain packs composed of uniform spherical grains. The packs consisted of randomly packed grains, with grain sizes set to represent a range corresponding to sands and gravels. The material properties for each pack were quantified through numerical analysis and the NMR response was simulated for a range of surface relaxivity values. The agreement between the numerically-derived permeability estimates and the permeability estimates derived using the Schlumberger-Doll Research (SDR) and Seevers equations was evaluated. Use of the SDR equation assumes that the relaxation of the bulk pore fluid can be neglected. The NMR-derived permeability estimates were calculated using each equation for the cases where relaxation was assumed to occur in one of the two major diffusion regimes. We found that permeability is most accurately estimated in all packs through use of the Seevers equation with the empirical constant n set equal to 1. We showed that the contribution of bulk fluid relaxation should be accounted for in materials with grain radii greater than 1.2e-4m (fine sand) and surface relaxivity values less than 1.0e-3 m s(-1). In practice, this range of surface relaxivity values and grain sizes corresponds to situations where the measured relaxation time T 2 is greater than approximately one-third the value of the bulk fluid relaxation time T-2B.
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.
© 2015, National Ground Water Association.
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.
The prediction of hydraulic conductivity K from nuclear magnetic resonance (NMR) measurements has been performed primarily in sandstones. In hydrogeological applications, however, unconsolidated material is more prevalent. Compared to sandstones, unconsolidated sediments can show pore sizes up to several millimeters. The known (semi-)empiric relations to estimate K from NMR have been applied on this material, but the underlying assumptions are not valid for large pores. We formulated a new model, called the Kozeny-Godefroy model. It is based on capillary pores with a single pore radius, and accounts for bulk water relaxation and relaxation in porous media under fast- and slow-diffusion conditions. The bulk-water relaxation and slow-diffusion conditions significantly affect the NMR measurements on coarse material. If the impact of the bulk-water relaxation is well known and small, a maximum K can be derived from NMR measurements by accounting for the slow-diffusion case. The model replaces the empirical factors in known relations with physical, structural, and intrinsic NMR parameters. Focusing the calibration on material-specific NMR parameters improves the prediction of K for similar material. Measurements on well-sorted glass beads and natural sands with different grain sizes are used for evaluation. These measurements confirm the applicability of the new model and, for coarse material, show the limit of the fast-diffusion-based Seevers and Schlumberger-Doll-Research equations. The application of our model is limited to (1) simple pore geometries, and (2) materials with a small range of pore sizes.
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.
Drilling and soil sampling for environmental site characterization and ground-water monitoring well installation utilizes much of the same technology used in geotechnical exploration, mineral exploration, oil and gas well drilling, and water well drilling. However, there are some very significant differences in how the technology is applied. For example, the primary purpose of most geotechnical exploration projects is to recover an intact physical specimen that can be tested for physical strength or inspected for material properties that may be indicative of the performance of the sampled material under projected conditions. For environmental site characterization, primary consideration must be given for collecting a sample that is representative of in situ physical conditions and valid for both chemical and physical analyses. The sample must not be and Ground-Water contaminated by drilling fluid or its physical properties altered by the drilling or sampling procedures. Care must be taken to preserve the sample in its natural state for on-site analysis or for transport to the laboratory.
The Treatise on Geophysics is a comprehensive and in-depth study of the physics of the Earth and of terrestriallike planets residing both within and outside our solar system. Its breadth and detail of coverage are beyond what any single geophysics text can provide. The first edition of the Treatise on Geophysics was published in 2007, nearly a decade ago. Of course, there is much progress in science in that length of time and the field of geophysics has grown rapidly and developed new lines of inquiry. Accordingly, we have brought the Treatise on Geophysics up-to-date with this second edition. The new edition will continue to provide students and professionals with fundamental and state-of-the-art discussion of all aspects of geophysics. In this new edition, the reader will find updates to all the chapters contained in the first edition and new chapters that discuss topics missing from the first edition. In a few instances, chapters from the first edition have simply been reprinted in order to avoid gaps in coverage. Chapters that emphasized fundamental physics often required little updating. A highlight of the second edition is a new volume on Resources in the Near-Surface Earth. The new volume discusses the role of geophysics in the exploitation and conservation of natural resources and the assessment of degradation of natural systems by pollution. The near surface is a zone where humans and natural systems interact. Understanding the effects of human impacts is a challenge particularly for the long term. © 2015 Elsevier B.V. unless otherwise stated. All rights reserved.
In Part I of this paper, the approach is taken, with the support of core and log studies, that a common value, ″w″ , can be adopted for both the saturation exponent and cementation exponent. A relationship between ″w″ , porosity, and formation resistivity at irreducible water saturation is proposed. A technique is also described for adapting this relationship to non-irreducible conditions. Variations in the value of this new exponent ″w″ , are found to be indicative of the nature of the formation matrix. In Part II, a new relation is proposed for obtaining a permeability value from logs. Based on the knowledge of the ″w″ exponent defined in Part I, this method yields permeability values in good agreement with core and production data in sandstone and carbonate formations. Field examples illustrate the mechanism of this new approach and the results obtained.
This paper presents two methods for joint inversion of aquifer test data, magnetic resonance sounding (MRS) data, and transient electromagnetic data acquired from a multi-layer hydrogeological system. The link between the MRS model and the groundwater model is created by tying hydraulic conductivities (k) derived from MRS parameters to those of the groundwater model. Method 1 applies k estimated from MRS directly in the groundwater model, during the inversion. Method 2 on the other hand uses the petrophysical relation as a regularization constraint that only enforces k estimated for the groundwater model to be equal to MRS derived k to the extent that data can be fitted. Both methodologies can jointly calibrate parameters pertaining to the individual models as well as a parameter pertaining to the petrophysical relation. This allows the petrophysical relation to adapt to the local conditions during the inversion. The methods are tested using a synthetic dataset as well as a field dataset. In combination the two case studies show that the joint methods can constrain the inversion to achieve estimates of k, decay times, and water contents for a leaky confined aquifer system. We show that the geophysical data can assist in determining otherwise insensitive k, and vice versa. Based on our experiments and results we mainly advocate the future application of method 2 since this seems to produce the most reliable results, has a faster inversion runtime, and is applicable also for linking k of 3D groundwater flow models to multiple MRS soundings.
We measured nuclear magnetic resonance (NMR) relaxation times on samples from Integrated Ocean Drilling Program (IODP) Expedition 333 Sites C0011, C0012, and C0018. We compared our results to permeability, grain size, and specific surface measurements, pore size distributions from mercury injection capillary pressure (MICP), and mineralogy from X-ray fluorescence (XRF). We found that permeability could be predicted from NMR measurements by including grain size and specific surface to quantify pore networks, and that grain size is the most important factor in relating NMR response to permeability. Samples within zones of anomalously high porosity from Sites C0011 and C0012 were found to have different NMR-permeability relationships than samples from outside these zones, suggesting that the porosity anomaly is related to a fundamental difference in pore structure. We additionally estimated the size of paramagnetic sites that cause proton relaxation and found that, in most of our samples, paramagnetic material is present mainly as discrete, clay-sized grains. This distribution of paramagnetic material may cause pronounced heterogeneity in NMR properties at the pore scale that is not accounted for in most NMR interpretation techniques. Our results provide important insight into the microstructure of marine sediments in the Nankai Trough.
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.
Free-precession signals were observed from fluids in samples containing randomly distributed ferromagnetic grains. The local free-precession phase shift was calculated by computing volumes of space for various ranges of perturbing field strength near individual grains. The frequency of occurrence of a given phase shift caused by individual grains is inversely proportional to the square of the phase shift, this distribution being a limiting case of the Cauchy form. The resultant distribution of phase shifts from effects of many grains is then still of the Cauchy form. This leads to an exponential signal decay, with the rate independent of diffusion. If M is the algebraic sum of the individual dipole moments of the individual magnetic grains per unit volume, and gamma the magnetogryic ratio, 1T2=(8pi293)Mgamma if all grains are magnetized parallel to the precession field; 1T2=(4pi3)Mgamma if perpendicular. Within 10%, 1T2=4.6Mgamma for any random or systematic orientation of the grains. Measurements on water containing magnetite powder stabilized by carboxymethylcellulose and on glycerine containing magnetite powder, as well as on sands containing magnetite powder and saturated with water or glycerine, verified the exponential decay, independence of decay rate on diffusion or viscosity, and the above numerical value of decay rate (with small geometrical correction applied to results for the sand system).
We develop a technique for extending nuclear magnetic resonance (NMR) permeability estimation to clay-rich sediments. Our technique builds on the Schlumberger-Doll Research (SDR) equation by using porosity, grain size, specific surface, and magnetic susceptibility data to yield more accurate permeability estimation in mudstones with large pore surface areas and complex mineralogies. Based on measurements of natural sediments as well as resedimented laboratory mixtures of silica, bentonite, and kaolinite powders, we find that our method predicts permeability values that match measured values over four orders of magnitude and among lithologies that vary widely in grain size, mineralogy, and surface area. Our results show that the relationship between NMR data and permeability is a function of mineralogy and grain geometry, and that permeability predictions in clay-rich sediments can be improved with insights regarding the nature of the pore system made by the Kozeny theory. This technique extends the utility of NMR measurements beyond typical reservoir-quality rocks to a wide range of lithologies.
Borehole measurements of the nuclear magnetic resonance (NMR) properties of rocks have been of interest for many years, especially for estimating permeability. This paper presents laboratory measurements of the NMR properties of water-saturated rocks and shows that permeability can be estimated well with expressions of the form ϕ4T12, where T1 is the relaxation time constant of the longitudinal nuclear magnetization of hydrogen nuclei. Different methods of representing the laboratory-measured T1 curves are shown, including a new one called the stretched-exponential representation. An improved method for estimating T1 parameters from borehole measurements that can be used with either old or new representations is presented.
Summary The water retention characteristic provides the traditional data set for the derivation of a soil's pore-size distribution. However, the technique employed to achieve this requires that assumptions be made about the way pores interconnect. We explore an alternative approach based on stray field nuclear magnetic resonance (STRAFI-NMR) to probe the water-filled pores of both saturated and unsaturated soils, which does not require information relating to pore connectivity. We report the relative size distributions of water-occupied pores in saturated and unsaturated samples of two sets of glass beads of known particle size, two sands, and three soils (a silty loam, a sandy loam and a loamy sand), using measurements of the NMR T1 proton relaxation time of water. The T1 values are linearly related to pore size and consequently measured T1 distributions provide a measure of the pore-size distribution. For both the sands and the glass beads at saturation the T1 distributions are unimodal, and the samples with small particle sizes show a shift to small T1 values indicating smaller voids relative to the samples with larger particles. Different matric potentials were used to reveal how the water-occupied pore-size distribution changes during drainage. These changes are inconsistent with, and demonstrate the inadequacies of, the commonly employed parallel-capillary tube model of a soil pore space. We find that not all pores of the same size drain at the same matric potential. Further, we observe that the T1 distribution is shifted to smaller values beyond the distribution at saturation. This shift is explained by a change in the weighted average of the relaxation rates as the proportion of water in the centre of water-filled pores decreases. This is evidence for the presence of pendular structures resulting from incomplete drainage of pores. For the soils the results are similar except that at saturation the T1 distributions are bimodal or asymmetrical, indicative of inter-aggregate and intra-aggregate pore spaces. We conclude that the NMR method provides a characterization of the water-filled pore space which complements that derived from the water retention characteristic and which can provide insight into the way pore connectivity impacts on drainage.
Nuclear resonance techniques involving free precession are examined, and, in particular, a convenient variation of Hahn's spin-echo method is described. This variation employs a combination of pulses of different intensity or duration ("90-degree" and "180-degree" pulses). Measurements of the transverse relaxation time in fluids are often severely compromised by molecular diffusion. Hahn's analysis of the effect of diffusion is reformulated and extended, and a new scheme for measuring is described which, as predicted by the extended theory, largely circumvents the diffusion effect. On the other hand, the free precession technique, applied in a different way, permits a direct measurement of the molecular self-diffusion constant in suitable fluids. A measurement of the self-diffusion constant of water at 25\ifmmode^\circ\else\textdegree\fi{}C is described which yields D=2.5(\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{-{}5} /sec, in good agreement with previous determinations. An analysis of the effect of convection on free precession is also given. A null method for measuring the longitudinal relaxation time , based on the unequal-pulse technique, is described.
On the basis of the nuclear magnetic resonance (NMR) relaxation of imbibed water, we evaluated the interparticle and intraparticle pore sizes in packed beds of silica materials of known particle sizes and microporous structure. The NMR relaxation distribution is scaled by the surface relaxivity parameter rho, which incorporates a surface area to volume ratio (So/Vo) term, to yield a corresponding pore size distribution. The NMR-derived pore sizes of nonporous silica sand agreed with the expected interparticle pore sizes estimated from the morphology of a packed bed of spheres of comparably sized particles. The NMR-derived intraparticle pore size for porous silica was also in good agreement with reported values for the silica materials studied. Scaling of the NMR relaxation corresponding to interparticle water by the same surface interaction parameter to yield interparticle pore size in high-surface area porous silica material, however, grossly underestimated interparticle pore size. In these high-surface area materials the intraparticle micropores provide a higher contribution to the N2 measured surface area relative to the contribution from interparticle macropores. When the NMR relaxation method was used to evaluate the pore space in the Borden Aquifer material, the NMR-derived pore sizes agreed with those observed in scanning electron micrographs as well as pore sizes estimated from the morphology of packed beds of comparably sized particles. For soils and aquifer materials of low to moderate surface area the NMR-derived porosity determination may be used to adequately evaluate both solute transporting and sorbing pore sizes.
Nuclear-magnetic-resonance measurements of the proton-spin relaxation for water in biological cells are known to exhibit a multiexponential decay. A theory, based on the diffusion equation using the bulk diffusivity of water, is developed to explain this phenomenon. It is shown that multiexponential decay arises simply as a consequence of an eigenvalue problem associated with the size and shape of the cell and that this multiexponential decay can only be observed for samples whose size is of the order of a biological cell. As an example, the theory is applied to a previously published data for rat gastronemius cells. Excellent agreement is obtained, and furthermore, the size of the cell is calculated by fitting the theory to the experiment.
a b s t r a c t We combine nuclear magnetic resonance (NMR) transverse relaxation time data and gamma ray data to estimate lithology-dependent permeability in silt-and clay-rich sediments. This approach extends the utility of the Schlumberger-Doll Research (SDR) permeability equation from reservoirs to aquicludes and seals, and thus improves the value and robustness of NMR data. Data from Keathley Canyon, northern Gulf of Mexico show that NMR data can be used to define permeability from 10 À18 to 10 À14 m 2 (0.001– 10 millidarcies) as calibrated and tested by direct measurements on core samples. We performed uniaxial, constant rate-of-strain consolidation experiments on sediments from Keathley Canyon to determine core-scale permeability. Permeabilities from these experiments were compared to perme-abilities calculated from logging-while-drilling data. A better fit between log-derived permeability and laboratory-measured permeability was obtained using the SDR equation with a variable coefficient A, rather than a constant A as is typically used. We show how A is a function of lithology and can be modeled from gamma ray data. The relationship between A and gamma ray values suggests that vari-ations in A are caused by platy clay minerals and the effect they have on the pore system. Our results provide improved means for permeability estimation for application in basin flow modeling, hydro-carbon migration modeling, and well completion design.
The resonant sonic drilling method offers unique capabilities to the environmental restoration market. By using a drill head that imparts high-frequency, high-force vibrations into a steel drill pipe, continuous, relatively undisturbed cores can be taken through virtually any formation. The resonant sonic method requires no mud, air, water, or other circulating medium for penetration: drills very fast; easily drills at any angle through formations such as rock, clay, sand, boulders, permafrost, or glacial till; and yields no cuttings in the drilling process. Case histories of projects using the method demonstrate excellent results but also indicate several problem areas with the method in its present state. Expanding research efforts to further develop the resonant sonic drilling method should help solve current drawbacks, and could produce a drilling technology for environmental work that significantly changes the way monitoring wells are drilled and constructed.
Borehole geophysics includes all methods for making continuous profiles or point measurements at discrete depth stations in
a borehole. These measurements are made by lowering different types of probes into a borehole and electrically transmitting
data in the form of either analog or digital signals to the surface, where they are recorded as a function of depth or distance
along the borehole. The measurements are related to the physical and chemical properties of the rocks surrounding the borehole,
the properties of the fluid saturating the pore spaces in the formation, the properties of fluid in the borehole, the construction
of the well, or some combination of these factors.
A spin echo method adapted to the measurement of long nuclear relaxation times (T 2 ) in liquids is described. The pulse sequence is identical to the one proposed by Carr and Purcell, but the rf of the successive pulses is coherent, and a phase shift of 90° is introduced in the first pulse. Very long T 2 values can be measured without appreciable effect of diffusion.
Precision NMR relaxation measurements of biological tissue frequently show complex multicomponent behavior. This paper presents methods for generating information about these relaxation spectra even when the original data are nonideal. A variety of acceptable solutions with differing types of simplicity are investigated. Ways of inferring trends in relaxation spectra that are independent of the particular analysis model are illustrated with examples using linear programming techniques. The question of the number, spacing, and signal-to-noise ratio of data for optimal experiments is also addressed.
Buried Quaternary valleys in Denmark are complex structures filled with various deposits consisting primarily of glacio-lacustrine clay, till and meltwater sand, and gravel. The valleys are important geophysical targets, because they often contain significant volumes of groundwater used for public water supply. About 700 km of buried valley structures have been imaged in the western part of Denmark by the transient electromagnetic (TEM) method. The ability to map the valleys depends primarily on valley geometry, infill architecture and the resistivity of the fill sediments as well as the substratum. One-dimensional (1-D) inversion models of the TEM soundings have been used to construct contour maps of 20 m average resistivities and depth to a good conductor, which provide images for geological interpretation. Images of buried valley morphology, fill properties, infill architecture, such as cut-and-fill structures, valley distribution and valley generations, are characterized for case studies from Hornsyld, Holstebro and the Vonsild/Agtrup areas of Denmark.
It is known that internal magnetic field gradients in porous materials, caused by susceptibility differences at the solid-fluid interfaces, alter the observed effective Nuclear Magnetic Resonance transverse relaxation times T2,eff. The internal gradients scale with the strength of the static background magnetic field B0. Here, we acquire data at various magnitudes of B0 to observe the influence of internal gradients on T2-T2 exchange measurements; the theory discussed and observations made are applicable to any T2-T2 analysis of heterogeneous materials. At high magnetic field strengths, it is possible to observe diffusive exchange between regions of local internal gradient extrema within individual pores. Therefore, the observed exchange pathways are not associated with pore-to-pore exchange. Understanding the significance of internal gradients in transverse relaxation measurements is critical to interpreting these results. We present the example of water in porous sandstone rock and offer a guideline to determine whether an observed T2,eff relaxation time distribution reflects the pore size distribution for a given susceptibility contrast (magnetic field strength) and spin echo separation. More generally, we confirm that for porous materials T1 provides a better indication of the pore size distribution than T2,eff at high magnetic field strengths (B0>1 T), and demonstrate the data analysis necessary to validate pore size interpretations of T2,eff measurements.
In porous media, magnetic susceptibility differences between the solid phase and the fluid filling the pore space lead to field inhomogeneities inside the pore space. In many cases, diffusion of the spins in the fluid phase through these internal inhomogeneities controls the transverse decay rate of the NMR signal. In disordered porous media such as sedimentary rocks, a detailed evaluation of this process is in practice not possible because the field inhomogeneities depend not only on the susceptibility difference but also on the details of the pore geometry. In this report, the major features of diffusion in internal gradients are analyzed with the concept of effective gradients. Effective gradients are related to the field inhomogeneities over the dephasing length, the typical length over which the spins diffuse before they dephase. For the CPMG sequence, the dependence of relaxation rate on echo spacing can be described to first order by a distribution of effective gradients. It is argued that for a given susceptibility difference, there is a maximum value for these effective gradients, gmax, that depends on only the diffusion coefficient, the Larmor frequency, and the susceptibility difference. This analysis is applied to the case of water-saturated sedimentary rocks. From a set of NMR measurements and a compilation of a large number of susceptibility measurements, we conclude that the effective gradients in carbonates are typically smaller than gradients of current NMR well logging tools, whereas in many sandstones, internal gradients can be comparable to or larger than tool gradients. Copyright 1998 Academic Press.
Magnetic resonance sounding (MRS) is distinguished from other geophysical tools used for ground water investigation by the fact that it measures a magnetic resonance signal generated directly from subsurface water molecules. An alternating current pulse energizes a wire loop on the ground surface and the MRS signal is generated; subsurface water is indicated, with a high degree of reliability, by nonzero amplitude readings. Measurements with varied pulse magnitudes then reveal the depth and thickness of water saturated layers. The hydraulic conductivity of aquifers can also be estimated using boreholes for calibration. MRS can be used for both predicting the yield of water supply wells and for interpolation between boreholes, thereby reducing the number of holes required for hydrogeological modeling. An example of the practical application of MRS combined with two-dimensional electrical imaging, in the Kerbernez and Kerien catchments area of France, demonstrates the efficiency of the technique.
Magnetic susceptibility differences in porous media produce local gradients within the pore space. At high magnetic fields, these inhomogeneities have the potential to greatly affect nuclear magnetic resonance measurements. We undertake a study using a new NMR technique to measure the internal gradients present in highly heterogeneous samples over a wide range of magnetic field strengths. Our results show that even at ultra-high fields there can exist signal at internal gradient strengths sufficiently small that techniques for suppressing unwanted side effects have the possibility to be used. Our findings encourage the use of these high and ultra-high field strengths for a broader range of samples. Our results also give experimental evidence to support the theory of internal gradient scaling as a function of field strength within pores.
BHMAR Project: Assessment of conjunctive water supply options involving Managed Aquifer Recharge options at Menindee Lakes (499 pp.). Geoscience Australia Record 2012/13
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Practical handbook of environmental site characterization and ground-water monitoring
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A nuclear magnetic method for determining the permeability of sandstones. Presented at the paper L presented at the SPWLA 7th annual logging symposium Soc
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SPWLA 7th annual logging symposium, Soc. of Petrophys. And well Logic and Analysis Retrieved from https://www.onepetro.org/conference-paper/SPWLA-1966-L