Nuclear magnetic resonance (NMR) relaxometry is routinely used to characterize the oil fraction in unconventional shale formations with low-field benchtop NMR hardware. However, organic phases with restricted mobility like kerogen and bitumen are typically not detectable with the standard Carr-Purcell-Meiboom-Gill (CPMG) method on such equipment, with the rapid spin-spin (T-2) signal decay of these solid/viscous components not visible to the measurement. The solid-echo (SE) and mixed-echo (ME) pulse sequences offer an alternative to the CPMG, extending the lifetime of the time-domain signal and allowing the capture of solid hydrogen-containing species. Accordingly, we combine NMR relaxation data generated from the application of the CPMG, SE, and ME experiments to a set of powdered oil- and brine-saturated Eagle Ford shale samples, with the aim of identifying and quantifing both the immobile (kerogen/bitumen) and mobile (oil/brine) components. Two-dimensional relaxometry correlating spin-lattice relation times T-1 to the effective decay times T-2 obtained using CPMG and SE techniques provides more complete information on the immobile phases in the shales, such as solid organic matter and clay-bound water. The three spin-echo techniques have similar efficiencies when detecting signal from mobile and low-viscosity fluids, while significant differences are seen in shale samples containing immobile organics. Overall, the combination of the three spin echo techniques provides an improved description of the solid, viscous, and liquid components in the investigated shales at low magnetic field.

A key factor for the petroleum potential of source rock is the degree of chemical and physical structure evolution of its kerogen fraction through a range of maturation processes. In this study, various high-field, solid-state NMR methods have been applied to a series of kerogen isolates (type I) over a defined maturity range (vitrinite reflectance R-0 from 0.98 to 1.86%). Results obtained from C-13 MAS NMR show that the sp(2)/sp(3)-hybridized carbon ratio of kerogen, here defined as the aromatic/aliphatic ratio, increases with increasing maturity. H-1 MAS NMR spectra contain partly overlapping aliphatic and aromatic resonances with distinct transverse relaxation behavior. In Hahn-echo experiments, the aromatic signal decays more slowly than the aliphatic signal, indicating that for these systems, transverse 'H relaxation is rather controlled by local distances between hydrogen atoms than by molecular mobility. Similar relaxation differences are also found in static (nonspinning) H-1 Hahn-echo NMR experiments, here used to discriminate between phases with different proton mobilities and/or densities in the kerogen samples and, ultimately, between aromatic and aliphatic fractions. The distributions of the static transverse relaxation time (T-2), extracted from the Hahn-echo decays, are characterized by a short-T-2 peak (similar to 10 /is) and a long-T-2 peak (similar to 100 mu s). The ratio between these two peaks correlates well with the aliphatic-to-aromatic signal intensity ratios in MAS NMR spectra of the corresponding kerogen samples, suggesting that a net decrease in kerogen proton density-occurring during maturation-is also reflected by H-1 NMR relaxation. For the investigated kerogen isolates, the long-T-2 peak in the T-2 distribution can be considered an indicator of aromatic content, which can be directly detected by measuring H-1 T-2 relaxation.

Nuclear magnetic resonance measurements of spin relaxation are used in studies of liquid-saturated porous media where multidimensional relaxation correlations probe interpore diffusion and other transport processes. However, the magnetic susceptibility contrast between the solid and liquid results in pore-scale field inhomogeneities that influence all measurements of transverse ${T}_{2}$ relaxation time. In a previous publication we conjectured that experimentally observed exchange phenomena can correspond to intrapore diffusion between localized volumes of coherent magnetization generated by these internal gradients, rather than interpore diffusion. Here, we use a finite-element method to explore the decay of magnetization in a single pore (interstice between spheres) for the Carr-Purcell-Meiboom-Gill and two-dimensional ${T}_{2}\text{\ensuremath{-}}{T}_{2}$ exchange experiments. These simulations permit direct visualization of the time-dependent distribution of magnetization in the pore. The chosen grain size and susceptibility contrast are matched to values for Bentheimer sandstone, allowing for a comparison of simulated and experimental results. Despite the simplicity of the model pore geometry, the salient features of the magnetization decay observed experimentally are reproduced in simulation. We demonstrate that intrapore magnetization transfer can explain observations of diffusive exchange in porous materials characterized by a monomodal pore size distribution and large internal gradients.

We explore, by direct numerical simulations of the Bloch-Torrey equation in a single model pore, NMR decay by "diffusion in a gradient". Illustrative results from the finite-element package COMSOL are shown for a single "pore" taken from a numerical model (unrelated) of a human vertebra reduced 3 orders of magnitude in size, with the internal field modelled by numerical solution of the local magnetostatic problem. Results are shown for a single Hahn echo in the known limits of "Short Time" (ST), "Motionally Averaged" (MAV) and "Localization" (LOC) behaviour. Computing time on a laptop is less than 1 min; the approach is entirely practical for obvious immediate extensions. The LOC regime is shown explicitly to be qualitatively different from both the ST and MAV limits, where the dominant diffusion eigenmode is uniform. In LOC, the intra-pore magnetization is rapidly distorted, and "Localized" to small pockets, either where restricted by the pore walls, or around internal field extrema. Surviving magnetization may comprise only a small and possibly unrepresentative fraction of the fluid-filled pore volume. These features are in agreement with the limited analytical results. (C) 2017 Published by Elsevier Inc.

Determination of petrophysical parameters from surface measurements of drill cuttings, rather than well logs, has long been a subject of interest in the petroleum industry. Analysis of cuttings is presently included in mud logging workflows, although the methods employed tend to be labor-intensive and inefficient with an emphasis on qualitative lithological and mineralogical descriptors. Nuclear magnetic resonance (NMR) has the potential to deliver rapid formation evaluation at the rig site for near-real-time completion decisions. Despite numerous efforts to demonstrate NMR cuttings analysis, dating back to the 1980s, the technique has yet to achieve commercial success. We suggest this failure is due not to the physics of the measurement, which is understood, but rather to inadequate sample preparation. By centrifuging brine-saturated cuttings from conventional formations under a per-fluorinated oil, and using multinuclear NMR to measure liquid volumes, we obtain accurate porosity and permeability values independent of other measurements. Our robust workflow removes the inconsistencies associated with manual sample preparation. We present two case studies and the important requirements for handling actual drill cuttings.

The enhancement in relaxation of 1H spins adsorbed at pore walls is well known, described by the famous solutions of Brownstein and Tarr to the Bloch-Torrey diffusion equation with a partially absorbing boundary. Sodium-23 is an alternative nucleus that can be readily accessed at low field, providing a robust discriminator of brine from organic phases in petrophysics and a marker for aqueous ingress in construction materials. We examine NaCl brine in a chalk selected for its monodispersed pore size distribution. The 1H T2 distribution reflects the pore size distribution as expected. In contrast, the 23Na T2 distribution is broad and multimodal, consistent with relaxation outside the motional narrowing regime. To understand the 23Na results, we determine the quadrupole coupling constant QCC and rotational correlation time τc, and show that well-defined single values of QCC and τc describe the observed relaxation behavior. Our observations call for a new interpretation for quadrupole relaxation in pores.

Time-domain NMR, in one and higher dimensionalities, makes routine use of inversion algorithms to generate results called \T2-distributions' or joint distributions in two (or higher) dimensions of other NMR parameters, T1, diffusivity D, pore size a, etc. These are frequently referred to as \Inverse Laplace Transforms' although the standard inversion of the Laplace Transform long-established in many textbooks of mathematical physics does not perform (and cannot perform) the calculation of such distributions. The operations performed in the estimation of a \T2-distribution' are the estimation of solutions to a Fredholm Integral Equation (of the First Kind), a different and more general object whose discretization results in a standard problem in linear algebra, albeit suffering from well-known problems of ill-conditioning and computational limits for large problem sizes. The Fredholm Integral Equation is not restricted to exponential kernels; the same solution algorithms can be used with kernels of completely different form. On the other hand, (true) Inverse Laplace Transforms, treated analytically, can be of real utility in solving the diffusion problems highly relevant in the subject of NMR in porous media.

Low-field nuclear magnetic resonance (NMR) displacement probability distributions (flow propagators) are presented for water flowing through heterogeneous porous materials. Four sedimentary rocks have been chosen as example systems: Dolostone, Bentheimer sandstone, Berea sandstone, and Indiana limestone (in order of decreasing permeability). The fluid displacement is characterized by pre-asymptotic Stokes' flow and so the probability distributions are bimodal, with peaks corresponding to stagnant fluid in dead-end pores and flowing fluid in the connected porosity. Cut-off Gaussian functions are used to fit the flowing and stagnant peaks independently. An effective dispersivity length scale Lv (also known as the mixing length scale) is estimated by fitting the portion of the probability distribution corresponding to the flowing fluid. For the relatively homogeneous Bentheimer sandstone, the ratio of effective dispersivity length scale to effective transport diameter dt is Lv/dt≈16, which is an order of magnitude larger than for randomly packed glass beads where Lv/dt≈1.8. We compare these dispersivity parameters to similar values extracted from a cumulant analysis of the entire propagator. Fitting a cut-off Gaussian avoids the usual complications of analyzing dispersion in the presence of the ubiquitous stagnant fluid, and results in a clear demonstration of the influence of long-range heterogeneities on the dispersivity for flow in real sedimentary rocks.

We review the theory and operation of digital filters in modern nuclear magnetic resonance (NMR) spectrometers with fully digital receivers. Custom digital filters tailored for particular experimental requirements offer substantial improvements in signal-to-noise ratio (SNR), sensitivity, pulse sequence timing, and rejection of heteronuclear contamination. Pass-band filters are designed and applied in the frequency domain. In high-field imaging and spectroscopy, the impact of the filter is straight forward to visualize. However, low-field NMR data acquired on bench-top magnets are typically analyzed in the time-domain where the influence of a frequency-domain filter is not obvious and largely overlooked by end-users. We provide practical guidance on the design and implementation of digital filters for bench-top NMR applications, with examples of data acquired at 2.4 and 12.9 MHz. We discuss the compromise between speed (filter settling time) and noise rejection, and consider the special case of F-19 signal contamination in H-1 measurements. We suggest filter designs for narrow-line liquid samples, broad-line samples, and imaging.

Nuclear magnetic resonance (NMR) provides a powerful toolbox for petrophysical characterization of reservoir core plugs and fluids in the laboratory. Previously, there has been considerable focus on low field magnet technology for well log calibration. Now there is renewed interest in the study of reservoir samples using stronger magnets to complement these standard NMR measurements. Here, the capabilities of an imaging magnet with a field strength of 0.3 T (corresponding to 12.9 MHz for proton) are reviewed in the context of reservoir core analysis. Quantitative estimates of porosity (saturation) and pore size distributions are obtained under favorable conditions (e.g., in carbonates), with the added advantage of multidimensional imaging, detection of lower gyromagnetic ratio nuclei, and short probe recovery times that make the system suitable for shale studies. Intermediate field instruments provide quantitative porosity maps of rock plugs that cannot be obtained using high field medical scanners due to the field-dependent susceptibility contrast in the porous medium. Example data are presented that highlight the potential applications of an intermediate field imaging instrument as a complement to low field instruments in core analysis and for materials science studies in general.

A laboratory study on core plugs from a carbonate field confirmed the efficacy of an alkaline surfactant (AS) formulation at reservoir conditions. Continuous monitoring of remaining oil saturation (ROS) in short core plugs using spatially resolved nuclear magnetic resonance (NMR) can provide insights into the processes by which surfactants release post-water-flood remaining oil. In single short plugs, volumetric averages do not correctly quantify remaining oil due to capillary end effects (high remaining oil saturation post water flood observed at the outlet face of the plug), oil banks, and other flow heterogeneities. These effects are all quantifiable by NMR, allowing accurate monitoring of ROS. Here, consistent ROS values are obtained in the laboratory, in single-well pilots, and in single-well chemical-tracer (SWCT) tests. Laboratory core floods with nuclear magnetic resonance (NMR) monitoring at low magnetic fields therefore provide a valuable calibration of the NMR logs, in addition to the direct assessment of ROS. Corroboration across multiple length scales, spatial resolution, and correspondence with preferred logging measurements, all contribute to the value of NMR-monitored core-floods as a complement to field pilot studies. Introduction Monitoring the efficacy of enhanced oil recovery (EOR) processes is an important step in the screening of new chemical agents. This screening process begins at the laboratory scale (Stoll et al. 2011). It is important to understand both the liquidliquid and liquid-rock interactions. A possible EOR agent is identified and then adjusted to provide the required fluid-phase behavior for a given set of reservoir conditions (temperature, salinity, crude composition, restore reservoir wettability). The role of surfactants is to reduce the interfacial tension (IFT) between oil and water; this is a critical requirement of the chosen formulation. The alkaline convert some of the oil into surfactant, further reducing the oil/water interfacial tension. Laboratoryscale core floods constitute the next stage of screening to explore liquid-rock interactions before reservoir pilots. Here, the alkaline alters the charge of the rock surface to reduce surfactant adsorption. Nuclear magnetic resonance (NMR) is a powerful tool for assessing remaining oil saturation (ROS) during EOR both in the reservoir—via NMR logging—and in the laboratory with a low-field bench-top instrument. Quantitative EOR evaluation has been demonstrated on short plugs at the laboratory scale using spatially resolved low-field NMR. The spatial dimension is important at all EOR piloting length scales, but is critical when studying short plugs where bulk volumetrics may be dominated by capillary or geometry end effects and yield inaccurate ROS values. At the laboratory scale, it is usual to construct long composite cores (length > 30 cm [12 in.]) from plugs to reduce the significance of end effects and other flow heterogeneities on bulk assays of remaining oil (e.g., effluent gravimetrics). Commercial NMR systems are not suited to bulk measurements of such long samples, thereby requiring some form of mechanical scanning to explore the entire core. Such methodology prevents continuous monitoring of ROS during a core flood. In the laboratory, NMR offers the capability to spatially resolve the distribution of ROS within a short core plug (length = 5 cm [2 in.]). Previously, we demonstrated the quantitative measurements of ROS, consistent with well logs, in such short plugs by monitoring a central portion of the plug expected to be free from capillary or geometry end effects. Magnetic resonance imaging (MRI) of core plugs has been unsuccessful as a probe of ROS; implemented at high field, the solid/fluid magnetic susceptibility contrast prevents quantitative assay of in-situ liquid volumes, and limits the practical spatial resolution, preventing pore-scale imaging. The history of MRI in special core analysis (SCAL) is given by Mitchell et al. (2013b). Here, we use a low-field permanent magnet (B0 = 50 mT [500 G]) to obtain a spatial distribution of oil saturation in a profile (single image axis). The signal-to-noise ratio

In this review we focus on the technology associated with low-field NMR. We present the current state-of-the-art in low-field NMR hardware and experiments, considering general magnet designs, rf performance, data processing and interpretation. We provide guidance on obtaining the optimum results from these instruments, along with an introduction for those new to low-field NMR. The applications of lowfield NMR are now many and diverse. Furthermore, niche applications have spawned unique magnet designs to accommodate the extremes of operating environment or sample geometry. Trying to capture all the applications, methods, and hardware encompassed by low-field NMR would be a daunting task and likely of little interest to researchers or industrialists working in specific subject areas. Instead we discuss only a few applications to highlight uses of the hardware and experiments in an industrial environment. For details on more particular methods and applications, we provide citations to specialized review articles.

Abstract A laboratory study on core plugs from a carbonate field confirmed the efficacy of an alkaline surfactant (AS) formulation at reservoir conditions. Continuous monitoring of remaining oil saturation (ROS) in short core plugs using spatially resolved nuclear magnetic resonance (NMR) can provide insights into the processes by which surfactants release post-water-flood remaining oil. In single short plugs, volumetric averages do not correctly quantify remaining oil due to capillary end effects (high remaining oil saturation post water flood observed at the outlet face of the plug), oil banks, and other flow heterogeneities. These effects are all quantifiable by NMR, allowing accurate monitoring of ROS. Here, consistent ROS values are obtained in the laboratory, in single-well pilots, and in single-well chemical-tracer (SWCT) tests. Laboratory core floods with nuclear magnetic resonance (NMR) monitoring at low magnetic fields therefore provide a valuable calibration of the NMR logs, in addition to the direct assessment of ROS. Corroboration across multiple length scales, spatial resolution, and correspondence with preferred logging measurements, all contribute to the value of NMR-monitored core-floods as a complement to field pilot studies.