Measurement of the true transverse nuclear magnetic resonance relaxation in the presence of field gradients

Nuclear spin relaxation as a probe of zeolite acidity: a combined NMR and TPD investigation of pyridine in HZSM-5

The relative surface affinities of pyridine within microporous HZSM-5 zeolites are explored using two-dimensional ¹H nuclear magnetic resonance (NMR) relaxation time measurements. The dimensionless ratio of longitudinal-to-transverse nuclear spin relaxation times T₁/T₂ is shown to exhibit strong sensitivity to the silica/alumina ratio (SAR) of these zeolites, which is indicative of material acidity. This trend is interpreted in terms of increased pyridine surface affinity with decreasing SAR. Temperature programmed desorption (TPD) analysis corroborates this observation, revealing a distinct increase in the heat of desorption associated with adsorbed pyridine as a function of decreasing SAR. A direct correlation between NMR and TPD data suggests NMR relaxation time analysis can be a valuable tool for the non-invasive characterisation of adsorption phenomena in microporous solids.

Multidimensional dynamic NMR correlations in sedimentary rock cores at different liquid saturations

In Nuclear Magnetic Resonance (NMR) spin echo measurements of confined liquids, the dynamic behaviour of liquid molecules are influenced by diffusion (D), translational displacement of molecules in internal gradients (G₀), and transverse surface relaxation (T₂). In this study, an experimental approach that enables characterisation of water and oil in rock core materials is presented. The approach is based on three-dimensional D-DG₀²-T₂ correlations, but the main focus is on the two-dimensional parts that involve DG₀²-T₂ and D-∣G₀∣. In order to evaluate potential signal loss that can be introduced when going from a two-dimensional to a three-dimensional experiment, D-T₂,DG₀²-T₂ and D-∣G₀∣ correlations derived from subsets of data obtained in the D-DG₀²-T₂ experiment are compared to directly obtained D-T₂ and DG₀²-T₂ correlations. The results show that when diffusion encoding is included in a multi-dimensional correlation experiment, it may lead to a significant loss of signals from liquids with relatively high diffusivity and which is located close to the mineral surface. Furthermore, a negative correlation between D and ∣G₀∣ is observed for the confined liquids in all the saturation states. Such correlations have not been measured previously, and they results in a more detailed description of the local distribution of the confined liquids. In particular, at significantly high water saturations, the surviving signal from water is found at lower values of internal gradients compared to the main part of the oil signal, indicating that this water is located further away from the surface compared to the oil. The study shows that the impact from heterogeneity in pore geometry and surface properties on the individual liquids is described in more detail in DG₀²-T₂ and D-∣G₀∣ correlations compared toD-T₂ correlations, but that potential signal loss during diffusion encoding intervals should be monitored and verified.

A fluid specific dimension of confinement as a measure of wettability in porous media

The wettability of a reservoir rock is among the most important factors influencing oil recovery. This study presents how the application of a modified CPMG pulse sequence can determine the wettability of porous samples containing oil and water in the same pore network. This method can be applied as an average of the signal over the entire sample as well as with spatial resolution using slice selection. Correlations between dephasing in internal gradients (G₀²D) and the spectral linewidth (Δν) were used to calculate a fluid specific dimension of confinement (DOC) distribution for oil and water separately during forced displacement experiments performed inside the bore of an NMR spectrometer. Two samples, one water-wet and one oil-wet, were investigated. Data averaged over the entire sample show oil and water distribution at endpoint saturations (S_w=1,S_w,i and S_or) while spatially resolved data show which pores are initially occupied by the displacing fluid as well as the displacing efficiency. Both the data averaged over the entire sample and the spatially resolved data can, by itself, indicate wettability. As opposed to relaxation times as an indirect indication of wettability, the DOC distributions provide a direct fluid dimension, which more carefully describes the distribution of oil and water in the pore network.

Characterising oil and water in porous media using correlations between internal magnetic gradient and transverse relaxation time

A method for characterising water and oil in a rock core plug using correlations between diffusion decay in internal magnetic gradient and transverse relaxation time (DG₀²-T₂) is presented. The method is evaluated at different saturation levels and is compared with the measurement of correlations between diffusion and transverse relaxation time (D-T₂). It is shown how signals from water and oil can be separated based on their difference in diffusion decay in internal gradients. The obtained results show that the impact from heterogeneity in pore geometry and mineralogy on the individual liquids is revealed in more detail in DG₀²-T₂ correlations compared to the more established D-T₂ correlations. Measurements of DG₀²-T₂ correlations should be included in the toolbox of NMR experiments performed in the laboratory analysis of rock core plugs, and could then potentially lead to more detailed estimations of saturation levels and surface wettability properties.

Investigating pore geometry in heterogeneous porous samples using spatially resolved G0-Δχapp and G0-Δν correlations

This study presents a two-dimensional NMR pulse sequence for obtaining spatially resolved correlations between magnetic susceptibility induced internal gradients (G₀), and both the apparent difference in magnetic susceptibility (Δχ_app) and spectral frequency (Δν). G₀-Δχ_app correlations were utilized to generate spatially resolved pore size distributions, while the G₀-Δν correlations were used to additionally evaluate sample heterogeneity. The spatially resolved measurements were performed on a water saturated heterogeneous porous sample which contains one layer of 5-50 μm glass spheres (top layer) and one layer of 140-165 μm glass spheres (bottom layer). The slice selection was validated by applying the pulse sequence on a liquid mineral oil and water sample as well as on the porous sample. The resulting spatially resolved pore size distributions show very good agreement with results from our previously published non slice selective pulse sequence in the 140-165 μm glass spheres and good agreement in the 5-50 μm glass spheres. The G₀-Δν correlations correctly indicate a slightly higher degree of heterogeneity in the 5-50 μm glass spheres compared to the 140-165 μm glass spheres.

Attenuation of the NMR signal in a field gradient due to stochastic dynamics with memory

The attenuation function S(t) for an ensemble of spins in a magnetic-field gradient is calculated by accumulation of the phase shifts in the rotating frame resulting from the displacements of spin-bearing particles. The found S(t), expressed through the particle mean square displacement, is applicable for any kind of stationary stochastic motion of spins, including their non-markovian dynamics with memory. The known expressions valid for normal and anomalous diffusion are obtained as special cases in the long time approximation. The method is also applicable to the NMR pulse sequences based on the refocusing principle. This is demonstrated by describing the Hahn spin echo experiment. The attenuation of the NMR signal is also evaluated providing that the random motion of particle is modeled by the generalized Langevin equation with the memory kernel exponentially decaying in time. The models considered in our paper assume massive particles driven by much smaller particles.

A multi-dimensional experiment for characterization of pore structure heterogeneity using NMR

In a liquid saturated porous sample the spatial inhomogeneous internal magnetic field in general depends on the strength of the static magnetic field, the differences in magnetic susceptibilities, but also on the geometry of the porous network. To thoroughly investigate how the internal field can be used to determine various properties of the porous structure, we present a novel multi-dimensional NMR experiment that enables us to measure several dynamic correlations in one experiment, and where all of the correlations involve the internal magnetic field and its dependence on the geometry of the porous network. (Correlations: internal gradient - pore size, internal gradient - magnetic susceptibility difference, internal gradient - longitudinal relaxation, longitudinal relaxation - magnetic susceptibility difference.) It is always a spatial average of the internal magnetic field, or one of the related properties, that is measured, which is important to take into consideration when analyzing the obtained results. We demonstrate how these correlations can be an indicator for pore structure heterogeneity, and focus in particular on how the effect from spatial averaging can be evaluated and taken into account in the different cases.

Contributed review: nuclear magnetic resonance core analysis at 0.3 T

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.

Rapid measurements of heterogeneity in sandstones using low-field nuclear magnetic resonance

Sandstone rocks can contain microscopic variations in composition that complicate interpretation of nuclear magnetic resonance (NMR) relaxation time measurements. In this work, methods for assessing the degree of sample heterogeneity are demonstrated in three sandstones. A two-dimensional T1-Δχapp correlation (where Δχapp is the apparent solid/liquid magnetic susceptibility contrast) reveals the microscopic heterogeneity in composition, whilst a spatially resolved T1 profile reveals the macroscopic structural heterogeneity. To perform these measurements efficiently, a rapid measure of longitudinal T1 relaxation time has been implemented on a low-field NMR spectrometer with a magnetic field strength B0=0.3 T. The "double-shot" T1 pulse sequence is appropriate for analysis of porous materials in general. Example relaxation time distributions are presented for doped water phantoms to validate the method. The acquisition time of the double-shot T1 sequence is equivalent to the single-shot Carr-Purcell Meiboom-Gill (CPMG) sequence used routinely in petrophysics to measure transverse T2 relaxation. Rapid T1 measurements enable practical studies of core plugs at magnetic field strengths previously considered inappropriate, as T1 is independent of molecular diffusion through pore-scale (internal) magnetic field gradients.