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 (Sw=1,Sw,i and Sor) 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 (Δχapp) and spectral frequency (Δν). G0-Δχapp 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.

Investigating mobility of crude oil adsorbates on mineral surfaces by NMR.

We have applied diffusion and relaxation Nuclear Magnetic Resonance experiments to investigate the translational and rotational mobility of adsorbents on quartz and calcite mineral surfaces. On both surfaces it was found that water is the dominant molecule. On the quartz surface the majority of water molecules have a relatively high degree of both rotational and translational mobility, while a minor fraction of water molecules, and all hydrocarbon molecules, have a significantly lower mobility. On the calcite surface the translational mobility is very low for all the adsorbed molecules, while there is a large diversity in rotational mobility, indicating that the hydrocarbon molecules are strongly attached to the surface, but that some part of each molecule still have a large degree of rotational mobility. Diffusion and relaxation experiments give a detailed description of both the molecular mobility of adsorbed species on these mineral surfaces, which leads to new insight with respect to aging processes on a molecular level.

Investigating structure-dependent diffusion in hydrogels using spatially resolved NMR spectroscopy.

HYPOTHESIS: Incorporation of the drug-loaded surfactant micelles into polymer hydrogels is a common method used to achieve controlled drug delivery. The characterization of the diffusion processes in drug delivery systems is critical in order to tune the drug loading and release. EXPERIMENTS: We present a simple and efficient NMR protocol to investigate the transport of the surfactant molecules in hydrogels on micro- and macroscale under non-equilibrium conditions. Our experimental protocol is based on a combination of 1H 1D NMR chemical shift imaging and slice-selective diffusion experiments, which enables determination of the mutual and self-diffusion coefficients of the surfactant in the non-equilibrium hydrogel-based system within the same short time frame. FINDINGS: Our results show that the self-diffusion coefficient of the positively charged surfactant in the hydrogel (Dsgel) decreases with the increasing surfactant concentration until it reaches a plateau value of 6.6±0.5×10-11m2s-1. The surfactant self-diffusion in the solution (Dssln) remains constant over the experiment with an average value of 6.7±0.3×10-11m2s-1. The surfactant mutual diffusion coefficient obtained from 1D chemical shift imaging in this hydrogel system (Dm) is 7.7±0.5×10-11m2s-1. Correlation of the localized Ds to the 1D chemical shift images gives insight into the structure-dependent diffusional behavior of surfactant molecules in the hydrogel. This NMR protocol will be of great value in studies of concentration dependent structures on the interfaces between two immiscible liquids.

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.

Characterising oil and water in porous media using decay due to diffusion in the internal field.

In the method Decay due to Diffusion in the Internal Field (DDIF), the diffusion behaviour of water molecules in the internal magnetic field makes it possible to determine a distribution of pore sizes in a sample. The DDIF experiment can also be extended to a DDIF-Carr-Purcell-Meiboom-Gill (DDIF-CPMG) experiment to measure correlations between the pore size and the transverse relaxation time, T2. In this study we have for the first time applied the DDIF experiment and the DDIF-CPMG experiment to porous materials saturated with both water and oil. Because of the large difference in diffusion rates between water and oil molecules, the DDIF experiment will act as a filter for the signal from oil, and we are left with the DDIF-signal from water only. This has been verified in model systems consisting of glass beads immersed in separate layers of water and oil, and in a sandstone sample saturated with water and oil. The results show that the DDIF and DDIF-CPMG experiments enable the determination of the confining geometry of the water phase, and how this geometry is correlated to T2. Data obtained in the sandstone sample saturated with water and oil also show that with the exception of the smallest pores there is no clear correlation between pore size and the relaxation time of water.

A comparison of retrospectively self-gated magnetic resonance imaging and high-frequency echocardiography for characterization of left ventricular function in mice.

Non-invasive imaging methods like echocardiography and magnetic resonance imaging (MRI) are very valuable in longitudinal follow-up studies of cardiac function in small animals. To be able to compare results from studies using different methods, and explain possible differences, it is important to know the agreement between these methods. As both self-gated high-field MRI and high-frequency echocardiography (hf-echo) M-mode are potential methods for evaluation of left ventricular (LV) function in healthy mice, our aim was to assess the agreement between these two methods. Fifteen healthy female C57BL/6J mice underwent both self-gated MRI and hf-echo during the same session of light isoflurane anaesthesia. LV dimensions were estimated offline, and agreement between the methods and reproducibility for the two methods assessed using Bland-Altman methods. In summary, hf-echo M-mode had better inter-observer repeatability than self-gated MRI for all measured parameters. Compared with hf-echo, systolic posterior wall thicknesses were significantly higher when measured by MRI, while diastolic anterior wall thicknesses were found to be significantly smaller. MRI measurements of diastolic LV diameter were also higher using MRI, resulting in larger fractional shortening values compared with the values obtained by hf-echo. In conclusion, hf-echo M-mode is easy to apply, has high temporal and spatial resolution, and good reproducibility. Self-gated MRI might be advantageous in cases of abnormal LV geometry and heterogeneous regional myocardial function, especially with improvements in spatial resolution. The moderate agreement between the methods must be taken into account when comparing studies using the two modalities.

Dynamic water changes in excised rat myocardium assessed by continuous distribution of T1 and T2.

Ischemic changes in excised rat myocardium were followed by series of T1 or T2 measurements from 1 to 60 min after isolated perfusion cessation, and the influence of manganese enhancement was investigated. An inverse Laplace transformation (ILT) of T1 or T2 data was used to resolve the number, time constants, and fractions of tissue water components in a continuous distribution. For T1 distributions, one single tissue component approximately 900 ms was significantly shortened and dispersed by manganese enhancement (25 and 200 microM MnCl2). For T2 distributions, three tissue components (approximately 30, approximately 100, and approximately 350 ms) were obtained initially. The two shortest components merged after approximately 10 min to one component (approximately 40 ms). Both T1 and T2 tissue components became shorter with time. In particular, the T2 distribution dynamics might be compatible with complex sequential changes in tissue water fractions during ischemia.

Analyzing equilibrium water exchange between myocardial tissue compartments using dynamical two-dimensional correlation experiments combined with manganese-enhanced relaxography.

Water compartments were identified and equilibrium water exchange was studied in excised rat myocardium enriched with intracellular manganese (Mn(2+)). Standard relaxographic measurements were supplemented with diffusion-T(2) and T(1)-T(2) correlation measurements. In nonenriched myocardium, one T(1) component (800 ms) and three T(2) components (32, 120, and 350 ms) were identified. The correlation measurements revealed fast- and slow-diffusing water fractions with mean diffusion coefficients of 1.2 x 10(-5) and 3.0 x 10(-5) cm(2) s(-1). The two shortest T(2) components, which had different diffusivities, both originated from water in intracellular compartments. A component with longer relaxation time (T(1) approximately equal 2200 ms; T(2) approximately equal 1200 ms), originating from extra-tissue water, was also observed. The presence of this component may lead to erroneous estimations of water exchange rates from multiexponential relaxographic analyses of excised tissues. The tissue T(1) value is strongly reduced with increasing enrichment of Mn(2+), and eventually a second tissue T(1) component emerges, indicating a shift in the equilibrium water exchange between intra- and extracellular compartments from the fast-exchange limit to the slow-exchange regime. Using a two-site water exchange analysis, the lifetime of intracellular water, T(ic), was found to be 475 ms, with a fraction, p(ic), of 0.71.

Determination of water compartments in rat myocardium using combined D-T1 and T1-T2 experiments.

We have used combined D-T1 and T1-T2 correlation experiments to explore water compartments in rat heart tissue (myocardium). The results show that two main compartments can be identified, which we assign to extracellular (ec) and intracellular (ic) water. The exchange rate of water across the cell membrane was found to be on the order of 0.1 Hz. In addition, the T1-T2 correlation measurements indicate that the ic compartment contain two T2 populations.

Correlations between diffusion, internal magnetic field gradients, and transverse relaxation in porous systems containing oil and water.

The main focus in this study is to investigate the correlations between internal magnetic field gradients (G0) and transverse relaxation times in liquid-saturated packings of glass beads of different wettabilities. We show how these correlations can be expressed as two-dimensional (2D) diagrams of distribution functions between internal magnetic field gradients and T2 values. In the case where it is difficult to distinguish the signals from oil and water, we separate them based on their difference in diffusivity. In addition to using such diffusion weighting in the G0-T2 diagrams, we also show results from experiments where the direct correlation between diffusion and T2 (D-T2) is determined. The overall results show that the wettability of the glass beads has a strong influence on the appearance of these diagrams, in particular on the location of the fast diffusing water molecules. However, due to their lower diffusivity, the transverse magnetization of the oil molecules is not so greatly influenced by either the presence of the glass beads or their wettability properties. Thus, the wettability properties of a liquid-filled porous material can be determined from the location of the water signal in such 2D diagrams. In particular, we show that this is the case not only for D-T2 diagrams, but also for G0-T2 diagrams.

Background gradient suppression in pulsed gradient stimulated echo measurements.

Pulsed gradient spin echo (PGSE) experiments can be used to measure the probability distribution of molecular displacements. In homogeneous samples this reports on the molecular diffusion coefficient, and in heterogeneous samples, such as porous media and biological tissue, such measurements provide information about the sample's morphology. In heterogeneous samples however background gradients are also present and prevent an accurate measurement of molecular displacements. The interference of time independent background gradients with the applied magnetic field gradients can be removed through the use of bipolar gradient pulses. However, when the background gradients are spatially non-uniform molecular diffusion introduces a temporal modulation of the background gradients. This defeats simple bipolar gradient suppression of background gradients in diffusion related measurements. Here we introduce a new method that requires the background gradients to be constant over coding intervals only. Since the coding intervals are typically at least an order of magnitude shorter than the storage time, this new method succeeds in suppressing cross-terms for a much wider range of heterogeneous samples.

A PFG-NMR Study of Restricted Diffusion in Heterogeneous Polymer Particles.

The diffusion resistance to monomers during heterogeneous polymerization of polyolefin particles may have a significant effect on the observed activity. This diffusivity is, in general, unknown. To gain more information on this diffusion resistance in such systems, PFG-NMR has been used to measure the diffusion of organic solvents in various systems of porous polymer particles. In such systems the complex morphology and geometry demands careful analysis of the PFG-NMR attenuation curve. In this study, effects from restricted diffusion, domains having different diffusivity, and internal magnetic field gradients are expected. Thus, the obtained diffusivities have to be considered carefully, and a way to analyze the data taking these effects into account is presented. Copyright 2001 Academic Press.