MAGNETIC RESONANCE IN POROUS MEDIA: Proceedings of the 9th International Bologna Conference on Magnetic Resonance in Porous Media (MRPM9), including 8th Colloquium on Mobile Magnetic Resonance (CMMR8)
1081(2008); http://dx.doi.org/10.1063/1.3058542View Description Hide Description
Initiated by the use of NMR for well logging, portable NMR instruments are being developed for a variety of novel applications in materials testing and process analysis and control. Open sensors enable non‐destructive testing of large objects, and small, cup‐size magnets become available for high throughput analysis by NMR relaxation and spectroscopy. Some recent developments of mobile NMR are reviewed which delineate the direction into which portable NMR is moving.
1081(2008); http://dx.doi.org/10.1063/1.3058553View Description Hide Description
Understanding transport processes in porous media is central to the design and operation of many chemical and physical processes. Applications of two magnetic resonance techniques implemented to study the operation of fixed‐bed catalytic reactors are reported here. First, gas and liquid velocity fields within a bead pack is reported, demonstrating that we can now image velocity fields within gas‐liquid‐solid reactors. In a separate experiment, correlations are shown to gain insight into liquid‐surface interactions within catalyst pellets. The two‐dimensional relaxation time correlation approach is then extended to the implementation of a chemical‐shift ‐resolved correlation, which has initially been applied to the identification of oil and water fractions in a permeable rock.
1081(2008); http://dx.doi.org/10.1063/1.3058534View Description Hide Description
We present in here a new 2D exchange experiment involving the measurement of local field distributions in porous media, associated with diamagnetic susceptibility inhomogeneity. The inverse Fourier method allows new analyses of exchange rates of fluids between pore spaces. When combined with inverse Laplace inversion it also permits the correlation of local fields and local magnetic field gradients.
1081(2008); http://dx.doi.org/10.1063/1.3058535View Description Hide Description
We present a unique approach for measuring the local diffusion coefficient of water inside or at the surface of soft matter systems, including lipid bilayer membranes, proteins, or protein assemblies. This is made possible by the use of dynamic nuclear polarization (DNP) of NMR signal of water via nitroxide based spin labels which are localized onto specific membrane or protein sites through covalent linkage. The DNP‐induced NMR signal enhancement critically depends on the diffusion coefficient of the solvent within approximately 10 Å distance of the spin labeled site. We discuss the usefulness of NMR in studying local water dynamics and accessibility in combination with electron spin spectroscopy and compare the results to the complementary technique of field cycling relaxometry (FC). Most importantly, the DNP approach allows us to work with sample volumes as small as 3–4 μL at spin label concentrations nearing 100 μM giving orders of magnitude higher sensitivity compared with field cycling approaches, making the technique well suited for biological samples.
1081(2008); http://dx.doi.org/10.1063/1.3058536View Description Hide Description
The experimental characterization of voidspaces in porous media generally includes measurements of volume averaged scalar properties such as porosity, dispersivity, or the hydrodynamic radius where V and S are the volume and surface area of the pore space respectively. Displacement encoding NMR experiments have made significant contributions to this characterization. It is clear, however, that NMR derived dispersivities in packed beds—the one random porous system for which there exist canonical but incompatible theoretical predictions with few or no adjustable parameters—can be affected by the same experimental complications which have substantially contributed to the puzzling scatter in published dispersion results based on elution experiments. Notable among these are macroscopic flow heterogeneities near walls, and inhomogeneous flow injection. Using the first three cumulants we delineate a transition from a pre‐asymptotic to a quasi‐asymptotic dispersion regime and determine the true dispersivity of the random pack of spheres.
1081(2008); http://dx.doi.org/10.1063/1.3058537View Description Hide Description
This paper examines the interplay of order and transport phenomena in porous media. Magnetic resonance (MR) measurements of porous media structure and displacement scale dependent dynamics characterize the variation of transport with structure. The mechanisms of transport which lead to scaling of hydrodynamic dispersion with Peclet number are discussed. The MR data are discussed relative to results from recent simulations that indicate the role of the distribution of velocity within pores on the dispersion scaling with Peclet number.
1081(2008); http://dx.doi.org/10.1063/1.3058538View Description Hide Description
The fields of MR in Porous Media and Neuroradiology have largely developed separately during the past two decades with little appreciation of the problems, challenges and methodologies of the other. However, this trend is clearly changing and possibilities for significant cross‐fertilization and synergies are now being realized.
Models and Applications of in Vivo Lung Morphometry with Hyperpolarized 3He MRI in a Mild COPD Population1081(2008); http://dx.doi.org/10.1063/1.3058539View Description Hide Description
Hyperpolarized diffusion MRI is increasingly used to non‐invasively quantify local alveolar structure changes, such as those from Chronic Obstructive Pulmonary Disease (COPD). Previously, we described an in vivo lung morphometry technique that decouples the helium apparent diffusion coefficient (ADC) into components oriented along the longitudinal and transverse axes of the acinar airways. Herein, we discuss our recent expansion of this theory, which relates the anisotropy of the MRI diffusion signal to the geometrical parameters of the acinar airways. We demonstrate the utility of this model in human studies and compare the measured airway radii with prior ex vivo experiments.
1081(2008); http://dx.doi.org/10.1063/1.3058540View Description Hide Description
Technical and economic projections strongly suggest that fossil fuels will continue to play a dominant role in the global energy market through at least the mid twenty‐first century. However, low‐cost conventional oil and gas will be depleted in that time frame. Therefore new sources of energy will be needed. We discuss two relatively untapped unconventional fossil fuels: heavy oil and gas hydrate. In both cases, nuclear magnetic resonance plays a key role in appraising the resource and providing information needed for designing production processes.
1081(2008); http://dx.doi.org/10.1063/1.3058541View Description Hide Description
This article provides an introduction to magnetic material properties and an overview of compact permanent magnet designs. The descriptions are focused on portable magnetic resonance instruments, including in‐magnet and open configurations. A few examples are presented for spectroscopy as well as for space resolved and relaxation measurements.
1081(2008); http://dx.doi.org/10.1063/1.3058543View Description Hide Description
We use the propagator resolved transverse relaxation exchange technique to look at the movement of fluid in three different types of rock samples. The two pore model previously used to fit molecular exchange simulations to the experimental data is expanded to accommodate the three site exchange seen in two of the samples. Estimated values for pore space characteristics from the simulations were compared to values calculated from X‐Ray CT data of the samples. While discrepancies exist between the NMR and X‐Ray CT results, the molecular exchange behavior estimated from the three samples reflects well with their morphology.
The Heterogeneity Spectrum: A Method for Quantifying the Extent of Spatial Heterogeneity as a Function of Length Scale in Complex Materials1081(2008); http://dx.doi.org/10.1063/1.3058544View Description Hide Description
Imaging measurements represent a powerful means of characterizing materials with complex spatial structure, such as naturally occurring porous media. In this contribution we present the application of the heterogeneity spectrum, which is a newly developed method for quantitative analysis of imaging data. The heterogeneity spectrum transforms imaging data into a measurement of the extent of spatial heterogeneity as a function of length scale. The inversion rigorously considers the support of the experiment, and the length scales of heterogeneity to which a measurement is sensitive are determined from the experimental voxel size and sample size. Heterogeneity spectra are shown for magnetic resonance imaging and thin section imaging data from several complex carbonate rock core samples. These measurements are sensitive to heterogeneity at length scales ranging many orders of magnitude and are shown to provide a useful characterization of these porous media.
Dynamics at Surfaces: Probing the Dynamics of Polar and A‐Polar Liquids at Silica and Vapour Surfaces1081(2008); http://dx.doi.org/10.1063/1.3058545View Description Hide Description
Recent studies by Nuclear Magnetic Resonance and Neutron Scattering of the dynamics and phase‐fractions of water/ice systems in templated porous silicas (SBA‐15) indicate that what was believed to be a non‐frozen surface water layer is actually plastic ice, the quantity varying (continuously and reversibly) with temperature, and converting to a brittle (mainly cubic) ice at lower temperatures.
Current research relates to the study the dynamics of polar water/ice and a‐polar organics at both solid and vapour interfaces. The polar results are significant for water/ice systems in the environment, where snow‐packs, glaciers and icebergs are common examples of water/ice systems with extensive air/vapour interfaces.
This research also points the way forward for wide‐range cryoporometric metrology in ‘difficult’ systems such as high iron content clays and rocks, as well as aged concrete. Results are presented for cryoporometric measurements on meteorite samples with a significant metallic content, exhibiting relaxation times down to 2.5 ms.
Direct Probing of the Wettability of Plaster Pastes at the Nanoscale by Proton Field Cycling Relaxometry1081(2008); http://dx.doi.org/10.1063/1.3058546View Description Hide Description
We present our last results on proton nuclear relaxation dispersion data of water in plaster pastes in various conditions of water to plaster ratio and temperature. These data have been interpreted with an intramolecular relaxation model involving elementary time steps of water near the interface (e.g. bulk bridges, adsorption trails and escaping tails). This leads to a direct probing of water adsorption time on solid surface and gives in consequence an original characterization of the surface nano‐wettability of such a porous material of great interest in the building industry.
1081(2008); http://dx.doi.org/10.1063/1.3058547View Description Hide Description
In this work we report on the direct measurement of the molecular diffusion coefficients of water, confined to purified bovine nuchal ligament elastin, by high resolution q‐space NMR imaging. The experimental data indicate that water trapped within an elastin fiber has two distinguishable molecular diffusion coefficients. The component with the slowest mobility has a diffusion coefficient on the order of that varies inversely with the diffusion time and is seen to reduce near 37 °C. The component with higher mobility has a diffusion coefficient reminiscent of free water but is observed to also behave similarly at 37 °C. From our experimental data we extract the surface‐to‐volume ratio of pores within elastin and associated changes as a function of temperature.
1081(2008); http://dx.doi.org/10.1063/1.3058548View Description Hide Description
This paper presents MRI measurements of a novel semi solid MR contrast agent to pressure. The agent is comprised of potassium chloride cross linked carageenan gum at a concentration of 2% w/v, with micron size lipid coated bubbles of air at a concentration of 3% v/v. The choice for an optimum suspending medium, the methods of production and the preliminary MRI results are presented herein. The carageenan gum is shown to be ideally elastic for compressions relating to volume changes less than 15%, in contrast to the inelastic gellan gum also tested. Although slightly lower than that of gellan gum, carageenan has a water diffusion coefficient of indicating its suitability to this purpose. RARE imaging is performed whilst simultaneously compressing test and control samples and a maximum sensitivity of 1.6% MR signal change per% volume change is found which is shown to be independent of proton density variations due to the presence of microbubbles and compression. This contrast agent could prove useful for numerous applications, and particularly in chemical engineering. More generally the method allows the user to non‐invasively image with MRI any process that causes, within the solid, local changes either in bubble size or bubble shape.
1081(2008); http://dx.doi.org/10.1063/1.3058549View Description Hide Description
In this contribution we present results obtained from monitoring the hydrogen peroxide decomposition in the presence of commercial porous particles, impregnated with different metals as catalytically active sites. The motivation in this study was to follow the decomposition and to establish a correlation between mass transport and reaction rate, by means of properties easily accessible with NMR methods, e.g., by monitoring the reactant concentration and the effective diffusion coefficient of the liquid in the vicinity of the pellet or in a defined closed volume.
1081(2008); http://dx.doi.org/10.1063/1.3058550View Description Hide Description
There are two distinct drawbacks to the measurement of fluid flow in porous media using Nuclear Magnetic Resonance (NMR) flow propagators. First, spin relaxation impairs the quantitative nature of the data. Second, the experiments can be considerably time consuming since both the Pulsed Field Gradient (PFG) strength g and observation time Δ need to be varied independently. Here we present two separate solutions to these problems and show how they can be applied to the study of fluid flow in permeable reservoir rocks. The accuracy of the measurements can be improved using a combination of and correlations. Under certain ideal conditions it is possible to use these data to remove both the and relaxation weighting from flow propagators [Mitchell et al. J. Magn. Reson. DOI:10.1016/j.jmr.2008.05.001 (2008)]. Statistical analysis of the resultant probability density distributions yields observed mean displacements equal to the expected mean displacements If speed is more important than accuracy, then the rapid Difftrain pulse sequence can be used to acquire multiple propagators with a range of observation times simultaneously. Difftrain has been validated for use in studies of fluid flow in reservoir rocks [Mitchell et al. J. Magn. Reson. 191 267–272 (2008)] and facilitates the observation of dynamic flowing systems, such as deposition or erosion.
1081(2008); http://dx.doi.org/10.1063/1.3058551View Description Hide Description
When a porous material is inserted in a uniform magnetic field, spatially varying fields typically arise inside the pore space due to susceptibility contrasts between the solid matrix and the surrounding fluid. Susceptibility contrast is present in many porous media of interest, such as fluid filled rocks, cements, granular media, colloids and trabecular bones. The presence of non‐uniform internal fields often interferes with NMR relaxation and diffusion measurements. Recently, the idea of utilizing internal gradients as a finger‐print of pore geometry for detailed pore structure information has been implemented and gave a new insight upon quantifying the structure related properties of internal gradients in porous media. [1,2] In this contribution, we further investigate and develop new NMR methods utilizing susceptibility induced internal gradient. First, the strength and orientation of internal gradients are visualized using diffusion based MR sequence for a 2D phantom of packed cylindrical capillaries. Second, it is shown that the structure factor of the granular and porous media can be approximated by the pair correlation function of inhomogeneous internal magnetic field, followed by experimental and numerical demonstrations using a randomly packed glass beads phantom .