Abstract:Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) allows recording of molecular diffusion paths (notably, the probability distribution of molecular displacements over typically micrometers, covered during an observation time of typically milliseconds) and has thus proven to serve as a most versatile means for the in-depth study of mass transfer in complex materials. This is particularly true with nanoporous host materials, where PFG NMR enabled the first direct measurement of intracrystalline diffus… Show more
“…Therefore, an accurate theory of configurational diffusion especially in geomaterials is incomplete thus warranting further research. As discussed earlier, new techniques such as PFG NMR 45 , 46 and compressed sensing MRI (CS-MRI) 39 have potential to assist and/or overcome X-ray and neutron imaging limitations and can thus play a role in further strengthening the configurational diffusion theory. Figure 12 ( a ) Neutron radiograph for ORS1 at 13 days showing water diffusion; the bright white color represents water inside the fracture and the less bright color represents diffused water in the matrix adjacent to the fracture, ( b ) non-homogenous water diffusion on either side of the fracture illustrated by the pixel profile of yellow line drawn across the fracture.…”
Section: Theoretical Expositionmentioning
confidence: 99%
“…In a more recent study, the dual potential of X-ray and nuclear techniques was highlighted using a novel data-driven approach to acquire quality resolution images using magnetic resonance imaging (MRI) 39 . There have been other novel attempts to characterize molecular diffusion paths using nuclear resonance techniques such as pulsed field gradient (PFG) nuclear magnetic resonance (NMR) in combination with magic-angle spinning (MAS) 45 , 46 . Such newer (NMR imaging) techniques have potential to overcome the limitations of independent X-ray or neutron imaging.…”
Understanding fluid flow in shale rocks is critical for the recovery of unconventional energy resources. Despite the extensive research conducted on water and oil flow in shales, significant uncertainties and discrepancies remain in reported experimental data. The most noted being that while oil spreads more than water on shale surfaces in an inviscid medium, its uptake by shale pores is much less than water during capillary flow. This leads to misjudgement of wettability and the underlying physical phenomena. In this study, therefore, we performed a combined experimental and digital rock investigation on an organic-rich shale including contact angle and spontaneous imbibition, X-ray and neutron computed tomography, and small angle X-ray scattering tests to study the potential physical processes. We also used non-equilibrium thermodynamics to theoretically derive constitutive equations to support our experimental observations. The results of this study indicate that the pre-existing fractures (first continuum) imbibe more oil than water consistent with contact angle measurements. The overall imbibition is, however, higher for water than oil due to greater water diffusion into the shale matrix (second continuum). It is shown that more water uptake into shale is controlled by pore size and accessibility in addition to capillary or osmotic forces i.e. configurational diffusion of water versus oil molecules. While the inorganic pores seem more oil-wet in an inviscid medium, they easily allow passage of water molecules compared to oil due to the incredibly small size of water molecules that can pass through such micro-pores. Contrarily, these strongly oil-wet pores possessing strong capillarity are restricted to imbibe oil simply due to its large molecular size and physical inaccessibility to the micro-pores. These results provide new insights into the previously unexplained discrepancy regarding water and oil uptake capacity of shales.
“…Therefore, an accurate theory of configurational diffusion especially in geomaterials is incomplete thus warranting further research. As discussed earlier, new techniques such as PFG NMR 45 , 46 and compressed sensing MRI (CS-MRI) 39 have potential to assist and/or overcome X-ray and neutron imaging limitations and can thus play a role in further strengthening the configurational diffusion theory. Figure 12 ( a ) Neutron radiograph for ORS1 at 13 days showing water diffusion; the bright white color represents water inside the fracture and the less bright color represents diffused water in the matrix adjacent to the fracture, ( b ) non-homogenous water diffusion on either side of the fracture illustrated by the pixel profile of yellow line drawn across the fracture.…”
Section: Theoretical Expositionmentioning
confidence: 99%
“…In a more recent study, the dual potential of X-ray and nuclear techniques was highlighted using a novel data-driven approach to acquire quality resolution images using magnetic resonance imaging (MRI) 39 . There have been other novel attempts to characterize molecular diffusion paths using nuclear resonance techniques such as pulsed field gradient (PFG) nuclear magnetic resonance (NMR) in combination with magic-angle spinning (MAS) 45 , 46 . Such newer (NMR imaging) techniques have potential to overcome the limitations of independent X-ray or neutron imaging.…”
Understanding fluid flow in shale rocks is critical for the recovery of unconventional energy resources. Despite the extensive research conducted on water and oil flow in shales, significant uncertainties and discrepancies remain in reported experimental data. The most noted being that while oil spreads more than water on shale surfaces in an inviscid medium, its uptake by shale pores is much less than water during capillary flow. This leads to misjudgement of wettability and the underlying physical phenomena. In this study, therefore, we performed a combined experimental and digital rock investigation on an organic-rich shale including contact angle and spontaneous imbibition, X-ray and neutron computed tomography, and small angle X-ray scattering tests to study the potential physical processes. We also used non-equilibrium thermodynamics to theoretically derive constitutive equations to support our experimental observations. The results of this study indicate that the pre-existing fractures (first continuum) imbibe more oil than water consistent with contact angle measurements. The overall imbibition is, however, higher for water than oil due to greater water diffusion into the shale matrix (second continuum). It is shown that more water uptake into shale is controlled by pore size and accessibility in addition to capillary or osmotic forces i.e. configurational diffusion of water versus oil molecules. While the inorganic pores seem more oil-wet in an inviscid medium, they easily allow passage of water molecules compared to oil due to the incredibly small size of water molecules that can pass through such micro-pores. Contrarily, these strongly oil-wet pores possessing strong capillarity are restricted to imbibe oil simply due to its large molecular size and physical inaccessibility to the micro-pores. These results provide new insights into the previously unexplained discrepancy regarding water and oil uptake capacity of shales.
“…The drawback of NMR, in general, and SSNMR techniques, in particular, is their low sensitivity. For this reason microporous materials like zeolites [ 54 , 60 , 61 , 62 , 63 ] or mesoporous materials like MCM-41 and SBA-15 derivatives with high specific surfaces are commonly employed as host materials for NMR-confinement studies [ 64 , 65 , 66 , 67 ]. To battle this drawback, indirect detection methods under MAS, where the X-nucleus of interest is detected via the far more sensitive protons, a technique originally developed by Ishii and Tycko [ 68 ], were successfully applied to porous systems by the Pruski group to achieve remarkably sensitivity enhancements [ 69 , 70 , 71 , 72 ].…”
This review gives an overview of current trends in the investigation of small guest molecules, confined in neat and functionalized mesoporous silica materials by a combination of solid-state NMR and relaxometry with other physico-chemical techniques. The reported guest molecules are water, small alcohols, and carbonic acids, small aromatic and heteroaromatic molecules, ionic liquids, and surfactants. They are taken as characteristic role-models, which are representatives for the typical classes of organic molecules. It is shown that this combination delivers unique insights into the structure, arrangement, dynamics, guest-host interactions, and the binding sites in these confined systems, and is probably the most powerful analytical technique to probe these systems.
“…A parallel and equally relevant aspect for separation processes involves the kinetics, governed by the diffusion and transport of the adsorbed molecules inside the pores. From an experimental point of view, it is challenging to measure diffusion coefficients of confined fluids with accuracy [17,18]. Molecular simulation, on the other hand, provides an unequivocal means of calculating transport coefficients, although the sparsity of molecules produced by the confinement can lead to poor statistics and hence large uncertainties.…”
We report on molecular simulations of model fluids composed of three tangentially bonded Lennard-Jones interaction sites with three distinct morphologies: a flexible “pearl-necklace” chain, a rigid “stiff” linear configuration, and an equilateral rigid triangular ring. The adsorption of these three models in cylindrical pores of diameters 1, 2, and 3 nm and with varying solid–fluid strength was determined by direct molecular dynamics simulations, where a sample pore was placed in contact with a bulk fluid. Adsorption isotherms of Type I, V, and H1 were obtained depending on the choice of pore size and solid–fluid strength. Additionally, the bulk-phase equilibria, the nematic order parameter of the adsorbed phase, and the self-diffusion coefficient in the direction of the pore axis were examined. It was found that both the molecular shape and the surface attractions play a decisive role in the shape of the adsorption isotherm. In general, the ring molecules showed a larger adsorption, while the fully flexible model showed the smallest adsorption. Morphology and surface strength were found to have a lesser effect on the diffusion of the molecules. An exceptional high adsorption and diffusion, suggesting an enhanced permeability, was observed for the linear stiff molecules in ultraconfinement, which was ascribed to a phase transition of the adsorbed fluid into a nematic liquid crystal.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
