Nuclear Magnetic Resonance Dipolar Cross-Relaxation Interaction between Nanoconfined Fluids and Matrix Solids.

Many different methods have been developed to investigate fluid-solid interactions in nanoporous systems. These methods either only work in the liquid phase or provide an indirect measurement by probing the fluid-solid interaction based on a measured property change of the fluid or solid under different sample conditions. Here, we report a direct measurement technique using NMR dipolar cross-relaxation between the nanoconfined fluids and the matrix solids. The method was tested using a methyl-functionalized mesostructured silica saturated with methanol as a model sample. A formal theory was established to describe the enhanced dipolar cross-relaxation interaction between the nanoconfined fluids and the matrix solids. Both the experiment and theory showed that nanoconfinement of the fluids enhances the dipolar cross-relaxation interaction between the fluid and the matrix solids, which can be applied to investigate the fluid-solid interaction for various materials of a similar nanostructure.

Specific-Ion Effects Unveil a Route for Modulating Oxidatively Triggered Acid Systems for Reservoir Applications.

On-demand in situ preparation of industrially relevant organic acids, namely, methanesulfonic acid, triflic acid, and trifluoroacetic acid, is demonstrated in this study. Sodium and potassium bromate were found to selectively oxidize a series of ammonium salts NH4X, where X = OMs, OTf, or OTFAc, with characteristic clock reaction behavior. The redox system undergoes rapid acid formation following an extended induction time at 150 °C and is identified as a potential candidate for high-temperature oil field chemistry applications where on-demand acid placement is required. Although the reaction kinetics for acid formation from NH4X salts where X = Cl, Br, F, or SO42- follows a pKa trend, the rates of formation of the organic acids are much slower and deviate from this trend. Furthermore, we demonstrate that the rate of acid formation can be modulated by the addition of alkali metal salts, with the strongest effect observed from LiBr. Spectroscopic studies implicate the formation of lithium bromate ion pairs that slow or altogether inhibit the oxidation of NH4+. Additionally, the presence of Br- alters the reaction path, eliminating the clock behavior and creating a pathway for Li+ to strongly inhibit the redox reaction. From these studies, a method for slowing ammonium oxidation under reservoir conditions to sufficiently delay acid formation until the precursors are placed in the zone of interest is identified.

Improved Methods for Determination of Petrophysical Properties of Unconventional Tight Rocks Using Particulate Samples.

Drill cuttings are available continuously over the entire depth of any drilled wells. The use of drill cuttings to obtain petrophysical data can add a significant value in formation evaluation. An update to the previous study is presented using NMR and Archimedes principle to determine petrophysical properties including porosity, bulk density, and matrix density from drill cuttings of organic-rich mudrock formations. In the original published method, sonication was used to clean and saturate the drill cutting samples. In this improved method, particulate samples were saturated using both sonication and pressure injection methods. The obtained results were compared with accepted lab measurement techniques. Results show that pressure saturation of particulate samples can provide accurate results of petrophysical properties. Obtaining reliable petrophysical data from drill cuttings can provide continuous and quasi-real-time data for the formation evaluation of reservoirs and can effectively reduce costs by reducing or eliminating expensive formation evaluation methods.

NMR Intermolecular Dipolar Cross-Relaxation in Nanoconfined Fluids.

Spin relaxation, a defining mechanism of nuclear magnetic resonance (NMR), has been a prime method for determining three-dimensional molecular structures and their dynamics in solution. It also plays key roles for contrast enhancement in magnetic resonance imaging (MRI). In bulk solutions, rapid Brownian molecular diffusion modulates dipolar interactions between a spin pair from different molecules, resulting in very weak intermolecular relaxations. We show that in fluids confined in nanospace or nanopores (nanoconfined fluids) the correlation of dipolar coupling between spin pairs of different molecules is greatly enhanced by the nanopore constraint boundaries on the molecular diffusion, giving rise to an enhanced correlation for the spin pair. As a result, the intermolecular dipolar interaction behaves cooperatively, which leads to a large intermolecular dipolar relaxation rate and opposite in sign to the bulk solution. We found that the classical NMR relaxation theory fails to capture these observations in a nanoconfined fluid environment. Hence, we developed a formal theory and experimentally confirmed that enhanced correlation and cooperated relaxation are ubiquitous in nanoconfined fluids. The newly discovered phenomenon and the developed NMR method reveal new applications in a broad range of synthesized and naturally occurring materials in the field of nanofluidics to study molecular dynamics and structure as well as for MRI image enhancement.

Structural Properties of Kerogens with Different Maturities.

The thermogenic transformation of kerogen into hydrocarbons accompanies the development of a pore network within the kerogen that serves as gas storage locations both in pore space and surface area for adsorbed gas within the source rock. Therefore, the successful recovery of gas from these rocks depends on the accessible surface area, surface properties, and interconnectivity of the pore system. These parameters can be difficult to determine because of the nanoscale of the structures within the rock. This study seeks to investigate the pore structure, surface heterogeneity, and composition of recovered kerogens isolated from source rocks with progressively increasing thermogenic maturities. Prompt gamma-ray activation analysis (PGAA), nitrogen and methane volumetric gas sorption, and small-angle neutron scattering (SANS) are combined to explore the relationship between the chemical composition, pore structure, surface roughness, surface heterogeneity, and maturity. PGAA results indicate that higher mature kerogens have lower hydrogen/carbon ratio. Nitrogen gas adsorption indicates that the pore volume and accessible specific surface area are higher for more mature kerogens. The methane isosteric heat at different methane uptake in the kerogens is determined by methane isotherms and shows that approximately two types of binding sites are present in low mature kerogens while the binding sites are relatively homogeneous in the most mature kerogen. The hysteresis effect of the structure during the adsorption and desorption process at different CD4 gas pressures are studied. An extended generalized Porod's scattering law method (GPSLM) is further developed here to analyze kerogens with fractal surfaces. This extended GPSLM quantifies the surface heterogeneity of the kerogens with a fractal surface and shows that kerogen with high maturity is chemically more homogeneous, consistent with the results from methane isosteric heat. SANS analysis also suggests a pronounced surface roughness in the more mature kerogens. A microporous region circling around the nanopores, which contributes to high surface roughness and methane storage, is shown to develop with maturity.

Indirectly detected heteronuclear correlation solid-state NMR spectroscopy of naturally abundant 15N nuclei.

Two-dimensional indirectly detected through-space and through-bond (1)H{(15)N} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse (1)H decoupling are evaluated. Remarkable efficiency of polarization transfer can be achieved at a MAS rate of 40 kHz by both cross-polarization and INEPT, which makes these methods applicable for routine characterizations of natural abundance solids. The first measurement of 2D (1)H{(15)N} HETCOR spectrum of natural abundance surface species is also reported.

Spectral editing in 13C solid-state NMR at high magnetic field using fast MAS and spin-echo dephasing.

A simple method is proposed for separating NMR resonances from protonated and non-protonated aromatic carbons in solids under fast magic angle spinning (MAS). The approach uses a MAS-synchronized spin-echo to exploit the differences in rotational recoupling of the dipolar interactions while fully refocusing the isotropic chemical shifts. This strategy extends the relevant time scale of spin evolution to milliseconds and circumvents the limitation of the traditional dipolar dephasing method, which in fast rotating solids is disrupted by rotational refocusing. The proposed approach can be used for quantitative measurement of carbon aromaticities in complex solids with poorly resolved spectra, as demonstrated for model compounds.