Modeling Molecular Interactions with Wetting and Non-Wetting Rock Surfaces by Combining Electron Paramagnetic Resonance and NMR Relaxometry

Abstract

Three types of natural rocks�Bentheimer and Berea sandstones, as well as Liege Chalk�have been aged by immersion in a bitumen solution for extended periods of time in two steps, changing the surface conditions from water-wet to oilwet. NMR relaxation dispersion measurements were carried out on water and oil constituents, with saturated and aromatic molecules considered individually. In order to separate the different relaxation mechanisms discussed in the literature, 1 H and 19 F relaxation times were compared to 2 H for fully deuterated liquids: while 2 H relaxes predominantly by quadrupolar coupling, which is an intramolecular process, the remaining nuclei relax by dipolar coupling, which potentially consists of intra-and intermolecular contributions. The wettability change becomes evident in an increase of relaxation rates for oil and a corresponding decrease for water. However, this expected behavior dominates only for the spin−lattice relaxation rate R 1 at very low field strengths and for the spin−spin relaxation rate R 2 , while high-field longitudinal relaxation shows a much weaker or even reverse trend. This is attributed in part to a change of radical concentration on the pore surface upon coverage of the native rock surface by bitumen as well as by the change of surface chemistry and roughness. EPR and DNP measurements quantify the change of volume vs surface radical concentration in the rocks, and an improved understanding of the role of relaxation via paramagnetic centers is obtained. By means of comparing different fluids and nuclei in combination with a defined wettability change of natural rocks, a refined model for molecular dynamics in conjunction with NMR relaxation dispersion is proposed.

Exploring the Interaction of Water with Open Metal Sites in MIL-101(Cr) by 1H NMR Relaxometry and ReaxFF Molecular Dynamics Simulations

“…The longitudinal relaxation of water protons in the MIL-101(Cr)-90w sample was modeled considering two phases in fast exchange one with the other: a liquid phase affected by surface effects and a bulk liquid phase. − In the surface-affected liquid phase, water molecules either belong to the coordination sphere of Cr 3+ paramagnetic centers, where they are characterized by the longitudinal relaxation time T 1 param , or diffuse on the surface around such centers, where they are characterized by T 1 surf . It must be noticed that the possible contribution to relaxation arising from reorientations of water molecules on the MOF surface without paramagnetic species was considered negligible with respect to those from water molecules interacting with Cr 3+ paramagnetic centers, as usually done. − In the bulk liquid phase, 1 H longitudinal relaxation is characterized by the relaxation time T 1 bulk , which is determined by the fast and isotropic tumbling of water molecules. In the fast diffusion limit between the two phases, the overall 1 H spin lattice relaxation rate, 1/ T 1 , is given by the following equation: 1 T 1 = N p a r a m N 1 T 1 p a r a m + N s u r f N 1 T 1 s u r f + N b u l k N 1 T 1 b u l k where N p a r a m N is the fraction of water molecules directly bound to Cr 3+ , …”

“…The contribution of bulk water protons, 1/ T 1 bulk , arising from the modulation of 1 H– 1 H dipolar interactions by molecular reorientation and fast translational diffusion motions in the frequency regime investigated by FFC NMR relaxometry, was considered to be frequency independent. In the literature, the contributions to relaxation of protons from water bound to (1/ T 1 param ) or diffusing in proximity of (1/ T 1 surf ) paramagnetic ions have been expressed using different models. − In particular, in a model devised by Korb, 1/ T 1 surf shows a bilogarithmic trend as a function of Larmor frequency and typically decreases when the temperature increases. ,, The well-known Bloembergen-Morgan theory of water relaxation in the presence of dissolved ions has been used to model 1/ T 1 param of water confined in rocks containing Mn 2+ , − leading to a double Lorentzian trend, where the two components are given by the respective contributions at the nuclear and electron Larmor frequencies.…”

“…31−37 In particular, in a model devised by Korb, 1/T 1surf shows a bilogarithmic trend as a function of Larmor frequency and typically decreases when the temperature increases. 32,34,35 The well-known Bloembergen-Morgan theory of water relaxation in the presence of dissolved ions 38 has been used to model 1/T 1param of water confined in rocks containing Mn 2+ , 32−34 leading to a double Lorentzian trend, where the two components are given by the respective contributions at the nuclear and electron Larmor frequencies.…”

Effect of Nanoconfinement on NMR Relaxation of Heptane in Kerogen from Molecular Simulations and Measurements

“…41 In general, a power-law dependence T 1 ∝ f 0 β suggests relaxation by RMTD (reorientations mediated by translational displacements). 42 Further investigations are underway to determine whether the dynamic molecular modes P(τ) can give insights into the RMTD mechanism and the surface topography.…”

Analysis of Adsorbed Polyphosphate Changes on Milled Titanium Dioxide, Using Low-Field Relaxation NMR and Photoelectron Spectroscopy

“…Nuclear magnetic resonance (NMR) relaxometry is also proving to be an invaluable technique to study dispersion surface properties, or otherwise applied industrially to analyze pore structures and wettability in reservoir rocks for enhanced oil recovery, − where the use of very low-field, desktop instruments has provided an avenue for inline or at-line analysis. Originally, the technique was largely developed for rapid surface area measurements of nanoparticle dispersions , but has advanced extensively to incorporate analyses of a number of colloidal surface chemistry effects − and even nanobubbles .…”