Quantifying Magnetic Resonance Effects Due to Solid–Fluid Interactions on Confined Water within Quartz-Lined Nanopores via Molecular Dynamics Simulations

Abstract

Nuclear magnetic resonance (NMR) relaxation time provides valuable information about properties of fluid and solid surfaces within rocks; however, the complexity of rock samples makes it difficult to quantify the effect of solid–fluid interactions on NMR measurements. Additionally, the ability of 2 MHz NMR equipment to probe nanopores is limited, while detailed analyses of fluid NMR properties in nanopores are possible through molecular dynamics (MD) simulations. We performed atomistic MD simulations to quantify the effect of solid–fluid interactions on confined water within hydroxylated slit and cylindrical quartz-lined rock pores (slit width and cylinder radius from 1 to 9 nm). NMR properties along with density profiles and mean-squared displacements (MSD) were calculated to quantify the effect of solids on relaxation times. MD results indicate that the surface chemistry of slit and cylindrical pores causes differences in density profiles and NMR properties of water confined in silica nanopores. Structural features present in density profiles are important to interpret both MSD and NMR relaxation properties. Intramolecular correlation times are shorter than intermolecular correlation times except when water is confined inside a 1 nm slit pore. These results highlight the importance of pore size on the solid–fluid interactions at the nanoscale. Layer analysis of confined water shows that NMR relaxation times of central layers in small nanopores are not the same as bulk relaxation time. This difference originates an overestimation of pore-size distributions of tight rocks from NMR measurements via Brownstein–Tarr’s equation. Furthermore, relaxation times of water layers indicate that the effect of solid surfaces on NMR relaxation times in a 3 nm slit pore is 15.32% of those in a 9 nm slit pore.

Structural and Nuclear Magnetic Resonance Relaxation Properties of Shale Condensate within Organic Nanopores via Molecular Dynamics Simulations

“…In Table , we describe the T 1 values for methane, n -pentane, n -hexane, and isopentane at Larmor frequencies (ω) of 2.3, 22, and 400 MHz, with a pressure of 300 bar and a temperature of 293.15 K. The contributions of intra and intermolecular components to the spin–lattice relaxation times indicate that intramolecular relaxation predominates over intermolecular mechanisms. This behavior is consistent with the findings reported for water ,, and hydrocarbons − under confinement and in the bulk phase. Table also shows that while the methane relaxation times remain almost constant as the Larmor frequency increases from 2.3 to 22 MHz, T 1 at 400 MHz is 50% longer than the value at 2.3 MHz.…”

Role of Paramagnetic Impurities and Surface Roughness on NMR Relaxation Times: Insights from Molecular Dynamics Simulations

“…However, NMR relaxation times of water within paramagnetic-free pores of known sizes, calculated based on MD simulations, exhibit relatively large values. Our previous results 8,11 show that NMR relaxation times of water within spherical pores are reduced compared to slit and cylindrical pores with periodic boundary conditions; however, the calculated relaxation times remain considerably longer than experimental measurements. This deviation could potentially arise from the absence of paramagnetic impurities.…”

“…Many researchers have utilized molecular dynamics to study the NMR properties. Several studies − employed molecular dynamics (MD) simulations to investigate the relaxation times of various confined fluids, such as alkanes and water. Previous MD studies − have revealed that water relaxation times are affected by pore size, pore geometry, ions in aqueous solution, the confinement caused by solids and fluids, etc.…”

“…Several studies − employed molecular dynamics (MD) simulations to investigate the relaxation times of various confined fluids, such as alkanes and water. Previous MD studies − have revealed that water relaxation times are affected by pore size, pore geometry, ions in aqueous solution, the confinement caused by solids and fluids, etc. However, NMR relaxation times of water within paramagnetic-free pores of known sizes, calculated based on MD simulations, exhibit relatively large values.…”

Axial Diffusion in Liquid-Saturated Cylindrical Silica Pore Models

“…Deviation from bulk diffusion can therefore only be due to confinement and can be conveniently probed by molecular dynamics simulations at atomistic resolution . Due to the controlled and well-defined environment, such simulations help in dissecting the complex interplay of various physical effects interfering each other and leading to the observed behavior. , In the present work, 14 different solvent molecules are simulated in cylindrical silica pores of 5 nm diameter. The main focus was to obtain reliable ratios for the densities and self-diffusion coefficients between the bulk and pore phases.…”