jp0c00711_si_001.pdf (519.27 kB)
Critical Role of Confinement in the NMR Surface Relaxation and Diffusion of n‑Heptane in a Polymer Matrix Revealed by MD Simulations
journal contribution
posted on 2020-04-24, 12:41 authored by Arjun Valiya Parambathu, Philip M. Singer, George J. Hirasaki, Walter G. Chapman, Dilipkumar AsthagiriThe mechanism behind the NMR surface-relaxation
times (T1S,2S) and the large T1S/T2S ratio of light hydrocarbons
confined in the nanopores of kerogen remains poorly understood and
consequently has engendered much debate. Toward bringing a molecular-scale
resolution to this problem, we present molecular dynamics (MD) simulations
of 1H NMR relaxation and diffusion of n-heptane in a polymer matrix. The high-viscosity polymer is a model
for kerogen and bitumen that provides an organic “surface”
for heptane. Diffusion of n-heptane shows a power-law
dependence on the concentration of n-heptane (ϕC7) in the polymer matrix, consistent with Archie’s
model of tortuosity. We calculate the autocorrelation function G(t) for 1H–1H dipole–dipole interactions of n-heptane
in the polymer matrix and use this to generate the NMR frequency (f0) dependence of T1S,2S as a function of ϕC7. We find that increasing molecular
confinement increases the correlation time, which decreases the surface-relaxation
times for n-heptane in the polymer matrix. For weak
confinement (ϕC7 > 50 vol %), we find that T1S/T2S ≃
1. Under strong confinement (ϕC7 ≲ 50 vol
%), we find that T1S/T2S ≳ 4 increases with decreasing ϕC7 and that the dispersion relation T1S ∝ f0 is consistent with previously
reported measurements of polydisperse polymers and bitumen. Such frequency
dependence in bitumen has been previously attributed to paramagnetism;
instead, our studies suggests that 1H–1H dipole–dipole interactions enhanced by organic nanopore
confinement dominate the NMR response in saturated organic-rich shales.