Critical Role of Confinement in the NMR Surface Relaxation and Diffusion of n-Heptane in a Polymer Matrix Revealed by MD Simulations

Abstract

The mechanism behind the NMR surface-relaxation times (T-1S,T-2S) and the large T-1S/T-2S 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 H-1 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 (phi(C7)) in the polymer matrix, consistent with Archie's model of tortuosity. We calculate the autocorrelation function G(t) for H-1-H-1 dipole-dipole interactions of n-heptane in the polymer matrix and use this to generate the NMR frequency (f(0)) dependence of T-1S,T-2S as a function of phi(C7). 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 (phi(C7) > 50 vol %), we find that T-1s /T-2s similar or equal to 1. Under strong confinement (phi(C7) less than or similar to 50 vol %), we find that T-1S/(2S) greater than or similar to 4 increases with decreasing phi(C7) and that the dispersion relation T-1S proportional to f(0) 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 H-1-H-1 dipole-dipole interactions enhanced by organic nanopore confinement dominate the NMR response in saturated organic-rich shales.