Molecular Dynamics Simulations of NMR Relaxation and Diffusion of Heptane Confined in a Polymer Matrix
Abstract
The mechanism behind the NMR surface relaxation and the large T1/T2 ratio of light hydrocarbons confined in the nano-pores of kerogen remains poorly understood, and consequently has engendered much debate. Towards bringing a molecular-scale resolution to this problem, we present molecular dynamics (MD) simulations of 1H NMR relaxation and diffusion of heptane in a polymer matrix, where the high-viscosity polymer is a model for kerogen and bitumen that provides an organic "surface" for heptane. We calculate the autocorrelation function G(t) for 1H-1H dipole-dipole interactions of heptane in the polymer matrix and use this to generate the NMR frequency (f0) dependence of T1 and T2 relaxation times as a function of φC7. We find that increasing molecular confinement increases the correlation time of the heptane molecule, which decreases the surface relaxation times for 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 the ratio T1S/T2S 4 increases with decreasing φC7, and that the dispersion relation T1S f0 is consistent with previously reported measurements of polymers and bitumen. Such frequency dependence in bitumen has been previously attributed to paramagnetism, but our studies suggests that 1H-1H dipole-dipole interactions enhanced by organic nano-pore confinement dominates the NMR response in saturated organic-rich shales, without the need to invoke paramagnetism.