Effect of Nano-Confinement on NMR Relaxation of Heptane in Kerogen from MD Simulations and Measurements

TL;DR

This study uses molecular dynamics simulations to reveal how nano-confinement in kerogen significantly reduces NMR relaxation times of heptane, providing insights into pore sizes and molecular behavior in shale reservoirs.

Contribution

It demonstrates the impact of nano-confinement on NMR relaxation times and introduces a method to predict pore-size distribution from simulated relaxation data without extra experiments.

Findings

Confinement reduces T1 by ~3 orders of magnitude, matching measurements.

Confinement causes residual dipolar coupling, reducing T2 by ~5 orders of magnitude.

Predicted pore-size distribution aligns with experimental data.

Abstract

Kerogen-rich shale reservoirs will play a key role during the energy transition, yet the effects of nano-confinement on the NMR relaxation of hydrocarbons in kerogen are poorly understood. We use atomistic MD simulations to investigate the effects of nano-confinement on the $^1$H NMR relaxation times $T_1$ and $T_2$ of heptane in kerogen. In the case of $T_1$, we discover the important role of confinement in reducing $T_1$ by $\sim$3 orders of magnitude from bulk heptane, in agreement with measurements of heptane dissolved in kerogen from the Kimmeridge Shale, without any models or free parameters. In the case of $T_2$, we discover that confinement breaks spatial isotropy and gives rise to residual dipolar coupling which reduces $T_2$ by $\sim$5 orders of magnitude from bulk heptane. We use the simulated $T_2$ to calibrate the surface relaxivity and thence predict the pore-size distribution of the organic nano-pores in kerogen, without additional experimental data.