Nuclear magnetic resonance relaxation and diffusion properties of confined fluids in organic nanopores: A molecular dynamics study

Abstract

A series of molecular dynamics (MD) simulations was conducted to investigate the nuclear magnetic resonance (NMR) relaxation properties of confined fluids in kerogen nanopores. We examined how the longitudinal (T1) and transverse (T2) relaxation times vary as a function of pore size, pore shape, the presence of paramagnetic impurities, and Larmor frequency (ω). Given the challenges of nanoscale experiments, this approach offers an in-depth analysis of how these petrophysical factors influence the measurements of T1 and T2 relaxation times. Water and oil were included in the simulations to assess how fluid type affects the results. The findings show that the presence of kerogen significantly impacts the diffusion of water and oil in organic nanopores compared to bulk. To quantify the influence of pore structure, we systematically analyzed the diffusion and NMR properties of fluids under nanoconfinement in rectangular nanopores of varying sizes. We observed that smaller pore sizes lead to a reduction in the diffusion coefficients. When considering a more complex pore network (kerogen matrix), the values of T1 and T2 relaxation times decreased by three and five orders of magnitude, respectively, compared to the values obtained for rectangular pores. At a Larmor frequency of 400 MHz, both n-pentane and water in kerogen exhibited longer relaxation times than at lower frequencies. The presence of paramagnetic impurities in the system allowed us to obtain relaxation times in the order of magnitude of the experimental data reported by other authors for hydrocarbons confined in kerogen matrices. This study provides a detailed guide to using MD simulations to investigate the behavior of nanoconfined fluids in kerogen.