Layer-Resolved T 1/ T 2 Relaxation Resolves Weakly Adsorbed Transition Layer of n -Octane in Hydroxylated Qua

Abstract

Nuclear magnetic resonance (NMR) is widely used to characterize fluids in shale oil reservoirs, yet resolving weakly adsorbed intermediate layers in mineral nanopores remains challenging. This difficulty arises from nanoscale confinement, signal overlap, and limited density contrast, which obscure the correspondence between NMR responses and local fluid occurrence. In this study, molecular dynamics simulations coupled with NMR relaxation theory were employed to investigate n-octane confined in hydroxylated quartz nanopores within the pore size and temperature ranges considered. This simplified model was designed to isolate the effect of silanol-bearing quartz surfaces, rather than to represent the full compositional and mineralogical complexity of shale oils. A layer-resolved T1/T2 relaxation framework was established to distinguish surface-affected, weakly adsorbed intermediate, and bulk-like regions beyond conventional bulk-averaged descriptions. The results show that quartz confinement produces distinct layer-specific T1/T2 signatures, reflecting differences in molecular packing, orientational ordering, and intra/intermolecular relaxation contributions. In particular, the fourth adsorption layer remains structurally identifiable as a near-wall density layer, but its T1/T2 values, relaxation characteristics, and diffusion coefficients closely approach those of the bulk-like region, indicating a structurally adsorbed but dynamically free-like intermediate layer. Temperature-dependent analysis further demonstrates that increasing temperature weakens the density contrast of this intermediate layer, while its evolution toward bulk-like relaxation behavior remains traceable by T1/T2. Quantitative relationships were further established among T1/T2, pore size, and the free-to-adsorbed oil ratio. Overall, these results demonstrate that, for n-octane confined in hydroxylated quartz nanopores, layer-resolved T1/T2 can serve as a sensitive molecular descriptor for linking density-defined occurrence states with dynamically defined relaxation behavior.