NMR longitudinal relaxation time for characterizing oil occurrence in shale organic nanopores: Insights from the molecular level

Low-field nuclear magnetic resonance (NMR) has proven to be a non-invasive and effective technique for characterizing shale oil occurrence. However, the abundant nanopores in shale and the limitations of experimental echo time hinder its further application in characterizing oil storage. Herein, the longitudinal relaxation time ( T 1 ) was used to investigate the occurrence states and characterization of n-octane in graphene nano-slits by molecular dynamics simulations. T 1 of different n-octane molecular layers indicates that the occurrence states of n-octane exhibit both adsorbed and free phases. The longitudinal relaxation of the first adsorption layer is dominated by intermolecular interactions, while the second adsorption layer's longitudinal relaxation in 6–12 nm slits is governed by intramolecular interactions. Notably, intramolecular interactions also mainly contribute to the longitudinal relaxation of the third adsorption layer and the free phase. Additionally, the relationship between the mass ratio of the free phase to the adsorbed phase ( m f / m a ) and T 1 is established. T 1 ranging from 0.24 to 1.62 s corresponds to m f / m a = 0, meaning that only adsorbed phase exists. When T 1 ranges from 1.62 to 5.94 s, m f / m a increases exponentially, showing coexistence of adsorbed and free phases. As T 1 exceeds 5.94 s, n-octane is predominantly in the free phase, and the corresponding mass ratio of the free phase to the adsorbed phase exceeds 174.64. Furthermore, T 1 is determined by slit size H and temperature T . For H ≤ nm, there is only the adsorbed phase, and for 3 < H < 50 nm, the adsorbed and free phases coexist. The slit size where the free phase dominates is larger than 50 nm, which decreases with increasing T . T 1 decreases with carbon chain length of n-alkanes, primarily dominated by intramolecular interactions. This study lays a theoretical foundation for using low-field NMR to quantitatively evaluate oil occurrence in shale nanopores. • The adsorbed and free phases of n-octane in the nano-slits are distinguished by T 1 . • An exponential model between free/adsorbed phase mass ratio and T 1 is established. • A method using T 1 to evaluate the slit size distribution is proposed. • Intramolecular interactions dominate the relaxation of n-alkanes in nano-slits.