Multidimensional dynamic NMR correlations in sedimentary rock cores at different liquid saturations

In Nuclear Magnetic Resonance (NMR) spin echo measurements of confined liquids, the dynamic behaviour of liquid molecules are influenced by diffusion (D), translational displacement of molecules in internal gradients (G₀), and transverse surface relaxation (T₂). In this study, an experimental approach that enables characterisation of water and oil in rock core materials is presented. The approach is based on three-dimensional D-DG₀²-T₂ correlations, but the main focus is on the two-dimensional parts that involve DG₀²-T₂ and D-∣G₀∣. In order to evaluate potential signal loss that can be introduced when going from a two-dimensional to a three-dimensional experiment, D-T₂,DG₀²-T₂ and D-∣G₀∣ correlations derived from subsets of data obtained in the D-DG₀²-T₂ experiment are compared to directly obtained D-T₂ and DG₀²-T₂ correlations. The results show that when diffusion encoding is included in a multi-dimensional correlation experiment, it may lead to a significant loss of signals from liquids with relatively high diffusivity and which is located close to the mineral surface. Furthermore, a negative correlation between D and ∣G₀∣ is observed for the confined liquids in all the saturation states. Such correlations have not been measured previously, and they results in a more detailed description of the local distribution of the confined liquids. In particular, at significantly high water saturations, the surviving signal from water is found at lower values of internal gradients compared to the main part of the oil signal, indicating that this water is located further away from the surface compared to the oil. The study shows that the impact from heterogeneity in pore geometry and surface properties on the individual liquids is described in more detail in DG₀²-T₂ and D-∣G₀∣ correlations compared toD-T₂ correlations, but that potential signal loss during diffusion encoding intervals should be monitored and verified.