Nuclear magnetic resonance (NMR) is sensitive to fluid properties such as viscosity and also to surface properties interactions as wettability. The challenge while interpreting NMR data is typically separating multiple effects on the overall signal. We investigated the sensitivity of the bulk oil T2 distribution with different fluid sample storage conditions in terms of temperature and seal imperfection.

Aim of the study was to prove the importance of extremely careful sample handling during laboratory studies, especially if the intention is to interpret NMR data in terms of wettability alterations. For example, T2 distributions can be used to extract information on the wettability of the rock surface. A wetting fluid experiences faster relaxation (and therefore shorter T2) compared with the non-wetting situation. However, the same effect can be originated by an increase in fluid viscosity. Generally, to interpret a T2 shift, either viscosity or wettability must be considered constant to address the T2 shift to the other cause.

The results of this study and lessons learned are as follows:

  • Even small evaporation of crude light components causes the NMR signal to shift considerably toward shorter T2 values. This effect, if wrongly assigned to surface interactions, leads to the wrong wettability considerations.

  • Food-grade cling film is permeable to oil vapour and should not be considered a good seal for crude oil storage while aging.

  • There was evidence of water volume reduction, even when the water was topped with crude oil.

Crude oils are complex fluids with thousands of components which include different surface-active species (e.g., polar species). Therefore, minute changes in the conditioning or handling of such fluids may cause considerable changes in rheological properties and therefore NMR properties.

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