A rock core wettability index using NMR T₂ measurements

Wettability is a critical parameter that controls fluid flow and distribution as well as recovery efficiency in reservoir rocks. The sensitivity of Nuclear Magnetic Resonance (NMR) relaxation measurements to wettability is well-known. Here we develop a new approach to calculate a wettability index based on low field NMR transverse (T₂) measurements which is simpler than existing models in the literature and still affords good agreement with Amott-Harvey and USBM wettability test results. The model relies on the emergence of a bulk like relaxation signal from the non-wetting phase in partially oil/water saturated cores. Unlike other models in the literature, which require T₂ measurements at up to four different saturation states, our model only requires measurements at two partial saturation states; irreducible brine (S_wi) and residual oil (S_or). The performance of the model was tested using experimental results from different core types aged with light and heavy oils containing varied amounts of asphaltene. Different wettability conditions with Amott-Harvey/USBM values ranging from −0.39 to 0.79 were thus tested. When compared with measured reference wettability indices from the Amott-Harvey or USBM method, the model prediction showed good agreement (the calculated mean absolute error was 0.10). Additionally, improved accuracy was achieved relative to existing comparable models in the literature for the range of samples considered.