Simultaneous assessment of wettability and water saturation using 2D NMR measurements

Nuclear Magnetic Resonance (NMR) measurements can be used for estimating porosity, fluid saturation, poresize distribution, and sometimes wettability. However, quantitative interpretation of rock wettability can be challenging in rocks with complex pore structures in the presence of different fluid types. Conventional NMR-based wettability models are applicable only for rocks with unimodal pore-size distributions. In this paper, we develop a new workflow for simultaneous assessment of fluid saturation and wettability in partially-saturated mixed-wet rock samples using 2D NMR measurements. We verify the reliability of this new workflow in three rock types, including unimodal and multi-modal pore-size distributions. The main inputs to the introduced workflow include the transverse relaxivity (T-2) and Diffusivity-T-2 (D-T-2) or longitudinal-T-2 (T-1-T-2) NMR measurements of the partially-saturated mixed-wet rock samples. Other input parameters include the bulk relaxivities of the saturating fluids and the NMR responses of fully water-saturated water-wet and fully oil-saturated oil-wet samples of the same rock type as the partially-saturated mixed-wet rock. In the verification step, we vary the fluid saturation by injecting brine, decane, or diesel into the rock sample using a core-flood setup. Then, we obtain NMR measurements at different fluid saturations to estimate the water/hydrocarbon saturation and wettability. Results demonstrated that the NMR-based wettability index has an average absolute difference of 0.18 when compared to the Amott Index, and the NMR-based water saturation estimates are within 18% average relative error of the gravimetric-estimates for the brine and diesel saturated rock samples.