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

Nuclear Magnetic Resonance (NMR) measurements can be used for estimating porosity, fluid saturation, pore-size 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 (T2) and Diffusivity-T2 (D-T2) or longitudinal-T2 (T1-T2) 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.