Experimental Workflow for Quantifying the Performance of Geophysics-Based and Conventional Core-Based Wettability Assessment Methods

Conventional wettability assessment methods (e.g., Amott-Harvey and USBM) are often time consuming and require core-scale measurements. We recently developed wettability models based on two-dimensional (2D) nuclear magnetic resonance (2D-NMR) and/or resistivity measurements, which can be applied to well logs for simultaneous assessment of water saturation and wettability. However, they require core-scale verification, which has been challenging due to the uncertainties in wettability distribution inside the core samples as well as the lack of a dependable ground truth on the wettability of a given sample. The objective of this paper is to develop a setup that can provide a wide range of ground-truth wettability indices to (a) test reliability of a variety of wettability assessment techniques and (b) enable the advancement of the geophysics-based methods for wettability assessment. Glass beads are used to create synthetic core samples. We use a siliconizing fluid to alter the wettability of the beads and confirm this alteration with sessile drop tests. Next, we aggregate beads of different wettability to create cylindrical artificial grain packs with a controlled wide range of wettability states. For this purpose, we design and fabricate an experimental fixture that tightly packs the beads between two electrodes. This fixture is designed in a way that enables performing both NMR and resistivity measurements. Then, we saturate the samples with a hydrocarbon/water mixture and perform 2D-NMR and resistivity measurements. We use 2D-NMR measurements to estimate water saturation. Then, we use our newly introduced resistivity-based wettability index model (which uses water saturation and resistivity as inputs) to quantify wettability. Finally, we test the reliability of the estimated water saturation and wettability. We observed that the average relative error between the estimated wettability indices and the fraction of water/hydrocarbon-wet beads is less than 20%. Moreover, we showed that water saturation and wettability could be simultaneously estimated by integration of 2D-NMR and electrical-resistivity measurements with average relative errors of less than 10% and 15%, respectively. The results clearly demonstrate that the introduced workflow can be reliably used in the simultaneous quantification of water saturation and wettability index. We also demonstrated that the experimentally obtained resistivity model parameters related to the shape of the grains (i.e., depolarization factor) are consistent with the one calculated from analytical solutions for spherical grain geometry. This observation verifies that the model parameters of the resistivity-based wettability index model are based on geometry and can be estimated via simplifying assumptions. The outcomes of this paper enable the detection of the most reliable geophysical-based wettability assessment method and the comparison of their performance against conventional methods. This comparison might suggest a need for redefining a standard wettability index that can be uniquely estimated. In this work, we used artificial rock samples to enable a direct comparison of the estimated wettability indices and actual wettability fractions, which is not possible in actual core samples. Moreover, the use of artificial samples enables testing of the wettability assessment methods in rocks with different pore-size distributions. Finally, results are promising for in-situ and real-time assessment of wettability using borehole geophysical measurements.