Quantification of Rock’s Wettability Index Using Dielectric Measurements

Wettability is a crucial parameter that governs several petrophysical attributes of oil- and gas-bearing rocks. However, the traditional methods to measure the wettability index are restricted to laboratory techniques, which makes that measurement expensive and time-consuming. Due to its sensitivity to fluid-solid interaction and the large contrast between the dielectric constant of oil and water, earlier studies investigated the use of dielectric measurements for wettability evaluation. Nevertheless, these studies mainly focused on the qualitative assessment of the wettability using the dielectric measurements and did not yield a practical and easy-to-implement dielectric-based wettability index correlation. Therefore, the objective of this study is to explore the response of the dielectric dispersion to wettability changes at two water saturation endpoints, full water saturation (S_w = 1) and irreducible water saturation (Swirr), and to develop a correlation for obtaining the wettability index from dielectric measurements. A pair (master and sister) of Berea sandstone (BS) and Fontainebleau (FB) sandstone and Indiana limestone (IL) samples were used in the study. The wettability of the sister samples was altered (to make them less water-wet) without impacting porosity before measuring their wettability index. The widely recognized US Bureau of Mines (USBM) wettability index (WI_USBM) served as the benchmark for evaluating the newly developed dielectric wettability index (WI_dielectric). Among all measured parameters, imaginary permittivity showed the most consistent and conclusive results, displaying (at frequencies below 200 MHz) a significant drop due to altering the wettability toward more oil-wet. This drop can be attributed to reduced electrical interactions between water and the grain surface due to wettability alteration. A strong linear relationship was also observed between the wettability index of each sample and the drop in its imaginary permittivity as saturation was changed from S_w = 1 to Swirr. Therefore, a WI_dielectric correlation was developed by incorporating the imaginary permittivity measured at both S_w = 1 and Swirr. The developed correlation was able to predict the WI_USBM of the tested samples with R² of 0.97 and a root mean square error of 0.066. Overall, this study offers an in-depth analysis of the dielectric response to various wettability conditions across different saturation levels, which has facilitated the formulation of an equation for determining the wettability index through dielectric measurements. The findings of this research lay the groundwork for broadening the application of dielectric measurements to additional petrophysical analyses and for more accurate characterization of the fluid-rock and fluid-fluid interactions.