Petrophysical models are used to estimate formation water saturation and salinity from dielectric dispersion data. These models are sensitive to other attributes of the formation such as wettability. Previous studies, analyzing different parameters of dielectric petrophysical models have shown how the dielectric dispersion is qualitatively influenced by rock wettability. Such analysis requires prior knowledge of the water saturation. To date, there is no direct method to quantify wettability from dielectric data.
In this work, we discriminate the effects of water saturation and wettability on dielectric dispersion data. We conditioned a 1.5-inch-diameter dolomite plug with solutions of different concentrations of stearic acid. In each set of experiments, we imbibed water into the sample and measured the dielectric dispersion in a frequency range of 10 MHz to 1 GHz using a reflection coaxial probe. This method enables us to obtain several dielectric measurements on a rock at defined wettability states and at different saturation levels. Then, the inverted dielectric interpretation answers such as water-phase tortuosity (MN) are analyzed for each dataset to distinguish their sensitivity to saturation and wettability. After each set of experiments, we measured the wettability index of the sample using US Bureau of Mines (USBM) and Nuclear Magnetic Resonance (NMR) methods and correlated it with dielectric inverted parameters.
Our findings show that the inverted interpretation parameters from dielectric data correlate well with the wettability index from USBM. These parameters include MN, the grain aspect ratio for the bimodal model, and the depolarization factors of water and oil-matrix of the textural model. We aged the sample into three wettability conditions: neutral-wet, strongly water-wet, and strongly oil-wet. For extreme wettability conditions, we observed a consistent trend of dielectric interpretation parameters with wettability during water flooding independent from saturation. For the neutral wettability case, the inverted dielectric parameters are constant up to a water saturation of about 50% and then starts to change gradually. For the strongly water-wet case, we observed a similar trend for depolarization factor as found in the literature, in which the nonwetting phase depolarization factor is close to the spherical geometry.
Our study illustrates how the inverted dielectric dispersion petrophysical parameters can correlate with well-established laboratory measurement of the wettability index, such as USBM method, for a specific core plug. This correlation can aid in estimating the wettability index directly from dielectric dispersion data for cores with similar rock textures. Different rock textures may impose different correlations, and further work is needed to establish a dielectric correlation for each rock type in a reservoir for application to downhole formation evaluation.
