Simultaneous Assessment of Water Saturation and Water Salinity From the Joint Multi-Frequency Interpretation of Real and Imaginary Parts of Dielectric Permittivity Measurements

Abstract

Fluid saturations and distributions, water salinity, pore structure, and porosity affect dielectric permittivity measurements. However, the conventional dielectric permittivity mixture models, such as Complex Refractive Index Model (CRIM) and Hanai-Bruggeman (HB) do not quantitatively include the cumulative effect of the aforementioned petrophysical properties. Moreover, the effect of salt concentration on multi-frequency dielectric permittivity measurements still needs to be investigated. The objectives of this paper are (a) to investigate the effect of salt concentration on complex multi-frequency dielectric permittivity responses in rocks with complex pore structure, (b) to develop a new workflow for estimating multi-frequency dielectric permittivity of rock samples taking into account the complexity of pore structure, different polarization mechanisms, porosity, water saturation, and salt concentration, and (c) to develop an inversion algorithm to simultaneously estimate water saturation and salinity from dielectric dispersion data. First, we conduct dielectric permittivity experiments on fully brine-saturated rock samples. Then, we change the salinity of the samples and perform dielectric permittivity experiments on the rock samples at different water salinity levels. Next, we develop a new rock-physics workflow which includes the combined effect of the aforementioned petrophysical properties. The new workflow calculates the multi-frequency complex dielectric permittivity responses of synthetic rock samples. Then, we use an automated inversion algorithm to simultaneously estimate water saturation and salinity of actual rock samples from the joint interpretation of the real and imaginary parts of multi-frequency dielectric permittivity measurements. We successfully verified the reliability of the new workflow in the core-scale domain using 12 different rock samples in the Barra Velha formation. The new workflow simultaneously estimated water saturation and salinity with average relative errors less than 12% and 14%, respectively. Moreover, we observed that the average relative errors between the experimentally observed and calculated dielectric permittivity that are obtained from the introduced mixture model, CRIM, and HB are 11%, 121%, and 26%, respectively. We demonstrated that the effect of salt concentration could have significant effects on dielectric permittivity responses up to 3 GHz and has to be reliably taken into account in interpretation of dielectric measurements. The multi-frequency joint interpretation of the real and imaginary parts of dielectric permittivity measurements makes the introduced workflow a robust interpretation technique in the presence of uncertainties in the estimates of the formation water salinity. Moreover, unlike the conventional dielectric mixture models, the introduced workflow honors the complexity of pore structure and composition, water salinity, and different polarization mechanisms.