The interaction between the solid and fluid phases, characterized by the wettability of the formation, strongly affects the fluid distribution in the pore-space and consequently the conductivity and the polarization mechanisms of the rock. Resistivity models that incorporate rock wettability for improved assessment of hydrocarbon reserves have been recently developed. However, wettability and pore structure in such models still have to be determined from core measurements and imaging methods. Dielectric dispersion measurements can simultaneously provide information about different polarization mechanisms at different frequencies, which are affected by porosity, grain-size distribution, and wettability. Thus, a multi-frequency interpretation of dielectric measurements can provide information about volume of fluids as well as solid-fluid interfacial properties, such as wettability and cation exchange capacity. In this paper, we introduce a new mechanistic model to characterize multi-frequency dielectric measurements of mixed-wet formations.
The objectives of this paper include
to quantify the influence of wettability on broadband dielectric dispersion mechanisms, and
to incorporate wettability and interfacial polarization at fluid-fluid and solid-fluid interfaces in a rock physics model for broadband characterization of dielectric measurements.
All the parameters required by the introduced model are associated to physical mechanisms at microscopic- and pore-scale or geometrical features of the rock. The induced polarization at the electrolyte-solid interface is characterized by a mechanistic model of Stern and Gouy-Chapman layers honoring the electrochemical interactions occurring at the surface of grains and clays. We also introduce a model to characterize the electrolyte-hydrocarbon interface, which originates from the adsorption of hydroxyl ions. We integrate the aforementioned solid-fluid interfacial mechanisms with the bulk properties of fluids and grains through the sequential application of an effective medium model, making explicit the dependence on wettability.
We successfully applied the introduced model to six sandstone core samples from two formations at different wettability conditions. The dielectric permittivity measured from 100 Hz to 1 GHz in the laboratory was in agreement with the permittivity obtained from the proposed analytical rock physics model model. The proposed model confirmed the experimental observations that the wettability has a detectable influence on dielectric permittivity. A significantly decrease in dielectric permittivity below 100 kHz was predicted by the model as the fraction of hydrocarbon-wet grains increases. The new model also explained the polarization experimentally observed around 1 MHz through the presence of a capacitive layer at the hydrocarbon-brine interface. The outcomes of this paper can enhance petrophysical interpretation of mixed-wet rocks by integrated interpretation of multifrequency dielectric dispersion measurements.