An experimental investigation of the dielectric properties of rocks
The objective of this research is to experimentally study the electrical response of rocks to changes in grain size, salinity of saturating fluid, and partial saturation. The use of electrical permittivity measurements has been proposed as a complement to resistivity measurements in formation evaluation. The combination of the two measurements has the potential to evaluate reservoir wettability, estimate pore size (or grain size), and estimate clay content. An Agilent 4192A LF Impedance Analyzer in combination with the Agilent 16451B Dielectric Test Fixture was used in conducting the dielectric measurements. Because complications arose with the electrodes in the original configuration of the fixture, five different methods were attempted to search for the most convenient, yet most accurate method. The method selected was a two-electrode method applied with silver plated electrodes. The powder is applied onto the electrode surfaces to reduce electrode polarization effects and to eliminate any chemical reactions of the electrode surfaces with the saturating fluid. Experiments were performed using different samples of various grain sizes fully saturated with de-ionized water. An increase in grain size causes the bulk conductivity and dielectric constant to decrease. The peak of the loss tangent decreases as the grain size increases. In experiments conducted with fully saturated Berea sandstone an increase in salinity causes both the conductivity and dielectric constant to increase. When plotting the reactance (out-of-phase electrical impedance) against frequency, a maximum is observed in the kHz to MHz frequency range. This maximum decreases in magnitude but increases in frequency when salinity increases. This change is a result of the double layer effect. Saturation profiles of drying samples were measured before making partial saturation dielectric measurements. CT (Computed Tomography) scans were used to analyze the saturation profile of the porous samples under drying conditions. The scans showed that the saturation profile of the disk shaped samples (Berea, Boise, and Texas Cream) were fairly uniform, suggesting a uniform electrical current distribution. In partially saturated cores, both the conductivity and dielectric constant decrease with decreasing saturation. The decrease in saturation also creates a second local maxima in a plot of reactance per unit length versus frequency. This new maxima increases in magnitude but decreases in frequency as saturation decreases. It is concluded that the effects of salinity of saturating fluid, partial saturation, and grain size of the porous medium cause changes in the dielectric measurements. The exact mechanism that caused these changes will require models that take into account the pore structure, clay content and grain size.