A Novel Analysis of the Electrical Transport Mechanisms in Porous Media

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Effects of water saturation and wettability on the dielectric constant are investigated experimentally using four-electrode impedance measurements, and theoretically using models that account for the electrical double layer polarization. Complex impedance measurements, performed on Berea sandstone and on Ottawa-sand packs in the frequency range 10 Hz to 1 MHz, appear to indicate that the dielectric constant varies linearly with water saturations above 50%. The rate of change of dielectric constant with saturation is found to be a function of frequency. As the frequency increases this rate of change decreases. The decrease in the slope of the dielectric constant-water saturation profile with frequency is not intuitively obvious, but has been proven theoretically in this work. The dielectric constant of water-wet samples is found higher than that of the oil-wet samples at all water saturations. The difference is more pronounced at high water saturations near unity. The wettability changes have been simulated using a generalized Maxwell–Wagner model by varying the amount of ionic surface charge of rocks. In general oil-wetting agents react with the formation matrix by connecting their positively charged tails to the negatively charged silica surfaces, lowering the surface charge density. Simulations show that the effect of wettability changes on the dielectric constant is very significant. These conclusions are consistent with the experimental results presented in this study.

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