Dielectric Properties of Fluid Saturated Rocks

The objective of this dissertation is to study the effect of wettability, clay content, water saturation and salinity on the electrical properties of hydrocarbon bearing rocks. The frequency range of interest is 10 Hz to 10 MHz. Measurements were made for the impedance of both fully and partially saturated rocks using the four-electrode method and for fully saturated rocks using the two-electrode method. These measurements include both clean and shaly sand samples. For fully saturated rocks, the dielectric constant is found to increase with the clay volume fraction, the cation exchange capacity and the electrochemical potential of the rock samples. It is found to decrease with increasing salinity, frequency, permeability, and porosity. Neither stress, nor wettability appear to significantly influence the dielectric constant of fully brine saturated Berea cores. Empirical correlations between the dielectric constant, frequency, permeability, cation exchange capacity, and porosity are presented for tight gas sands used in this study. These correlations provide a means of estimating important petrophysical parameters such as the permeability and the clay content from a non-destructive complex impedance sweep of tight gas sands fully saturated with brine. A lower critical frequency is found to characterize the geometry of the pore space. For partially saturated rocks, the dielectric constant appears to depend linearly on water saturation above 0.1 MHz and to have a power law dependence on water saturation below 0.1 MHz. This frequency appears to be the limit below which the resistivity index was invariant to frequency and at which all experimental runs displayed a peak in the reactivity index. Wettability effects were pronounced below 10 KHz. The effects observed experimentally are explained on the basis of the Generalized Maxwell-Wagner Theory. It is shown that the model presented by Lima and Sharma agrees quantitatively with the measured effects of porosity, clay content, saturation, grain size, and frequency. The Lima-Sharma theory for modeling the complex impedance of partially saturated shaly sands has been inverted. This inversion allows a log analyst to use low ( < lKHz) and high (> lMHz) complex impedance data from well logs to calculate reservoir petrophysical parameters such as water saturations in the virgin formation and in the flushed zone, porosity, clay volume fraction, clay surface charge density, and grain size.