ABSTRACT

We have measured the dielectric constant of twelve sandstones as a function of saturation and frequency from 5 Hz to 13 MHz. The dielectric constant has been found to be strongly frequency dependent in saturated samples, decreasing with increasing frequency. The dielectric constant of dry samples shows little dependence on frequency. When the data from saturated samples are presented as a complex impedance plot, a low frequency and high frequency region can be identified; within these two regions the dielectric constant shows two different power law dependence upon frequency. The focus of this study is the effect of saturation on the dielectric constant at frequencies above10 kHz. The dielectric constant shows a large increase with increasing saturation at low levels of saturation and then a more gradual, linear increase with saturation at higher saturation levels. At high saturations we find that the change in the dielectric constant can be simply related to the change in the volume of water. The change in the dielectric constant at low saturations appears to be related to the roles of bound and free water in the pores of the rock. The ratio of surface area to volume of the pore space has been determined from nitrogen adsorption isotherms for five of the samples. This surface area to volume ratio appears to be a dominant factor in the change in the dielectric constant at low saturations and also in determining the absolute value of the dielectric constant. Modeling of the electrical response of a rock by a Debye circuit shows that the dielectric constant as calculated from a parallel capacitance is increasingly affected by the conductance of the system as the frequency decreases.

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