L. C. Shen, W. C. Savre, J. M. Price and K. Athavale

Conventional resistivity and induction tools measure the electrical conductivity of the formation. Interpretation of these logs is difficult in situations where the formation water resistivity is variable or unknown as a result, for example, of water, steam, or chemical flooding. Recent introduction of several dielectric tools offers a new technique in well logging. These sondes measure the relative dielectric permittivity of the formation at very-high and ultra-high radio frequencies. Because water has a much higher relative dielectric permittivity (about 80) than oil (about 2) or gas (about 1), the dielectric tool can distinguish hydrocarbon-bearing zones from water-bearing zones even when the formation fluid is nonconducting. However, in order to quantify the oil saturation in the formation, one needs an accurate formula that can relate the measured relative dielectric permittivity of the rock to the oil saturation in the rock. Present interpretation formulas have only a limited range of applicability. Therefore, our study was undertaken to answer the following question. Whereas Archie's relationship relates the resistivity to oil saturation for resistivity logs, what is the corresponding saturation formula for dielectric logs?The approach we take is to measure core samples and obtain a broad data base from which we derive an interpretation formula. This paper describes how we developed a laboratory technique to measure reservoir core samples at ultra-high frequencies, how the data are processed, and how an interpretation formula for water saturation is found. The data are obtained in the frequency range 800 to 1 200 megahertz (MHz), with the porosity of the rock ranging from 6 to 42 percent. The rocks are saturated with NaCl solution with salinity ranging from 0 to 182 000 ppm. Our study enabled us to develop a new and accurate interpretation technique for the dielectric tool called EPT (Electromagnetic Propagation Tool) manufactured by Schlumberger.