Measurement of Core Electrical Parameters at Ultrahigh and Microwave Frequencies

Abstract

Summary The Electromagnetic Propagation Tool (EPT) has demonstrated that the electrical conductivity and dielectric permittivity of rocks at microwave frequencies are different from those at low frequencies. These differences are helpful in determining the properties of the rocks and the fluids they contain and thus contribute to improved formation evaluation. Electrical parameters of rocks are found generally complex. A technique has been developed for measuring the complex dielectric permittivity, conductivity, and magnetic permeability over a range of frequencies in the ultrahigh frequency (UHF) and microwave frequency bands (100 MHz to 2.0 GHz). The sample core constitutes the dielectric of a coaxial transmission-line sample holder. With an S-parameter setup, two-port S-parameter measurements are made. The complex constituent parameters ( ', '', sigma, mu) of the core samples then are computed from the S-parameters. Measurements on bulkwater of various salinities demonstrate the salinity dependence of the dielectric constant (E'/Eo) of water at high salinities. The total measured loss is separated into a conductivity loss (sigma) and a dielectric loss (E"/Eo), since the frequency dependence of conductivity loss is well-known. Computed conductivity of bulk water shows little dispersion and is in good agreement with measured low-frequency (400-Hz) values. Introduction The microwave EPT has produced a need for laboratory measurement of the electrical parameters of various rocks at UHF and microwave frequencies and a need to deduce their relationship to formation parameters such as lithology, water saturation, water salinity, etc. This paper describes the first results of our laboratory investigation of the formation constituents (rock matrix and borehole fluids) and the composite formation. Measurements are made over a wide frequency range, from 100 MHz to 2 GHz. A special experimental setup, built for this purpose, also is described. The principal quantities of interest are the (complex) dielectric constant and the electrical conductivity. Measurements are made by using samples in a coaxial transmission line and the well-known S-parameter method of data analysis. Measurements have been made on bulk water of various salinities (NaCl) and on carbonate and silicate rock samples, both dry and impregnated with waters of various salinities. Measurements also have been made on fused glass beads impregnated with waters of several salinities. Measured values of dielectric constant are compared with values computed for clean formations. Measurement Theory Type of Measurement Measurements of electrical parameters at high frequencies can be made by reflection, transmission, or resonant-cavity methods. The reflection method is a one-port measurement. (The other port is terminated successively in an open circuit and short circuit at each frequency). This method gives good results for low- and medium-loss samples but poor results for samples with high losses. The transmission method is ideal for high-loss samples. However, in samples that are too short, end reflections will produce spurious measurements. The resonant-cavity technique gives good results for medium- and low-loss samples. For high-loss samples, the sample size has to be reduced drastically to maintain the accuracy of the measurements. When measurements are made over a wide frequency range, the cavity dimensions must be changed with each resonant frequency, a rather cumbersome procedure. JPT P. 2689^