Low frequency complex conductivity measurements of microcrack properties

Abstract

The frequency dependence of complex electrical conductivity in the IP frequency range (10−3 to 103 Hertz) has been investigated for a variety of microcracked rocks from the German continental deep drilling project (KTB), Northern Bavaria. The laboratory measurements were made with a computer controlled four-electrode system on plugs saturated with brine of different salinity. It has been found that the complex nature of the conductivity is caused solely by the capacitive behaviour of the interlayer region between the solid matrix and the electrolytic pore solution. The resulting main feature of the conductivity spectra is a constant phase angle over the investigated frequency range combined with a nearly identical power law frequency dependence of the real as well as the imaginary parts. The low-frequency exponent is in the order of about 0 to 0.05. It is related to common IP-parameters. The relationships between the frequency exponent and microcrack properties are of special interest. The results of the study show that the frequency exponent is (1) proportional to the surface area to porosity ratio, (2) inversely proportional to water salinity, and (3) dependent on water composition. Complex conductivity measurements allow an uncomplicated separation of electrical volume and interface effects. Moreover, the results suggest that determination of specific surface area of microcracked rocks directly from complex electrical measurements can be made.