Characterization of Fractures via Electrical Impedance

Abstract

This paper investigates the relationship between electrical impedance and resistance and rock fractures. The ultimate goal of this research is to create a down-hole tool for detecting and characterizing the fractures created or existing in Engineered Geothermal Systems (EGS) and other geothermal reservoirs Previous studies have identified a frequency dependence in the electrical properties of rocks partially saturated with water with marked difference between the frequency response of rock cores with varying fracture density. The research presented in this paper offers a geophysical explanation for this phenomenon rooted in the capacitor-like nature of fractures in partially desaturated rock. Laboratory experiments and theoretical work were designed to explore the complex impedance and resistance response to alternating current over a frequency range covering three orders of magnitude. A core-scale experiment measured the impedance and resistance of rock samples initially saturated with saline solution and subjected to evaporative desaturation. Theoretical analysis attempted to match the rock response to a model electrical circuit to help prediction of the response in subsquent experiments. The goal of these experiments was to verify previously-found relationships between frequency and resistance as well as to establish a correspondence between fractures in rock and the capacitors in a representative circuit. The research indicates this relationship and suggests possible methods of extracting fracture information from electrical response. This paper concludes with planned improvements to the experimental setup and future avenues of experimentation.