Relationship between Complex Resistivity, Elastic Wave and Water Saturation of Cracked Andesite under Laboratory Fluid-Flow Test

Abstract

Estimating the underground water distribution by means of geophysical explorations (e.g., seismic and electromagnetic explorations) is essential to develop fluid resources (e.g., geothermal fluid). To improve the accuracy of data that were obtained from geophysical explorations, we should experimentally investigate the physical properties experimentally. In this study, we conduct a fluid-flow test on a rock core sample to elucidate the relationship between complex resistivity, elastic wave, and water saturation of the geothermal reservoir rock. The reservoir rock core sample (andesite) contains an initial macro crack, and we further induced thermal cracks in the rock core sample. The fluid-flow test was conducted using this sample (10.5% of porosity) under a confining pressure of 20 MPa and a temperature of 25 °C. In this test, initially, the sample was filled with nitrogen gas under 20 MPa of confining pressure. The gas is observed to emulate the superheated steam that is observed in geothermal fields. Further, brine (1wt%-KCl, 1.75 S/m), which emulates the artificial recharge into the reservoir, is injected into the sample. During the test, water saturation, permeability, complex resistivity (in the frequency range of 10^－2–10⁵ Hz), and elastic wave (P-wave, 250 kHz) are measured. As a result, complex resistivity was observed to dramatically decrease from 10⁴ to 10² Ωm owing to brine injection. After starting the brine injection, complex resistivity decreased continuously with the increasing water saturation. The relationship between complex resistivity and water saturation exhibited almost no frequency dependence. P-wave velocity almost remained constant against the increasing water saturation, whereas the amplitude of P-wave decreased continuously with the increasing water saturation. These results indicate that complex resistivity varies with minor changes in water saturation, whereas P-wave velocity does not depict any variation. In other words, complex resistivity can be potentially used to monitor the changes in water saturation in geothermal reservoirs. Based on the changes in complex resistivity, P-wave velocity, and amplitude against water saturation, the two−phase (nitrogen−brine) fluid flow form of in the sample is suggested to be divided into two stages.