Laboratory measurements on electric and elastic properties of fractured geothermal reservoir rocks under simulated EGS conditions

For developing the technique of Enhanced Geothermal System (EGS), the estimation of injected water distribution is essential. To estimate water saturation changes deep under the ground, geophysical explorations (e.g., seismic and electromagnetic explorations) have been applied; however, the relationship between the electric/elastic properties and the water saturation in reservoir rocks has not been well known. Our goal is to elucidate this basic relationship as well as effects of salinity and fracture porosity on it via fluid-flow experiments for more quantitative interpretation of geophysical explorations. In this research, we prepared two types of specimen from geothermal reservoir rocks; A) contains artificially induced thermal cracks (porosity = 10.5%) and B) initially contains a single fracture (porosity = 3.8%). In fluid-flow tests, specimens were initially filled with nitrogen gas (10 MPa of pore pressure) under 20 MPa of confining pressure; the gas emulates the superheated steam that is observed in the geothermal fields. Then, brine (0.1 and 1wt%-KCl) which emulates the artificial recharge to the reservoir, was injected into the samples (11-16 MPa of injection pressure). During the tests, water saturation, permeability, complex resistivity (in the frequency range of 10-2-105 Hz) and elastic wave velocity were simultaneously measured. As a result of Type A specimen, resistivity dramatically decreased from 104 to 102 Ω due to the brine injection. However, P-wave velocity was almost constant (the difference was less than 1%) at that time. These results indicate that resistivity could be sensitive to minor changes in water saturation in the reservoirs compared with P-wave velocity. As a result of Type B specimen, we observed salinity effect on resistivity (the resistivity difference between 0.1 and 1wt%-KCl brine was almost twice) and its decreasing trend against the water saturation was different from the result of Type A specimen. These dependencies (water saturation, salinity, and porosity) on resistivity could be explained by the total ion content within a specimen.