Laboratory measurements of the electrical resistivity of intact and fractured representative geothermal reservoir rocks were performed to investigate the resistivity contrast caused by active boiling. Measurements were performed to simulate reservoir conditions with confining pressures up to 100 bars and temperatures to 181?C. Measurements presented are a first step toward making the search for fractures using electrical methods quantitative. Intact samples showed a gradual resistivity increase when pore pressure was decreased below the phase-boundary pressure of free water. When fluid-filled, the prepared fractured samples were 25-50% less resistive than the intact samples and showed larger increases in resistivity when boiling commenced. The resistivity ratio between boiling samples and liquid-saturated samples ranged between 2 and 5. Both intact and fractured samples demonstrated vapor pressure lowering. Analysis of a field test provided the opportunity to evaluate fracture detection using electrical methods at a large scale. Interpretation of electrical resistance tomography (ERT) images using resistivity contrasts determined by laboratory experiments indicates that actively boiling fractures can be identified. These results will lead to improved geophysical diagnostics for locating fractures and regions of active boiling in geothermal reservoirs.

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