Laboratory and Field Measurements of Electrical Resistivity to Determine Saturation and Detect Fractures in a Heated Rock Mass

摘要

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 and to infer saturation and fracture location in a large-scale field test. Measurements were performed to simulate test conditions with confining pressures up to 100 bars and temperatures to 145 degrees C. Measurements presented are a first step toward making the search for fractures using electrical methods quantitatively. Intact samples showed a gradual resistivity increase when pore pressure was decreased below the phase-boundary pressure of free water, while fractured samples show a larger resistivity change at the onset of boiling. The resistivity change is greatest for samples with the most exposed surface area. 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 of resistivity contrasts, aided by laboratory derived resistivity-saturation-temperature relationships, indicates that dynamic saturation changes in a heated rock mass are observable and that fractures experiencing drying or resaturation can be identified. The same techniques can be used to locate fractures in geothermal reservoirs using electrical field methods.