Use of Electrical Surveys for Geothermal Reservoir Characterization: Beowawe Geothermal Field

Abstract

The STAR geothermal reservoir simulator was used to model the natural state of the Beowawe geothermal field, and to compute the subsurface distributions of temperature and salinity which were in turn employed to calculate pore-fluid resistivity. Archie’s law, which relates formation resistivity to porosity and pore-fluid resistivity, was adopted to infer formation resistivity distribution. Subsequently, DC, MT and SP postprocessors were used to compute the expected response corresponding to available survey data. The measured apparent resistivity distribution from a dipole-dipole DC resistivity survey is in good agreement with the computed values. The computed self-potential distribution reproduces the main features of an available SP survey. Although the computed MT apparent resistivity sounding curves reproduce the shapes of the measured MT sounding curves, an overall scale factor exists between the measured and computed MT responses, and similarly with the computed dipole-dipole resistivity model. Possible reasons are static shifts in the coarsely sampled MT stations, and resitivity anisotropy due to the stratigraphy. Taken as a whole, the results of present work indicate that a suite of carefully designed electrical surveys (DC, MT, and SP) may be employed to infer subsurface geothermal reservoir characteristics.