Wireline Borehole Geophysical Measurements in Geothermal Exploration – A Few Fundamentals & Examples Getting into Hot Water

Abstract Geothermal development involves harvesting the earth's natural heat. Early exploration involved multiple airborne & surface geophysical measurements. However, the most successful technique has involved patterns of successively deeper boreholes, for monitoring the stabilized subsurface temperature, with depth. While direct steam and surface flash projects have been developed in California, El Salvador, Iceland, Indonesia, Italy, Mexico, New Zealand, and elsewhere, the reservoir temperatures for these sites greatly exceed the safe operating temperatures, for even Hostile Environment Wire Line (W/L) sondes. Most Geothermal developments in the Western US involve relatively lower temperature resources, where the geothermal waters are never exposed to the atmosphere but utilized instead with a low-boiling-point (i.e., Isobutene) working fluid, in closed surface or subsurface heat exchangers, to drive the surface electrical generators. Another lower temperature application is to utilize lower temperature geothermal fluids to heat buildings such as workshops, offices, greenhouses, and homes. While multiple surface and borehole geophysical measurements have been utilized, the most successful evaluation tool has been successively deeper arrays of temperature gradient holes to locate the subsurface source of near-surface heat anomalies. However, porosity and permeability measurements have been useful if a development involved producing geothermal fluids, instead of simply mining the heat. The current review paper describes the early through current and proposed Western US Geothermal E&P development and the role of surface and borehole (BH) geophysics, or WL measurements, as well as a novel concept for repurposing idle oil and gas wells as low temperature industrial heating sources.