Using Slim Holes for Long-Term Monitoring of Geothermal Reservoir Performance at Steamboat Springs, Nevada, U.S.A.

Abstract

Four slim holes were drilled into the producing geothermal reservoir, a highly fractured granodiorite, at the Steamboat Hills Geothermal Field in northwestern Nevada, U.S.A. These slim holes have been used for monitoring subsurface geothermal reservoir pressures and temperatures as a function of time. Static spinner surveys have also been performed. Since the slim holes are not produced, accurate water level elevations versus time can be measured. From these data, pressure in the slim holes exhibit strong correlation with local rainfall, and thus, imply a hydrological connection between the shallow groundwater system and the geothermal reservoir. The produced geothermal fluid temperatures versus time data from Steamboat Hills indicate a gradual reservoir cooling. Periodic temperature versus depth measurements, in the slim holes, indicate that there has been a gradual cooling, followed by a gradual heating, as a function of time in the shallow portion of the reservoir. Deeper portions of the geothermal reservoir exhibit temperature decreases. These temperature decreases are related to injection of cooled geothermal fluids into the geothermal reservoir. Because of the areal distribution of the slim holes relative to the production and injection wells, it has been possible to determine the relative temperature decrease and fluid flow patterns in the reservoir. These slimhole data have been used to recommend appropriate modifications to some well completions and suggest modifications to the injection system.