High Angle Fracture-Controlled Permeability at Upper Steamboat Hills Geothermal Field, NV

Abstract

Geothermal wells at Upper Steamboat Hills, Nevada produce ~395° F water from nearly orthogonal high angle fractures trending NE-SW and WNW-ESE and dipping between 55 and 85 degrees. The interpretation of high angle fractures is based on 3D modeling of depths of lost circulation zones, formation microscanner (FMS) logs, formation microimaging (FMI), air photo analysis, gravity survey, and field investigations. Previous workers have suggested both high angle and nearly horizontal fracturing to explain high permeability and flow rates, but 2 or 3 high angle fractures are sufficient to delineate successful an d unsuccessful drilling results. It is difficult to explain drilling results with horizontal fractures, and none have been observed in logs. The producing lithologies are metasedimentary and granodiorite rocks, in some cases near the boundary between the two. Surficial alteration at Upper Steamboat Hills is minor, in sharp contrast to the highly altered volcanic and intrusive rocks found at lower Steamboat. Ormat Nevada Inc. has operated the entire Steamboat Hills field since 2004. The resource has traditionally been divided into upper and lower Steamboat projects with Upper Steamboat having higher temperatures and lower flow rates. Lower Steamboat is characterized by moderate temperatures (~330° F), higher flow rates, and intense surface alteration indicative of a relatively high temperature outflow plume from the Upper Steamboat portion of the field.