Roosevelt Geothermal Field, Utah – Reservoir Response after more than 25 years of Power Production

Abstract

After more than 25 years of production, Roosevelt geothermal field continues to produce in excess of 400 kp/h of steam from much the same borefield area as drilled during the late 1970s. Initial tests of the two new wells drilled east of the production borefield indicate that the high-temperature reservoir may be more extensive than previously thought. The plant was originally built as single stage flash with 23MW net installed capacity. In 2007 a binary plant was installed to generate 10 MW of additional power from the separated hot brine. Studies indicate the reservoir pressures are presently declining at about 20 psi/year, after declining at three times this rate during the early production years. The total pressure decline is about 600 psi. Deep reservoir temperatures appear to be declining at less than 1.5 ºF/year, and a substantial thermal resource remains after > 25 years of production. Shallow temperatures have increased along the Opal Mound fault zone due to steam zone formation over the hot liquid reservoir. Pacificorp Energy is initiating an investigation of fluid flow paths between injection and production wells so that future heat extraction from the reservoir can be optimized as it considers expansion of power generation from the field at some point in the future.