ROOSEVELT HOT SPRINGS GEOTHERMAL FIELD, UTAH - RESERVOIR RESPONSE AFTER MORE THAN 25 YEARS OF POWER PRODUCTION

摘要

After more than 25 years of production, Roosevelt Hot Springs geothermal field continues to produce in excess of 400,000 pounds/h (about 200 metric tons/hour) of steam from much the same borefield area as drilled during the late 1970s. Initial tests of two wells drilled in 2008 east of the production borefield indicate that the high-temperature reservoir may be more extensive than previously thought. The plant was originally built as a single-stage flash plant with a 23 MW net installed capacity. In 2007 a binary plant was installed to generate 10 MW of additional power from the separated hot liquid. The reservoir pressures are presently declining at about 0.3 bar/year (5 psi/year), after declining at a much higher rate during the early production years. The total pressure decline is about 40 bars (600 psi). Deep reservoir temperatures appear to be declining at about 0.7 deg C /year (<1.5 deg F/year), and a substantial thermal resource remains after more than 25 years of production. Temperatures of 260 deg C (500 deg F) still exist on the east side of the production borefield. Shallow temperatures have locally increased along the Opal Mound fault zone due to formation of a steam zone over the hot liquid reservoir. Based on the original heat flow, the undisturbed liquid inflow to the field was about 50 kg/s (approx400 kp/h), and this will have increased due to the reservoir pressure decline with development. Past reservoir modeling efforts appear to have been too conservative in predicting development potential. PacifiCorp Energy is initiating an investigation of fluid flow paths between the injection and production wells so that future heat extraction from the reservoir can be optimized. Expansion of power generation from the field in the future is anticipated.