Discovery of a new type of geothermal energy resource in Utah offers hope for significantly more potential across the western U.S., and a boost in geothermal power production.
In 2011 and 2012, Utah Geological Survey geoscientists, in partnership with a U.S. Geological Survey research drilling crew, drilled nine temperature gradient holes in Utah’s Black Rock Desert basin south of Delta to test a new concept that high temperature geothermal resources might exist beneath young sedimentary basins. Preliminary results show that near-surface temperature gradients in the basin vary from about 60⁰C/km (33⁰F/1000 feet) to 100⁰C/km (55⁰F/1000 feet). This implies temperatures of 150 to 250⁰C (300 to 500⁰F) at 3 – 4 km depth (10,000 to 13,000 feet) beneath the basin. An abandoned oil exploration well drilled near Pavant Butte in the central part of the basin in 1981 confirms these exceptionally high temperatures. Seven of the drill holes were funded by the U.S. Department of Energy as part of a National Geothermal Data System project, managed by the Arizona Geological Survey. The new holes also confirm the results from three other research holes that were drilled in the basin over the past few years; these were funded by the Utah State Energy Program and the Utah Division of Wildlife Resources.
The 1,000 square kilometer Black Rock Desert basin is filled with unconsolidated sediments to a depth of 3 km, while the underlying basin floor comprises a variety of Paleozoic and older bedrocks. In some parts of the basin, porous and permeable carbonates (limestones and dolomites) are known to be present and these would be natural hosts for a geothermal reservoir. Using the drilling results, a reservoir modeling team at the University of Utah estimates a basin-wide power density of about 3 to 10 MWe/km2, (megawatts of power per square kilometer) depending on reservoir temperature and permeability. Given the large area of this basin, the power potential is conservatively estimated to be hundreds of megawatts, and preliminary economic modeling suggests a cost of electricity of about 10c per kilowatt-hour over the life of a geothermal power project. The modeling assumes air-cooled binary power generation with all produced water injected back to the reservoir so that there would be no emissions or consumption of water. The heat in the produced water would be exchanged at the surface in an air-cooled binary power plant. Such power plants are common these days in geothermal power developments. The cool, injected water would move laterally in the reservoir between injection and production wells, and can be considered as heat-farming at depth.
This basin is especially attractive for geothermal development because of the existing nearby infrastructure ─ it is next to a large coal-fired power plant, a 300 MWe wind farm, and a major electrical transmission line to California. [Right, location of the new temperature gradient wells in the Black Rock Desert, and the inferred temperature at 3 kilometer depth (10,000 feet; 150⁰C = 300⁰F; 200⁰C = 400⁰F). The contours of gravity outline a basin coinciding with the region of high temperature. Credit, UGS]
Geothermal exploration in the Basin and Range Province of western Utah and Nevada has traditionally focused on narrow, hydrothermal upwelling zones along bounding faults of mountain ranges. Most current power developments have reservoir areas of less than 5 km2 (2 square miles). However basins within the Basin and Range usually have areas of many hundreds of square kilometers. Although the depth to potential reservoirs beneath these basins is deeper than the geothermal industry is used to, the large reservoir area offers economies of scale. Drilling to depths of 3 – 4 km is not unusual in oil and gas developments.
Dr. Rick Allis, Director of the Utah Geological Survey and joint lead scientist of the sedimentary basin geothermal research project, said that existing heat flow maps of the Basin and Range don’t have the resolution to identify this type of geothermal energy resource. “There are other potentially hot basins across the Basin and Range province that need to be investigated using this exploration model. We have identified the Steptoe Valley and Mary’s River –Toano basins in northeast Nevada as obvious geothermal targets. There may also be hot basins across the western U.S. that have similar unrecognized geothermal energy potential.”
The project findings are being presented at 2:30pm on Monday, October 1, at the annual meeting of the Geothermal Resources Council in Reno, Nevada. A question and answer period with Dr. Allis will take place following the close of the session at 3:45pm at the Department of Energy Geothermal Technologies Program booth, 610-612.
The National Geothermal Data System is in operational test mode, integrating large amounts of information from all 50 states to enhance the nation’s ability to discover and develop geothermal energy. Visit the State Contributions site at www.stategeothermaldata.org.
A 3 minute video with and without subtitles is available at http://geology.utah.gov/blog/.