Conventional geothermal reservoirs are characterized by a heat source, hydrothermal convection, and sufficient natural permeability to allow for fluid migration. Recognizing the geologically restricted occurrence of natural sites, additional opportunities have been sought. Enhanced geothermal systems are reservoirs in hot rock that lack the natural permeability required for fluid movement. In 2014, the U.S. Department of Energy initiated a program to test and develop new technologies for characterizing, creating and sustaining EGS reservoirs under natural field conditions. A site approximately 350 kilometers south of Salt Lake City Utah was selected for the Frontier Observatory for Research in Geothermal Energy, or “FORGE” laboratory. The ultimate goal of the Utah FORGE project is to demonstrate to the public, stakeholders and the energy industry that EGS technologies have the potential to contribute significantly to future power generation.
The FORGE footprint covers an area of about 5 square kilometers within Utah’s Renewable Energy Corridor. The site lies adjacent to a 306-megawatt wind farm, a 240-megawatt solar field and PacifiCorp Energy’s 38-megawatt Blundell geothermal plant at Roosevelt Hot Springs. Cyrq Energy’s 10.5-megawatt geothermal field and a biogas facility are located 40 kilometers to the south.
Geological Studies
The area around the FORGE site has been the focus of numerous geoscientific studies over the last 40 years, starting with intensive geothermal exploration during the late 1970s. More than 80 shallow (less than 500 meters) and 20 deep (more than 500 meters) wells were drilled and logged, providing a very complete picture of the thermal structure of the region. The deepest well, Acord-1-26, reached a depth of 3.8 kilometers (11,811 feet). Recent geological mapping, 2 and 3-D seismic reflection, gravity and geochemical surveys, and the drilling, logging and testing of well 58-32 to 2,296 meters (7,536 feet) have added significantly to our understanding of the FORGE site.
The FORGE laboratory lies near the eastern boundary of the Basin and Range Province. The geology of the site and the adjacent Mineral Mountains is dominated by a composite Tertiary pluton composed of diorite, granodiorite, quartz monzonite, syenite, and granite. Development of the plutonic complex began at 25 Ma, and continued until 8 Ma.
The Tertiary plutonic rocks intrude Paleozoic and Mesozoic sedimentary sequences exposed at the northern and southern ends of the Mineral Mountains and a thin belt of Precambrian gneiss. Quaternary (less than 1 Ma) rhyolite lava flows originating from domes along the crest of the range partially cover the plutonic rocks. Temperatures near 250 degrees C in the Roosevelt Hot Springs geothermal reservoir suggest the presence of a magmatic heat source, presumably at a depth more than 5 kilometers.
Sedimentary and volcanic rocks filling the Milford basin beneath the FORGE footprint lie unconformably on the plutonic basement. The contact between the overlying gently dipping and undeformed alluvial deposits and the plutonic rocks is interpreted to represent a rotated and eroded Basin and Range fault dipping 20-30 degrees to the west. The most prominent of the younger Basin and Range structures is the Opal Mound Fault, which dips steeply to the east and offsets surficial deposits of alluvium and silica sinter, with a down-dip displacement of at least 15 meters. Temperature and pressure data demonstrate the Opal Mound Fault forms a hydraulic barrier separating the convective thermal regime of the Roosevelt Hot Springs geothermal system from the conductive thermal regime beneath the Utah FORGE site to the west.
The Utah FORGE reservoir will be created in the plutonic rocks where temperatures range from approximately 175 to 230 degrees Celsius at 2 to 4 kilometers depth. Three vertical wells have been drilled for reservoir characterization and seismic monitoring. The deepest, well 58-32, reached a depth of 2,291 meters GL (7,515 feet GL) and encountered a temperature of 199 degrees Celsius. Well 68-32 was drilled to 303 meters (994 feet) and well 78-32 to 998 meters (3,274 feet). A fourth monitoring well will be drilled to about 2,286 meters GL (7,500 feet GL) in 2020. Although image logs from well 58-32 suggest the presence of numerous fractures in the plutonic rocks, permeabilities are low, less than 30 microdarcies.
Two hydraulic injection tests were conducted in well 58-32. In 2017, a series of microhydraulic fracturing, and other diagnostic fracturing injection tests was performed to estimate stress magnitudes in the open hole section of the well. A maximum wellhead pressure of 27.6 megapascals (4,000 pounds per square inch gauge) was measured at an injection rate of about 1,431 liters per minute (9 barrels per minute). Repeat logging indicated significant enhancement of a drilling-induced vertical tensile fracture had occurred during the stimulation. A second series of injections was performed in 2019. The openhole tests were repeated and a region of critically stressed fractures were broken down and propagated in a perforated section of the cemented casing.
Future Research
In 2020, full deployment of the laboratory will be initiated. The centerpiece of the laboratory will consist of a production and injection well pair. The wells will be deviated 65 degrees from vertical and drilled to the east-south-east, perpendicular to the maximum horizontal stress. The injection well will be drilled first to a measured depth of approximately 3,333 meters (10,938 feet) and a true vertical depth of 2,603 meters (8,540 feet). The well will be cased, perforated and the toe of the well will be hydraulically stimulated using zonal isolation and stimulation techniques adapted from the oil and gas industry. A second well will be drilled to intersect the microseismic cloud recorded during the stimulation at the toe of the first well.
Seismicity in the region surrounding the Utah FORGE site has been monitored since 1981. There is no record of any events greater than magnitude 1.5 beneath the Utah FORGE site between 1981 and 2016. In late 2016, the network was upgraded to improve detection of the microseismic events. The current estimate of magnitude of detection is close to zero.
An extensive network of surface, shallow borehole and deep borehole instruments will be used to monitor microseismicity during the creation and growth of the Utah FORGE reservoir. Surface and shallow borehole instrumentation will include seismometers and strong motion detectors centrally located above the reservoir and in rings 3 and 8 kilometers from the center. During injection activities, the network will be augmented with a nodal array of seismometers. A broadband sensor and a geophone are deployed in well 68-32 at depths between 281 - 282 meters GL (921 - 924 feet GL).
Two of the monitoring wells, well 78-32 and well 56-32, will be instrumented with distributed acoustic sensing fiber-optic cables cemented behind casing. The DAS cable in well 78-32 extends from the surface to 994 meters GL (3,262 feet GL). The cable in 56-32 will extend to a depth of about 1,525 meters. Strings of high-temperature geophones will be deployed in wells 58-32, 78-32 and 56-32 during periods of stimulation and during flow testing between the injection and production wells. Analysis of the seismic data and faults surrounding the Utah FORGE site suggests the risk of induced seismicity and seismic hazards is low.
All of the data collected at Utah FORGE is available in the public domain through the Geothermal Data Repository at and the Utah FORGE website at UtahForge.com/data-dashboard.
Conclusions
The Utah FORGE site in central Utah is an ideal field laboratory for developing next-generation technologies capable of producing geothermal power from low permeability crystalline rock. The site is easily accessible all year, there are no limiting environmental constraints, temperatures suitable for enhanced geothermal system development can be reached at shallow depths of 2 to 4 kilometers, the risks of induced seismicity are low, and injection testing indicates the stress characteristics are suitable for reservoir development.
Acknowledgements
Funding for this work was provided by U.S. Department of Energy under grant DE-EE0007080 “Enhanced Geothermal System Concept Testing and Development at the Milford City, Utah FORGE Site.” We thank the many stakeholders who are supporting this project, including Smithfield, Utah School and Institutional Trust Lands Administration, and Beaver County. A grant from the Utah Governor’s Office of Energy Development has provided support for educational outreach activities. The Bureau of Land Management and the Utah State Engineer’s Office have been very helpful in guiding the project through the permitting processes. Gosia Skowron’s help preparing the figures and manuscript is greatly appreciated.
Editor’s Note: Other contributors to this article were: Stuart Simmons and Philip Wannamaker of the University of Utah Energy and Geoscience Institute, John McLennan of the University of Utah Department of Chemical Engineering, Kristine Pankow of the University of Utah Seismograph Stations, Robert Podgorney of the Idaho National Laboratory, and William Rickard of Geothermal Research Group.