Raft River Geothermal Area
Geothermal Area Profile
|Exploration Region:||Northern Basin and Range Geothermal Region|
|GEA Development Phase:||Operational"Operational" is not in the list of possible values (Phase I - Resource Procurement and Identification, Phase II - Resource Exploration and Confirmation, Phase III - Permitting and Initial Development, Phase IV - Resource Production and Power Plant Construction) for this property.|
|Mean Reservoir Temp:||195°C468.15 K
|Estimated Reservoir Volume:||4.69 km³4,690,000,000 m³
|Mean Capacity:||13 MW13,000 kW
|USGS Mean Reservoir Temp:||145°C418.15 K
|USGS Estimated Reservoir Volume:||5 km³|||
|USGS Mean Capacity:||47 MW|||
The Raft River Geothermal Area is located in southern Idaho, approximately 200 miles southeast of Boise and 150 miles northwest of Salt Lake City near the Sawtooth National Forest (Figure 1). The field was initially explored and developed between 1974 and 1982 by the Energy Research and Development Administration (ERDA) and later the US Department of Energy (DOE) which was formed by joining the Federal Energy Administration and ERDA in 1977. The Raft River site was identified as an area of geothermal potential in the mid-20th century when ranchers discovered hot water while drilling irrigation wells. Beginning in 1974 and lasting until 1982, ERDA/DOE invested over $40 million in the Raft River demonstration project. Efforts during this period included: exploration, field development which consisted of drilling five production and two reinjection wells, emplacement of production infrastructure, construction of a 7 MW demonstration binary cycle power plant, and a direct-use system that supplied a fish farm and an orchid greenhouse with warm water. DOE’s Idaho National Laboratory, located approximately 50 miles from the site, administered the project during this time.
A major achievement of the project was the first demonstration of binary cycle geothermal technology in 1979 when its technical feasibility was proven. The binary power plant utilized isobutene as the binary fluid. At the time of first development, the field was not considered commercially viable due to adequate supplies of low cost power from other energy sources in the area. The binary plant was relocated to a proven geothermal field in Nevada where there was more opportunity for geothermal power production in the market. The seven wells drilled on the site were capped and the field was closed in 1982.
The Raft River site remained closed until 2002, when U.S. Geothermal Inc. acquired the Raft River property from Vulcan Power Company. U.S. Geothermal Inc. engaged GeothermEx, Inc. to analyze the field and estimate its production capacity. GeothermEx Inc. estimated a sustainable production capacity of 14-17 MW from the existing wells that were drilled during the initial development period from 1974-1982. They estimated a power production potential of up to 110 MW if the field was extended over U.S. Geothermal Inc.’s entire land holding of 8.2 square miles. U.S. Geothermal Inc.’s ultimate project plan was to develop 30 MW in two phases. First, U.S. Geothermal Inc. signed a 20-year, 10 MW power purchase agreement (PPA) with Idaho Power in January 2005 for output from Raft River Unit 1 (Phase I of development plans at Raft River). In May of 2005, it signed two additional 10 MW PPAs with Idaho Power for its planned $70 million Phase II development. In 2006, construction began on the 13 MW (net) capacity Raft River Unit 1 power plant that was completed and made operational by December, 2007. In January of 2008 a new PPA was approved with Idaho Power for the full 13 MW power output and Raft River Unit 1 began commercial operations (Figure 2). The two PPAs for the original Phase II development were mutually cancelled by Idaho Power and U.S. Geothermal Inc. in 2008.
History and Infrastructure
Operating Power Plants: 1
|Add a new Operating Power Plant|
Developing Power Projects: 2
|Add a new Developing Power Project|
Power Production Profile
|Gross Production Capacity:||11.5 MW11,500 kW
|Net Production Capacity:||10.5 MW10,500 kW
|Power Purchasers :|
The Raft River geothermal power plant that was successfully constructed and tested in 1979 was the first binary-cycle power plant of commercial-size in the US. This 7 MW (gross) dual-pressure demonstration plant used isobutane as the binary fluid and only produced electricity for several months on a test basis. The binary cycle technology, however, has since advanced to become the leading, proven technology for producing electrical power from moderate temperature geothermal resources in the world. As of 2005, there were 117 operating binary cycle units in the United States, which produced 193.8 megawatts of electrical power. Since this time, the number of binary cycle units has continued to increase. According to U.S. Geothermal Inc.: “The site is attractive because of the proven 300°F hot water resource that has been developed and tested, and because of the significant infrastructure facilities that are currently in place.” Geoscientific data collected from the Raft River Geothermal Area provides abundant evidence that confirms the existence of a large, moderate-temperature geothermal resource. The Raft River geothermal reservoir has a proven 135-146°C (275-300°F) hot water resource. The total flow rate is about 400 kg/s (6,320 gpm) of brine from fractured Precambrian basement rocks and immediately overlying fractured Tertiary sediments at relatively shallow depths of 1,500-2,000 meters (4,500 to 6,000 ft). The geothermal fluids from Raft River have salinities between 1,200 to 6,800 parts per million and low non-condensable gas content. The field contains nine deep geothermal wells to depths greater than 1,500 m. Of these nine wells, four are currently used for production (RRG-1, RRG-2, RRG-4, and RRG-7) and three are used as reinjection wells (RRG-3, RRG-6 and RRG-11). The Raft River geothermal field is currently the site of an Enhanced Geothermal System (EGS) demonstration project funded by DOE that aims to stimulate a sidetrack of well RRG-9 to increase permeability and flow capacity.
The site of the Raft River project was originally the location of a DOE geothermal demonstration project. Between 1974 and 1982, $40 million was invested in geothermal studies and production infrastructure at the demonstration site. More recently, the Raft River plant received $10 million in funding from DOE as part of the EGS grant program. See “Future Plans” below for more information.
1974: Idaho National Laboratory began research on geothermal resources in Cassia County Idaho, Snake River Plain.
1974-82: Raft River is the location of a DOE demonstration site with $40 million invested in geothermal studies and production infrastructure.
1980-82: Five production wells and two injection wells drilled; 4500-6000 ft total depth. Fluid temperatures between 275-300 °F.
1980-82: 7 MW (gross) dual pressure binary plant constructed and tested.
1982: DOE withdrew from project due to sub-economic cost of power. Sold and removed equipment which was sent to site in Nevada. Capped the producing and injection wells.
2002: U.S. Geothermal Inc. acquires the site and field assets of the Raft River Geothermal Project.
2004: U.S. Geothermal Inc. conducts flow tests and work overs of the existing wells with DOE funding support. Plans developed to deepen and upgrade existing wells and drill additional wells.
2006: U.S. Geothermal Inc. reentered well RRG-4, drilled sidetrack and completed in several fracture zones. Well RRG-3 deepened from 5937 to 6195 ft and new sidetrack drilled to 5735 ft. Both RRG-3 and RRG-4 flowed at more than 1000 gpm. Drilled new reinjection well.
2006: U.S. Geothermal Inc. and Goldman Sachs formed Raft River Energy I LLC. Raft River Energy I LLC built, owns, and operates the 13-MW Phase I project. U.S. Geothermal Inc. contributed $5 million and transferred ownership of the five production and two reinjection wells, as well as other geothermal rights and leases covering 1,800 of the 5,200 acres of rights now held, to Raft River Energy I. Goldman Sachs matched with the $34 million needed to construct Phase I of the project.
2006: Ormat began construction of 13 MW (Net) binary plant.
2007: The electrical interconnection completed without incident. A 3.2-mile 34.5-kV extension was built to connect the geothermal power with the Bonneville Power Administration Bridge Substation, where the voltage is kicked up to 138-kV transmission line voltage. The power is then delivered to Idaho Power Co. at its Minidoka Dam substation, approximately 45 miles away.
2007: U.S. Geothermal Inc. envisions Phase Two 26 MW unit by 2009 and Phase Three 50 MW by 2012.
2007: Raft River Field began generating approximately 10 MWe in January.
2008: Idaho Public Utilities Commission approved new PPA, nullifying the original 2005 PPA with Idaho Power.
2010: RRG-2 and RRG-7 wells shut down for repairs.
2011: RRG-7 back in service July 2nd.
2011: DOE Enhanced Geothermal System (EGS) demonstration project started on RRG-9.
2012: RRG-2 back in service January 6th.
Regulatory and Environmental Issues
While regulatory disagreements between U.S. Geothermal Inc. and Idaho Power required involvement of the Idaho Public Utilities Commission (PUC) before the PPAs were signed, other aspects of developing the RRG site, such as acquiring permits, went very smoothly.
Development at Raft River was delayed due to disputes between U.S. Geothermal Inc. and the local utility, Idaho Power. The disagreement rested on the “qualifying facilities” class distinction under the Public Utility Regulatory Policies Act (PURPA) of 1978. The law incentivizes “qualifying facilities” – smaller (10 MW or less), independent, and renewable power producers – by requiring utilities to buy power from them at a rate known as “avoided cost.” The avoided cost rate is based on how much it would cost the utility to produce the extra power. The qualifying facility can usually produce this power at a lower cost than the utility could have. PURPA also guarantees this favorable rate structure for 20 years, with yearly inflation adjustment increases. Idaho Power argued that, although the geothermal power plant was rated as a 10 MW plant, this would be the exact power production only for moderate temperature and humidity conditions. The plant could produce as much as 12.9 MW under more favorable ambient atmospheric conditions such as on cold winter days. U.S. Geothermal Inc. assumed that the definition of 10 MW of delivered power referred to the average over the year, but Idaho Power argued that the 10 MW referred to the delivered output for any given hour. In addition, Idaho Power wanted what is known as a firm “90/110 band,” ensuring that the amount of power delivered would be between 90 % and 110 % of the agreed upon 10 MW.
To settle these disputes the parties turned to the Idaho Public Utilities Commission (PUC) which heard the arguments and decided on a compromise: the three 10 MW generation plants planned at Raft River would be considered qualifying facilities, with the associated avoided cost rate, but they must produce 10 MW of power average for every month, and if they delivered power outside of the 90/110 band then the price would be either 85% of the wholesale market price or of the contract rate, whichever is less. With this new compromise, Idaho Power and U.S. Geothermal Inc. were able to sign the power purchase agreement and the development process continued. But due to the stringent regulation on power, U.S. Geothermal Inc. decided to alter their design to water-cooled condensers, allowing better control of power output, from the planned air-cooled system, which is more susceptible to ambient air conditions. This design change also forced U.S. Geothermal Inc. to negotiate for water rights needed for the new water-cooling towers.
There are many aspects of the Raft River Geothermal field that made the permitting, transmission leasing, and land purchases very quick and easy. Getting permission to build plants, roads, and transmission lines for any generating plant, geothermal included, can often take years, but for U.S. Geothermal Inc. it took only three or four months. The speed of the permitting decisions and public hearings was due largely to: 1) The existing DOE research development on the site (meaning little added “footprint” or competing uses for the land) 2) The support of local and national government policies for renewable energy 3) The need for electricity generation and job growth in the area 4) The fact that the company already owned the prime property to be developed 5) The relative remoteness of the area.
There was also a nearby 138-kVA (kilovolt-ampere) transmission line running along the northern boundary of the property and a substation—the Bridge Substation—only two miles from the planned Phase I plant. The line has the capacity to transmit 120 MW of power; Bonneville Power Administration (BPA) currently leases 60 MW, allowing for 60 MW of excess capacity. After completing a study on interconnection and transmission that demonstrated no technical problems, U.S. Geothermal Inc. was able to purchase 12 MW of point-to-point power transmission and reserve an additional 24 MW of capacity for Phase II of the project. These negotiations were particularly straightforward due to the BPA’s policy of encouraging renewable energy.
Raft River Unit II (26 MW) and Raft River Unit III (32 MW)
U.S. Geothermal Inc.’s Annual Report for the fiscal year ending March 31, 2012 states that the capital expenditure for the Raft River Unit II development is estimated to be $134 million and for Raft River Unit III it is estimate at $166 million. They anticipate that up to 75% of these expenditures may be funded by loans, with the remainder funded through equity financing. The PPA for Raft River Unit II with Eugene Water and Electric Board was terminated on May 16, 2011 since the Notice to Proceed had not been issued on or before the required date. Based on the annual report, both the projected commercial operation date and the power purchaser for both Raft River Unit II and Raft River Unit III are still to be determined.
Enhanced Geothermal System Demonstration
The EGS demonstration project team, with the Energy & Geoscience Institute at the University of Utah as program lead, along with U.S. Geothermal Inc., Geothermal Resources Group, APEX Petroleum Engineering Services and HiPoint Reservoir Imaging, received a $7.39 grant from DOE in September, 2009 that was announced in October, 2008. The total EGS program cost is up to $10.21 million including the DOE cost share. The project will use one of Raft River’s existing Phase II production wells (RRG-9), which is not currently in adequate contact with the hydrothermal system, to test thermal and hydraulic fracturing of the Precambrian reservoir rocks 6,000 feet below the surface. This well is an ideal candidate for the thermal and hydraulic stimulation demonstration project for multiple reasons. Due to difficulties recompleting well RRG-9, it was sidetracked and a new leg was drilled to 5932 MD.
The first phase of the program that started early in 2010, measured and modeled in-place characteristics of the reservoir rock, including borehole imaging, and fracture analysis. This phase also included construction of injection pipelines and installation of a liner in the wellbore to prepare the well for the first simulation using “cold” water injection. The first two simulations will use water of different temperatures to pre-condition and thermally fracture the reservoir. It is anticipated that some of these tensile thermal fractures will intersect existing weak areas, which will fail during the next high rate, large volume conventional stimulation. Thus it may be possible to access a larger volume of the target region by taking advantage of the thermoelastic stress alteration. Microseismic activity and pressure transient evaluations will be used to monitor the effects of each stimulation stage on the fracture volume and interconnectivity. Lawrence Berkeley National Labs is monitoring induced seismicity at the field. In the end, the goal of this project is to develop and demonstrate the techniques needed to form, sustain, and monitor EGS reservoirs. Furthermore, there is hope that this work will help improve the performance of the Raft River geothermal field by improving production or injectivity from the sidetrack of RRG-9.
First Discovery Well
|Initial Flow Rate:||
|Flow Test Comment:|
The Raft River site was identified as an area of geothermal potential in the 1950s when the Bridge and Crank wells that were drilled for irrigation purposes encountered boiling water. Altered alluvium near the Bridge well indicates a former surface hot spring. There are no tufa or sinter mounds in the area. On the basis of the hot water wells, the US Geological Survey (USGS) designated the Raft River area a Known Geothermal Resource Area (KGRA) in 1971. The boiling wells and geochemical thermometry suggested temperatures at depth exceeding 150°C, very close to the actual temperature measured in the RRGE-1 discovery well.
ERDA began an exploration program in the area in 1973. Under this effort, 84 wells were drilled by DOE at Raft River, including 34 auger holes to 30 m (100 ft) depth, 5 core holes from 80-400 m (250-1,423 ft) depth, 7 monitoring wells from 150-450 m (500-1,300 ft) depth, and the 7 full diameter deep wells in excess of 1,500 m (5,000 ft) wells that delineated the reservoir and ultimately produced fluids for the binary demonstration plant.
The USGS conducted geophysical surveys over the Raft River area in the early 1970s, including gravity, aeromagnetics, seismic refraction, magnetotellurics (MT), DC resistivity, and self potential (SP) surveys. These surveys indicated the presence of about 2,000 m (6,500 ft) of Cenozoic sedimentary and volcanic rocks underlain by Precambrian basement. The gravity, seismic refraction and DC resistivity surveys revealed the gross structure of the Raft River basin, its Cenozoic sedimentary fill, and the location of major faults. Magnetic anomalies indicated the distribution of volcanic rocks (Figure 3). The SP survey provided clues to near surface circulation of geothermal waters along faults. However, the geophysical surveys, individually and together, did not yield a method to directly detect thermal waters.
From surface water sampling, it was determined that occurrences of thermal waters above 100°C are located at the intersection of north-trending Bridge normal fault with the east-to-northeast trending Narrows Zone fault. The first deep exploratory well, RRG-1, was drilled at this fault intersection and targeted the Bridge fault zone at ~1,400 m (4,600 ft). RRG-1 encountered the Bridge fault zone at 1,240-1,320 m (4,070-4,330 ft), which flowed 40 kg/s at subsurface temperatures of 140°C. Core analysis indicates that the productive fractured reservoir of the Raft River geothermal field is lithologically complex and includes fractured quartz monzonite, schist, quartzite and siltstone in several formations near the sediment/basement interface. RRG-1 encountered Precambrian basement rocks at 1,390 m and was completed at total depth of 1,526 m (5,000 ft). 
Tracer tests have also been conducted the Raft River Geothermal Field between injection well RRG-5 and production wells RRG-1 which is at a distance of 790 m (2,591 ft) and RRG- 4 at a distance of 740 m (2,497 ft) to understand fluid movement in the reservoir. Tracer arrival data from these tracer tests indicate a highly fractured reservoir consistent with televiwer logs and core data. Also, seismic activity in the area was measured before production, during the testing phase, and once production began. There was little seismic activity prior to production at the Raft River geothermal field. A small peak in seismic activity was observed during the testing phase and an increase was seen in seismic activity during fluid production. Seismic activity is a strong indicator of induced seismicity present at the Raft River geothermal system. The seismic activity is interpreted to be related to local events because the wave forms contain high frequencies. There is currently insufficient data to determine the exact source locations of these events.
Well Field Description
Well Field Information
|Number of Production Wells:||5|
|Number of Injection Wells:||4|
|Number of Replacement Wells:|
|Average Temperature of Geofluid:||270 140°C413.15 K
|Sanyal Classification (Wellhead):||Very Low Temperature|
|Reservoir Temp (Geothermometry):|
|Reservoir Temp (Measured):||195°C468.15 K
|Sanyal Classification (Reservoir):||Moderate Temperature|
|Depth to Top of Reservoir:||1400m1.4 km
|Depth to Bottom of Reservoir:||1750m1.75 km
|Average Depth to Reservoir:||1575m1.575 km
At the Raft River geothermal field, five deep production wells and two intermediate depth injection wells were drilled between 1975 and 1978. Since U.S. Geothermal acquired the lease, two more wells were drilled. All of the production wells penetrate the Cenozoic sediments and reach their total depth in the Precambrian basement. The injection wells were completed in Salt Lake Formation sediments. The intention was to reinject in the shallower horizon to avoid temperature drawdown of the deeper hydrothermal resource.
- RRGE-1 was one of several studied by the Idaho National Engineering Laboratory (INEL) and the United States Geological Survey (USGS) in 1974. The RRGE-1 well was designed to confirm the presence of a commercial hydrothermal system suitable for power production. RRGE-1 was completed at a total depth of about 1,521 m (4,990 ft) on April 1, 1975. Limited flow testing of the zone 1,128-1,372 m (3,700-4,500 ft) resulted in a rate of approximately 37.8 kg/s at a down hole fluid temperature of 150°C.
- RRGE-2 was located to confirm deep geothermal circulation and to confirm the hydrothermal resource discovered in RRGE-1. Drilling operations were started in late April 1975 and were suspended at a depth of 1,825 m (5,988 ft) on June 26, 1975. Flow testing of the interval 1,295-1,463 m (4,249-4,800 ft) resulted in 50.4 kg/s with maximum down hole temperature of 147°C. RRGE-2 was later deepened 169 m to a total depth of 1,994 m.
- RRGE-3 was spudded on March 28, 1976 and has three legs drilled to depths between 1675 m and 1830 m (5,500-6,000 ft).
- RRGI-4 was drilled as an injection well but was later re-designated as production well RRGP-4. RRGP-4 was a poor producer and became a reservoir monitoring well. In August 1979, well RRGP-4 was cased to 1,433 m (4,701 ft) and hydraulically fractured. The hydraulic stimulation was the first such treatment of a geothermal well in the world. The stimulation consisted of pumping 1.276 million liters of treated water and 49,169 kg sand at rates of up to 137.8 liters per second. After the treatment, the well was tested at flowing rates of 3.8-14.2 liters per second.
- RRGP-5 was designed as a production well with three barefoot legs. Only one leg was completed as a production interval. RRGP-5 was hydraulically fractured in November 1979.
- RRGI-6 and RRGI-7 are injection wells into an intermediate depth zone in the top 1,000 m (3,281 ft).
Two Raft River Field wells were repaired in 2010 and 2011. Production well RRG-2 was shut down due to pump failure. RRG-7 required a cement squeeze when cooler fluids entered the well bore through a cement leak and the flowing temperature dropped from 148° C to 115° C (299°F to 240°F).
Technical Problems and Solutions
Two geothermal wells at Raft River required repairs in the period between June 2010 and May 2011. Due to a pump failure, production well RRG-2 was shut down. The 13 3/8" steel production casing was removed and replaced from 585 MD in the well back up to surface, in order to repair a separation in the casing that prevented installation of the production pump. RRG-2 returned to production on January 6, 2012. A leak in a cement seal of well RRG-7 allowed cooler geothermal fluid to enter the well bore. This failure caused the flowing production temperature to drop from 299°F down to 240°F. RRG-7 was placed back in service on July 2, 2011 upon completion of a “squeeze” job, where grout was injected directly into the leaking lap joint which repaired the leak.
Geology of the Area
|Tectonic Setting:||Extensional Tectonics|
|Controlling Structure:||Fault Intersection|||
|Topographic Features:||Horst and Graben|
|Brophy Model:||Type E: Extensional Tectonic, Fault-Controlled Resource|
|Moeck-Beardsmore Play Type:||CV-3: Extensional Domain|
|Modern Geothermal Features:||Blind Geothermal System|||
|Relict Geothermal Features:|
|Host Rock Age:||1- Archean; 2-Proterozoic|||
|Host Rock Lithology:||1- quartz monzonite; 2-schist; quartzite|||
|Cap Rock Age:|
|Cap Rock Lithology:|
The Raft River Valley lies near the geologic province boundary between the Basin and Range and south of the Snake River Plain. Figure 4 shows the location of the thermal anomaly area within the Raft River Valley. The Raft River Valley is about 60 km long and 20 km wide, and lies at an elevation of approximately 1,400 m (4,600 ft) above sea level. The geology of the valley is complex reflecting the combined influences of the Basin and Range and Snake River geologic terrains.
The Raft River Valley is a north–trending, Cenozoic, fault-bounded, downward-filled valley with Tertiary and Paleozoic sediments, metasediments, and volcanics that overlie Precambrian basement rocks. It is bordered on the west by the Jim Sage and Cotterel Mountains that expose Tertiary rhyolitic volcanic rocks and volcaniclastic sediments. The Black Pine and Sublett Mountains to the east are composed of allochthonous Paleozoic rocks. To the south, in the Raft River Range, lies a Precambrian autochthonous gneiss-dome complex consisting of quartz monzonite overlain by metasediments. There are indications such as overturned folds and local imbrications that the allochthonous sheets were transported westward and northward during two instances of metamorphic deformation potentially caused by gravity acting on a broadly heated dome. Also, there are indications that they were transported as much as 30 km eastward after the period of metamorphism ended. The absence of Paleozoic rocks beneath the Raft River Valley has been attributed to tectonic denudation prior to the Cenozoic basin fill.
Surface geologic mapping and geophysics indicate that the prominent structure in the Raft River Valley is the Narrows structure. The Narrows fault, which appears to have right lateral displacement, passes through the Raft River Field and is likely to provide significant structural control of the hydrothermal system (Figure 5).
The Narrows Fault has been interpreted as a near-vertical detachment surface and as a right-lateral, strike-slip basement shear. The Narrows structure is interpreted to offset the Precambrian basement rocks. The western side of the Raft River Valley has been downdropped along the listric Bridge and Horse Wells fault zones, which dip 80° east at the surface, sole out in the Tertiary sediments, and apparently do not displace the basement rocks. The Bridge and Horse Wells Faults terminate at the Narrows Fault, and are thought to play a major role in the Raft River geothermal system by creating highly permeable vertical fractures in the base of the Tertiary rocks and underlying metasediments that provide permeability for upflow of hydrothermal fluids. Movement on the faults in the Raft River area is as recent as post middle Pleistocene, several thousand years before present.
Raft River Formation
The Raft River formation is made up of non-indurated Pleistocene deposits consisting of conglomerates, sandstones and siltstones composed of angular, poorly-sorted lithic clasts. Lithic clasts include quartzites, granites, lava flows, ash-flow tuffs and fragments of calcite. The Raft River Formation reaches thicknesses of up to 300 m and overlies the Salt Lake Formation and (980 ft). The Raft Formation - Salt Lake Formation contact is gradational and difficult to distinguish. A cross section of a portion of the Raft River Geothermal field is shown in Figure 6.
Salt Lake Formation
Mid-Tertiary rocks of the Salt Lake Formation comprise a thick, up to 1600 m (5,250 ft), sequence of tuffaceous sedimentary rocks of largely fluviatile and possibly lacustrine origin. Fine grained tuffaceous lithologies predominate in the section with conglomerate as a minor rock type. Siltstone and sandstone of the Salt Lake Formation have been deformed by numerous high angle microfaults and bedding convolutions. Bedding is commonly inclined 10 to 30°. The Salt Lake Formation has been subdivided into three members: an Upper Tuffaceous Member, the Jim Sage Volcanic Member, and the Lower Tuffaceous Member.
Precambrian rocks penetrated by deep test wells in the Raft River geothermal resource area are comprised of Quartz Monzonite basement overlain by a series of schists and quartzites. The Quartz Monzonite is overlain locally by the Lower Narrows Schist which is also referred to as the Older Schist. The Lower Narrows Schist is a discontinuous biotite-chlorite-muscovite-rich quartz schist. The Elba Quartzite, a muscovite bearing quartzite, rests locally upon the Lower Narrows Schist. Where the Lower Narrows Schist is absent, the Elba Quartzite rests directly upon the Quartz Monzonite. The Upper Narrows Schist, a biotite-muscovite-quartz schist, and the discontinuous Quartzite of Yost overlies the Elba Quartzite.
The likely mechanism for the Raft River hydrothermal system is deep circulation of groundwater and upwelling of heated water along faults in a region of relatively high heat flow from conduction with an absence of an igneous intrusive heat source. Raft River waters contain only crustal helium, indicating no active volcanic sources. Tritium analysis of the geothermal reservoir fluids suggests the fluid is at least 60 to 70 years old.
The hydrologic recharge of the fractured crystalline basement reservoir rocks probably occurs where they outcrop in the Albion- Raft River Range. Before development of the geothermal reservoir, static water levels were interpreted to be approximately 100 m above the surface elevation. This helps explain the leakage of the underlying geothermal water along the faults in the Salt Lake Formation and the heating of the aquifers encountered in shallow water wells.
Conceptual models for the recharge of the system, such as one depicted in Figure 7, include vertical recharge through fractures in the quartz monzonite basement rocks and horizontal recharge from the Jim Sage Mountains through faulted metamorphic rocks overlying the quartz monzonite.  More recent modeling of the Raft River reservoir indicates high permeability zones related to fault intersections in the metasediments, and a combination of lateral and horizontal recharge. Work done by Dolenc and others (1981) indicate that the geothermal resource produces from fractures that are found in a zone near the top of the metamorphic basements. This zone is near the intersection of the Narrows Zone and the Bridge Fault Zone. Anomalous hot productive zones were found in many shallow wells. These hot zones originate from upward leakage and lateral spreading of the hydrothermal fluid through fractures in the Narrows Zone, the Bridge Fault Zone and possibly fractures in the Salt Lake Formation. In general, the sediments in the Salt Lake Formation have relatively low permeability and porosity. This indicates that the hydrothermal fluids are produced from zones where fractures exist. The well testing undertaken at Raft River in the 1970s showed that the general anisotropy of the geothermal reservoir and that the major axis of hydraulic conductivity follows the northeast-southwest direction of the Bridge Fault Zone. The complexity of the reservoir is typical of Basin and Range systems. No distinct reservoir boundaries have been determined from well testing, but thermal boundaries exist where the upward flow of hot water through fractures does not occur. There may also be areas of lower hydraulic conductivity that are caused by the anisotropic nature of the fractured system.
Some researchers have suggested a significant contribution to hydrothermal flow from fractures in the Salt Lake Formation sedimentary strata overlying the basement rocks. The most intense fracturing is located at the base of the Salt Lake Formation at the listric detachment between the Cenozoic rocks and the underlying Paleozoic and Precambian rocks.
Geothermal fluids in the Raft River Valley are most likely derived from two sources of water. Meteoric water from the northwest is heated through conduction from the earth’s core at depths on the order of 2,000 m and then moves upward along the Bridge Fault Zone to the surface. Water from the central valley region is drawn into the valley and then through convection rises upwards to the productive zone. The geothermal fluid heated by these two processes is found at depths from 1,524-1,829 m (5,000-6,500 ft), and is known to have temperatures ranging from 135-146°C (275-295°F). Figure 8 shows the temperature distribution within the geothermal field with the highest temperatures reaching up to 320 °F. The geothermal reservoir consists of fractured Precambrian schist, quartzite, and monzonite which are cut by younger diabase intrusions. High fluid flow rates of over 40 kg/s from these basement rocks suggest an open fracture system. Open fractures have been identified on borehole televiewer logs and core samples taken from drilling. The listric fault and fracture conduits are believed to increase in frequency with depth.
Clay mineralogy in the Raft River system shows temperature with depth relationships common to other liquid-dominated hydrothermal systems (Figure 9). Throughout the thickness of the Tertiary Salt Lake Formation there is kaolinite and illite. Montmorillonite which is seen in shallower layers is replaced by mixed layer clays (corrensite). Finally, chlorite is found deeper, directly above the Upper Narrows Schist. In hydrothermal systems montmorillonite and kaolinite tend to occur in lower temperature zones up to 180°C (356°F), mixed layer clays are found between 150-220°C (302-428°F), and chlorite occurs in higher temperature zones at temperatures greater than 200°C (392°F). The clay mineralogical data for the Raft River system suggest that temperatures may have been higher in the past than presently observed.
NEPA-Related Analyses (1)
Below is a list of NEPA-related analyses that have been conducted in the area - and logged on OpenEI. To add an additional NEPA-related analysis, see the NEPA Database.
|DOI-BLM-ID-220-2009-EA-3709||EA||Agua Caliente, LLC||12 February 2009||BLM||Geothermal/Exploration||Exploration Drilling|
Exploration Activities (77)
Below is a list of Exploration that have been conducted in the area - and cataloged on OpenEI. Add a new Exploration Activity
- Jacob Bradford, John McLennan, Joseph Moore, Douglas Glasby, Douglas Waters, Richard Kruwell, Alan Bailey, William Rickard, Kevin Bloomfield6, and Dennis King. 2013. Recent developments at the raft river geothermal field. Proceedings of thirty-eighth workshop on geothermal reservoir engineering; Stanford University: thirty-eighth workshop on geothermal reservoir engineering.
- U.S. Geological Survey. 2008. Assessment of Moderate- and High-Temperature Geothermal Resources of the United States. USA: U.S. Geological Survey. Report No.: Fact Sheet 2008-3082.
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List of existing Geothermal Resource Areas.