An Integrated Approach to a Geothermal Resource Assessment
Geothermal resource studies focused on the Imperial Valley of California indicate that there is plenty of room to expand production of a clean, renewable, and abundant energy source.
Global energy use during the past 20 years has increased 70 percent and is expected to climb as developing countries continue to grow and industrialize. Such increasing demand and high consumption rates can threaten U.S. access to energy and raw materials, with potential implications for national security. Developing renewable energy from natural sources within the United States can reduce that risk.
In California, interest in geothermal resources has been growing because of the state’s mandate to generate 33 percent of its power from renewable sources by the end of 2020 and the availability of federal funds for energy sector renewable infrastructure. As a result, the Imperial Valley of Southern California has become the focus of investigation for renewable energy and the target of numerous solar and geothermal energy projects.
The hot and arid Imperial Valley produces more than $1 billion in crops annually. These crops are fed by the Colorado River, which is routed to the valley through the All American Canal. In 1936, after discovering the potential for low-cost hydroelectric energy from the canal’s five waterfalls, the Imperial Irrigation District joined the power industry, becoming the sixth largest utility in California.
Seeking to expand its service and capacity, the Imperial Irrigation District asked The Aerospace Corporation to provide an assessment of its lands near geothermal areas of interest. The goal was to help establish a cost-effective development strategy for locating and tapping sources of geothermal energy. Working from multiple sources, Aerospace compiled information on surface and subsurface geology and organized the geospatial data into a Geographic Information System, which the district now uses as the basis for its exploration and land-management strategy.
Since the early twentieth century, geothermal heat has been exploited to produce energy. Geothermal power plants operate in nine states: Alaska, California, Hawaii, Idaho, Nevada, Utah, New Mexico, Oregon, and Wyoming, and produce approximately 3,102 megawatts—or enough energy to power 2.4 million homes. A recent U. S. Geological Survey assessment of the nation’s geothermal resources found that:
- The mean estimated power from undiscovered geothermal resources could provide an additional 30,033 megawatts.
- Another 517,800 megawatts could be generated from enhanced geothermal systems by fracturing and stimulating hydrothermal reservoirs.
Typically, geothermal prospectors seek three critical elements—heat, a heat-transfer medium (usually water), and a permeable reservoir that can produce hot water. Earth’s temperature increases with depth from the surface, and the change in temperature per unit distance is called the geothermal gradient. When the geothermal gradient is high and there is sufficient water to transfer the heat, energy can be generated by drilling and producing hot water.
This water is usually brine, having high concentrations of total dissolved solids. The hot brines can be brought to the surface, where they flash (burst into steam) and drive a turbine to generate electricity—this is called a flash steam plant. If the brine is insufficiently hot, the heat can be transferred to a secondary liquid that can flash at a lower temperature—this is known as a binary plant. The brine can be reinjected into the reservoir to ensure that the resource is sustainable and that surface water, aquifers, and soils are not contaminated. (see sidebar, Why Geothermal Energy?)
The Salton Trough
The Imperial Valley lies within an area known as the Salton Trough, a topographic and structural feature that extends from southeastern California into Mexico. The area is located in a complex tectonic environment where the North American plate transitions from a transform boundary (an area where Earth’s plates slide in opposite directions—e.g., the San Andreas fault) to an extensional boundary (an area where Earth’s plates spread apart from each other, allowing new crust to form between them, e.g., the East Pacific Rise in the Gulf of California).
The Salton Trough, which is about 130 miles long and up to 70 miles wide, is a landward extension of the Gulf of California, which began forming by seafloor spreading approximately four million years ago. In fact, the trough was once part of the Gulf of California. The Salton Sea now occupies the lowest part of the trough, which sits more than 200 feet below sea level.
Within the Salton Trough, several geothermal fields (designated as known geothermal resource areas) situated over local zones of extension allow magmas from Earth’s mantle to intrude and heat up the sedimentary strata. Reservoir temperatures within these zones are often greater than 260°C (500°F).
Although some geothermal exploration occurred between 1927 and 1954, the actual emergence of geothermal energy within the Imperial Valley began in the 1960s and 1970s with the drilling of several wells leading to private generating plants in the early 1980s. The first commercial geothermal well was commissioned on January 1, 1964, near Niland, California.
Twenty geothermal energy plants from five geothermal fields currently produce hot brine, which flows to the surface and powers electrical generators. As a result, the Imperial Valley receives more than 617 megawatts of geothermal electricity—enough energy to power more than 600,000 homes.
Aerospace and the Imperial Irrigation District
Working closely with the Imperial Irrigation District, Aerospace formed an assessment team with Clear Creek Associates, a groundwater consulting firm based in Scottsdale, Arizona. The team’s ability to assimilate surface and subsurface data from multiple sources and generate a three-dimensional interpretation of the hydrothermal system provided the basis for the district’s geothermal energy exploration strategy.
Each township (6 × 6 miles) was divided into 36 cells, with each cell covering a square mile. The cells were ranked on a scale of 2 to 31, with the higher scores having the greatest geothermal potential. The scores were based on four criteria, arranged according to priority: surface manifestations (garnered from satellite and airborne remote sensing data and field observations), Bouguer gravity data, geothermal gradients, and presence within a known geothermal resource area. Resistivity data were also considered in the scoring.
Surface Data—Airborne and Satellite Imagery
A linear string of mud pots and mud volcanoes on the southeastern coast of the Salton Sea displays evidence for a southern extension of the San Andreas Fault that runs through the Salton Sea. The mud volcanoes are cones of mud built up by viscous hot mud that bubbles up through vents. Mud pots occur in the same area as mud volcanoes, but they are enclosed basins and appear as depressions with bubbling hot muddy water. Both geothermal features result when water and gas are forced upward through sediments. These features are strongly associated with volcanic and seismic activity, active plate boundaries, and hot spots-key indicators for subsurface hydrothermal systems.
Other visible signs of active hydrothermal systems are the fumaroles or gas vents throughout the Salton Trough area. These fumaroles are the result of hot gases (primarily carbon dioxide) migrating to the surface by way of active faults and fractures. A series of small fumaroles are also located near the eastern edge of the Salton Sea—a marsh area that has become a bird sanctuary. These fumaroles were observed in the field as continuous bubbles emerging from the shallow waters of the marsh. Many similar geothermal indicators lie under the shallow waters of the Salton Sea and are thus surveyed only with difficulty; however, airborne hyper- spectral thermal-infrared imagery provides a means for readily locating and monitoring such features.
Aerospace provided airborne hyperspectral data and analysis over a portion of the district’s area of interest using the SEBASS (Spatially Enhanced Broadband Array Spectro- graph System) hyperspectral sensor. Aerospace captured evidence of a large fumarole field within the Mullet Island Thermal Anomaly at the edge of the Salton Sea. Some of these fumaroles exhibited core temperatures of more than 60°C and were well resolved by the SEBASS sensor. The team also assembled available imagery from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), an instrument aboard a satellite in NASA’s Earth Observing System. ASTER is a multispectral system and captures imagery in 14 bands from visible to thermal infrared wavelengths. In general, the ASTER thermal maps indicate that surface temperatures reflect general land-use patterns. Further investigation is needed over areas with little human interference to determine the utility of ASTER data for detecting thermal anomalies associated with subsurface hydrothermal systems.
Aerospace also used the SEBASS hyperspectral data to identify minerals, which could indicate previous geothermal activity; however, most of the minerals seem to be associated with human-made features—for example, roads, berms, and agricultural tracts. Moreover, surface drainage and shallow sea deposits influence the distribution of minerals. Because of these significant and natural surface-level disturbances, garnering as much information as possible from subsurface indicators is critical.
Subsurface Data—Bouguer Gravity, Geothermal Gradients, and Resistivity
Clear Creek Associates compiled data using available reports, files, maps, well logs, and other geologic and hydro- geologic information to evaluate geothermal potentials of land owned by the district. Bouguer gravity data were obtained from Shawn Biehler at the University of California-Riverside. High Bouguer gravity values have been shown to be associated with geothermal resources in the Salton Trough and may be related to upwelling of denser materials from Earth’s mantle; low values are associated with thicker crust that does not allow for heat transfer. Clear Creek Associates noted that the lower land elevations in the Salton trough and the thinner crust is indicative of dense, oceanic style crust—perhaps associated with mantle upwelling and emplacement of intrusive rocks. Such movement was consistent with the tectonic setting of the Salton Trough as an area with both rifting (pulling apart) and transform (moving horizontally) faulting.
Geothermal gradients for approximately 200 wells in the areas were calculated using data from the California Department of Conservation. The relative thermal gradients were recorded, scored, and arranged according to priority within a matrix to evaluate the geothermal resource potential of the area. Typically throughout Earth, the geothermal gradient is 25–30°C per kilometer of depth; however, the Salton Sea is characterized by unusually high geothermal gradients, in excess of 200°C per kilometer. For the matrix scoring, the team gave greater weight to high geothermal gradients.
The geothermal assessment team also acquired and incorporated resistivity survey data, which became critical input for the delineation of geothermal resource areas. Resistivity surveys have been the traditional method of exploring for geothermal resources in other areas, but this method has been secondary to gravity surveys in the Salton Trough. The resistivity survey is a geophysical measurement to locate subsurface features by mapping the way Earth conducts an electrical current. The principle is that resistivity of solution-saturated rocks will decrease as the salinity of the solution increases. Salinity generally increases with temperature, so resistivity generally decreases (or conductivity increases) as temperature rises. A particular type of resistivity survey, natural source magnetotelluric survey, has been successful in identifying hot, briny water that is more conductive than the background water. Schlumberger Ltd., an oilfield services company, conducted a magnetotelluric survey in 2009 and completed four resistivity lines over the area of interest on behalf of the California Energy Commission. The resistivity survey found that the upper three kilometers consisted of three layers: a thin (300–600 meters) surface cap of approximately 1 ohm meter (i.e., cap rock); a low-resistance layer that is extremely conductive at approximately 0.2–1.0 ohm meter (i.e., hot brine reservoir); and a relatively resistive basement at greater than 1.0 ohm meter (i.e., dense deep basement rock).
The conductive second layer appears to be the reservoir rock for supersaline thermal fluids, while the thin upper layer is composed of surficial trough sediments. The resistivity data further helped define the depth and geometry of the hot supersaline reservoir.
Geographic Information System Integration
The geothermal assessment team integrated the multimodal geospatial information (satellite, airborne, resistivity, and gravity data as well as field observations and thermal gradients) into the Geographic Information System. The team created a matrix that arranged in priority the weighted values for surface and subsurface survey data. The weighted values were assigned based upon the relevancy and accuracy of the data. This priority-coded matrix has become the basis for the Imperial Irrigation District’s land management and geothermal exploration and drilling strategy.
The team also worked with the district to develop a priority matrix that provided weighted factors for various data inputs and cumulative scores. The district was then able to rank its land parcels and develop land management and drilling strategies as part of its geothermal exploration plan. For instance, high scores could indicate a drillable prospect, and low scores would indicate areas that the district might want to lease or farm out. (see sidebar, Utility-Scale Geothermal Production at Military Bases)
The Aerospace Corporation is in a unique position to apply its natural-resource-assessment technologies and expertise to evaluate the feasibility and value of geothermal energy resources. Beyond the priority matrix and geothermal assessment of the Imperial Irrigation District’s lands, this project demonstrated the effectiveness of developing a geological understanding of a region by compiling independent surface and subsurface data from a variety of sources. Multiple layers of geospatial data in a geographic information system can be integrated, analyzed, and prioritized to provide actionable information to decision makers. Aerospace maps and priority matrices were integrated with existing data to allow the district to continue to manage, analyze, and work with the information in the future.
The author would like to thank Clear Creek Associates; Patrick Johnson, Stephen Young, and David Tratt of Aerospace; Shawn Biehler; The California Division of Oil and Gas and Geothermal Resources; and Schlumberger.
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