Draft:Brawley Geothermal Field

Brawley Known Geothermal Resource Areas

The Brawley Known Geothermal Resource Areas (KGRA), located approximately two miles north of Brawley in Imperial County, California, is one of the United States' notable geothermal energy developments. The field, originally explored in the 1970s, has undergone multiple phases of development and innovation. Despite significant early challenges related to scaling and corrosion, modern technologies have enabled its revitalization. Today, the Brawley field contributes to California’s renewable energy portfolio through the operations of Ormat Nevada Inc., which has successfully implemented a 50 MWe binary power plant.

Geological Background

The Brawley Geothermal Field is located within the Salton Trough, a tectonically active region formed by the spreading of the Earth’s crust. The Salton Trough contains a thick sequence of sediments, up to 15,000 feet, composed of marine and non-marine deposits. The geothermal resources in this area are attributed to the heat flow associated with the tectonic activity, which results in elevated temperature gradients. Deep magmatic activities elevate the geothermal gradient, with heat flow densities reaching more than twice the average value, providing favorable conditions for geothermal resource formation.

The Brawley field itself is underlain by a geothermal reservoir characterized by high temperatures and complex hydrothermal systems. Early exploration revealed deep reservoirs with temperatures reaching 525°F at depths of 5,000 to 7,000 feet. These reservoirs demonstrated exceptional productivity, with flow rates exceeding 1,000,000 pounds per minute at pressures of 450 psig. However, the fluid's salinity approximately 125,000 parts per million (ppm)—and its heavy metal content posed significant operational challenges.

Historical Development

Early Exploration and Challenges

The initial exploration of the Brawley Geothermal Field began in the 1970s, led by companies such as Union Oil Company (Unocal), Chevron, and Grace Geothermal Company. Temperature gradient wells and geophysical surveys identified promising geothermal anomalies. In 1982, Unocal, in partnership with Southern California Edison, constructed and operated a 10 MWe power plant. However, the project encountered severe issues with scaling and corrosion caused by the high salinity and heavy metal content of the geothermal fluids. Additionally, lease disputes exacerbated operational difficulties, leading Unocal to dismantle the power plant and abandon the project in 1985.

Revival by Ormat Nevada

In 2006, Ormat Nevada revisited the Brawley field, focusing on shallow sands instead of the deep, hypersaline reservoirs targeted by earlier developers. These shallow sands, with temperatures ranging from 300°F to 400°F and moderate salinity levels, offered a more manageable resource for geothermal energy production. By leveraging modern drilling techniques and advanced reservoir analysis, Ormat drilled and tested five wells in 2007, confirming the feasibility of constructing a binary power plant capable of producing 50 MWe of electricity.

Technological Innovations

Addressing Scaling and Corrosion

Ormat's development strategy involved a departure from traditional geothermal systems that relied on deep, highly saline reservoirs. By focusing on the shallow sands, the company avoided many of the scaling and corrosion issues that plagued earlier efforts. These sands exhibited matrix permeability with porosity values exceeding 25%, enabling fluid production through pumping methods similar to those employed at the East Mesa and Heber geothermal fields.

The fluids from these sands had significantly lower salinity levels, with Total Dissolved Solids (TDS) ranging between 15,400 and 18,600 mg/l, and heavy metal concentrations below 0.1 mg/l. This reduced the potential for equipment damage and operational interruptions, making the project more economically viable.

Binary Power Plant Design

Ormat adopted a binary power plant design, which transfers heat from the geothermal fluid to a secondary working fluid with a lower boiling point. This working fluid vaporizes and drives turbines to generate electricity. Unlike flash systems, binary systems do not require fluids with extremely high temperatures and avoid emissions of non-condensable gases, making them an environmentally friendly and efficient choice for moderate-temperature geothermal resources.

Well Design and Testing

The wells drilled by Ormat were completed to depths of up to 4,500 feet, with production intervals carefully selected based on electric wireline logs and porosity measurements. Each well demonstrated flowing temperatures between 315°F and 366°F and Productivity Indices (PI) ranging from 4.5 to 19.0 gallons per minute per psi. These results confirmed the commercial viability of the shallow Brawley reservoir.

Discussions

Operation Challenges

In 2010, Ormat Technologies' North Brawley plant faced significant operational hurdles due to high levels of undissolved solids in the geothermal fluids. These solids complicated the reinjection process, limiting the plant's output to 17 MW, well below its 50 MW capacity. The company implemented temporary measures to manage the solids and planned to install permanent equipment to enhance efficiency and output.

Concerns About Induced Seismicity

The Brawley Geothermal Field is in a tectonically active region within the Salton Trough, an area naturally prone to seismic activity due to its geological setting. The potential relationship between geothermal energy production and induced seismicity has been a subject of scientific interest and public discussion. While the region has experienced seismic events, including the 2012 earthquake swarm, some studies suggest that activities such as fluid injection may alter subsurface stress conditions, potentially influencing fault dynamics. However, it is important to note that such correlations remain under investigation. The ongoing research underscores the importance of careful monitoring and modeling to understand these complex interactions and mitigate potential risks.

Community Sounds

Local communities have engaged in discussions about balancing the economic benefits of geothermal development with environmental stewardship and public safety. The California Energy Commission emphasizes the importance of community involvement in geothermal project planning to ensure that development aligns with local interests and environmental considerations.

See Also

• Geothermal Energy
• Salton Sea Geothermal Field
• Imperial Valley
• Energy in California

References

• 1. Geothermal resource investigations, Imperial Valley, California. Status report. United States: N. p., 1971. Web.
• 2. Matlick, S., and T. Jayne. "Brawley—Resurrection of a previously developed geothermal field." GRC Transactions 32 (2008): 159-162.
• 3. Delph, J., et al. "The Salton Seismic Imaging Project: Seismic velocity structure of the Brawley Seismic Zone, Salton Buttes and Geothermal Field, Salton Trough, California." AGU Fall Meeting Abstracts. Vol. 2011. 2011.
• 4. Matek, Benjamin, and K. Gawell. "Report on the state of geothermal energy in California." February. Washington, DC: Geothermal Energy Association (2014).
• 5. Im, Kyungjae, and Jean-Philippe Avouac. "On the role of thermal stress and fluid pressure in triggering seismic and aseismic faulting at the Brawley Geothermal Field, California." Geothermics 97 (2021): 102238.
• 6. Materna, Kathryn, et al. "Detection of aseismic slip and poroelastic reservoir deformation at the North Brawley Geothermal Field from 2009 to 2019." Journal of Geophysical Research: Solid Earth 127.5 (2022): e2021JB023335.
• 7. Cheng, Feng, et al. "Using dark fiber and distributed acoustic sensing to characterize a geothermal system in the Imperial Valley, Southern California." Journal of Geophysical Research: Solid Earth 128.3 (2023): e2022JB025240.