Geothermal Case Study

The Geysers

The Geysers Geothermal Field

Calpine Corporation owns and operates The Geysers, the largest complex of geothermal power plants in the world. The Geysers include 45 square miles in the ­Mayacamas Mountains in northern California. The network contains 13 power plants, 321 steam wells, and 72 injection wells with a net generating capacity of about 725 megawatts of electricity (enough to power a city the size of San Francisco). The Geysers provide 9% of California’s green power.1,2

The Geysers Timeline

A million years ago 1,400˚F molten magma penetrated near the Earth’s surface and recrystallized the overlying rocks, making them hard and brittle and caused fracturing and permeability. Hot water geothermal reservoirs were formed over the next half-million years through continued high temperatures from magmatic activity and water seeping through fractures. The caprock fractured about 250,000 years ago, which allowed steam to erupt. The high temperatures from the eruptions caused water to boil down and form the current steam reservoir. The heat is close enough to the surface that the deep, fractured rock layers allow water to permeate through the system.3

Native Americans regarded the thermal areas for their healing powers and used them for ceremonial purposes. Tourists made it famous starting in the 1840s and continued to call it The Geysers, although no actual geysers are found at the site. Now it is the top producer of sustainable geothermal power by using steam from beneath the Earth’s surface.3

Geothermal Power Plant

A geothermal power plant contains a steam turbine, generator, condenser, cooling towers, gas removal system and hydrogen sulfide abatement system. Pressurized steam flows from wells, through pipelines, and to power plants producing thermal energy. At The Geysers the steam enters the turbine at 40-100 psi, and as it expands through the turbine, thermal energy is converted into mechanical shaft energy. The steam turbine is connected to the generator, which converts mechanical energy to electricity.1

Sonoma Plant at the Geysers
The Sonoma Calpine 3 geothermal power plant at The Geysers field in the Mayacamas Mountains of Sonoma County, Northern California

Sustainability of Geysers Power Generation

Only 20% of the expended steam at The Geysers can be cost-effectively condensed and put back into the ground. After the first thirty years of dry steam power production, the steam supply had depleted and generation was substantially reduced. To restore some of the former capacity, supplemental water injection was developed during the 1990s and 2000s, including utilization of effluent from nearby municipal sewage treatment facilities.4


During the 1970s and 1980s, more steam was produced from the reservoir than was replaced by the injection of power plant steam condensate. By 1989, steam pressure had decreased in the reservoir, resulting in lower steam production rates. To counter this, in 1990, The Geysers operators, Lake County and the California Energy Commission decided that Lake County wastewater would be injected to sustain reservoir pressure and steam production.5

Delivery of wastewater began in 1997. Today, the pipeline has been lengthened to 40 miles and now includes effluent from additional communities in the Clear Lake area. This Southeast Geysers Effluent Pipeline Project now delivers approximately 9 million gallons per day of secondary treated wastewater for injection into the reservoir.5 The project is a good example of a private/public partnerships to address the convergence of two problems: 1) the need for augmented injection to mitigate declining reservoir productivity at The Geysers and 2) the need for a new method of wastewater disposal for Lake County communities near The Geysers.6

An additional project begun in 2003, Santa Rosa Geysers Recharge Project, delivers an additional 11 million gallons per day of tertiary treated wastewater to replenish The Geysers’ geothermal reservoir.4 The project boosts The Geyser’s electrical output by 100 megawatts – enough to meet the energy needs of up to 100,000 households.7

Public Concerns

Pipeline Right-of-Way

Projects involving the movement of large volumes of water inevitably involve the construction of a pipeline. Construction for the Santa Rosa Geysers Recharge Project affected (passed through or near) approximately 624 parcels. Two hundred property owners provided right-of-way for the project to cross their parcels.7

Natural Earthquake Hazards

As with most projects, natural hazards have been assessed with mitigation measures in place. The City of Santa Rosa reports that there is a 70% chance that a magnitude 7.0 or greater earthquake could occur along the nearby Rodgers Creek/Hayward fault over the next 30 years. A quake of 5.5 or greater would cause eight 48-in automatic isolation valves (weighing 15 tons each) to close to prevent flooding in the unlikely occurrence of a pipeline rupture.5

Induced Seismicity

The Geysers are in a seismically active area, but over 99% of the seismic events are 3.0 magnitude or smaller. Calpine uses a vast network of seismometers to detect minute levels of micro-seismicity. Local reservoir temperatures or pressures at depth can cause slight changes in stress and produce detectable seismicity. Geoscientists and engineers record the magnitude, location, and depth of these seismic events to study how water injection and low magnitude seismicity are correlated. Though there is a direct correlation, the frequency of events greater than 3.0 magnitude has decreased since 1990.8

The Seismic Monitoring Advisory Committee was formed in 1998 and meets twice per year to provide the community with regular updates and information on the subject. The committee is appointed by the Lake County Board of Supervisors and administered by the Lake County Special Districts Department.9

To reduce the frequency and impact of these events, Calpine views the most promising approaches involve a more uniform distribution of water injection, reducing injection rates for certain areas, adding more low-rate injectors, and relocating water injections further away from residential areas.8 We talk more about this challenge in our lesson on Induced Seismicity.


1. Calpine Corporation. (n.d.). Welcome to The Geysers. Retrieved 12/30/2022 from

2. Calpine Corporation. (n.d.). Geysers by the numbers. Retrieved 12/30/2022 from

3. Calpine Corporation. (n.d.). The Geysers: A very special place. Retrieved 10/30/2020 from

4. Geothermal Power. (n.d.). In Wikipedia. Retrieved 11/6/2020 from

5. Calpine Corporation. (n.d.). The Water Story. Retrieved 12/30/2022 from

6. Dellinger, M. (n.d.). Southeast Geysers Effluent Pipeline Project. U.S. Department of Energy. Geothermal R&D Program. Retrieved 11/6/2020 from

7. City of Santa Rosa. (n.d.). Geysers Recharge Project- Facts and Stats. Retrieved 12/30/2022 from—Facts-and-Statistics-PDF.

8. Calpine Corporation. (n.d.). Seismicity at The Geysers. Retrieved 12/30/2022 from

9. Calpine Corporation. (n.d.). Seismic Monitoring Advisory Committee. Retrieved 12/30/2022 from

Images: “Power plant” by Calpine; “The Geysers Geothermal Field” by Julie Donnelly-Nolan; “Sonoma Plant at the Geysers” by Stepheng3, CC0, via Wikimedia Commons