An enhanced geothermal system is a man–made geothermal reservoir, created where there is hot rock but insufficient or little natural permeability or fluid saturation.
To create a man-made geothermal reservoir, fluid is injected into the subsurface under carefully controlled conditions, re-opening preexisting fractures, creating permeability and fluid saturation. With permeability and fluid saturation now in place with hot rock, the production of electricity is possible.
EGS could conceivably be created anywhere there is hot rock underground.
EGS emits little to no greenhouse gases beyond small amounts of water vapor. They also have the potential to fulfill the U.S. energy need for firm, baseload power, while being flexible enough to respond to demand. Because of the size of the resource, it is difficult to quantify the untapped potential beneath the Earth's surface. Currently, 100+ gigawatts of EGS power in the U.S. is a well-cited, near-term estimate based on work from the National Renewable Energy Laboratory and the U.S. Geological Survey.
Steps to Develop Power Production at an EGS Site
Step 1: Identify/Characterize a Site
- Develop a geologic model of a potential site via surface, geologic, geophysical, and remote sensing exploration.
- Assess the temperature gradient, permeability, in-situ stress directions of the resource, rock mechanical properties, and whether fluid is present.
- Determine if the necessary characteristics to create an EGS reservoir are present.
Step 2: Create a Reservoir
- Drill an injection well into hot rock with limited fluid content and/or permeability.
- Inject water at sufficient pressure (or temperature differential) to create a fracture network.
- Continue operation until there is enough fractured volume to create a reservoir (flow rate, temperature, volume, and sustainability).
- Drill a production well into the fracture network, intersecting the created flow paths.
- The resulting circulation loop allows water to flow through the enhanced reservoir, picking up in situ heat. The hot water is then pumped to the surface through the production well (see diagram below).
Step 3: Operate the Power Plant and Maintain the Reservoir
- At the surface, the water flashes to steam, or it heats a working fluid that produces vapor.
- The steam/vapor turns a turbine to create electricity.
- The original geothermal water is recycled into the reservoir
Want to learn more about how an EGS System works?
*This EGS animation is a Calpine Corporation and Lawrence Berkeley National Laboratory collaborative project co-funded by the U.S. Department of Energy. It was produced by Baker Hughes in 2012.