Conventional geothermal systems are located in areas where high subsurface heat, permeable rock, and underground fluid all naturally coexist. These three conditions interact to create a natural underground heat exchanger (transferring heat from the rock to the moving fluids) that allows recovery of the earth’s energy (by drilling wells and producing hot water, steam, or both) to generate electricity. Nearly all geothermal power produced worldwide is supplied by conventional geothermal reservoirs.
By contrast, EGS are hot, but with low permeability and a low fluid content. Once an EGS heat source is located—typically in deep, hard rock— researchers drill deep wells and hydraulically stimulate the underground rock to increase permeability, thus creating a geothermal reservoir. Water injected into one or more wells passes through the zone of enhanced permeability, picking up heat along the way, and is extracted through a production well. After reaching the surface, the hot water and/or steam is used to produce power in the same way as in conventional geothermal systems.
The practice of manipulating pre-existing fractures in the subsurface to enhance permeability, which is key to EGS, is a subject of active research in the United States and other countries.