Drilling, Fracturing, and Characterizing Enhanced Geothermal Systems

New technology is making it possible to build geothermal energy systems in locations that previously lacked accessible and extractable natural heat sources.

The Earth has the capacity to provide an unlimited supply of heat for electricity generation, heating, and cooling. However, accessing this heat in a way that provides sufficient energy to those who need it brings many technical challenges. The ideal technology would allow one to drill deep wells to access this heat and enhance the native geological permeability by creating fractures in an otherwise non-convective formation so that a working fluid could be introduced and circulated, after which the acquired heat is recovered. This type of engineered thermal reservoir is commonly called an enhanced geothermal system (EGS). However, creating geothermal reservoirs in low-permeability rocks (absolute permeability in the range of

10^–17 to 10^–19 m²) where none exist naturally has proven challenging. This article will discuss the historical promise of geothermal energy, the state of EGS technology, and the strategies the authors and their team are using to transition EGS technology from field-lab to commercial scale.

Past geothermal technologies

Geothermal plants have been producing usable electricity for more than a century (1). Natural (conventional) geothermal systems are characterized by a heat source, water to extract the heat from the rocks, and sufficient fracture permeability to allow for fluid convection. These sites can be identified by the presence of hot springs or fumaroles (i.e., vents through which hot volcanic gases are emitted). The heat source may be volcanic or the Earth’s natural thermal gradient. The hottest geothermal systems typically occur adjacent to volcanoes that form along tectonic plate boundaries, such as those surrounding the Pacific Ocean. Temperatures of 350°C can frequently be encountered in these volcanic systems. Lower-temperature geothermal systems capable of electric generation using organic Rankine cycle (ORC) plants are found in extensional geologic terrains where faults are formed naturally, such as the Basin and Range Province of the western U.S. In these environments, active faults allow groundwater to circulate to depths of three or more miles. However, at depths greater than 10,000–12,000 ft in both volcanic and extensional environments, permeability and productivity typically decrease, which makes conventional geothermal plays economically unfeasible.

Unlike most geothermal systems in which liquid fills the fractures, dry steam systems make use of existing subterranean steam pockets. These types of plants are some of the oldest operating systems; examples include The Geysers, CA; Larderello, Italy; and a few reservoirs in West Java, Indonesia. The Geysers in northern California ...