Enhanced geothermal system

An enhanced geothermal system (EGS) generates geothermal electricity without the need for natural convective hydrothermal resources. Until recently, geothermal power systems have exploited only resources where naturally occurring heat, water, and rock permeability are sufficient to allow energy extraction. However, by far most of geothermal energy within reach of conventional techniques is in dry and impermeable rock. EGS technologies enhance and/or create geothermal resources in this hot dry rock (HDR) through 'hydraulic stimulation'.

When natural cracks and pores do not allow economic flow rates, the permeability can be enhanced by pumping high-pressure cold water down an injection well into the rock. The injection increases the fluid pressure in the naturally fractured rock, triggering shear events that enhance the system's permeability. As long as the injection pressure is maintained, a high matrix permeability is not required, nor are hydraulic fracturing proppants required to maintain the fractures in an open state. This process is termed hydro-shearing, perhaps to differentiate it from hydraulic tensile fracturing, used in the oil and gas industry, which can create new fractures through the rock in addition to expanding the existing fractures.

Hot dry rock geothermal energy

Hot dry rock (HDR) is by far the most abundant source of geothermal energy available to mankind. A vast store of thermal energy is contained in the hot—but essentially dry—impervious crystalline basement rocks found almost everywhere deep beneath the earth’s surface. A concept for the extraction of useful amounts of geothermal energy from HDR originated at the Los Alamos National Laboratory in 1970, and was so groundbreaking that Laboratory researchers were awarded a U.S. patent covering it.

Overview

Although often confused with the relatively limited hydrothermal resource already commercialized to a large extent, HDR geothermal energy is very different. Whereas hydrothermal energy production can only exploit hot fluids already in place in the earth’s crust, an HDR system (consisting of the pressurized HDR reservoir, the boreholes drilled from the surface, and the surface injection pumps and associated plumbing) recovers the earth’s heat from hot but dry regions via the closed-loop circulation of pressurized fluid. This fluid, injected from the surface under high pressure, opens pre-existing joints in the basement rock, thus creating a man-made reservoir that may be as much as a cubic kilometer in size. The fluid injected into the reservoir absorbs thermal energy from the high-temperature rock surfaces and then serves as the vehicle for transporting that heat to the surface for practical use.