Geothermal energy is the heat that originates within the earth. It is the thermal energy contained in the rock and fluid (that fills the fractures and pores within the rock) in the earth’s crust.
Many of the large-scale geological processes that have helped to form the earth’s surface features are powered by the flow of heat from inner regions of higher temperature to outer regions of lower temperature. Generation of new oceanic crust at spreading centers such as the mid-Atlantic ridge, motion of the great lithosphere plates, uplifting of mountain ranges, release of stored strain energy by earthquakes and eruption of volcanoes are all powered by the outward transport of internal heat. Plastic, partially molten rock is postulated to exist everywhere beneath the earth’s surface at depths of 100 km (60 mi) or less. By comparison, using present technology applied under favorable circumstances, holes can be drilled to depths of about 10 km (6.2 mi), where temperatures range upward from about 150°C (300°F) in average areas to perhaps 600°C (1,100°F) in exceptional areas.
Exploitable geothermal resources originate from transport of heat to the surface through several geological and hydrological processes. Geothermal resources commonly have three components: 1) a heat source, 2) relatively high permeability reservoir rock, and 3) water to transfer the heat.
In general, the heat source for most of the high-temperature resources (>150°C [300°F]) appears to be a molten or recently solidified intrusion, whereas many of the low-temperature (<100°C [212°F]) and moderate-temperature resources (between 100° and 150°C [212° and 300°F]) seem to result from deep circulation of meteoric water with heating due to the normal increase in temperature with depth.
A number of high-temperature resources also occur in the Basin and Range Province of the western U.S. as the result of deep circulation along major faults in a region of high heat flow.
In most geothermal systems, fracture permeability controls water movement, but inter-granular permeability is also important in some systems. Water is, of course, the ideal heat transfer fluid because it has a high heat capacity and high heat of vaporization, and can therefore transport more heat per unit volume that any other common fluid.
A free convective circulating system is set up with the heated water ascending in the center of the system along zones of permeability, spreading outward in the shallow subsurface or discharging to the surface, and with cool water descending along the margins and recharging the system. Rapid convection produces nearly uniform temperatures over large volumes of the reservoir.
Geothermal Areas in Utah
With few exceptions, the higher temperature geothermal areas in Utah occur either in the Basin and Range Province or within the Basin and Range-Colorado Plateau transition zone.
In central and western Utah, most thermal areas are located in valleys near the margins of mountain blocks, and are probably controlled by active basin-and-range faults. Other geothermal systems occur in hydrologic discharge zones at the bottom of valleys. A few thermal areas are situated in mountainous regions.
The most significant known occurrence of geothermal water in eastern Utah is from oil wells of the Ashley Valley oil field, which yield large volumes of nearly fresh water at temperatures between 43°C and 55°C (109°F and 131°F) as a byproduct of oil production. In 1981, the Ashley Valley field yielded 5.42 million m3 (26.1 million barrels) of water.
Known high-temperature systems include the Roosevelt Hot Springs and Cove Fort-Sulphurdale Known Geothermal Resource Areas (KGRA). KGRA is a federal classification pertaining to geothermal areas where federal lands have competing leasing interests.
Other potential high-temperature systems are Thermo Hot Springs, Joseph Hot Springs, the Newcastle area, and the Monroe-Red Hill area.
In and around the Sevier, Black Rock, and Escalante Deserts in southwestern Utah, a number of geothermal systems have estimated reservoir temperatures greater than 100°C (212°F).