Geothermal fields are formed when water from Earth’s surface is able to seep through faults and cracks within rock, sometimes to depths of several kilometers, to reach hot regions within the crust. As the water is heated is rises naturally back toward the surface by a process of convection and may appear there again in the form of hot springs, geysers, fumaroles, or hot mud holes. These are particularly common along tectonic plate boundaries.

Sometimes the route of the ascending water is blocked by an impermeable layer of rock. Under these conditions the hot water collects underground within the porous rock beneath the impermeable barrier. This water can reach a much higher temperature than the water that emerges at the surface naturally. Temperatures as high as 350 oC have been found in such reservoirs. This geothermal fluid can be accessed by boring through the impermeable rock. Steam and hot water will then flow upwards through the borehole under pressure and can be used at the surface.

Most of the geothermal fields that are known today have been identified by the presence of hot springs. In the United States, Italy, New Zealand, and many other countries the springs led to prospecting using boreholes drilled deep into the earth to locate the underground reservoirs of hot water and steam that were feeding them. More recently, geological exploration techniques have been used to try and locate underground geothermal fields where no hot springs exist. Sites in Imperial Valley in southern California have been found in this way.

Some geothermal fields produce simply steam, but these are rare. Larderello in Italy and the Geysers in California are the main fields of this type in use today though others exist in Mexico, Indonesia, and Japan. More often the field will produce either a mixture of steam and hot water or hot water alone, often under high pressure. All three can be used to generate electricity.

Deep geothermal reservoirs, as much as 2 km or more below the surface, produce fluid at the highest temperature. Typically, they will produce water with a temperature of 120–350 oC. High-temperature reservoirs of this type are the best for power generation, and the higher the temperature, the more energy can be extracted by a turbine. Shallower reservoirs may be a little as 100 m below the surface. These are cheaper and easier to access but the water they produce is cooler, often less than 150 oC. This can still be used to generate electricity but is more often used for heating.

The fluid emerging from a geothermal reservoir, at a high temperature and usually under high pressure, contains enormous quantities of dissolved minerals such as silica, boric acid, and metallic salts. Quantities of hydrogen sulfide and some carbon dioxide are often present too. The concentrated brine from a geothermal borehole is often corrosive and if allowed to pollute local groundwater sources can become an environmental hazard. This problem can be avoided if the brine is reinjected into the geothermal reservoir after heat has been extracted from it.

Geothermal reservoirs are all of limited extent and contain a finite amount of water and energy. As a consequence, both can become depleted if over- exploited. When this happens either the pressure or temperature (or both) of the fluid from the reservoir declines.

In theory, the heat within a subterranean reservoir will continuously be replenished by the heat flow from below. This rate of replenishment may be as high a 1000 MW, though it is usually smaller. In practice, geothermal plants have traditionally extracted the heat faster than it is replenished. Under these circumstances the temperature of the geothermal fluid falls and the practical life of the reservoir is limited.

Reinjection of the brine after it has passed through the power plant helps maintain the fluid in a reservoir. However, reservoirs such as the Geysers in the United States where fluid exiting the boreholes is steam have proved more difficult to maintain since the steam is generally not returned after use. This has led to a marked decline in the quantity of heat from the Geysers. In an attempt to correct this, waste water from local towns has been reinjected into the reservoir. Some improvement has been noted.

Estimates for the practical lifetime of a geothermal reservoir vary. This is partly because it is extremely difficult to gauge the size of the reservoir. While some may become virtually exhausted over the lifetime of a power plant, around 30 years, others appear able to continue to supply energy for 100 years or more. Better understanding of the nature of the reservoirs and improved management will potentially help maintain them for longer in the future.

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