Geothermal fields are formed when water from rain or snow is able to seep through faults and cracks within rock, sometimes for several kilometres, to reach hot rock beneath the surface. As the water is heated it rises naturally back towards the surface by a process of convection and may appear there in the form of hot springs, geysers, fumaroles or hot mud holes.
Sometimes the route of the ascending water is blocked by an impermeable layer of rock. Under these conditions the hot water collects underground in the cracks and pores of the rock beneath the impermeable barrier. This water can reach a much higher temperature than the water which emerges at the surface naturally. Temperatures as high as 350°C have been found. Such geothermal reservoirs can be accessed by boring through the impermeable rock. Steam and hot water will then flow upwards 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 California, Italy, New Zealand and may other countries the presence of these springs led to prospecting usage of boreholes drilled deep into the earth to locate the underground reservoirs of hot water and steam. 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 may be 2 km or more below the surface. These can produce water with a temperature of 120-350°C. High-temperature reservoirs are the best for power generation. Shallower reservoirs may be as little as 100 m below the surface. These are cheaper and easier to access but the water they produce is cooler, often less then 150°C. 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 sulphide and some carbon dioxide are often present too. The concentrated brine from a geothermal borehole is usually corrosive and if allowed to pollute local groundwater sources can become an environmental hazard. This problem can be avoided if the brine is re-injected into the geothermal reservoir after heat has been extracted from it.
Geothermal reservoirs are not limitless. They 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 the temperature - or both - of the fluid from the reservoir declines.
In theory the heat within a subterranean reservoir will be continuously replenished by the heat flow from below. This rate of replenishment may be as high as 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.
Re-injection of brine after use helps maintain the fluid in a reservoir. However reservoirs such as the Geysers in the USA, 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, wastewater from local towns is now being re-injected 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 help maintain them for longer.
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