Survey of Energy Resources 2007
Geothermal energy, in the broadest sense, is the natural heat of the Earth. Immense amounts of thermal energy are generated and stored in the Earth's core, mantle and crust. At the base of the continental crust, temperatures are believed to range from 200 to 1 000°C, and at the centre of the earth the temperatures may be in the range of 3 500 to 4 500°C. The heat is transferred from the interior towards the surface mostly by conduction. This conductive heat flow means that temperatures rise with increasing depth in the crust by on average 25-30°C/km. Geothermal production wells are commonly over 2 km deep, but at present rarely much over 3 km. With an average thermal gradient of 25-30°C/km, a 1 km well in dry rock formations would have a bottom temperature near 40°C in many parts of the world (assuming a mean annual temperature of 15°C) and a 3 km well 90-100°C.
Exploitable geothermal systems occur in a number of geological environments. They can be divided broadly into two groups, depending on whether they are related to young volcanoes and magmatic activity or not. High-temperature fields used for conventional power production (with temperatures above 150°C) are mostly confined to the former group, but geothermal fields exploited for direct application of the thermal energy can be found in both groups. The temperature of the geothermal reservoirs varies from place to place, depending on the local conditions.
Volcanic activity mainly occurs along so-called plate boundaries (Figure 11.3 Geo ). According to the plate tectonics theory, the Earth's crust is divided into a few large and rigid plates which float on the mantle and move relative to each other at average rates counted in centimetres per year (the actual movements are highly erratic). The plate boundaries are characterised by intense faulting and seismic activity, and in many cases volcanic activity. Geothermal fields are very common on plate boundaries, as the crust is highly fractured and thus permeable, and sources of heat are readily available. In such areas magmatic intrusions, sometimes with partly molten rock at temperatures over 1 000°C, situated at a few kilometres depth, heat the groundwater. The hot water has lower density than the surrounding cold groundwater and therefore flows up towards the surface along fractures and where there is permeability.
Most of the plate boundaries are beneath the sea, but in cases where the volcanic activity has been intensive enough to build islands or where active plate boundaries transect continents, there are commonly high-temperature geothermal fields scattered along the boundaries. A spectacular example of this is the 'ring of fire' that circumscribes the Pacific Ocean (the Pacific Plate) with intense volcanism and geothermal activity in New Zealand, Indonesia, Philippines, Japan, Kamchatka, Aleutian Islands, Alaska, California, Mexico, Central America, and the Andes mountain range. Other examples are Iceland, which is the largest island on the Mid-Atlantic Ridge plate boundary, and the East African Rift Valley with impressive volcanoes and geothermal resources in e.g. Djibouti, Ethiopia and Kenya.
Low-temperature fields. Geothermal resources unrelated to volcanoes can be divided into four types:
a) resources related to deep circulation of meteoric water along faults and fractures;
b) resources in high-porosity rocks at hydrostatic pressure;
c) resources in high-porosity rocks at pressures greatly in excess of hydrostatic pressure (i.e. 'geopressured');
d) resources in hot but dry (low-porosity) rock formations.
These four types are in fact end members, with most natural systems displaying some intermediate characteristics. All of these types, with the exception of type c), can also be associated with volcanic activity. Types c) and d) are not commercially exploited as yet. A comprehensive description of the nature of geothermal systems is given (with diagrams) on the homepage of the International Geothermal Association (www.geothermal-energy.org).
Type a) is probably the most common type for warm springs in the world. These can occur in most rock types of all ages, but are most visual in mountainous regions where warm springs appear along faults in valleys. Warm springs of this type are of course more numerous in areas with a high regional conductive heat flow (with or without volcanic activity), but are also found in areas of normal and low heat flow. The important factor here is a path for the meteoric water to circulate deep into the ground and up again. Areas of young tectonic activity are commonly rich in this type of geothermal spring, such as in Turkey and the Balkan Peninsula.
Type b) is probably the most important type of geothermal resource not associated with young volcanic activity. Many regions throughout the world are characterised by deep basins filled with sedimentary rocks of high porosity and permeability. If these are properly isolated from surface ground water by impermeable strata, the water in the sediments is heated by the regional heat flow. The age of the sediments makes no difference, so long as they are permeable. The geothermal reservoirs in the sedimentary basins can be very extensive, as the basins themselves are commonly hundreds of kilometres in diameter. The temperature of the thermal water depends on the depth of the individual aquifers and the geothermal gradient in the area concerned, but is commonly in the range of 50-100°C (in wells less than 3 km deep) in areas that have been exploited (such as the Paris Basin in France, the Pannonian Basin in Hungary, and several areas in China). Geothermal resources of this type are rarely seen on the surface, but are commonly detected during deep exploration drilling for oil and gas. The widespread low-temperature geothermal resources of China are divided between types a) and b).