Geothermal systems in global perspective

Abstract

Geothermal resources are distributed throughout the world. They are classified in various ways on the basis of heat source, heat transfer, reservoir temperature, physical state, utilization and geological settings. Common classification of geothermal systems is: (a) volcanic systems with the heat source being hot intrusions or magma chambers in the crust, (b) convective systems with deep water circulation in tectonically active areas of high geothermal gradient, (c) conductive sedimentary systems with permeable layers at great depth (2-5 km), (d) geopressured systems often in conjunction with oil resources, (e) hot dry rock or EGS systems where abnormally hot masses of low permeability rocks are found at drillable depths, (f) shallow resources in normal geothermal gradient areas utilized with ground-source heat pump applications. In most of these classes the energy transport medium is the water within the geothermal system and such systems are therefore called hydrothermal systems, exceptions being the EGS systems and the shallow, ground-source heat pump resources. The geothermal systems are suitable for various applications depending on the reservoir temperature and fluid type. The hot volcanic systems are utilized primarily for electric power generation and the lower temperature systems for space heating and other direct uses.

Some 50 years ago a classification was proposed in which geothermal fields in Iceland were divided into high- and low-temperature hydrothermal fields or areas. This division was based on (arbitrarily) inferred temperature at 1 km depth, high temperature fields where a temperature of 200°C is reached at 1 km depth and low temperature fields where temperature is below 150°C in the uppermost km. The HT-fields are all related to volcanism whereas the LT-fields draw heat from the general heat content of the crust and the heat flow through the crust. Other temperature subdivisions have been proposed by adding intermediate or medium temperature systems in-between the two main categories. There are several types of systems in each of the two main groups.