Geothermal Systems

The word geothermal comes from the Greek words geo (earth) and therme (heat), and means the heat of the earth.

Geothermal energy is ultimately derived from the heat contained in the core of the earth and from radioactive decay within its mantle. At high temperatures and pressures within the mantle, melting of mantle rock forms magma which rises towards the surface carrying the heat from below.

Geothermal hot pool at Waiotapu In some regions where the earth's crust is thin or fractured, or where magma bodies are close to the surface, there are high temperature gradients. Deep faults, rock fractures and pores allow groundwater to percolate towards the heat source and become heated to high temperatures. Some of this hot geothermal water travels back to the surface through buoyancy effects to appear as hot springs, mud pools, geysers, or fumaroles. If the ascending hot water meets an extensively fractured or permeable rock zone, the heated water will fill pores and fractures and form a geothermal reservoir. These reservoirs are much hotter than surface hot springs, reaching temperatures of more than 350°C, and are potentially an accessible source of energy.

These high temperatures can be achieved in liquid-dominated reservoirs because increasing hydrostatic pressure with depth allows elevated temperatures without boiling. Many undisturbed geothermal reservoirs in New Zealand have temperature and pressure profiles such that the fluid is close to boiling point to depths of more than 1 km.

Geothermal areas are commonly close to the edges of continental plates, and New Zealand's location on an active plate boundary (between the Indo-Australian and Pacific Plates) has resulted in the development of numerous geothermal systems and a world-class geothermal energy resource.

The characteristics of geothermal systems vary widely, but three components are essential:

  • a subsurface heat source that may be igneous magma bodies or heat stored in other rocks
  • fluid to transport the heat
  • faults, fractures or permeability within sub-surface rocks that allow the heated fluid to flow from the heat source to the surface or near-surface.

Geothermal resources can be classified into three categories:

  • High temperature, usually magmatic-related resources. These have temperatures of 200- 350°C at economically-drillable depth. They are of limited occurrence, and form individual convective geothermal systems of up to 50 sq km in area. Technologies may be developed in the future to exploit even deeper, hotter resources.
  • Moderate to low temperature resources, of non-magmatic origin, usually associated with deep faults. Maximum temperatures at drillable depth do not exceed 140°C, and are often less. These are more widespread than the high temperature resources, but the individual systems are no larger. The distinction between these and the first kind is not entirely clear-cut, as cooled outflows from hotter resources can also fall into the same temperature range.
  • Very low temperature resources, which are widespread but close to ambient temperature.

Links

Generalised Geothermal System of the Taupo Volcanic Zone

(After Henley & Others, 1986)

Schematic cross-section showing the main features and geochemical structure of geothermal fields in the Taupo Volcanic Zone