4 Locating high-enthalpy geothermal fields
The search for potentially useful geothermal fields focuses initially on locating rocks that have been chemically altered by natural geothermal fluids, as well as looking for obvious surface features of geothermal activity such as geysers and hot springs. Measurements of fluid flow through the field allow estimation of its likely economic potential.
When a promising resource has been located exploration wells are drilled. However, given the high pressures and temperatures typical of a geothermal resource, special precautions need to be taken. For example, once the drill stem penetrates into a zone saturated with superheated water, the liquid will flash to steam as the pressure drops. This is more dangerous than simply venting the 'head' of pressure in an oil or gas field. The well head itself is capped with valve gear to regulate the flow of fluids, and the whole assembly is then 'plumbed' to an electricity generating plant.
Exploration for HDR fields involves checking exposed granites for their content of radioactive heat-producing elements, and assessing subsurface heat production by measuring surface heat flow. Outlining the buried granite intrusions that are targets for HDR development depends on modelling their extent, depth and volume from gravity surveys; granites have a lower density relative to their surrounding rocks, and produce gravitational 'lows'.