Geothermal handbook: PlanninG and Financing power Generation t e c h n I c a L r e p o r t 2 / 2

• 
Geothermal SyStem 
refers to all parts of the hydrological system involved, including the recharge zone, 
all subsurface parts, and the outflow of the system.
• 
Geothermal reServoir 
indicates the hot and permeable part of a geothermal system that may be 
directly exploited. For a geothermal reservoir to be exploitable, it needs to have sufficient natural heat that 
transforms to pressure and brings the steam to the surface.
• 
Geothermal Field
i
s a geographical definition, usually indicating an area of geothermal activity at 
the Earth’s surface. In cases without surface activity, this term may be used to indicate the area at the surface 
corresponding to the geothermal reservoir below.
7
A production well used to withdraw geothermal water/steam, combined with a reinjection well to return the water back into the reservoir, is 
called a doublet.
Several EGS pilot projects have had problems with induced seismicity, which created minor 
earthquakes, and the commercial viability of the technology has not been successfully proven yet. The 
EGS technology will not be discussed in detail in this handbook. 
Shallow resources
refer to the normal heat flux through near surface formations (< 200 m deep) and 
to thermal energy that is stored in the rocks and warm groundwater systems near the surface of the 
Earth’s crust. Recent developments in the application of ground source heat pumps have opened up 
new possibilities for utilizing these resources.

---

19
High temperature fields are all related to volcanism whereas low temperature fields draw heat from 
the general heat content of the crust and from the heat flow through the crust. Another temperature 
subdivision has been proposed, an intermediate or medium temperature system between the two main 
categories. Medium temperature fields have temperatures between 150° and 200°C and are included 
in this guide because they can be utilized for power generation by binary power plants, which are 
discussed later in this chapter. 
Following a similar resource classification based on temperature, Table 1.1 summarizes their most 
likely uses and the technologies involved.