To make use of thermal energy the heated water is pumped out of a rock. This rock is named reservoir or aquifer. The aquifer has to be highly permeable to allow sufficient water flow. To extract and inject the water from the rock a production and injection well are drilled into it. The combination of a production and injection well is called a doublet. Via the production well warm water is pump out of the aquifer.
A heat exchanger extracts the heat from the water which is used for heating. The cooled down water is injected back into the aquifer via the injection well. The re-injection of water into the aquifer is necessary to prevent earthquakes or subsurface subsidence. Moreover, the water from the aquifer may contain heavy metals or minerals (like salts). Water discharge at the surface is therefore prohibited.
Due to the injection of cool water a cold front will develop around the well head. Furthermore, cold water will migrate towards the production well. Within a couple of decades the temperature of the produced water will decline. This reduces the efficiency of the geothermal installation. If possible a new production and/or injection well is drilled in the aquifer so that the power generation can continue. In the meanwhile the cooled down area of the aquifer van slowly regenerate to its initial temperature.
A suitable aquifer for geothermal energy production should meet the following requirements:
Sufficiently porous and highly permeable
Obviously an aquifer should contain a large quantity of water. The volume of water in rock is determined by its porosity. Porosity is a measure of the percentage of small pores present in the rock. In addition, the pores should be interconnected so that water can easily flow through the rock. Permeability is a measure for the ability of material to transmit fluid (or gases). High permeable rock is therefore essential.
Temperature increases with depth. This is called the temperature gradient. On average, the temperature gradient of the Dutch subsurface is roughly 31 °C per kilometre (0,031 °C per meter). Due to differences in thermal conductivity of rock and the geological setting the gradient can vary per region.
Continuous rock layer
The number of natural barriers, like faults, should be minimal to maintain a proper flow of fluids. A fault in an aquifer may act as a barrier, preventing the water to continue flowing. Preferably, there should not be a fault between the production and injection well.
Rock should be as homogeneous as possible
It is important that the composition and properties of an aquifer is homogeneous. Inhomogeneous zones within an aquifer may influence the amount of flow. This will decrease the amount of power that can be produced.