Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 444

production pipe and the production level have to be taken into consideration, so
that the required flow throughout the aboveground part of the system is ensured.
If the produced geothermal fluid contains large gas quantities, the bubble point
must not be below the pump installation depth. Depending on geological condi-
tions, well completion and the pump type, the installation depth could vary be-
tween 100 and 400 m and even more.
In order to adapt the production of geothermal fluid to a given heat demand, the
pump is controlled by varying the rounds per minute (rpm) of the electro motor.
Even though geothermal heating plants should preferably be operated at base
load, i.e. with a continuous volume flow, the described control technique is rec-
ommended
− to allow for a reaction to changing reservoir conditions,
− to ensure a smooth start-up and shutdown operation of the plant, and
− to consider the aboveground plant characteristic changes according to the in-
creasing pressure losses due to an increasing filter load.
Quality assurance of re-injected water. Re-injection of cooled geothermal fluid is
comparable to filtering (i.e. the geothermal fluid is pumped through the injection
well, the reservoir rocks influenced by drilling operations and the undisturbed
reservoir rocks). Hence, particulate matter must necessarily either be prevented
from entering the reservoir, or its entering must at least be delayed.
Within an uninfluenced reservoir there is a chemical balance between geother-
mal fluid and reservoir rocks. However, within the geothermal fluid circuit real-
ised within a geothermal heating plant, this balance could be influenced by pres-
sure and temperature changes. This is true for the following aspects:
− pressure relief during extraction,
− temperature reduction and/or a possible oxygen entry into the aboveground
geothermal fluid circuit,
− pressure increase during re-injection,
− mixture of the geothermal fluid from the production well with those of the in-
jection well that may not necessarily be identical in terms of their chemical
characteristics,
− temperature increase.
Due to pressure relief, additional degasification may occur during production,
leading to changes of the pH-value and the redox potential. The redox potential is
additionally increased by oxygenation.
The predefined quality of the re-injected geothermal fluid could be ensured by
avoiding possible contamination within the geothermal fluid circuit through the
overall geothermal heating plant and by filtration of re-injected water. Both op-
tions are briefly explained below.
− Avoiding contamination of re-injected geothermal fluid. Most geothermal flu-
ids are highly complex, highly saline, undersaturated solutions with low con-
tents of dissolved gases. In most cases, precipitations of soluble salts are
unlikely. Geochemical models have shown that, provided that a mixture of the