Intensive investigations into the utilisation of the geothermal potential of the North German Basin began in the early 1980s. The first production and reinjection tests from/into sandstone reservoirs started in 1982 and led to the commissioning of the first geothermal heating plant for heat supply to a residential area in the town of Waren (Müritz) in 1984. More plants were put into operation in Neubrandenburg, Neustadt-Glewe and Berlin. The use of these sandstone reservoirs for heat storage produced new technical solutions. A precise knowledge of the geological and geochemical conditions forms an essential prerequisite for the successful planning, construction and operation of geothermal plants. This paper describes the geological and geochemical conditions, as well as the technical solutions and practical experience acquired so far.

Results from two onsite core flood experiments are presented addressing permeability development of a clastic reservoir to brine flooding. Sandstone samples were percolated with anoxic thermal brines to simulate injection during geothermal exploitation. Petrographic studies of the cores before and after the experiment and chemical investigations of the thermal water have shown that despite good initial petrophysical properties, injection of geothermal brines led to a permeability decrease of the core plugs. The experiment with oxidized water showed a permeability decrease resulting in a total loss of permeability due to precipitation of Fe hydroxide. The decrease is dependent on particle size and concentration, pore throat size and flow rate. The decline of permeability during the second percolation experiment with original thermal water can be explained by deposition of small particles such as fragments of feldspars and quartz into the porous medium.

Wie geochemisch-thermodynamische Modellierungen gezeigt haben, können während des Betriebs einer geothermischen Heizanlage durch Druck- und Temperaturveränderungen Baryt, Anhydrit/Gips oder Cölestin ausfallen. Das Fällungsverhalten der Erdalkalisulfate wurde unter In-situ-Bedingungen (Druck, Temperatur, Salinität 216 g/l) in einem Durchströmungsversuch bestimmt. Es wird eine Durchströmungsanlage vorgestellt, die Drucke bis zu 250 bar und hohe Temperaturen bei der Durchströmung eines Sandsteinkerns zuläßt.