In recent years, geothermal energy use from low-temperature sandstone reservoirs has sharply increased. Nonetheless, the injection of heat-depleted geothermal fluids has not been an easy task because of well/formation damage and operational/economic issues. Sønderborg geothermal plant is a case example of heat-mining from a low-temperature reservoir. It is in the northeast of Sønderborg towards Augustenborg Fjord. The present work takes into consideration the regional and local geology of the Sønderborg area, construction of the wells, field experience and water chemistry. The main issues of the geothermal plant appear to be related to the construction of the wells and reinjection of the heat-depleted brine. Our water chemistry analysis and PHREEQC simulations indicate that geothermal brine was saturated with respect to carbonate and barite minerals. The excess of Ca2+ and SO4 2– ions could have led to the formation and precipitation of carbonate and sulfate scales. Moreover, the increment of iron concentration over time could suggest the ingress of oxygen and pitting corrosion due to the presence of halide ions.
Bibliografisk noteFunding Information:
The Danish Oil and Natural Gas A/S (DONG) group (now Oersted A/S) was granted with a license to explore and exploit the Danish geothermal resources in 1977. The outcomes from the first drilling activities in Års, Farsø in Himmerland were disappointing. This is because a poor rock conductivity made it impossible to produce 100 ◦C geothermal water discovered at 3000 m. However, a sandstone layer (Gassum formation) with good rock properties at 1250 m was found in Thisted. A pilot project in that location was established in 1984 by DONG, which was expanded to the current plant in 1988 . A renewed campaign of DONG VE A/S assisted by GEUS was initiated with financial support from the European Union, which resulted in the Margretheholm and Sønderborg geothermal plants . The license for exploration and production of geothermal energy in the Sønderborg area was granted to Sønderborg Fjernvarme and DONG Energy in 2007 . The same year, as the first phase of the project, a new seismic survey was completed, which indicated that the two wells (injector and producer) should be placed outside the town .
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