Energy Systems and Technologies for the Coming Century ...

concludes that the subsurface contains very large geothermal resources(Mathiesen et al 2009). However, despite the good understanding of thesubsurface a number of challenges remains for a successful geothermalexploitation owing to the great geological heterogeneity related to faults,lateral and vertical lithology variations, occurrence of pervasive diagenesisreducing porosity and permeability – all factors that decrease the reservoircontinuity and performance. Variations in temperature gradient and highsalinity of the formation water also impose challenges. This paper reviewsthe geothermal activities and potential, the geological challenges andpresents a stepwise maturation procedure involving a number of decisiongates connected to data acquisition and analyses.Geothermal activities in DenmarkDuring the late seventies the geothermal potential was investigated by DGU(now GEUS) together with the University at Aarhus (Michelsen 1981). In1979 and 1982 two wells were drilled by DONG. The wells encountered theGassum Formation as anticipated but flow rates were not economic. In 1982the Thisted-2 well successfully tested the Gassum Formation. The EUcommission initiated an investigation of the geothermal resources in Europeand the results were presented in a geothermal atlas in 1988, which laterwas updated (Hurter & Haenel, 2002).Existing and planned geothermal plantsTwo geothermal plants are currently operating in Denmark, the Thisted andMargreteholm plants (Fig. 1). They have demonstrated that it is possible toproduce large amounts of heat for district heating purpose using only aminimum of power to extract the heat from the subsurface water. Both plantsapply the concept of two wells, a production well pumping warm formationwater to the surface and an injection well re-circulating the cooled formationwater in a closed system back to the geological reservoir at some distancefrom the production point to avoid mixing of warm and cold water. TheThisted plant was established in 1984 and has produced water with atemperature of 45ºC from the Gassum Formation at a depth of c. 1250 m formore than 25 years without notable production or injection problems. TheMargreteholm plant began production in 2006 producing water of c. 73ºCfrom the Bunter Sandstone Formation at a depth of c. 2600 m. For bothplants exists plans for further expansion. In Sønderborg initial investigationsbased on existing data suggested a geothermal potential. After acquisitionand interpretation of new seismic data two explorations wells were drilledproving the presence of the Gassum Formation. A geothermal plant is underconstruction and is expected to begin operations in 2012 utilising 48ºC warmwater from the Gassum Formation at c. 1200 m of depth.2Risø International Energy Conference 2011 Proceedings Page 128