Geothermal Energy Use, Country Update report for Denmark

The deep Danish onshore subsurface contains huge geothermal resources, but only a very limited fraction of these resources are utilized in three existing geothermal heating plants. At the three plants deep situated warm formation water is pumped to the surface from a production well and, after heat is extracted and distributed to the district heating system, the cooled water is returned to the reservoir through injection well(s).

To stimulate the exploitation of the geothermal resource and thus the transformation to a more sustainable energy mix in Denmark a recently completed research project (GEOTHERM, under the Innovation Fund Denmark) has thoroughly evaluated seismic reflection surveys and well data acquired during former hydrocarbon and geothermal exploration activities. The results of the last years research and geological assessments presented in a public available WebGIS portal have reduced the exploration risks significantly and has stimulated the interest from the industry leading to newly awarded exploration licenses. This development in combination with the large distribution of district heating in Denmark is promising for a much larger utilization of geothermal heat in Denmark.

The Danish basins are classic low enthalpy sedimentary basins characterized by long-term subsidence and infilling by sediments. The widely distributed fluviatile Lower Triassic Bunter Sandstone and the mainly marginal marine Upper Triassic-Lower Jurassic Gassum formations constitute the most important geothermal reservoirs and are utilized in the present geothermal plants. Furthermore, formations with more local distribution also have geothermal potentials. In many areas, where existing detailed geological subsurface data are limited, predrilling reservoir prognosis are associated with large uncertainties, especially regarding the reservoir permeability. The temperature gradient of typical 2530 °C/km in the Danish subsurface implies that at depths shallower than 800 m the temperature is generally too low, whereas at depths greater than 3000 m, diagenetic alterations related to high pressuretemperature conditions reduce the porosity and permeability of the reservoir sandstones. Pronounced temperature anomalies are absent and variations in the temperature gradients are mainly due to differences in the thermal conductivity of the geological strata.

Shallow geothermal energy (down to c. 250 m) has been utilized in Denmark since the late 1970's following the oil crisis and is commonly described as Ground Source Heating and Cooling. Energy extraction by heat pump technology from shallow geological formations is beginning to play a significant role in Denmark in the transition towards a sustainable heat supply, especially in areas without district heating. The shallow geothermal resources have become more attractive as there are now nine collective 5th generation district heating and cooling grids based on borehole heat exchangers (BHE) and aquifer thermal energy storage (ATES) in Denmark, all being economically feasible when compared to alternative means of supply. A pilot borehole thermal energy storage system (BTES) for storing seasonal heat from solar thermal was operated successfully for several years and proved to live up to the expected storage efficiency but is no longer active.