The potential and geological requirements of using geothermal energy in Denmark

The Danish subsurface contains deep geothermal resources that can contribute for hundreds of years to the energy mix by supplying heat for the district heating networks. Successful geothermal exploitation from the deep subsurface requires the presence of thick and laterally continuous sandstone reservoirs with high porosity and permeability, which can ensure effective and long-term extraction of warm formation water and re-injection of the cooled water. A thick and coherent reservoir that is not hydraulically compartmentalized by faults, by lateral lithological differences in e.g. grain size or by diagenetic features, implies that a large volume of warm water may be accessible. Thus, the production and injection wells can be placed at appropriate distances from each other in the reservoir while being hydraulically connected. With the Danish temperature gradient of 25–30°C/km in the subsurface, the heat yield is generally too low for district heating at depths shallower than c. 800 m. At depths greater than c. 3000 m, diagenetic alterations with cement precipitation related to the high pressure and temperature conditions reduce the porosity and permeability of the sandstones too much for geothermal production. In Denmark, several sandstone reservoirs are present at depths appropriate for geothermal exploitation. The Triassic–Jurassic sandstones, comprising the Bunter Sandstone, Skagerrak, Gassum, and Haldager Sand Formations, are the primary geothermal reservoirs in Denmark. In some areas, some of these sandstone reservoirs are located too shallowly, too deeply or missing, but one or more reservoirs are present in most parts of Denmark. The geothermal resource is huge, sustainable, and ready for extraction.