Denmark to embark on an innovative geothermal drilling project

Green Therma, in collaboration with Aalborg Forsyning (the local utility company), is cur­rently preparing to drill a single well to deliver geothermal heat directly to Aalborg’s district heating network. While Denmark is recognized for its ad­vanced district heating systems and strong renewable en­ergy profile, geothermal has yet to play a major role in the national mix. The well will extend about 7 km in total: 4-5 km vertically, followed by a 2-3 km horizontal sec­tion. The concept draws on proven technology from the oil and gas industry, where long horizontal well sections are standard.

In the Aalborg area, subsurface temperatures increase by roughly 30° C/km. At depths of 4-5 km, rock tempera­tures are thereby expected to reach 120-150° C, providing a stable, long-term heat source suitable for direct use in district heating.

A closed and insulated system

Unlike conventional doublet geothermal systems, which re­quire at least two separate wells, the Heat4Ever^TM system operates with a single well closed circuit. Inside the well runs a double pipe with vacuum insulation (DualVac^TM) between the inner and outer tubes. Water circulates continuously within this sealed system. It is pumped down in the annulus between the well casing and the outer pipe. Once it reaches the horizontal section, the water will then absorb heat from the surrounding rock and return, now heated, to the surface through the inner insulated pipe.

The system minimizes thermal losses to just a few % and eliminates the need to extract formation water from the reservoir. The closed system also prevents mineral scaling and corrosion issues often encountered in open-loop hydrothermal systems. At the surface, the heat is transferred through a heat exchanger directly to Aalborg Forsyning’s district heating system.

Because of the depth and efficient insulation, the sys­tem is expected to deliver water at temperatures of around 85-90° C to the surface for more than 50 years, while the return temperature after heat exchange will be around 40° C. These temperatures allow the heat to be used directly in the district heating network without the need for a heat pump to boost the temperature, offering far higher overall efficien­cy when compared to conventional hydrothermal systems. The only energy demand in the Heat4Ever^TM solution is a 45 kW circulation pump for a 1.5-2 MW well.

From demonstration to scalable deployment

The plant will supply up to 2 MW of geothermal heat to Aalborg’s district heating grid. Aalborg Forsyning has agreed to purchase heat from the facility for 30 years, pro­viding a long-term framework for operational experience and data collection.

Green Therma views this project as a stepping stone toward broader deployment. While the operations in Aal­borg involve a single well, the plan for future projects is to take a “multi-well” approach across global sites, drill­ing a sequence of 5-10 wells from a single surface loca­tion, each with horizontal sections extending in different directions.