Geothermal well design is about more than drilling a hole in the ground – it’s about creating a sustainable, efficient connection between the Earth’s heat and the communities that use it. By carefully planning depth, flow, and reinjection, district heating operators can secure decades of clean, local, and dependable heat.
The wells are at the heart of geothermal heating
Every geothermal district heating system begins with a set (minimum two, called a doublet) of wells – carefully designed and drilled to reach the geothermal reservoir deep underground. These wells are the essential link between the Earth’s natural heat and the district heating network above.
In most projects, water is extracted from reservoirs – layers of permeable rock, often sandstone or limestone – that hold naturally heated water at depths of 1,000 to 3,000 metres below the surface. The temperature depends on the local geology, but for district heating it typically ranges from 30°C to 90°C.
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The doublet system – production and reinjection
The simplest and still most common configuration in district heating is the doublet system:
- Production well – Pumps hot water from the underground reservoir to the surface.
- Reinjection well – Returns the cooled water to the same reservoir after heat extraction.
This open-loop approach maintains the aquifer’s pressure and water balance, ensuring that the geothermal resource can be used sustainably for decades.
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Drilling and construction
Geothermal wells are drilled using techniques adapted from the oil and gas industry, but with designs tailored for the production of water rather than hydrocarbons.
Key steps include:
- Drilling to the target depth using drilling rigs.
- Casing the well to prevent collapse and protect surrounding rock layers.
- Well completion with screens or perforations to allow exchange of water with the reservoir.
- Pump installation in the production well, using electrical submersible pumps (ESPs) to lift water efficiently to the surface.
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Temperature and flow considerations
For district heating, both temperature and flow rate matter:
- Higher temperatures reduce the need for additional heating or heat pumps.
- Higher flow rates increase the total energy available to the network.
Design teams use reservoir modelling to predict how much water can be sustainably extracted without cooling the aquifer too quickly.
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Maintaining long-term performance
Proper well design is critical for system longevity:
- Reinjection maintains reservoir pressure.
- Regular maintenance avoids scaling or clogging in the well.
- Monitoring systems track flow, temperature, and water chemistry over time.
With good design and management, geothermal wells can operate for 30 years or more, making them a reliable long-term investment.
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Examples of well design in practice
- Paris Basin, France – Doublet wells at 1,800–2,000 metres depth deliver water at around 70°C to urban heating networks.
- Tianjin, China – Multiple well pairs operate in parallel to serve large urban areas, often integrated with heat pumps.
- The Netherlands – Shallower wells (1,500–2,000 metres) supply 70°C water for low-temperature district heating and greenhouse heating.
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