Understanding the geology beneath a site is the first and most crucial step in developing geothermal district heating. With the right reservoir, temperature, permeability, and water chemistry, a project can operate successfully for decades – as proven in geothermal hubs around the world.
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Why geology matters
Not every location is suitable for geothermal district heating. The success of a project depends heavily on what lies deep beneath the surface. While both sufficient heat demand and access to a suitable plot are critical factors, geology is the foundation – it determines whether there’s access to enough heat to make a system technically and economically viable.
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The role of reservoirs
Most geothermal district heating projects draw heat from deep reservoirs – underground layers of water-bearing rock. In Europe and much of Asia, these reservoirs are often made of sandstone or limestone, which combines good permeability (allowing water to flow) with the ability to store heat.
A productive geothermal reservoir must:
- Contain water at a sufficient temperature for heating (typically 30–90°C).
- Have high enough permeability to allow sustainable pumping without major pressure drops.
- Be located deep enough to maintain stable temperatures year-round.
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Temperature and depth
The temperature of geothermal water increases with depth, but the rate of increase – known as the geothermal gradient – varies by region.
- Sedimentary basins like the Paris Basin or the North German Basin often reach 60–80°C at depths of 1,500–2,500 metres.
- Volcanic regions like Iceland can have much higher temperatures at shallower depths.
For district heating, higher temperatures are desirable, but low- to medium-temperature reservoirs can still be used effectively when paired with large-scale heat pumps.
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Permeability and flow rate
Even if temperatures are ideal, a reservoir must deliver enough flow rate to meet heating needs. Permeability is often measured in Darcy or millidarcy units and indicates how easily water can move through the rock. Higher permeability reduces the energy required for pumping and supports long-term, sustainable, and economical operations.
Reservoir engineers use test pumping and reservoir modelling to predict performance before a project moves into full-scale development.
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Water chemistry
The mineral content of geothermal water can affect system design:
- High content of dissolved minerals (e.g., calcium carbonate, silica) can cause scaling in wells and heat exchangers.
- Slight acidity or certain dissolved gases (e.g., COâ‚‚, hydrogen sulphide) may require materials that resist corrosion.
Proper water chemistry analysis helps select the right design, materials, and treatment systems, reducing maintenance needs.
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Examples of suitable geology worldwide
- Paris Basin, France – Thick, permeable limestone layers at 1,800–2,000 metres depth, delivering water at 70°C.
- North German Basin – Multiple sandstone formations offering a range of depths and temperatures.
- Tianjin, China – Deep aquifers with good flow and moderate temperatures, ideal for integration with heat pumps.
- The Netherlands – Shallower formations providing moderate temperatures for low-temperature heating systems.
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When geology is a challenge
In some areas, geology limits geothermal potential:
- Hard, impermeable rock makes water extraction impossible.
- Shallow reservoirs may be too cool without significant heat pump use.
- Complex faulting systems can complicate drilling and operations.
In such cases, hybrid systems or alternative heat sources may be more cost-effective.
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