Aulnay-sous-Bois Advances Geothermal Heating Project After 2,000m Drilling

Aulnay-sous-Bois Advances Its Geothermal Heating Project After Completing 2,000-Metre Drilling

Aulnay-sous-Bois, in the Seine-Saint-Denis department of France’s Île-de-France region, has reached an important milestone in the development of its local geothermal heating network. After approximately four months of work, drilling for the city’s geothermal project was completed during the summer of 2026.

The well reaches nearly 2,000 metres beneath the surface, where the project will access the Dogger aquifer. At this depth, naturally occurring geothermal water reaches temperatures of approximately 70°C. Once the facility becomes operational, the heat extracted from the aquifer is expected to supply most of the energy required by Aulnay-sous-Bois’s district heating network.

The project is scheduled to enter service before the end of 2026. According to Groupe Coriance, the geothermal system is expected to cover 93% of the network’s requirements, produce approximately 140 GWh of heat each year, and avoid around 28,000 tonnes of carbon dioxide emissions annually. The system is designed to meet the heating and domestic hot-water needs of approximately 14,000 homes. [1]

## A Major Milestone for Aulnay-sous-Bois

The completion of drilling marks one of the most technically important phases in the development of a geothermal heating project. Drilling to a depth of nearly 2,000 metres requires detailed geological planning, specialised equipment, continuous monitoring, and close coordination between engineering teams, local authorities, contractors, and residents.

For Aulnay-sous-Bois, the end of drilling brings the city closer to operating a locally sourced renewable-energy system. The project will use heat naturally stored underground rather than relying primarily on imported fossil fuels or conventional gas-fired heating systems.

Although drilling is now complete, additional work is required before the network enters commercial service. The well and surface facilities must be equipped, connected, tested, and integrated into the existing district heating infrastructure. Operators will also need to verify the performance of the geothermal resource and ensure that the system can deliver heat safely and consistently throughout the year.

The planned commissioning before the end of 2026 would represent the transition from construction to operation. Once connected to the heating network, the geothermal plant will become a central component of Aulnay-sous-Bois’s strategy to reduce emissions and strengthen the role of renewable energy in local heating.

## Accessing Heat from the Dogger Aquifer

The project targets the Dogger aquifer, a deep geological formation widely used for geothermal heating in the Paris Basin. The aquifer contains naturally heated water that can be brought to the surface through a production well and used to transfer heat to a district heating network.

In a conventional deep-geothermal heating system, operators typically extract hot water through one well. The thermal energy is transferred through heat-exchange equipment before the cooled water is returned underground through a reinjection well. Reinjection helps maintain pressure in the reservoir and supports the long-term management of the geothermal resource.

The Aulnay-sous-Bois project will use the natural temperature of the Dogger resource, estimated at approximately 70°C. This temperature is suitable for district heating applications, particularly when the network is designed or upgraded to operate efficiently with moderate-temperature heat.

Geothermal projects based on deep aquifers differ from shallow ground-source heat-pump systems. Shallow systems generally use the relatively stable temperature of the near-surface ground and require heat pumps to raise or lower the temperature. Deep geothermal projects instead access naturally hot water at several kilometres below ground and can deliver large quantities of heat directly to an urban network.

The geology of the Paris Basin has made the region one of Europe’s most important areas for deep geothermal heating. Aulnay-sous-Bois is part of a broader movement across the Île-de-France region to use the Dogger aquifer as a source of low-carbon heat for municipalities, residential areas, public buildings, and commercial users.

## Supplying 140 GWh of Heat Each Year

The completed project is expected to deliver approximately 140 GWh of heat annually. This is a substantial volume for a municipal heating network and demonstrates the potential of geothermal energy to serve large urban communities.

The planned output is equivalent to the annual heating and domestic hot-water demand of approximately 14,000 homes. Rather than supplying electricity, the facility will primarily produce thermal energy for direct use in buildings connected to the district heating network.

This distinction is important. Much of the energy transition focuses on renewable electricity, but heating represents a major share of energy consumption in homes, public buildings, and businesses. Geothermal district heating can address this demand directly by supplying hot water through a network of insulated pipes.

The system can serve multiple types of customers, including apartment buildings, schools, healthcare facilities, municipal properties, and other connected users. District heating allows a single energy centre to supply many buildings, reducing the need for each property to maintain an individual boiler or heating system.

The expected annual output will depend on factors such as weather conditions, network demand, plant availability, maintenance schedules, and the operating temperature of the distribution system. Even so, the project’s planned capacity positions geothermal energy as a major source of heat for Aulnay-sous-Bois rather than a minor supplementary technology.

## Covering 93% of Network Demand

Groupe Coriance says the geothermal system is expected to cover 93% of the heating network’s requirements. This high percentage would make geothermal energy the dominant source of heat for the network once the project enters service. [1]

A geothermal share of this scale can significantly reduce exposure to fossil-fuel price volatility. Gas prices can fluctuate because of international markets, geopolitical events, supply constraints, and changes in demand. A locally produced geothermal resource is not entirely immune to operating costs or electricity prices, but its energy source is available within the region and is not purchased as a conventional fuel.

The remaining share of the network’s energy requirements may be supplied by complementary sources, backup equipment, or auxiliary heating systems. These systems are important because geothermal production can be affected by maintenance, peak winter demand, technical interruptions, or temporary changes in resource availability.

A diversified district heating network can combine geothermal energy with backup boilers, heat pumps, waste heat, biomass, or other low-carbon sources. Such a configuration improves reliability while allowing geothermal energy to provide the majority of annual heat production.

The 93% target also highlights the importance of matching geothermal production with network demand. Because the resource provides a steady flow of heat, the network must be properly managed to absorb and distribute that energy. Building-level efficiency improvements, weather compensation, thermal storage, and careful temperature control can all help maximise the value of the geothermal system.

## Reducing Carbon Emissions

The Aulnay-sous-Bois project is expected to avoid approximately 28,000 tonnes of carbon dioxide emissions each year. [1] This reduction will result primarily from replacing higher-emission heating sources with geothermal energy.

The exact climate benefit of a geothermal heating project depends on the energy sources it replaces, the electricity required to operate pumps and equipment, the design of the wells, and the emissions associated with construction and maintenance. However, deep geothermal heating generally offers a low-carbon alternative to fossil-fuel-based heat when the resource is managed efficiently.

The largest emissions savings are achieved when geothermal energy displaces natural gas or other fossil fuels in large, continuously operating heating networks. Because the same geothermal facility can supply many buildings, the emissions reduction is concentrated in one coordinated project rather than spread across thousands of individual heating-system upgrades.

The project may also reduce local air pollution associated with combustion. A geothermal plant does not burn fuel at the point of heat production, which can help lower emissions of pollutants linked to combustion-based heating. The environmental performance of the complete system still depends on the design of the plant, pumping requirements, water chemistry, and the treatment of geothermal fluids.

For Aulnay-sous-Bois, the projected emissions reduction supports both local climate objectives and France’s wider effort to decarbonise the heating sector. The project demonstrates how municipal infrastructure can contribute to emissions reductions while continuing to provide reliable heat to residents.

## Why District Heating Matters

District heating networks can play an important role in urban decarbonisation because they allow energy sources to be managed at system level. Individual buildings may have limited space for equipment or may be difficult to retrofit. A district network can centralise heat production and distribute it through underground pipes.

This model is particularly useful in dense urban areas with apartment blocks and public facilities. Instead of installing separate heating systems in every building, the network connects customers to a central energy source. When that source changes from fossil fuel to geothermal energy, many buildings can benefit from the transition at the same time.

District heating also creates opportunities to combine different energy sources. A network can use geothermal energy as its baseload supply and retain other technologies for peak demand and emergency backup. This approach avoids requiring the geothermal plant to meet every hour of maximum demand while still maximising annual renewable-energy production.

The performance of a district heating network depends on more than the energy centre. Pipe insulation, network temperatures, customer substations, metering, maintenance, and building efficiency all influence the amount of heat that can be delivered. Lower distribution losses improve the environmental and economic performance of the entire system.

Aulnay-sous-Bois’s geothermal project therefore represents both a drilling project and a broader infrastructure transition. Its success will depend on the integration of the underground resource with the city’s heating network and the buildings that receive the heat.

## Four Months of Drilling

The drilling campaign lasted approximately four months before reaching the target depth. [1] This timeframe reflects the complexity of drilling a deep geothermal well in an urban environment.

Deep drilling involves several stages, including site preparation, installation of the drilling rig, drilling through successive geological layers, casing installation, cementing, geological monitoring, and well testing. Each stage must be completed carefully because the well will remain a critical part of the heating system for many years.

The drilling operation must also manage noise, traffic, safety, lighting, logistics, and communication with nearby residents. Urban geothermal projects take place close to homes, roads, schools, and businesses, making public information an important part of project delivery.

The completion of drilling indicates that the project has successfully passed one of its most visible and technically demanding stages. It also provides a foundation for the next phase: installing and commissioning the equipment needed to transfer geothermal heat into the district network.

For project developers, drilling success is especially significant because the underground resource determines whether the facility can achieve its planned output. Temperature, flow rate, chemistry, pressure, and reservoir characteristics all influence the project’s operational performance.

## Managing Technical and Geological Risks

Like all deep-geothermal developments, the Aulnay-sous-Bois project has had to address geological and technical risks. The resource is located underground, so developers must rely on geological data, regional experience, exploration studies, and drilling results to confirm the conditions at the project site.

Important considerations include whether the aquifer contains sufficient water, whether the water can flow at the required rate, whether the temperature matches expectations, and whether the chemistry of the geothermal fluid is compatible with long-term operation.

Geothermal water can contain dissolved minerals and gases that may contribute to corrosion, scaling, or equipment wear. Operators therefore need suitable materials, filtration systems, monitoring procedures, and maintenance plans. Heat exchangers are commonly used to isolate the geothermal fluid from the clean water circulating through the district heating network.

Well integrity is another major consideration. Casings and cement must protect the wellbore, separate geological formations, and support safe operation over the project’s lifetime. Regular monitoring can help identify changes in pressure, temperature, flow, or chemical composition.

Reinjection is also important for reservoir management. Returning cooled water underground can help sustain the resource and reduce the need to dispose of geothermal fluids at the surface. The reinjection system must be designed to avoid adverse effects on the reservoir and maintain reliable production.

## Community Engagement During Construction

The project included public information and educational activities for residents of Aulnay-sous-Bois. During the construction period, residents could learn about geothermal energy and receive information about the work through an educational module called “À la découverte de la géothermie.” [1]

Activities were organised in different neighbourhoods, and presentations were also provided to CM1 pupils at Les Petits Ormes primary school. [1] These initiatives helped explain how geothermal energy works, why drilling was required, and how the project could benefit the city.

Community engagement is particularly important for urban infrastructure projects because construction can affect daily life. Residents may experience vehicle movements, noise, temporary access restrictions, or changes to local public spaces. Clear information can help communities understand the purpose and timetable of the work.

Educational programmes can also make geothermal energy more accessible to younger audiences. Concepts such as underground heat, aquifers, wells, and district heating can be difficult to visualise. Demonstrations and teaching modules provide an opportunity to connect local construction activity with wider climate and energy issues.

Public communication is likely to remain important as the project moves toward commissioning. Residents and network customers will want to understand when the facility will begin operating, how the system will affect heating services, and how the project’s expected environmental benefits will be measured.

## The Role of Groupe Coriance

Groupe Coriance is involved in the development and operation of district heating and cooling networks in France. In Aulnay-sous-Bois, the company has communicated the completion of the geothermal drilling campaign and the expected benefits of the project.

The company’s role places it within the wider development of renewable and low-carbon heating infrastructure in French cities. District energy operators are increasingly combining geothermal resources with other technologies to reduce emissions while maintaining reliable service.

Operating a geothermal network requires long-term management rather than one-time construction. The operator must coordinate the wells, heat exchangers, pumps, control systems, distribution network, customer substations, and backup equipment. Performance monitoring will be essential once the Aulnay-sous-Bois plant begins supplying heat.

The project’s planned commissioning before the end of 2026 will provide an important test of how effectively the geothermal resource can be integrated into the local network. Operational data will help demonstrate whether the facility achieves its expected heat production, coverage rate, and emissions reductions.

## What Happens Before Commissioning?

With drilling completed, the project enters a phase focused on completing surface infrastructure and preparing the system for operation. The exact sequence will depend on the project’s construction programme, but typical activities include:

- Completing the wellhead and surface connections.

- Installing pumps, heat exchangers, valves, controls, and monitoring systems.

- Connecting the geothermal facility to the district heating network.

- Testing production and reinjection performance.

- Checking water chemistry and equipment protection systems.

- Commissioning backup and auxiliary heating equipment.

- Carrying out safety inspections and operational trials.

- Gradually increasing production before full service.

Commissioning must confirm that the facility can deliver heat under different operating conditions. Engineers may test the system at varying flow rates and temperatures while monitoring pressure, vibration, water chemistry, and heat-transfer performance.

The network must also be balanced so that the heat produced at the energy centre reaches connected buildings efficiently. Customer substations may require adjustments to ensure that heating and domestic hot-water demand can be met without unnecessary energy losses.

Once testing is complete, the operator can move toward regular commercial operation. The first winter season will provide particularly valuable information about peak demand, backup requirements, network behaviour, and the practical performance of the geothermal installation.

## A Model for Low-Carbon Urban Heat

Aulnay-sous-Bois’s geothermal project illustrates how cities can use local geological resources to reduce the carbon intensity of heating. The project combines deep drilling, reservoir management, district energy infrastructure, and community engagement in one integrated development.

Its expected figures are significant: approximately 70°C geothermal water from the Dogger aquifer, a well depth of nearly 2,000 metres, 140 GWh of annual heat production, coverage of 93% of network demand, and an estimated reduction of 28,000 tonnes of carbon dioxide emissions each year. [1]

The project also shows why geothermal heating can be valuable in densely populated areas. It can provide continuous thermal energy, use existing network infrastructure, and serve many buildings from a central facility. Unlike intermittent renewable sources, geothermal heat can operate day and night, subject to resource and equipment availability.

The planned start of service before the end of 2026 will be the next major milestone. If the facility performs as expected, it will strengthen the role of geothermal energy in Aulnay-sous-Bois and contribute to the expansion of low-carbon district heating across the Île-de-France region.

For residents, the project’s success will ultimately be measured through reliable heating, stable service, and improved environmental performance. For the wider energy sector, it will provide another example of how deep geothermal resources can support the transition away from fossil-fuel-based urban heating.

Source : Groupe Coriance