Aulnay-sous-Bois Geothermal Breakthrough: Drilling Begins for a Cleaner, Local Heat Future in France’s Paris Basin
By Alphaxioms Geothermal News | Published: March 28, 2026
In a landmark moment for urban renewable energy, the French city of Aulnay-sous-Bois has officially launched the drilling phase of its ambitious geothermal project. Announced by Groupe Coriance on March 26, 2026, this initiative marks a decisive step in the city’s energy transition, harnessing the Earth’s natural heat to deliver sustainable, low-carbon heating to thousands of residents. Located in the heart of the Île-de-France region, just northeast of Paris, Aulnay-sous-Bois is joining a growing network of communities tapping into the vast geothermal potential of the Paris Basin’s legendary Dogger aquifer.
This isn’t just another infrastructure project—it’s a blueprint for how cities worldwide can decarbonize heating, reduce reliance on volatile fossil fuels, and build resilient local energy systems. With drilling underway and a new geothermal heating plant under construction, the project promises to supply 93% renewable energy to an expanded district heating network, slashing annual CO₂ emissions by a staggering 28,000 tonnes. As global attention turns to geothermal as a baseload renewable solution, Aulnay-sous-Bois stands as a shining example of innovation meeting practicality.
In this in-depth 2500-word feature, we dive into every aspect of the project: its technical details, historical context within France’s geothermal leadership, the role of Groupe Coriance, environmental and economic benefits, the science of deep drilling, community engagement efforts, and what this means for the future of geothermal energy in Europe and beyond. Whether you’re a policymaker, energy enthusiast, environmental advocate, or simply curious about how the ground beneath our feet can power our homes, this article delivers the full story.
The Project at a Glance: Timeline, Scale, and Ambition
The Aulnay-sous-Bois geothermal initiative is a comprehensive overhaul of the city’s heating infrastructure. Construction of the central heating plant began in March 2026 and is slated for completion by early 2027. At the heart of the facility will be a geothermal doublet—two boreholes drilled to approximately 1,700 meters depth into the Dogger limestone formation. These wells will extract hot water at around 70°C, which will then be integrated with heat pumps and gas boilers for peak and backup supply.
Drilling operations themselves are expected to run for four months, from late March through early July 2026. The full geothermal system is targeted for commissioning in January 2027, aligning perfectly with the winter heating season. Once operational, the network will transition dramatically: 93% of the heat supplied will come from renewable geothermal sources, with natural gas reserved strictly for supplementary and emergency use.
The existing district heating network, managed by Aulnay Énergie Services (a Coriance subsidiary since 1999), currently spans 15 kilometers of pipes and serves the equivalent of 5,500 dwellings. The project includes a major expansion—adding 22 kilometers of new piping—bringing the total network length to 37 kilometers. This upgrade will extend reliable, affordable heat to more homes, public buildings, schools, and potentially future developments like the Val Francilien business zone.
Early public engagement is already underway. Coriance is hosting a series of interactive workshops titled “À la découverte de la géothermie” (Discovering Geothermal Energy) for children and adults alike. Running for three months across municipal venues, these sessions use simple explanations, models, and visuals to demystify how geothermal works and its role in cutting environmental impact. It’s a smart move that builds trust and educates the community during the visible construction phase.
Artist renderings released by Coriance depict a modern, compact heating plant designed by Hurtaux architecte, blending seamlessly into the urban landscape while showcasing sustainable engineering.
Understanding Geothermal Energy: Why the Paris Basin is a Geothermal Goldmine
To appreciate the significance of Aulnay-sous-Bois, we must first understand geothermal energy itself. Unlike solar or wind, which are intermittent, geothermal provides constant, 24/7 baseload power by tapping heat stored in the Earth’s crust. In deep geothermal systems like this one, hot water or steam is extracted from underground aquifers, used for heating (or electricity in higher-temperature cases), and then reinjected to maintain reservoir pressure and sustainability.
The Paris Basin, underlying much of the Île-de-France region including Paris and its suburbs, is one of Europe’s premier geothermal hotspots. At its core is the Dogger aquifer—a Middle Jurassic limestone formation roughly 160 meters thick, lying 1,500–2,000 meters below the surface. Naturally heated by the Earth’s geothermal gradient (approximately 3°C per 100 meters in this area), the Dogger yields water at 55–85°C, ideal for district heating after minimal processing.
France pioneered large-scale geothermal district heating here in the 1970s, driven by the 1973 oil crisis and a national push for energy independence. The first plants came online in the late 1970s and early 1980s, using the now-standard “doublet” technology: one production well to extract hot water and one injection well to return cooled water, creating a closed-loop system that minimizes environmental footprint and prevents depletion.
Over the past 40+ years, more than 40 geothermal plants have been developed in the Paris Basin, collectively supplying heat to over 250,000 households plus hospitals, schools, and businesses. This represents one of the largest urban geothermal clusters on the planet. The Dogger aquifer has proven remarkably resilient, with careful management preventing issues like scaling, corrosion, or thermal breakthrough (where cold reinjected water cools the production zone). Operators monitor reservoir performance meticulously, and historical data from the French Geological Survey (BRGM) shows stable long-term output.
Aulnay-sous-Bois builds directly on this legacy. The city’s existing network was originally gas-fired, but the new geothermal doublet will transform it into a predominantly renewable system. This shift aligns with France’s national goals: the government aims to quadruple deep geothermal deployment by 2035, growing from 2.3 TWh of heat in 2021 toward 6 TWh by 2030. Recent years have seen a surge in permit applications, with Coriance at the forefront of execution.
Groupe Coriance: France’s Geothermal Leader Accelerating the Transition
Groupe Coriance, a specialist in urban heating and cooling networks, operates 46 district systems across France totaling over 557 km of pipes. With 70% of its energy mix already from renewables and recovery sources, the company positions itself as one of the greenest operators in the sector—and the third-largest overall.
Coriance has deep roots in Aulnay-sous-Bois through its subsidiary Aulnay Énergie Services (AES), which has managed the local network since 1999. The group is no stranger to ambitious projects in Seine-Saint-Denis and neighboring suburbs. Recent milestones include the inauguration of a second geothermal plant in Champigny-sur-Marne (April 2025), completed drilling in Garges-lès-Gonesse (August 2025), and ongoing work in Dugny-Le Bourget, Plessis-Robinson, Châtenay-Malabry, and Alfortville. In 2025 alone, Coriance advanced four major geothermal forages, underscoring its role as Île-de-France’s geothermal champion.
What sets Coriance apart? A commitment to public-private partnerships (often via mixed-economy companies like AULNERGIE, the Aulnay joint venture with SEMAD), rigorous risk mitigation (France’s historic geothermal insurance scheme has de-risked projects since the 1980s), and a focus on community benefits. The Aulnay project exemplifies this: stable heat prices decoupled from fossil fuel volatility, job creation during construction and operation, and educational outreach.
Technical Deep Dive: How the Aulnay Drilling Works
Drilling a geothermal doublet is a marvel of modern engineering. Here’s a step-by-step look at what’s happening on-site in Aulnay-sous-Bois:
1.Derrick Installation: A tall metal tower (derrick) is erected to support the drill string.
2.Bit and Rod Assembly: A rotary drill bit (trépan) with teeth is attached to the first hollow steel rod. Additional rods are screwed together as depth increases.
3.Drilling Process: The bit rotates under its own weight and the force of the rig, cutting through soil, rock, and finally the Dogger limestone. Drilling mud—a specialized fluid—is pumped down the hollow rods under pressure. It cools the bit, lubricates the borehole, carries rock cuttings (déblais) back to the surface, and stabilizes the walls to prevent collapse.
4.Mud Management: At the surface, cuttings are separated in settling tanks (décantation), and the cleaned mud is reused in a closed circuit.
5.Geological Monitoring: A team of geologists works 24/7, analyzing cuttings and real-time data to ensure the borehole hits the target aquifer precisely. Sensors track pressure, temperature, and formation properties.
Once both production and injection wells are complete (typically 1,700 m deep here), surface piping connects them to the heating plant. Heat exchangers transfer energy from the 70°C geothermal water to the district heating loop without mixing fluids. Heat pumps boost efficiency for lower-temperature needs, while gas boilers provide redundancy.
This doublet design ensures sustainability: extracted heat is balanced by reinjection, maintaining aquifer pressure and temperature over decades.
Environmental and Economic Wins: Quantifying the Impact
The numbers tell a compelling story. By displacing fossil-based heat, the project will avoid 28,000 tonnes of CO₂ emissions annually—equivalent to removing roughly 6,000 cars from the road each year. At 93% renewable supply, Aulnay’s network will rank among France’s cleanest.
Economically, residents gain price stability. District heating prices are regulated and insulated from oil and gas market swings, offering predictability for households and businesses. Construction and expansion create local jobs—in drilling, engineering, pipe-laying, and ongoing operations. Long-term, lower maintenance costs for geothermal (compared to aging gas boilers) benefit taxpayers.
Broader ripple effects include improved air quality (fewer combustion emissions) and alignment with EU Green Deal targets. For a suburb like Aulnay-sous-Bois—home to diverse communities and facing urban heat demands—this project delivers tangible climate justice.
Challenges Overcome and Lessons from the Paris Basin
Geothermal isn’t without hurdles. Early Paris Basin projects in the 1980s faced scaling from mineral-rich waters, corrosion, and occasional thermal short-circuiting. Decades of experience have refined solutions: advanced materials, chemical inhibitors, regular well workovers, and sophisticated 3D reservoir modeling (tools like Seequent Leapfrog now optimize well placement).
France’s unique geothermal risk guarantee fund (established in the 1980s and evolved since) has insured exploration risks, encouraging investment. Public inquiries, like the 2025 one for Aulnay, ensure transparency on environmental impacts.
Looking Ahead: Geothermal’s Role in France and Global Energy Transition
Aulnay-sous-Bois is part of a renaissance. With permit applications doubling in recent years and Coriance leading execution, France is poised to meet—and exceed—its 2035 targets. Similar projects in nearby suburbs demonstrate a domino effect: once one city proves the model, neighbors follow.
Globally, geothermal is gaining traction for its high capacity factor (>90%) and minimal land use. Cities from Nairobi (Kenya’s Olkaria fields) to Reykjavik (Iceland’s district heating) show what’s possible. In Europe, the Paris Basin model is studied for replication in other sedimentary basins.
For Aulnay, success here could unlock further expansions—perhaps integrating cooling networks, green hydrogen production from excess heat, or even shallow geothermal for individual buildings.
Conclusion: Heat from the Earth, Hope for the Future
As the drill bit bites into the Dogger beneath Aulnay-sous-Bois, France reaffirms its position as a geothermal pioneer. This project isn’t merely technical—it’s transformative. It delivers clean heat, economic resilience, environmental stewardship, and community empowerment.
For geothermal advocates worldwide, the message is clear: the technology works, the geology is proven, and the benefits are immediate. Cities everywhere should look to Aulnay-sous-Bois as inspiration.
Stay tuned to Alphaxioms Geothermal News for updates on commissioning, performance data, and the next wave of Coriance projects. The ground is warming up—literally—and the future of urban energy is brighter (and greener) than ever.
Source: Gruope Corian
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