The inhabitants of the Paris basin (France) have two deep aquifers under their feet that are worth gold: hot underground water tables, so an article in French publication Revolution Energetique. The article describes the utilisation of geothermal energy for heating in the Greater Paris area
Since the first drilling carried out in Melun in 1969, geothermal projects have followed one another. Operated today by some fifty heating networks, they supply the equivalent of 250,000 homes. In Bobigny a drilling rig has been at work for a few months. Four experimental wells are drilled there. If the project is successful, it will open up new prospects for French geothermal energy.
Located between 1,500 and 2,000 meters deep, the Dogger is the main aquifer exploited in the Paris region, the most productive in Europe in terms of geothermal potential. This limestone geological formation, 150 to 175 million years old, contains a fossil layer whose temperature varies between 60 and 80 degrees Celsius. Highly loaded with mineral salts, the brackish water of the Dogger is unfit for consumption, but the heat it contains can be harnessed to supply district heating networks. It is indeed a renewable energy since, after having given up its calories in an exchanger, the water is reinjected into the basement where it is heated by circulating in the geological layers. On a human scale, this energy is also inexhaustible because it is fueled mainly by the natural disintegration of radioactive elements contained in the earth’s crust, such as uranium and thorium. Continuously exploitable, geothermal energy does not depend on weather conditions and therefore does not require storage. In addition, its operation does not emit greenhouse gases. In short, it is “perfect” energy … or almost.
This is why since the first drilling carried out in the Dogger at Melun in 1969, geothermal projects have followed one another in Ile-de-France (so the French name for the greater Paris area). Operated today by around fifty heating networks, they supply the equivalent of 250,000 homes. The conditions are in fact met there to make it the largest geothermal operation in Europe: a large sedimentary basin with a deep hot water aquifer and, at the surface, a high population density allowing economic exploitation by local people. district heating networks
Avoiding overexploitation of the deposit
But the danger that threatens a geothermal deposit is that of its overexploitation. The water, when it is reinjected into the water table, is cooled to around 40 degrees Celsius. The result of the operation is therefore to create a cold bubble around the reinjection well. If at the surface the head of the pumping well and that of the reinjection well are close to each other, the latter is drilled obliquely so that at depth a distance of several kilometers separates the extraction from the reinjection.
Despite everything, a growth of this cold bubble can lead to the cooling of the resource in the long term and put its exploitation in question. In the Paris basin, the risk has increased in recent years due to the increase in new wells.
In 1985, specialists in the operation of the Ile-de-France Dogger estimated that the extension of the cold zones would lead to the closure of the water supply networks. geothermal heat around 2005. But 30 years later, new studies based on monthly readings carried out by operators have shown that geothermal energy in Ile-de-France still has a bright future ahead. Apart from the case of the Alfortville drilling where a drop of 3 degrees Celsius was observed, the simulations predict a thermal decrease which should not be felt in the aquifer before the 2040s.
However, to avoid overexploitation of the deposit and delay the deadline, Sipperec (Paris intercommunal union for energy and communication networks) decided to experiment in Bobigny with an extraction in a deeper geological layer, that of the Triassic , located under the Dogger, 2,100 meters from the surface. The water in this aquifer is naturally warmer: 80 degrees Celsius compared to 60 degrees Celsius in the Dogger at this location.
The exploitation of the Triassic, if it is successful, would be a first in France and it would open up new perspectives for French utilisation of geothermal energy. In particular, it would make it possible to prospect in the west of the Paris region, where the temperature of the water pumped into the Dogger is not sufficient to ensure economic exploitation. Today, two-thirds of the Ile-de-France wells are concentrated in the east, south and north, mainly in Val-de-Marne. Going out to conquer the west, however, would require the green light from the state and mining authorities.
Descending into the Triassic increases the risks
The cost of deeper drilling is obviously higher: going down in the Triassic amounts to 9 million euros per well compared to 5 million for those that stop in the Dogger. But the use of warmer water gives rise to hopes of lower operating costs. According to Sipperec, the price charged to network users could even be reduced. At the surface, the water pumped into the borehole is not immediately usable in the network. It transfers its heat, in an exchanger, to a secondary circuit, the temperature of which must be raised by heat pumps, which results in electricity consumption. Extracting hotter water is therefore the assurance of reduced energy costs.
The Triassic rock being more friable, the risks are also higher and, in the past, two attempts have already failed. As in oil exploration, the success of geothermal drilling is never guaranteed: at the end of the day, the water temperature may be too low or its flow insufficient.
The experiment is of great interest to Ademe, the French agency for ecological transition, and the [Paris] Region, which respectively finance EUR 17 and EUR 4 of the EUR 78 million euros of planned investments. Sipperec brings almost 50 million.
In Bobigny, below the Parc de la Bergère, a drilling rig began drilling four wells on November 21, 2019: two for pumping and two for reinjection. Work continued throughout the confinement. The wells have been drilled to the Dogger aquifer to a depth of about 1,800 meters. It is a success: the tests carried out have shown that the temperatures and flow rates are in line with what was expected. Continued drilling to the Triassic at 2,100 meters began on May 25, 2020.
If all goes well, the Genyo network will see the light of day there by 2021. Thirty kilometers long, it will supply green heat to the equivalent of 20,000 homes in the town and in Drancy, its neighbor. The project will prevent the emission of 30,000 tonnes of CO2 annually.
Source: Revolution Energetique