A geothermal resource?
A high-temperature geothermal resource is a renewable and exploitable quantity of heat that is naturally present underground.
As the temperature in the underground increases with depth, one could say that it is enough to drill deep enough to find a geothermal resource. However, this is not necessarily true since temperature is not the only parameter to be taken into account.
Indeed, the amount of exploitable heat also depends on the presence of natural fluid in the subsoil, its volume and its capacity to circulate in the rocks.
For example, there is much more heat in a swimming pool at 25°C than in a cup of tea at 80°C. This is due to the difference in the volume of hot water. A high temperature without the presence of a fluid does not necessarily constitute a geothermal resource.
Why in the Massif Central?
So why is the Massif Central a favorable region for high temperature geothermal energy? First of all, it is linked to the large amount of heat naturally present in its subsoil. Under the Massif Central, the earth’s mantle, which is very hot, rises slightly and brings with it large quantities of heat. This rise of the mantle explains the bulging of the regional topography that can be clearly seen on a map of France from the altitude.
Without this bulge, the Massif Central would have the same altitude as Brittany since these two regions have a similar geological history.
This phenomenon, which makes the Massif Central a region with a particularly hot subsoil on the scale of Europe, has also resulted in the recent volcanic activity, on a geological scale, that can be observed throughout the region.
The other reason that makes the Massif Central a region of interest for geothermal energy is the composition of its rocks and the structures that cross them. Indeed, the rocks of the Massif Central are essentially “basement” rocks like granite, and are very rigid. These rocks generally appear in red on geological maps.
Moreover, the rocks of the Massif Central are intersected by what are called faults. These geological structures are the result of the forces at work during the movement of tectonic plates. The faults are zones where the rock is naturally very fractured. Fluids will therefore be able to fill these fractures and circulate naturally along them both horizontally and at depth.
Why the Sioule PER ?
In 2015, TLS Geothermics applied to the State for a PER for Exclusive Permit to Search for a high temperature geothermal deposit in the Sioule region. This permit, which only grants an exclusive right to explore and not to drill was validated in 2017. It covers a large area in which there are elements favorable to the presence of a geothermal resource. The region is indeed located at the plumb of the heat rise under Massif Central. It is also crossed by several large faults including the Pontgibaud fault which was exploited until the end of the 19th century for silver lead. Finally, the Sioule region is also of interest due to its proximity to the Chaîne des Puys, which could imply a surplus of heat in the deep underground linked to its recent volcanic activity.
Joined by Storengy in 2017, TLS Geothermics has since conducted studies to understand the subsurface and assess the geothermal potential of the region. The analysis methods, often deployed in partnership with universities, have consisted of: analyzing rocks and faults on the surface, obtaining images of the subsoil through what is called geophysics and analyzing the composition of the fluids in certain springs such as in Ceyssat or Pranal.
All the results obtained during these analyses have confirmed the strong geothermal potential of the area. A very favourable site for the presence of a high temperature geothermal resource has been identified in the commune of Saint-Pierre-Roche thanks to a number of favourable elements resulting from the exploration.
First of all, the identified site is located at the intersection between two fault zones: the Pontgibaud fault and the Miouze fault. This leads to an increase in the number of fractures and therefore in the volume of fluid available and its capacity to circulate in the rock.
Other elements indicate the presence of fluid circulation in the fault zone, from deep hot levels to shallower depths of about 3 km.
The subsurface images taken during the exploration have allowed us to locate these circulations at the very intersection between the two identified faults and at a depth of 3 to 4 km.
The geophysical images also showed the deep rooting of these structures in what could be an old solidified but still hot magma chamber located between 12 and 15 km deep.
How to extract the heat?
An exploration drilling is therefore proposed on this site to first verify the favorable elements observed from the surface, then to acquire new in-situ data and finally to validate or not the presence of an exploitable geothermal resource. The diameter of the deepest terminal part of the borehole is about 20 centimeters.
The results of the first drilling, called SIM1, will be considered positive if the temperature and the fluid flow found are sufficient. In this case, a second borehole will be drilled to constitute what is called a geothermal doublet. In a doublet, the hot geothermal fluid is pumped through the first well. Part of its heat is then recovered at the surface in a geothermal power plant and used to produce electricity and heat. The fluid is then completely reinjected into the subsoil through the second well. The two wells in a doublet must be spaced apart to allow the fluid to warm up as it circulates through the rock.
If, and only if, the data from the boreholes are very positive, it may be possible to build a second doublet from the same site to increase the production capacity of the geothermal plant. This is why the Geopulse project is requesting a total of 4 boreholes.
If the results of the first well are not good, the power plant project will be abandoned and the drilling site will be returned to its initial state.
For which uses?
The geothermal power plant will have an electrical capacity of 5 MW or even 10 MW if there are two doublets. It will allow for the continuous production of clean, renewable electricity to supply the equivalent of 9,000 French homes, in the case of a single doublet. This corresponds to about ten wind turbines. The land area of the plant will be reduced to 2 hectares, and the landscape impact will be low, especially compared to wind turbines. In addition to electricity, the plant will produce heat that can be used locally to meet needs yet to be identified. In addition, during the operation phase, the plant will employ 5 people full time.