Flagship project / 2007-2018: results and prospects for heat production from deep geothermal energy
In a report submitted to the French Agency for the Ecological Transition (ADEME) in March 2020, BRGM provides a detailed analysis of more than ten years of deep geothermal energy operations This substantial study, funded under the national agreement linking the BRGM and ADEME, draws lessons from the operations and studies conducted since 2007 when they were resumed, with the aim of consolidating the revival of geothermal activity and encouraging new projects. As summarised by project leader Virginie Hamm, "We used the information collected in the Sybase database on the characteristics of the engineering works, the geothermal reservoirs and geothermal energy production to review all deep geothermal operations conducted in France to produce heat for all end uses, whether domestic, industrial or other." The analysis included impacts on urban heat distribution networks, particularly in the Paris region, in terms of creation, extension, interconnection and densification.
We also synthesised the R&D studies on deep geothermal energy co-funded by the ADEME to facilitate the use of their results concerning the conditions required for carrying out and monitoring these operations. "These research projects were aimed at overcoming certain technical or scientific obstacles, from resource characterisation to exploitation and including modelling, construction technologies and aquifer access conditions, as well as operational monitoring and decision-support tools," says Virginie Hamm. The aim is to share the experience gained from the different geological targets, the carbonated Dogger and Lusitano reservoirs and the clastic reservoirs of the Albian/Neocomian and Triassic in the Paris Basin.
The review also included an economic analysis. A study of the investment and operating costs of deep geothermal operations was carried out based on data provided by the prime contractors for 23 geothermal sites with a total of 43 boreholes drilled into the Dogger from 2008 to 2018. "This inventory should enrich the current debate on the development of the geothermal guarantee fund and its linkage with the renewable heat fund, with a view to improving the implementation of explorations, particularly in so far little-known basins and aquifers with unused or underused geothermal potential, which require more costly upstream studies," announces Virginie Hamm.
A rational approach to projects
To promote the use of new aquifers, an inventory of available data for the main sedimentary basins in mainland France has demonstrated the need for additional exploration programmes to limit the risk of failure of geothermal projects. A technico-economic analysis of the various solutions that could be implemented validated the relevance of an integrated approach involving multiple disciplines, from geology, geophysics and hydrogeology to thermal, engineering. Among these solutions, BRGM recommended the use of seismic reflection to explore new basins and aquifers. This indirect acoustic-imaging method, first used by the oil industry, provides a three-dimensional view of geothermal reservoirs and the geological horizons surrounding them. "With this method, We can thus visualize variations in physical properties of interest for geothermal energy, such as the porosity of target reservoirs, and consequently quantify the resource and evaluate its geothermal potential before drilling," explains Mathieu Darnet, Head of the Geophysical Imaging unit. "Given the information it can provide, the cost of implementing this very reliable technology when compared to the price of drilling makes a risk assessment prior to investment very much worthwhile."
Studying a resource before implementing explorations and subsequent operations: this rational approach is gradually coming into use in deep geothermal projects, as in a recent Électricité de Strasbourg project in Alsace. This is an example to be followed in the light of the objectives set for the multi-annual energy programme, which projects a fourfold increase in final heat consumption from deep geothermal energy from 2017 to 2028.