New prospecting methods for deep geothermal resources
Sampling fluids from the geothermal well at Litomerice in the Czech Republic.
© BRGM - F. Gal
The Image project (Integrated Methods for Advanced Geothermal Exploration), completed in 2017, was a 7th FPRD programme that involved over twenty public and private partners across the EU. The BRGM, which chaired the Executive Board, was specifically responsible for piloting the sedimentary basin and underlying bedrock component and also contributed to two components on magmatic contexts: development of high-temperature tracers and hydrodynamic modelling.
Sedimentary basins and their underlying bedrock are very promising contexts for deep geothermal energy. In mainland France and in Europe, human activities tend to concentrate in these basins, resulting in high demand for energy (heat and power). Although little studied to date, they are known to contain extractable resources. The scientific challenges are to locate them and capture the geothermal fluids efficiently.
Geological model of the Rhine Graben at Obernai in the Strasbourg sector, showing faulting accross the Tertiary formations. Blue dots: resistivity data acquisition. Pink dots: passive seismic data acquisition.
© BRGM - C. Dezayes
In the Rhine Graben, the Eger region in the Czech Republic and the molasse basin in Switzerland, the BRGM’s multidisciplinary teams have been focusing on zones of interest in terms of permeability. These are deep fault zones that interface between the basement and the sedimentary overburden.
Several prospecting methods have been significantly improved. By transposing a technique used in volcanic environments to measure the maximum temperature encountered by fluids, the project succeeded in developing and validating auxiliary chemical geothermometers capable of determining the temperature at equilibrium of fluids in deep reservoirs, within a range of 50 to 320°C.
A seismic probe for data acquisition in boreholes.
An active electromagnetism method, using sensors on the surface, was also developed to produce 3D imagery of deep geological objects (such as large faults) lying below urban areas. This highly innovative method required the development of specific expertise on implanting electrodes and interpreting the signal.
In order to represent the deep structures of these basins, our researchers also applied the passive seismics principle to capture ambient “noise” over long periods (vibrations due to human activities and geological events, even at a considerable distance) before analysing the responses of the subsoil.
As regards modelling, finally, in-depth studies were conducted on the integration of dynamic features (status of constraints and flow rate of fluids, for example) into static (geometric) geological models. Mechanical and hydraulic models incorporating a complex network of faults in 3D were developed at the regional scale (Rhine Graben) in order to model fluid circulation and identify the most suitable sectors for geothermal fluid extraction.
Image is a highly innovative project which, by identifying the most relevant techniques for acquiring useful data for the different models that help to define the boundaries of zones of geothermal interest, has successfully developed prospecting procedures suited to the types of targets sought. These methods can now be applied to other areas such as the Paris Basin and the Rhône Valley.