GERESFAULT

The objective of GERESFAULT is to contribute to increase of the amount of energy extracted from the subsurface in France and Europe, by exploring new geothermal systems: crustal fault zones. Usually, geothermal exploration focus on areas that are well known for their elevated temperatures at shallow depths. If the subsurface is hot, but not sufficiently permeable, artificial techniques (with more or less success) can increase fluid circulation within the hot medium, thus creating so-called “Enhanced Geothermal Systems”. GERESFAULT, by contrast, will focus on the exploration of highly permeable fault zones (allowing for high flow rates) rooted at the depth of the brittle-ductile transition (350-400°C). Naturally, high permeability zones necessarily implies hot fluid ascent up to an economically depth level of about 2-3 km. The Pontgibaud fault zone (French Massif Central) is a relevant study case and is currently being explored by TLS-Geothermics, a geothermal exploration company that have acquired geological, geochemical and geophysical data for more than three years. In this framework, GERESFAULT propose a new way to explore potential geothermal crustal fault zones through a multi-scale combination of field studies, experimental petrophysics, geophysics and numerical modelling. The consistency and integration of the results from one scale to the other (upscaling) will be particularly addressed since the newly acquired petrophysical properties and geological models will help constrain the final 3D numerical hydrothermal system, from the fault to the crustal scale. The input of geophysical data into GERESFAULT should permit the construction of a 3D numerical, geological, static model, which will be constrained by additional field data. Petrophysical properties (porosity, permeability, density, electrical conductivuty, heat production rate, thermal conductivity) will be measured on core samples and on newly sampled rocks in order to better constrain the hydrothermal system within the fault zone. The associated scale transfer problems between different approaches will be addressed through geophysical modelling and percolation theory. The numerical modelling of the hydrothermal system will be performed at the scale of the fault zone but also at the crustal scale, for which a specific numerical code (ComPASS) will be used. Finally, a large-scale geodynamic approach will include the last 40 million years of trench retreat and should lead to the prediction of anomalously hot and permeable zones, at the scale of Europe. This innovative geodynamic approach has recently demonstrated that, in the case of a slab retreat, some thermal undulations that develop in the middle ductile crust also localize the damage zones in the upper brittle crust, and thus the associated geothermal system. A European view of these thermo-mechanical processes should help to assess European geothermal potential. During this 4-year GERESFAULT project, a 3 km deep borehole is planned - independently of GERESFAULT. Obviously, the first results from the project will be used to refine the location of the geothermal target, and similarly, the use of borehole data will provide additional data and key parameters of the hydrothermal system (productive zones and temperature distribution). However, GERESFAULT does not depend on the implementation of the drilling project. To reach the objectives of the project, the GERESFAULT team is made of 9 partners: 6 academic partners and 3 industrial partners. Further, project involves 26 scientists, and 4 master students, 4 post-docs and one research engineer will be hired in the framework of the project. A PhD thesis, co-funded by an industrial-academic partnership between TLS-Geothermicsn BRGM and ISTO has begun in March 2019 and is focused on one part of a subtask of GERESFAULT.