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Geothermal resources of crustal-scale fault zones: exploring new systems for competitive geothermal power production – GERESFAULT

Geothermal resources of crustal fault zones: exploring new systems for competitive geothermal power production

Crustal fault zones (CFZ) correspond to permeability anomalies of crustal rocks. Instead of looking for temperature anomalies and artificially enhance the rock permeability, this project is focused on anomalously high permeability zones, where deep hot fluids naturally rise up to the subsurface. With a multidisciplinary approach on the Pontgibaud CFZ (Massif Central), the understanding of this hydrothermal system will be applied to european CFZ.

Characterize Crustal Fault Zones (CFZ) as new high-temperature geothermal systems, in France (CFZ of Pontgibaud, French Massif Central) and in Europe

The objective of GERESFAULT is to enhance the amount of energy extracted from the subsurface in France and Europe, by exploring new geothermal systems: crustal fault zones. Usually, geothermal exploration is focused on areas where subsurface temperatures are anomalously high. Here, GERESFAULT focus on areas where subsurface permeability is anomalously high, as crustal fault zones. These fault zones, allowing elevated flow rates, are rooted around the brittle-ductile transition (350-400°C). The high permeability makes it possible to get rising deep hot fluids up to an economic depth level, around 2-3 km.<br /><br />The Pontgibaud fault zone (French Massif Central) is an exceptional case study on which TLS-Geothermics, a small exploration company, gain knowledge for more than four years (Geology, Geochemistry, Geophysics) to demonstrate this concept validity. In this framework, GERESFAULT suggests a new exploration approach for defining the geothermal potential of crustal fault zones, through a multi-scale combination of geology, experimental petrology, geophysics and numerical modeling.<br /><br />Hence, the main objective aims at a complete understanding of the dynamics of these hydrothermal systems, from their geology, geometry, petrophysics, geophysical signatures and from numerical models reproducing field data. Simulations of power production will then be performed, and this concept will be extrapolated to recognized CFZ in Europe.

Field work: structural analyses to define the architecture of the Pontgibaud Fault Zone and its embeddings; geology (rock type identification, sampling, in situ radiometry to quantify heat production); geophysical surveys (magnetics, electro-magnetics, airborne radiometry) to delineate possible envelope of the geothermal reservoir.A final 3D crustal-scale model will be built and reproduced in a digital format.

Laboratory work: measurements on rock samples of density, thermal conductivity, heat capacity, permeability and electrical conductivity. Measurements at room pressure and high pressures corresponding to a depth of a few kms.

Geophysical modeling and scale transfer: from measurements in the laboratory and from geophysical surveys, a scale transfer will be defined. Upscaling laws from the laboratory to the rock scale, and from the rock to the crustal scale will be established, in particular with the percolation theory.

Numerical modelling will be applied to three scales: the fault zone-scale, the crustal scale and the lithospheric scale. Hydrothermal processes will be investigated at the fault zone scale, with the Comsol Multiphysics tool. Heat and mass transfer at the crustal scale will be tackled with the Compass code, and lithospheric processes will be investigated with the pTatin3D code.

Finally, simulation of geothermal power production at Pontgibaud will be investigated, through modeling of fluid circulation between 2 boreholes. Extrapolation of the entire methodology to European crustal fault zones will be guided by european dataset such as structural datasets available on onegeology.org.

Results from field work suggest that partial melting of metasedimentary rocks of Pontgibaud area are genetically linked to migmatites by partial melting, and to granitic plutons by magma extraction. This allows us to suggest a geological model for Variscan crust structure in the Pongibaud areawith redistribution of heat producing elements from partial melting zone at depth to superficial part of the crust.

Results obtained in the Malzieu basin emphasize the influence of the CFZ width on potential convective fluid circulations.

Results obtained with clinopyroxenes suggest a magma dynamics marked by the presence of paleo-reservoir at a depth of 45 km in the mantle, at the base of the crust around 30 km and within the upper crust around 10 km.

The statiscical study on ground-measured radiometric data and airborne data demonstrated the influence of measurement scale on the observed distribution of radiogenic elements. This study also shows some important differences in absolute values which are not linked to a scale factor, but which can be associated to the the ground cover (organic matter). This cover would control the airborne data and must be investigated to better interpret concentration maps of K, Th and U.

Primary laboratory experiments have shown that the permeability of a fracture within a granite, after a significant sliding on the fracture, does not decrease below that of the host rocks. It does not represnt a barrier to fluid flow.

Thermo-mechanical models have shown that 1/ mantle dynamics helps to maintain the 1300°C isotherm at shallow depth and thus limits the conductif cooling and that 2/ heat sources locations control the deformation location and this could have a positive feedback on geothermal potential by allowing a local mantle uprise.

Numerical models of hydrothermal fluid flow around CFZ confirm that 1/ thermal anomalies are more important and shallower when fault zone is vertical, and that 2/ a strike-dlip tectonic regime will favor high amplitude thermal anomalies as well as their lateral diffusion.

Development of numerical solutions for propagating electric properties from cm/dm to km.

Development of a new methodology for sensibility analysis ((CPSO/Baysian) applicable to numerous problems

A next field campaign where several GERESFAULT partners will be involved is planned in spring 2022. The objective is to constrain the 3D geological model (geometry and petrophysics).

The used methodology for the Malzieu basin will be applied to the Chaudes-Aigues hydrothermal system (Cantal) during a master 2 training stage (6 months)

Estimate thermo-barometric informations and acquire geochronological data

To complet analyses on clinopyroxene, analyses on amphiboles will be performed on different volcanic systems

Another field campaign in 2022 will be focused on ground measurement of radiogenic signatures to identify relationships between ground and rock measurements.

Experimental data on permeability (post-doc in 2022) will be used in large-scale models of fluid circulation in fractured reservoirs.

Numerical models at lithospheric scale with the pTatin3D code are still ongoing. A first manuscript will be written after the post-doc (january 2022).

Models at the crustal scale with the Compass code will begin in spring 2022 (new post-doc).

Measure of electric conductivity will be performed on Pontgibaud rock samples and will be compared to permeability measurements (common project between ISTO and ITES in 2022).

Numerical methods on scale transfer will be applied to the petrophysical and geological models developed in the other tasks of GERESFAULT.

Guillou-Frottier L., H. Duwiquet, G. Launay, A. Taillefer, V. Roche, G. Link. On the morphology and amplitude of 2D and 3D thermal anomalies induced by buoyancy-driven flow within and around fault zones, Solid Earth, 11, 1571-1595, 2020. doi.org/10.5194/se-11-1571-2020

Duwiquet H., L. Guillou-Frottier, L. Arbaret, M. Bellanger, T . Guillon, M.J. Heap. Crustal Fault Zones (CFZ) as Geothermal Power Systems: A Preliminary 3D THM Model Constrained by a Multidisciplinary Approach. Geofluids, 2021, article ID 8855632, 24 p., 2021.
doi.org/10.1155/2021/8855632

Duwiquet H., L. Guillou-Frottier, M. Bellanger, L. Arbaret, M.J. Heap. Crustal Fault Zones: New targets for geothermal exploration? Insights from the Pontgibaud Fault Zone in the French Massif Central. Proceedings, World Geothermal Congress 2020, Reykjavik, Iceland, April 26 – May 2, 2020, 10p.

Ars, J-M., P. Tarits, S. Hautot, M. Bellanger, O. Coutant, M. Maia. Geothermal Exploration With Ambient Noise Tomography, Gravity Data And a 3-D MT Resistivity Model In a Joint Inversion Approach. World Geothermal Congress 2020, Reykjavik, Iceland, April 26 – May 2, 2020, 7p.

Duwiquet H., L. Guillou-Frottier, L. Arbaret, T. Guillon, M. Bellanger, M.J. Heap. Characterization of Crustal Fault Zones as geothermal power systems: a multidisciplinary approach. Réunion des Sciences de la Terre, RST 2021, Lyon, November 01-05.

Jolivet L., C. Allanic, T. Becker, N. Bellahsen, J. Briais, A. Davaille, C. Faccenna, C. Homberg, E. lasseur, B. Romanowicz. Continental rifts and mantle convection: Insights from the East African Rift and a new model of the West European Rift System. Réunion des Sciences de la Terre, RST 2021, Lyon, November 01-05

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.

Project coordination

Laurent Guillou-Frottier (BUREAU DE RECHERCHE GEOLOGIQUE ET MINIERE)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

IPGS IPGS Institut de Physique du Globe de Strasbourg (UMR 7516)
BRGM BUREAU DE RECHERCHE GEOLOGIQUE ET MINIERE
TLS TLS GEOTHERMICS
CNRS-ISTO Institut des sciences de la Terre d'Orléans
ISTEP Institut des sciences de la Terre Paris
STORENGY STORENGY
IMAGIR IMAGIR
UBO-LGO Université Bretagne Occidentale (UBO), Laboratoire Géosciences Océan (LGO)
GET Géosciences Environnement Toulouse

Help of the ANR 767,563 euros
Beginning and duration of the scientific project: February 2020 - 48 Months

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