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Pré-publication, Document de travail

Advances on Electro-Magnetic Imaging for De-Risking Enhanced Geothermal System Prospects

Abstract : Exploiting geothermal resources at temperatures between 120 and 200°C in sedimentary and basement rocks, in rifts or in flexural basins, to produce electricity or heat is now possible because of the development of Enhanced Geothermal System (EGS) technology. Reaching such temperature range is a major challenge for mainland France and Europe as it usually requires drilling at more than 2km depth. Aside from temperature, two other conditions are required to allow the exploitation of the geothermal energy: the presence of fluid, which is the heat vector, and sufficient permeability to allow the production and re-injection of the natural fluid. This translates into the reservoir being located in the deep layers of sedimentary basins and the upper part of the Paleozoic basement, including the transition zone between the two. The reality is however even more complex as the geothermal potential of this zone is also strongly influenced by large heterogeneities comprising lithology of the cover/basement, the internal architecture of the transition zone, the structural history of the batholith and the presence of natural faults and fracture networks. It is therefore clear that the characterization of the transition zone and its heterogeneity in the deeper part of sedimentary basins constitutes one of the most challenging problems for the development of geothermal resources. Within the frame of several national and European research projects (e.g. FP7-funded IMAGE, ANR-funded CANTARE and ADEME-funded DEEP-EM projects), we have undertaken to develop geophysical techniques capable of imaging the electrical resistivity of the transition zone, as this physical parameter is highly sensitive to the presence of geothermal fluids and associated hydrothermal alterations. In this paper, we report out our analysis of core, well and field-scale resistivity measurements to establish the feasibility of electromagnetic (EM) imaging for de-risking EGS prospects in the Upper Rhine Graben. Data comes from the Soultz-sous-Forêts and Rittershoffen producing geothermal plants, an EM field trail near the Strasbourg city but also from EM measurements performed on an analogue found on exhumed crystalline basement in the Vosges Mountains. This work shows that the knowledge of the electrical properties of the transition zone can help assess its porosity/permeability and hence guide the exploration and development of deep geothermal resources. It also demonstrates that the main challenges for imaging with EM methods such a zone reside in the ability of imaging with high accuracy resistivity variations within the thick sedimentary cover but also in the ability of acquiring EM datasets with high signal to noise ratios despite the presence of strong anthropogenic noise. It also showed that the integration of the EM data with other datasets, especially structural constraints (e.g. horizons from seismic imaging), will be key to achieve sufficient resolution at target depth.
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https://hal-brgm.archives-ouvertes.fr/hal-02268520
Contributeur : Mathieu Darnet <>
Soumis le : mercredi 21 août 2019 - 09:56:18
Dernière modification le : mardi 26 mai 2020 - 11:53:56

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  • HAL Id : hal-02268520, version 1

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Mathieu Darnet, Pierre Wawrzyniak, Nicolas Coppo, J Porte, S Neeb, et al.. Advances on Electro-Magnetic Imaging for De-Risking Enhanced Geothermal System Prospects. 2019. ⟨hal-02268520⟩

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