LES MASSIFS ROCHEUX DU CRETACE SUPERIEUR DU LABOURD OCCIDENTAL : PROCESSUS D’ALTERATION ET INSTABILITES LITTORALES

Coastal erosion threatens human lifes, private properties and public substructures and buildings. Along the coast of Labourd (France), it is mainly the result of landslides. Previous studies show that most of the slidings occur in weathered rock (weathering of cretaceous marls and limestones with flysch facies), called alterites. In order to understand failure processes in these rocks, geological, geotechnical and hydrogeological context is studied. Weathered rocks are very heterogeneous silty clays with flints. The parent rock structure has been conserved although being compressed. At the base, the limestones are characterized by a karstic channels system containing in situ alterite. This level, called karstified level, constitutes the « roots » of the weathered rocks. It is formed by the particular weathering process called « ghost rocks alteration », which consists in the dissolution and the oxidation of the rock mass by very slow percolation water. According to the parent rock facies, that induce variations in calcareous contents, the depth of the seepage of the weathering front is variable. The alterites thickness can reach 50 m, and reaches its peak in the most calcareous parent rocks. The fluctuation in the altitude of the footwall of the alterites is the roots of the coastal morphology evolution. Erosion is easier in alterites leading to a faster retreat of the coast in this rock and, so, to the formation of coves. Nowadays, erosion is the consequence of numerous landslides in alterites: in fact, weathering process generates a reduction of rock strength. Moreover the weathered rock structure, and in particular the karstified level, induce an easier water circulation which is canalized, in one hand, by permeable levels of alterites (silty and sandy levels and fractured flint beds) and, in the other hand, by karstic channels that have been recently hollowed out. Water creates destabilizing strengths in the weak rock mass. Thus, water circulation is the main factor inducing the failure in the rock mass. These analyses and observations allowed to create a tool to estimate the hazards in the alterites. Based on the presence of mains triggering factors of landslides, it will be used to make the hazards mapping evolve with new fact that can appear subsequently.L’érosion du littoral est un aléa qui menace les vies, les biens personnels et les infrastructures publiques. Sur la côte labourdine (Pays Basque français), elle est en partie le fait de nombreuses instabilités de versant. Les études préliminaires menées par le BRGM, dans le cadre des travaux de l’Observatoire de la Côte Aquitaine, ont mis en évidence la présence et l’implication de roches altérées, les altérites, dans ces mouvements de terrain. Dans la continuité de ces observations, l’étude du contexte environnemental des instabilités qui mobilisent ces roches méconnues est réalisée afin de mieux appréhender in fine les processus de rupture. Elle est focalisée sur les altérites des marno-calcaires à faciès flysch du Crétacé supérieur, qui forment les deux tiers des reliefs côtiers basques. Les produits d’altération de ces roches sont des argiles silteuses au sein desquelles perdurent des bancs de chailles fracturés. Ces altérites ont conservé la structure de la roche-mère, litage et déformations tectoniques, bien qu’elles se soient tassées. A leur base, l’horizon karstifié, caractérisé par un réseau de conduits formés dans les bancs calcaires pleins d’altérites en place, forme les « racines » de l’altération dans les roches-mères. Le processus qui a conduit à cette transformation a consisté en une oxydation et une dissolution des carbonates, probablement par une masse d’eau inerte, par « fantômisation ». De ce fait, il apparait une altérabilité croissante avec la teneur en carbonates des marno-calcaires : dans les faciès de flysch caractérisés par une dominance des bancs calcaires sur les niveaux marneux, le front d’altération s’est enfoncé plus profondément. L’épaisseur d’altérite peut alors, aujourd’hui, atteindre plus de 50 m.La variabilité dans l’altérabilité des roches du Crétacé supérieur est en partie à l’origine du développement de baies. Ces dernières, creusées dans les altérites, correspondent à des secteurs où l’érosion a progressé plus rapidement que sur le reste du littoral. Aujourd’hui, cette désagrégation des reliefs littoraux est le fait de nombreux glissements circulaires. L’altération des marno-calcaires est, en effet, à l’origine d’une dégradation des propriétés mécaniques de la roche. L’étude de ces dernières montre, par ailleurs, leur extrême dispersion du fait de l’hétérogénéité des altérites héritée de celle de la roche-mère. De plus, la structure des massifs altérés facilite les circulations hydrogéologiques ; en effet, d’une part, l’hétérogénéité lithologique de l’altérite permet des circulations d’eau, chenalisées par les bancs de chailles fracturés et les niveaux les plus silteux, et d’autre part, la formation de conduits dans les marno-calcaires de l’horizon karstifié, récemment vidangés, a permis la formation d’un aquifère de type karstique, fréquemment captif sous les altérites. En créant des contraintes déstabilisatrices dans le massif rocheux aux propriétés dégradées, les circulations hydrogéologiques représentent le principal facteur déclenchant les mouvements de terrain comme cela a été montré par le test de différents scénarios par modélisation numérique (FLAC ©Itasca). Ces analyses et observations ont permis l’élaboration d’un outil d’estimation de l’aléa dans l’altérite. Basé sur la présence des facteurs de prédisposition et déclenchants, cet outil permettra de faire évoluer la cartographie de l’aléa en fonction des nouvelles connaissances hydrogéomécaniques

Numerical modeling of regional stress distributions for geothermal exploration

International audienceAny high-enthalpy unconventional geothermal projectcan be jeopardized by the uncertainty on the presence of the geothermal resource at depth. Indeed, for the majority of such projects the geothermal resource is deeply seated and, with the drilling costs increasing accordingly, must be located as precisely as possible to increase the chance of their economic viability. In order to reduce the " geological risk " , i.e. the chance to poorly locate the geothermal resource, a maximum amount of information must be gathered prior to any drilling of exploration and/or operational well. Cross-interpretation from multiple disciplines (e.g., geophysics, hydrology, geomechanics. . .) should improve locating the geothermal resource and so the position of exploration wells ; this is the objective of the Euro-pean project IMAGE (grant agreement No. 608553), under which the work presented here was carried out. As far as geomechanics is concerned, in situ stresses can have a great impact on the presence of a geothermal resource since they condition both the regime within the rock mass, and the state of the major fault zones (and hence, the possible flow paths). In this work, we propose a geomechanical model to assess the stress distribution at the regional scale (characteristic length of 100 kilometers). Since they have a substantial impact on the stress distributions and on the possible creation of regional flow paths, the major fault zones are explicitly taken into account. The Distinct Element Method is used, where the medium is modeled as fully deformable blocks representing the rock mass interacting through mechanically active joints depicting the fault zones. The first step of the study is to build the model geometry based on geological and geophysical evidences. Geophysical and structural geology results help positioning the major fault zones in the first place. Then, outcrop observations, structural models and site-specific geological knowledge give information on the fault zones family sets and their priority rule. In the second step, the physical model must be established, including constitutive equations for the rock mass and the fault zones, initial state and boundary conditions. At such large scales, physical laws and parameters are difficult to assess and must be constrained by sensitivity analysis. In the last step of the study, the results can be interpreted to highlight areas where the mechanical conditions favor the presence of a geothermal resource. The DEM enables accounting for the strong stress redistributions inherent to highly-segmented geometries, and to the dilational opening of fault zones under shearing. A 130x150 square-kilometers region within the Upper Rhine Graben is used as a case-study to illustrate the building and interpretation of a regional stress model

Numerical assessment of the near-wellbore rock matrix permeability gain due to thermal stimulation

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3D Groundwater flow model at the Upper Rhine Graben scale to delineate preferential target areas for geothermal projects

International audienceAny deep unconventional geothermal project remains risky because of the uncertainty regarding the presence of the geothermal resource at depth and the drilling costs increasing accordingly. That's why this resource must be located as precisely as possible to increase the chances of successful projects and their economic viability. To minimize the risk, as much information as possible should be gathered prior to any drilling. Usually, the position of the exploration wells of geothermal energy systems is chosen based on structural geology observations, geophysics measurements and geochemical analyses. Confronting these observations to results from additional disciplines should bring more objectivity in locating the region to explore and where to implant the geothermal system. The Upper Rhine Graben (URG) is a tectonically active rift system that corresponds to one branch of the European Cenozoic Rift System where the basin hosts a significant potential for geothermal energy. The large fault network inherited from a complex tectonic history and settled under the sedimentary deposits hosts fluid circulation patterns. Geothermal anomalies are strongly influenced by fluid circulations within permeable structures such as fault zones. In order to better predict the location of the geothermal resource, it is necessary to understand how it is influenced by heat transport mechanisms such as groundwater flow. The understanding of fluid circulation in hot fractured media at large scale can help in the identification of preferential zones at a finer scale where additional exploration can be carried out. Numerical simulations is a useful tool to deal with the issue of fluid circulations through large fault networks that enable the uplift of deep and hot fluids. Therefore, we build a numerical model to study groundwater flow at the URG scale (150 x 130km), which aims to delineate preferential zones. The numerical model is based on a hybrid method using a Discrete Fracture Network (DFN) and 3D elements to simulate groundwater flow in the 3D regional fault network and in sedimentary deposits, respectively. Firstly, the geometry of the 3D fracture network and its hydraulic connections with 3D elements (sedimentary cover) is built in accordance with the tectonic history and based on geological and geophysical evidences. Secondly, data from previous studies and site-specific geological knowledge provide information on the fault zones family sets and on respective hydraulic properties. Then, from the simulated 3D groundwater flow model and based on a particle tracking methodology, groundwater flow paths are constructed. The regional groundwater flow paths results are extracted and analysed to delineate preferential zones to explore at finer scale and so to define the potential positions of the exploration wells. This work is conducted in the framework of the IMAGE project (Integrated Methods for Advanced Geothermal Exploration, grant agreement No. 608553), which aims to develop new methods for better siting of exploitation wells

Adapted numerical modelling strategy developed to support EGS deployment.

International audienceThe exploitation of Enhanced/Engineered Geothermal Systems (EGS), for electricity and/or heat production, is a promising way to increase the amount of renewable energies contribution in the energetic mix in Europe. In regard to the required production characteristics (production temperature and flowrate) for the economical viability of EGS, the favourable targeted geological systems are deep and fractured. In order to reduce the risks and the prohibitive costs linked to the depth of such geothermal systems, numerical modelling is a useful tool to understand such deep fractured systems and to help in the construction and in the management of the deep infrastructures (wells architecture, stimulation of wells, implementation of adapted network of wells). Nevertheless, this forces to a change of paradigm in comparison to « classical » reservoir modelling based on mechanics of continuum media. Indeed 3D Discrete Fracture Network (DFN) approach looks fairly adapted to catch the mechanical and hydraulic phenomena in the fractured rock mass around wells and to understand the global systems in the network of wells. The conceptualisation of the fractured rock mass is a crucial step for such DFN models not only for the geometry but also to constrain the constitutive behaviour of singularities (fault zones, fractures etc.), depending on the tectonic context. We present some results illustrating how DFNs can be used to study the EGS behaviour at several scales

Borehole damaging under thermo-mechanical loading in the RN-15/IDDP-2 deep well: towards validation of numerical modeling using logging images

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Study of Thermo-Mechanical Damage around Deep Geothermal Wells: from the Micro-Processes to Macroscopic Effects in the Near Well

International audienceThe different processes involved in the life of a geothermal well, from drilling to exploitation, can damage the rock mass in the near well area. In this paper, we propose to study the potential damage linked to the mechanical and thermo-mechanical effects of the well drilling, the well development and the well exploitation. The cooling of the rock mass of the near well pre-damaged by drilling process is a complex phenomenon with the superimposition of different kind of loadings at different scale that lead us to use modeling with a micro-macro approach. To confront the results of the modeling with the reality, we propose to base our study on real cases. For studying mechanical and thermo-mechanical loadings due to drilling and development of the well, we focus our study on the granitic reservoir exploited in the framework of the enhanced geothermal system (EGS) of Soultz-sous-Forêts (France). The study of the thermo-mechanical loading due to well exploitation is performed for a sandstone in the conventional heat exploitation of Melleray (Loiret, France). These simulations highlight the thermo-mechanical damage of a geothermal well linked to the different steps of its life

Modélisations mécanique et hydraulique pour la compréhension des interactions fluide/roche en fracture : Mechanical and hydraulic modelling for the understanding of fluid/rock interactions in fracture

Les modèles hydraulique et mécanique mis en œuvre pour aider à la compréhension et à l'interprétation des essais de percolation réactive en fracture sont présentés. Ces modèles sont basés sur la construction d'un modèle géométrique à partir de données morphologiques de la fracture. Hydraulic and mechanical models implemented to improve the understanding of reactive percolation tests in a fracture are presented. Both models are based on the construction of a geometrical model built on the base of morphological data