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We study the physical mechanisms that drive alpine slope deformation during water infiltration and depletion into fractured bedrocks. We develop a fully coupled hydromechanical model at the valley scale with multiscale fracture systems ranging from meter to kilometer scales represented. The model parameterized with realistic rock mass properties captures the effects of fractures via an upscaling framework with equivalent hydraulic and mechanical properties assigned to local rock mass blocks. The important heterogeneous and anisotropic characteristics of bedrocks due to depth-dependent variations of fracture density and stress state are taken into account and found to play a critical role in groundwater recharge and valley-scale deformation. Our simulation results show that pore pressure actively diffuses downward from the groundwater table during a recharge event, rendering a critical hydraulic response zone controlling surface deformation patterns. During the recession, the hydraulic front migrates downwards and the deformation recorded at the surface (up to ~4 cm) rotates accordingly. The most essential parameters in our model are the fracture network geometry, initial fracture aperture (controlling the rock mass permeability), and regional stress conditions. The magnitude and orientation of our model’s transient annual slope surface deformation are consistent with field observations at our study site in the Aletsch valley. Our research findings have important implications for understanding groundwater flow and slope deformations in alpine mountain environments., ESS Open Archive

The success of geological carbon storage depends on the assurance of permanent containment for injected carbon dioxide (CO2) in the storage formation at depth. One of the critical elements of the safekeeping of CO2 is the sealing capacity of the caprock overlying the storage formation despite faults and/or fractures, which may occur in it. In this work, we present an ongoing injection experiment performed in a fault hosted in clay at the Mont Terri underground rock laboratory (NW Switzerland). The experiment aims to improve our understanding of the main physical and chemical mechanisms controlling (i) the migration of CO2 through a fault damage zone, (ii) the interaction of the CO2 with the neighboring intact rock, and (iii) the impact of the injection on the transmissivity in the fault. To this end, we inject CO2-saturated saline water in the top of a 3 m thick fault in the Opalinus Clay, a clay formation that is a good analog of common caprock for CO2 storage at depth. The mobility of the CO2 within the fault is studied at the decameter scale by using a comprehensive monitoring system. Our experiment aims to close the knowledge gap between laboratory and reservoir scales. Therefore, an important aspect of the experiment is the decameter scale and the prolonged duration of observations over many months. We collect observations and data from a wide range of monitoring systems, such as a seismic network, pressure temperature and electrical conductivity sensors, fiber optics, extensometers, and an in situ mass spectrometer for dissolved gas monitoring. The observations are complemented by laboratory data on collected fluids and rock samples. Here we show the details of the experimental concept and installed instrumentation, as well as the first results of the preliminary characterization. An analysis of borehole logging allows for identifying potential hydraulic transmissive structures within the fault zone. A preliminary analysis of the injection tests helped estimate the transmissivity of such structures within the fault zone and the pressure required to mechanically open such features. The preliminary tests did not record any induced microseismic events. Active seismic tomography enabled sharp imaging the fault zone.

International audience; Groundwater (GW) and streams are intimately linked. Transient groundwater storage is the main driver for streamflow during extended periods of droughts. The connection between GW and streams is more resilient when the drainage timescale is longer, i.e. with slower GW flows and low hydraulic conductivity. Eventually, a gaining stream can be transformed into a losing one when the GW table level drops below the streambed elevation, in which case surface and subsurface bodies become disconnected. Identifying the processes controlling fluxes of GW into stream boundaries has been identified as one of the main challenges by the hydrological community (Blöschl et al., 2019). The emergence of new surface observations of stream network dynamics, provides new opportunities to address this challenge. In this work, we explore the information content of surface stream mapping (perennial and intermittent rivers) to characterize subsurface hydraulic properties. To this end, we set up a process-based parsimonious model to relate the recharge assessed independently by climate models and the subsurface hydraulic properties to the structure of the stream network and its evolutions. The model uses the water balance of the SURFEX regional land surface model as an input of a 3D groundwater flow model. The river networks directly result from the interception of the GW table with the topography. We investigate about thirty watersheds in Brittany and Normandy covering a variety of climatic, geomorphological and geological conditions by calibrating observed perennial stream referenced in the national river database (BD Topage). Results are twofold: First, the perennial stream network is highly sensitive to the ratio of hydraulic conductivity to recharge. For each lithology, estimated permeabilities are found consistent with previous estimates reported in similar context. Secondly, hydraulic conductivities defined in the previous steady-state approach are used to simulate observed transient stream discharge and expansion/contraction dynamics of the stream network (Onde database). This surface-based information on the seasonal expansion and contraction of intermittent stream network contributes to constrain the aquifer storage capacity. The use of surface observations on stream to characterize near subsurface properties might bring up new opportunities to provide predictions for ungauged basins with absence of stream discharge data. Applied with other remote sensing products, this method could contribute to identify the spatial distribution and dynamic reorganization of intermittent rivers. It would be especially useful in a context of climate change, as reduced recharge would changes the organization, fragmentation and hierarchy of groundwater flow systems.

International audience; We simulate non-Newtonian flow in rough fractures. Random aperture fields with isotropic spatial correlations are generated using an FFT-based algorithm. We first solve the flow with a finite-difference code based on the lubrication approximation for power law and Carreau rheologies (2D approach). We then solve a modified Navier-Stokes equation incorporating both rheologies, in the 3D fracture space (3D approach). The 2D approach's efficiency to predict the flow distribution in the fracture plane depends on the fracture closure and correlation length.

International audience; The characterization of thermal transport in fractured rocks is crucial for understanding numerous systems of environmental, geological, and industrial importance. Fracture wall roughness exhibits long range spatial correlations and induces a heterogeneous aperture field, thus promoting the formation of preferential flow channels within fracture planes. Here, we develop a modelling approach to identify geometrical parameters of individual fractures that control the heat exchange at the fluid/rock interface and the advection of heat along the fracture.

International audience; Assessing the potential for leakage through faults plays a key role for the long-term containment of CO2. Faults through a caprock represent one of the possible pathways for CO2 to escape deep storage reservoir. Thus, the presence of faults in caprocks will strongly affect the site characterization process, and consequently the monitoring, verification, and risk management plans.Here we present the results of an experiment performed at the Mont Terri underground rock laboratory (NW Switzerland) located in a clay formation that is a good analogue of common caprock for CO2 storage. The experiment goal was to improve our understanding on the main mechanisms controlling the migration of CO2 through a fault, and the impact of a prolonged injection on the transmissivity in the fault. To this end, we injected for more than one year CO2-saturated saline water in the top of a 3 m thick fault in the clay. The mobility of the CO2 within the fault was studied at decameter scale, by using a comprehensive monitoring system. The experiment offered a unique opportunity to develop improved, and more integrated monitoring technologies. In particular, a multi-component monitoring network integrates strain monitoring, micro-seismicity and active seismic monitoring, electrical resistivity monitoring, and in situ dissolved gas monitoring via a portable mass spectrometer.The in situ monitoring was complemented by lab tests on geochemistry of fluid samples and geomechanical tests on rock samples. The monitoring program will be continued well after the end of injection in order to allow for studying not only the short-term poro-visco-elastic response, but also the geochemical and mineralogical changes within the damaged zone. This experiment will help improving the methods for monitoring and imaging fluid flow.

International audience; L’alimentation en eau potable du bassin rennais repose sur un dispositif complet de 12 ressources et 7 usines. Ce dispositif permet de faire face aux divers aléas pour garantir en permanence une production d’eau potable pour les usagers du bassin rennais mais également pour des Collectivités plus éloignées dans le cadre d’interconnexions. Néanmoins, au cours des trois dernières années, Eau du Bassin Rennais a dû faire face à des évolutions inhabituelles de ses ressources, aussi bien souterraines que celles de surface (rivières ou barrages). Dans ce contexte, des schémas nouveaux de fonctionnement (réalimentation de retenues, arrêts d’usines,..) ont été mise en œuvre pour faire à l’ensemble des demandes. L’alimentation en eau potable est confrontée aux variations climatiques et aux inégalités de répartition de la ressource. L’impact direct ou indirect du changement climatique pose la question de la résilience des systèmes d’alimentation face aux évolutions futures. Face à cette situation, Eau du Bassin Rennais et Rennes Métropole ont pris la décision de financer la Chaire Eau et Territoire, dans le cadre de la Fondation de l’Université Rennes 1. Le pilotage de cette étude a été confié à l’Observatoire des Sciences de l’Univers de Rennes. L’objectif de cette Chaire est notamment d’évaluer l’impact du réchauffement climatique sur les ressources (quantité et qualité). La modélisation numérique, basée sur les lois physiques de l’hydrologie et l’hydrogéologie, simule les processus régissant le cycle de l’eau afin de fournir des moyens de compréhension et de prédiction de la ressource en eau sur un territoire donné. Les simulations réalisées dans le cadre de la chaire Eaux et Territoires permettront d’analyser les effets potentiels du changement climatique à travers des scénarios futurs. Des indicateurs seront définis pour comprendre comment les forçages climatiques modifient les différents termes du bilan hydrologique (évapotranspiration, ruissellement, infiltration, recharge, stockage, qualité) tant géographiquement que temporellement. Un site d’intérêt d’une superficie totale d’environ 680 km² a été sectionné à l’ouest de Rennes (bassin du Meu et de la Chèze-Canut) étant donné son importante contribution à l’alimentation en eau potable de la Collectivité Eau du Bassin Rennais. Un modèle couplé surface-souterrain en 3D des bassinsversants sera développé puis calibré à partir des données disponibles. Des projections basées sur des scénarios climatiques préalablement sélectionnés seront réalisées. L’influence d’autres forçages, par exemple l’utilisation du sol, pourra être analysée grâce à ce type de modèle. Les résultats devraient permettre d’identifier l’évolution spatio-temporelle des ressources en eau sous l’impact du changement climatique, en particulier l’impact de sécheresses répétées ou de redistribution temporelle des précipitations. Le but final est de contribuer à l'élaboration de stratégies adaptatives futures pour l’optimisation de la gestion durable des ressources en eau du bassin rennais.