Hydrogéologie en milieu fissuré et thermalisme- Etude du massif de Bramefarine et du nord du massif d'Allevard- La source thermominérale d'Allevard (Isère-France) - Alpes françaises.

L'hydrogéologie en milieu fissuré nécessite, plus encore que dans d'autres milieux, une parfaite connaissance de la géométrie et de la structure du magasin. Des études poursuivies dans ce sens par de nombreux auteurs ont été effectuées en milieu carbonaté karstique ( L.KIRALI, 1969, J.C. GRILLOT , 1979) ou en milieu granito-gneissique ( J.AVIAS, 1967, F.O. FRANCISS, 1970, O.JAMIER, 1975), mais peu de chercheurs se sont penchés sur le problème des écoulements souterrains dans des milieux où, comme les schistes et les micaschistes, la présence d'une schistosité ou d'une foliation fine rend très difficile l'étude de la fissuration. Cette difficulté de connaissance du milieu doublée de la difficulté d'observation d'écoulements profonds à partir de la surface, entraine la nécessité de surprendre les écoulements lors de leur trajet, à l'occasion de l'exécution de travaux souterrains. C'est ce que nous avons pu réaliser grâce au percement de la galerie Arc-Isère. L'étude qui nous a été confiée comportait donc deux buts: - la mise en évidence des directions et des modes d'écoulements des eaux dans des milieux dont rien à priori ne permettait de penser qu'ils puissent être favorables à des circulations profondes. Pour cela, nous nous devions de connaître la structure intime des milieux considérés et la nature des circulations aquifères qui les parcourent; - la connaissance hydrogéologique d'un secteur de la bordure occidentale du massif de Belledonne, dont l'étude globale entreprise par le laboratoire d'Hydrogéologie de l'Institut Dolomieu comptait deux précédents volets avec les travaux de M. MICHEL (1971) et G.PAPPINI (1976). Nous n'avons pas poursuivi ici un but strictement monographique, mais tenté d'établir une méthodologie d'étude comparative surface-profondeur du milieu fissuré avec, en conclusion ou plutôten application, l'étude d'un cheminement thermo-minéral.Pas de résum

Hydrogéologie en milieu fissuré et thermalisme- Etude du massif de Bramefarine et du nord du massif d'Allevard- La source thermominérale d'Allevard (Isère-France) - Alpes françaises.

L'hydrogéologie en milieu fissuré nécessite, plus encore que dans d'autres milieux, une parfaite connaissance de la géométrie et de la structure du magasin. Des études poursuivies dans ce sens par de nombreux auteurs ont été effectuées en milieu carbonaté karstique ( L.KIRALI, 1969, J.C. GRILLOT , 1979) ou en milieu granito-gneissique ( J.AVIAS, 1967, F.O. FRANCISS, 1970, O.JAMIER, 1975), mais peu de chercheurs se sont penchés sur le problème des écoulements souterrains dans des milieux où, comme les schistes et les micaschistes, la présence d'une schistosité ou d'une foliation fine rend très difficile l'étude de la fissuration. Cette difficulté de connaissance du milieu doublée de la difficulté d'observation d'écoulements profonds à partir de la surface, entraine la nécessité de surprendre les écoulements lors de leur trajet, à l'occasion de l'exécution de travaux souterrains. C'est ce que nous avons pu réaliser grâce au percement de la galerie Arc-Isère. L'étude qui nous a été confiée comportait donc deux buts: - la mise en évidence des directions et des modes d'écoulements des eaux dans des milieux dont rien à priori ne permettait de penser qu'ils puissent être favorables à des circulations profondes. Pour cela, nous nous devions de connaître la structure intime des milieux considérés et la nature des circulations aquifères qui les parcourent; - la connaissance hydrogéologique d'un secteur de la bordure occidentale du massif de Belledonne, dont l'étude globale entreprise par le laboratoire d'Hydrogéologie de l'Institut Dolomieu comptait deux précédents volets avec les travaux de M. MICHEL (1971) et G.PAPPINI (1976). Nous n'avons pas poursuivi ici un but strictement monographique, mais tenté d'établir une méthodologie d'étude comparative surface-profondeur du milieu fissuré avec, en conclusion ou plutôten application, l'étude d'un cheminement thermo-minéral.Pas de résum

Sequence stratigraphy as an inter-disciplinary tool for karst network forecast. Multiphase and multiprocess karst genesis in the Urgonian Formation

International audienceCommonly, in carbonate reservoirs, karst systems result from long-term, multiphase and multiprocess speleogenesis that can occur from very early after the sedimentation of the limestone to presentday geomorphological events. This leads to the construction of inherently complex karst networks with a piece of architecture difficult to understand and to accurately predict. The dynamic appraisal of these reservoirs gives greater insight into the overall organization of the actual karst systems, while totally ignoring the detailed features that impact the local fluid flow behavior. In order to reduce the uncertainties in predicting the behavior of complex systems, a reconstruction of the development of the karst conduits is needed. It requires that, for each speleogenesis stage, the processes and their controlling parameters are correctly assessed.The issue of defining the prevalent parameters for the karst development is usually one on which the different specialists involved in the study of karst systems disagree. The difference in views is all the more important in the fact that specialists do not necessary address the question according to the karst origin (epigenic, hypogenic, island).For geologists, lithology and geological structure impose a general control on most cave genesis. The karst is considered to be facies-controlled, with some rock fabrics and textures described as more favorable for the development of conduits than others. Faults and fracture zones influence the secondary porosity development, through karst enhanced fractures and highly developed cavern systems. However, lithologic and structural conditions for speleogenesis evolve throughout time and change from one phase to another: progressively, the importance of fabric-selective porosity and stratigraphic elements diminishes in favor of fracture porosity.For hydrologists, hydraulic boundary conditions play as a controlling factor in karst genesis, as conduits develop mainly vertically in vadose zone and horizontally along the groundwater table, following the hydraulic gradient. Moreover, the hydraulic gradient can limit the extension or completely inhibit the development of horizontal conduits even if lateral flow path was structurally preferred. In the Urgonian Formation, a cretaceous series outcropping in south-eastern France, the development of karst conduits has been considered through geological time, and examined from many different perspectives. For several stages of speleogenesis, the relationships between various geological and hydraulic drivers with the formation of karst features have been established. An integration of most drivers has been possible through sequence stratigraphy, which influenced both the development of pore and fracture systems, the extension of seepage areas, the evolution of water table, the organization of drainage networks, and the location of inception surfaces on sequence boundaries. Sequence stratigraphy appears as an impressive tool for proposing an integrated and predictive understanding of caves development. More generally, this study highlights how much karst genesis is a multi-factorial process, which requires an inter-disciplinary approach

Dynamic outcrop analogues in groundwater reservoirs. A disruptive approach for reservoir characterization and modelling

International audienceThe usual workflow for delivering production forecasts for oil-bearing subsurface reservoirs consists in the building of very detailed geological and petrophysical models on which fluids flows are simulated. Populating models with heterogeneous distributions of reservoir properties is a very challenging issue because of the lack of data in typical datasets, that comprise boreholes with sparse lateral sampling, and seismic data with low vertical resolution.Against this background of limited data, reservoir analogues have long been used to improve understanding of subsurface fields. From outcrops, geological data have been collected. Their interpretation has been used to build conceptual static models, and to run sensitivity studies of dynamic response to geological and petrophysical heterogeneity. Although quite advanced, this linear workflow does not achieve the objective of reservoir characterization as there is not any hydraulic information for calibration of the outcrop interpretation.In the same time, hydrogeologists have developed observatories, dedicated to the observation and monitoring of groundwater reservoirs, for the exploitation of water resources and the prevention of environmental risks. These experimental sites provide data relevant to the understanding of the water cycle and the behavior of the aquifers. However, despite the acquisition of geological, geophysical and petrophysical data in boreholes, the description of geology is usually lacking in detail, thereby compromising the quality of the coupling between measurements, theories, and models.A new concept of dynamic outcrop analogue arose from a double need: on one side, the integration of hydrodynamics and geophysics in outcrops studies, on the other side the use of detailed geology in observatories studies. The dynamic outcrop analogue allows the characterization of the reservoir at different scales and from an integrated point of view. The analogue is assessed through multiphysics experiments on various supports : outcrops, but also plugs, cores, boreholes and between wells, caves, tunnels… Coupling the hydrodynamic response of groundwater reservoir to a detailed static description of its heterogeneity very early before the building of models provides a key knowledge on the understanding of reservoir architecture. It contributes to improve the way models are populated and to enhance the forecast of reservoir.This new methodology has been developed and tested through a pilot project, ALBION. The demonstration project is dedicated to the Urgonian carbonates (a barremo-aptian series outcropping in South-East France) and proposes a multi-scales set of analogues in the Fontaine de Vaucluse catchment area. On other sites, further projects are dedicated to various topics: multi-scales permeability in tidal sandstones, anisotropy of permeability in anastomosed fluviatile channels, heterogeneity in unconsolidated alluvial coarse gravels, fractures and karstic networks, and permeability in a fault zone

The relationship between joint aperture, spacing distribution, vertical dimension and carbonate stratification: An example from the Kimmeridgian limestones of Pointe-du-Chay (France)

International audienceJoint aperture and joint development have been studied in the Kimmeridgian limestones of the Pointe-du-Chay, at the northern boundary of the Jurassic Biscay Basin (France). At Belette outcrop, in some layers the mean joint spacing of the N120° joint set is close to mean layer thickness. There, the classical spacing to thickness relationship appears to be valid in the competent carbonate layers that are included in a more argillaceous matrix. At Pillar outcrop, the N10° joint set is characterised by a high level of joint density and a non-saturated spacing distribution as indicated by the mode/mean ratio values and the Cv values; Cv is the ratio of standard deviation to mean fracture spacing. The classical relationship between layer thickness and fracture spacing has not been observed at the Pillar outcrop. Joint aperture reaches larger values at the Pillar outcrop than at the Belette outcrop where aperture is more homogeneously distributed. Almost all the joints are opened with moderate aperture values at Belette outcrop whereas most joints with large vertical dimension have large apertures at Pillar outcrop, and smaller fractures are closed or poorly opened. From two outcrops that have been subjected to the same geological conditions, apertures of non-stratabound joints appear to be controlled by the vertical dimension of the joints whereas stratabound joints are more regularly spaced and opened

Caractérisation des relations entre réseau de fractures et réseau d'écoulement dans les réservoirs fracturés et karstiques (Modélisation numérique et étude in situ (Aquifère du Lez, France))

Bien que le comportement et la modélisation des écoulements dans les réservoirs fracturés constituent une problématique de recherche abordée depuis le milieu des années soixante, la caractérisation des réservoirs fracturés soulève encore de nombreuses incertitudes. Dans le contexte du développement et de l'exploitation des réservoirs fracturés, le problème principal le plus souvent cité comprend la définition des propriétés du réseau de fractures qui détermine le réseau d'écoulement ainsi que les modalités de l'écoulements inter-porosité. L'essai de puits est l'un des outils le plus utilisé pour caractériser les propriétés géométriques et hydrodynamiques des réservoirs fracturés sur la base de solution analytique simple. La considération d'une solution analytique de type double porosité pour l'interprétation de l'essai de puits permet ainsi de renseigner le coefficient d'échange inter-porosité, ainsi que le rapport entre les propriétés hydrodynamiques des fractures et celles du réservoir. Ces informations, qui concernent un réservoir fracturé donné, sont généralement issues de l'interprétation, à l'échelle régionale, de la réponse hydrodynamique obtenue sur quelques puits seulement. Néanmoins, tous les puits d'observation n'ont pas la même réponse hydrodynamique à un essai de puits ; ceci pose la question de l'impact de la localisation du puits de pompage et du piézomètre sur la détermination des propriétés hydrodynamiques du réservoir lors de l'interprétation de l'essai de puits. Pour évaluer les différents effets liés à l'emplacement du puits de pompage sur la réponses hydrodynamique, une série d'essais de puits a été simulée dans des réseaux de fractures synthétiques et leur réponse hydrodynamique a été interprétée à l'aide d'un modèle de type double porosité L'impact de la localisation du puits de pompage et des propriété géométriques du réservoir fracturé, telles que l'espacement et la longueur des fractures, sur le comportement des écoulements et la valeur du coefficient d'échange inter-porosité est examiné dans le premier chapitre. Dans le deuxième chapitre, la relation entre réseau de fractures et réseau d'écoulement est examinée dans le cas d'un aquifère fracturé et karstique (aquifère du Lez), en considérant le comportement hydrodynamique à l'échelle locale et le comportement hydrodynamique à l'échelle régionale. De cette façon, sont appréhendés les paramètres principaux qui conditionnent le réseau d'écoulement préférentiel, et ses liens avec le réseau de fractures. Différentes propriétés géométriques, telles que la densité, l'orientation, et la longueur des fractures sont déterminées à différentes échelles : échelle du puits, échelle locale (site expérimental), et échelle régionale (aquifère). A cette fin, différentes techniques d'acquisition (imagerie de puits, mesures à l'affleurement, photographie aérienne) sont utilisées pour caractériser la fracturation. Des mesures géophysique réalisées à l'échelle locale (site expérimental) permettent d'obtenir des cartes de résistivité 2D et 3D qui illustrent la grande hétérogénéité du site, caractéristique des réservoirs fracturés. L'investigation du réseau d'écoulement est réalisée au moyen de diagraphies de puits, de tests d'interférences et de tests d'impulsions, à l'échelle locale et à l'échelle régionale. Ces différents outils nous renseignent sur la connectivité des différents puits avec le réseau d'écoulement ainsi que sur les propriété hydrodynamiques de différentes zones du réservoir fracturé. Dans le troisième chapitre, un modèle hydrogéologique conceptuel est proposé à l'échelle locale et à l'échelle régionale. Ces modèles servent de fondement à deux modèles numériques 2D basé pour voir si la geomodélisation numérique et les résultats de simulation d'écoulements de fluides confirment notre hypothèse intégrée dans les modèles conceptuelsAlthough the specific flow behaviour of fractured reservoirs has been identified and modelled since relatively long time, there are still lots of problem concerning the fractured reservoir characterisation. The main problem, often cited in the context of the development and exploitation of fractured reservoirs, is the difficulty in defining the fracture network properties that impact the flow path-network and the inter-porosity flow transfer between the fracture continuum and matrix continuum. Well test is one of the tools that are used for the characterisation of geometrical and hydrodynamic properties of fractured reservoir, such as inter-porosity flow exchange coefficient, fracture storativity to whole reservoir storativity ratio, and reservoir permeability. But all these information applied to characterise a fractured reservoir at regional scale are inferred from only a few boreholes. One may thus wonder if all observation boreholes have the same hydrodynamic response to a well test. In order to answer this question a series of numerical well tests are conducted in synthetic fracture networks to evaluate the effect of the pumping well location on the dual porosity pressure responses. The impact of geometrical parameters, such as fracture spacing and mean length, on fluid flow behaviour and inter-porosity exchange coefficient are investigated for different pumping well location in the first chapter. Well test simulations in the synthetic fractured reservoirs, with a non uniform distribution pattern, highlight the dependency of the measured inter-porosity flow exchange coefficient to the pumping test location. The mean fracture length (which controls connectivity) is assessed to be the main geometrical parameter which influences the reservoir hydrodynamic response during well test. In the second chapter, the relationship between fracture network and flow-path network are investigated in a real fractured and karstic carbonate aquifer (Lez aquifer, southern France). A multi-disciplinary case study, which aims to assess the main parameters which affect the preferential flow-path network, is thus performed. It comprises geological, geophysical and hydrodynamic data acquisition and interpretations. The collection of geometrical parameters such as fracture density, orientation, length is done at different scales (borehole, field site, aquifer). Applying different data acquisition techniques such as borehole image, outcrop observation and aerial photography are claimed to characterise the fracture network. In this experimental part, it is shown that geological and geophysical data such as fracture data acquisition, borehole imagery (OTV), reservoir electrical resistivity imaging are the minimum requested data for characterizing the fracture network of a given reservoir. Investigation of flow-path network is done through conducting a series of well logging, interference tests and pulse tests at two scales. This step gives some clarifications about the quality of connectivity and conductivity of fractures at different zones. Then, we assess the relationship between fracture network and flow-path network by integrating all geological, geophysical and hydrodynamic data. It is shown that conducting well test series at two different scales within the same fractured and karstic reservoir allow verifying the scale dependency of permeability within a same system; it also shows that other important factors than the scale of observation affect these permeability variations. Combination of all data (geological, geophysical and hydrodynamic) illustrates that the borehole connectivity to the flow-path network and water table level during well test operation are the parameters which mainly control the permeability value estimation, whatever the scale of observation. Finally, in the third chapter, we define the hydrogeological conceptual models at local and regional scales. Then a simple 2D numerical model based on the hydrogeological conceptual model at each scale is proposed and the numerical geomodelling and fluid flow simulation are compared to the observed hydrodynamic behaviour in the site. Results of fluid flow simulation in the numerical models at local and regional scales confirm our hypothesis about effect of borehole location and its connectivity with the flow-path network on the recorded hydrodynamic responses at boreholeMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Characterization of horizontal transmissivity anisotropy using cross-hole slug tests

International audienc

Sensitivity analysis of hydrodynamics response to structure and flow parameters in a karstic reservoir

International audienc

Taking into Account both Explicit Conduits and the Unsaturated Zone in Karst Reservoir Hybrid Models: Impact on the Outlet Hydrograph

The main outlets of karst systems are springs, the hydrographs of which are largely affected by flow processes in the unsaturated zone. These processes differ between the epikarst and transmission zone on the one hand and the matrix and conduit on the other hand. However, numerical models rarely consider the unsaturated zone, let alone distinguishing its subsystems. Likewise, few models represent conduits through a second medium, and even fewer do this explicitly with discrete features. This paper focuses on the interest of hybrid models that take into account both unsaturated subsystems and discrete conduits to simulate the reservoir-scale response, especially the outlet hydrograph. In a synthetic karst aquifer model, we performed simulations for several parameter sets and showed the ability of hybrid models to simulate the overall response of complex karst aquifers. Varying parameters affect the pathway distribution and transit times, which results in a large variety of hydrograph shapes. We propose a classification of hydrographs and selected characteristics, which proves useful for analysing the results. The relationships between model parameters and hydrograph characteristics are not all linear; some of them have local extrema or threshold limits. The numerous simulations help to assess the sensitivity of hydrograph characteristics to the different parameters and, conversely, to identify the key parameters which can be manipulated to enhance the modelling of field cases

Modeling the Matrix-Conduit Exchanges in Both the Epikarst and the Transmission Zone of Karst Systems

Usual conceptual models of karst hydrodynamics highlight the important role of unsaturated subsystems in recharge repartition. However, few of them have been compared with scarce suitable physically-based numerical models. Hybrid models that couple single continuum medium with discrete features promise an improved consideration of karst specificities. Here we evaluate their capability to properly reproduce interactions between a vertical conduit and the surrounding unsaturated matrix. We simulate the response of such a configuration to a single recharge event for various sets of parameters. We show the ability of hybrid models to reproduce the most significant behaviors described in the literature, i.e., transient storage and distribution of recharge, flow concentration towards conduits in the epikarst, and matrix-conduit exchanges varying in time and space. In addition to the explicit conduits, simulating variably saturated flows with the Richards equation and distinguishing the epikarst and the transmission zone are key elements to reproduce most processes. The contrasts between subsystems necessary to observe desired behaviors have been quantified. They are reinforced by the varying matrix saturation that causes realistic competition between matrix and explicit conduits. The study also highlights the need to deepen knowledge of the scaled medium properties we need to know to apply such models to actual cases