Non-Fickian dispersion in porous media : 1. Multiscale measurements using single-well injection withdrawal tracer tests

International audienceWe present a set of single-well injection withdrawal tracer tests in a paleoreef porous reservoir displaying important small-scale heterogeneity. An improved dual-packer probe was designed to perform dirac-like tracer injection and accurate downhole automatic measurements of the tracer concentration during the recovery phase. By flushing the tracer, at constant flow rate, for increasing time duration, we can probe distinctly different reservoir volumes and test the multiscale predictability of the (non-Fickian) dispersion models. First we describe the characteristics, from microscale to meter scale, of the reservoir rock. Second, the specificity of the tracer test setup and the results obtained using two different tracers and measurement methods (salinity-conductivity and fluorescent dye­optical measurement, respectively) are presented. All the tracer tests display strongly tailed breakthrough curves (BTC) consistent with diffusion in immobile regions. Conductivity results, measured over 3 orders of magnitude only, could have been easily interpreted by the conventional mobile-immobile (MIM) diffusive mass transfer model of asymptotic log-log slope of 2. However, the fluorescent dye sensor, which allows exploring much lower concentration values, shows that a change in the log-log slope occurs at larger time with an asymptotic value of 1.5, corresponding to the double-porosity model. These results suggest that the conventional, one-slope MIM transfer rate model is too simplistic to account for the real multiscale heterogeneity of the diffusion-dominant fraction of the reservoir

Rochechouart 2017-Drilling Campaign: First Results

No abstract available

Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, II: fracturing and alteration. An integrated study from core, downhole measurements and borehole wall images

This paper is part of the special publication Core-log integration (eds P.K. Harvey and M.A. Lovell). We used a database derived from the integration of core material and geophysical downhole measurements in order to investigate the relationships between fracturing and alteration in the volcanic section of DSDP/ODP Hole 504B. The studied crustal section (from top of the basement to 1000 mbsf (metres below sea floor)) consists of low resistivity/high porosity pillow lavas associated with breccias and rubble material, alternating with high resistivity/low porosity massive basalt flows. A positive correlation between DLL (Dual Laterolog)-derived porosity and occurrence of breccias in the core suggests that breccias more than fractures contribute to the electrical resistivity signal. A structural analysis performed from core suggests that most fractures and veins are steeply dipping, and may represent tectonic features or cracks due to contractional cooling of the crust, the latter being more abundant in pillows. Fractures and veins recorded on core tend to be clustered in massive units or thin flows. This result may derive from criteria adopted during structural measurements and must be taken with care. The natural radioactivity (GR) profile delineates two main alteration zones in the volcanic section: an oxidizing zone with increased potassium above, and a reducing one without K gain below. Most of the GR maxima are found to be correlated with celadonite-bearing alteration halos. GR minima are frequently located at the boundaries between domains of contrasting fracture orientation, where metasomatic reactions may have occurred due to contrasting permeability

Projet « DIGUINTELLIGENT » Recherche et développement d'une technologie de détection et de quantification d'écoulements dans les digues

L’érosion interne, qui est définie comme l’entraînement progressif des particules constituant un ouvrage sous l’action de l’écoulement qui le traverse, est à l’origine d’un grand nombre de ruptures de digues. De 2012 à 2016, le projet DIGUINTELLIGENT a mis au point un système de surveillance des écoulements dans les ouvrages hydrauliques de type digue. Le système de surveillance visait notamment à réaliser un suivi intégré et continu de l’état hydrique des massifs, à localiser les écoulements anormaux et à tenter de quantifier les débits de fuite en temps réel. Le projet vise plus largement à développer une stratégie opérationnelle de mesures intégrées permettant d’évaluer en temps réel les risques de rupture d’ouvrages, et à proposer une aide à la décision adaptée quant au déclenchement d’opérations de réparation. Le principe général est d’augmenter la réactivité des équipes de surveillance en cas de dysfonctionnement des ouvrages et de minimiser grandement les risques de rupture. Le projet a consisté en la conception, le test sur un ouvrage expérimental et la validation d’un système innovant de surveillance d’une digue avec la conception d’un ensemble de capteurs expérimentaux parmi lesquels la fibre optique (réseaux de Bragg) pour la localisation des écoulements, des capteurs de potentiel électrique spontané (PES) pour la quantification des écoulements, la construction d’un site d’expérimentation grandeur nature, la mise en place d’un protocole de tests et l’analyse des résultats. Après avoir exposé les principaux enjeux et objectifs du projet, l’article présentera la méthodologie appliquée pour la conception du démonstrateur (réalisation d’une digue « neuve » et d’une digue « ancienne »), et la conception de l’instrumentation mise en place (réseaux de fibres optiques, capteurs de potentiel électrique spontané (PES), mesures de résistivité électrique). Dans un second temps, le protocole expérimental et les différentes manipulations seront décrits, avant de présenter les résultats issus de l’interprétation des données acquises lors des tests et de conclure avec les perspectives se dessinant à l’issue de l’expérimentation

Comparison of alternative methodologies for identifying and characterizing preferential flow paths in heterogeneous aquifers

International audienceOne of the main difficulties encountered when characterizing the hydrodynamic properties of a fractured aquifer is to identify the preferential flow paths within it. Different methods may be applied to determine the variability of the permeability at the borehole scale and to image the structure of the main flow zones between boreholes. In this paper, we compare the information obtained from different measurement techniques performed in a set of three 100 m depth wells (well-to-well spacing: 5­10 m) in a fractured crystalline rock setting. Geophysical logging and borehole-wall imaging are used to identify open and closed fractures intersecting the boreholes and their orientation. The comparison with flowmeter and single packer tests shows that few of the fractures interpreted as open from geophysical logs are significantly transmissive. Cross-borehole connectivity is first investigated from single packer tests with pressure monitoring in adjacent boreholes. To determine fracture zone connectivity, we propose a methodology simply based on the variation with packer depth of the ratio of the drawdown in the observation well and the drawdown in the pumping well. The results are compared to the analysis of cross-borehole flowmeter tests. We show that both methods provide consistent results with a similar level of information on connectivity

The French network of hydrogeological sites H+

1 p.For groundwater issues (potential leakages in waste repository, aquifer management ...), the development of modeling techniques is far ahead of the actual knowledge of aquifers. This raises two fundamental issues: 1) which and how much data are necessary to make predictions accurate enough for aquifer management issues; 2) which models remain relevant to describe the heterogeneity and complexity of geological systems. The French observatory H+ was created in 2002 with the twofold motivation of acquiring a large database for validating models of heterogeneous aquifers, and of surveying groundwater quality evolution in the context of environmental changes. H+ is a network of 4 sites (Ploemeur, Brittany, France; HES Poitiers, France; Cadarache, France; Campos, Mallorca, Spain) with different geological, climatic, and economic contexts. All of them are characterized by a highly heterogeneous structure (fractured crystalline basement for Ploemeur, karstified and fractured limestone for Poitiers, Cadarache and Mallorca), which is far to be taken into account by basic models. Ploemeur is exploited as a tap-water plant for a medium-size coastal city (15,000 inhabitants) for 20 years. Each site is developed for long term investigation and monitoring. They involves a dense network of boreholes, detailed geological and geophysical surveys, periodic campaigns and/or permanent measurements of groundwater flow, water chemistry, geophysical signals (including ground motions), climatic parameter, etc. Several large-scale flow experiments are scheduled per year to investigate the aquifer structure with combined geophysical, hydrogeological, and geochemical instruments. All this information is recorded in a database that has been developed to improve the sustainability and quality of data, and to be used as a collaborative tool for both site researchers and modelers. This project lasts now for 5 years. It is a short time to collect the amount of information necessary to apprehend the complexity of aquifers; but it is already enough to obtain a few important scientific results about the very nature of the flow heterogeneity, the origin and residence time of water elements, the kinetic of geochemical processes, etc. We have also developed new methods to investigate aquifers (in-situ flow measurements, flow experiment designs, groundwater dating, versatile in-situ probes, etc.). This experience aiming at building up long term knowledge appears extremely useful to address critical issues related to groundwater aquifers: the structure and occurrence of productive aquifer in crystalline basement, the assessment of aquifer protection area in the context of highly heterogeneous flow, the biochemical reactivity processes, the long term evolution of both water quantity and quality in the context of significant environmental changes, for instance

Probing connections between deep earth and surface Processes in a land-locked ocean basin transformed into a giant saline basin: the Mediterranean DREAM-GOLD Project

During the last decade, the interaction of deep processes in the lithosphere and mantle with surface processes (erosion, climate, sea-level, subsidence, glacio-isostatic readjustment) has been the subject of heated discussion. The use of a multidisciplinary approach linking geology, geophysics, geodesy, modelling, and geotechnology has led to the awareness of coupled deep and surface processes. Deep earth dynamics (topography, erosion, tectonics) are strongly connected to natural hazards such as earthquakes, landslides, and tsunamis; sedimentary mass transfers have important consequences on isostatic movements and on georesources, geothermal energy repartitions. The ability to read and understand the link between deep Earth dynamics and surface processes has therefore important societal impacts. Ground-truthing at carefully-selected sites of investigation is imperative to better understand these connections. Due to its youth (<30 Ma) and its subsidence history, the almost land-locked Gulf of Lion-Sardinia continental margins system provides a unique record of sedimentary deposition from the Miocene to present. Due to its high subsidence rate, palaeoclimatic variations, tectonic events and vertical evolution are all recorded here at very high resolution. The late Miocene isolation and desiccation of the Mediterranean, the youngest and most catastrophic event, the Messinian Salinity Crisis (MSC), induced drastic changes in marine environments: widespread deposition of evaporite (gypsum, anhydrite and halite) in the central basin, and intense subaerial erosion along its periphery. These extraordinary mass transfers from land to sea induced strong isostatic re-adjustments that are archived in the sedimentary record and represent a window to the lithospheric rheology and the deep processes. The GOLD (Gulf of Lion Drilling) project, proposes to explore this unique sedimentary record as well as the nature of the deep crustal structure, providing valuable information about the mechanisms underlying vertical motions in basins and their margins.The work in the Gulf of Lion and long-term drilling project, including offshore cruises, has been supported by CNRS, IFREMER, IFPEN, BRGM, EDROME, IUEM, the french alliance (Allenvi and Ancre), CNFH (Commission National de la Flotte Hauturi ere Française) and different national and international Programs: Actions Marges, MISTRALS-Termex, Labex Mer (ANR-10-LABX-19), CNRS-INSU SYSTER, CNRS-INSU INTERRVIE, ARTEMIS, MAGELLAN þ European Program, TOPO-EUROPE and ILP (International Lithosphere Program), TECLA CGL2011-26670, RCMNS, IODP-France, ECORD, JSPS, JAMSTEC and CDEX and IODPPeer Reviewe