Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Advances in groundwater storage monitoring are crucial for water resource management and hydrological processes understanding. The evaluation of water storage changes (WSC) often involve point measurements (observation wells, moisture probes, etc.), which may be inappropriate in heterogeneous media. Over the past few years, there has been an increasing interest in the use of gravimetry for hydrological studies. In the framework of the GHYRAF (Gravity and Hydrology inAfrica) project, 3 yr of repeated absolute gravity measurements using a FG5-type gravimeter have been undertaken at Nalohou, a Sudanian site in northern Benin. Hydrological data are collected within the long-term observing system AMMA-Catch. Once corrected for solid earth tides, ocean loading, air pressure effects, polar motion contribution and non-local hydrology, seasonal gravity variations reach up to 11 μGal, equivalent to a WSC of 260-mm thick infinite layer of water. Absolute temporal gravity data are compared to WSC deduced from neutron probe and water-table variations through a direct modelling approach. First, we use neutronic measurements available for the whole vertical profile where WSC occur (the vadose zone and a shallow unconfined aquifer). The RMSD between observed and modelled gravity variations is 1.61 μGal, which falls within the error bars of the absolute gravity data. Second, to acknowledge for the spatial variability of aquifer properties, we use a 2-D model for specific yield (Sy) derived from resistivity mapping and Magnetic Resonance Soundings (MRS). The latter provides a water content (θMRS) known to be higher than the specific yield. Hence, we scaled the 2-D model of θMRS with a single factor (α). WSC are calculated from water-table monitoring in the aquifer layer and neutronic measurements in the vadose layer. The value of α is obtained with aMonte-Carlo sampling approach, minimizing the RMSD between modelled and observed gravity variations. This leads to α = Sy/θMRS = 0.63 ± 0.15, close to what is found in the literature on the basis of pumping tests experiments, with a RMSD value of 0.94 μGal. This hydrogeophysical experiment is a first step towards the use of time-lapse gravity data as an integrative tool to monitor interannual WSC even in complicated subsurface distribution.
ISSN: 0956-540X

hal-00821181
https://hal.archives-ouvertes.fr/hal-00821181
DOI : 10.1093/gji/ggt146

Éditeur(s) : HAL CCSD
Résumé : The heterogeneous nature and sometimes karstified carbonate environments has a direct impact on many environmental issues, such as coastal aquifers affected by saltwater intrusion. At the south of the island of Mallorca (Spain), the Llucmajor carbonate platform offers a privileged site for the study of such intrusions including the experimental site of Campos who meets the intrusion at shallow depth. The characterization of heterogeneous properties passes through a preliminary description of the properties of carbonate reservoirs, including those of the porosity. According to the wide range of sizes of heterogeneities from micrometer to hundreds of meters, the use of multiscale methods is required. This thesis focused on the determination of multiscale porosities from the microporosity to karstic porosity, and of relationships between observation scale changes of these properties. At the micron scale, high-resolution X-ray microtomography allows to extract the microporosity and the microporous network connectivity. At lower resolution (mm), X-ray tomography is also used to study the micro- and macro-porous network at the core scale which is already allocated to number of observations and measurements, including petrophysical. At the borehole scale, the wall images are used to identify the dissolved zones from centimeter macropores to plurimetric karstic cavities. At the larger scale, surface geophysics study the lateral extension of the karsts and heterogeneities. Finally, the porous structure of the studied site is integrated with its wide range of scales into the regional geology of the Llucmajor platform. Thus, the hydrogeophysical observatory set up on the site of Campos to monitor the spreading of salt water in the aquifer is integrated in its geological and petrophysical context, in order to refine the interpretation of its variations of salinity measurements during the time.
La nature hétérogène et parfois karstifiée des milieux carbonatés a un impact direct sur nombre de problématiques environnementales, comme celle des aquifères côtiers affectés par une intrusion d'eau salée. Au sud de l'ile de Majorque (Baléares, Espagne), la plate-forme carbonatée de Llucmajor offre un site privilégié pour l'étude de telles intrusions notamment avec le site expérimental de Campos, qui rencontre l'intrusion à faible profondeur. La caractérisation des propriétés hétérogènes du réservoir passe par une description préalable des propriétés des réservoirs carbonatés et notamment celle de la porosité. Au regard de la large gamme d'échelles d'hétérogénéités allant du micromètre à la centaine de mètre, l'utilisation de méthodes multi-échelles est nécessaire. Ce travail de thèse a porté sur la caractérisation multi-échelle de la porosité, allant de la microporosité à la porosité karstique, et l'établissement de relations entre les changements d'échelles d'observation de ces propriétés. A très petite échelle, la microtomographie haute-résolution à rayons X extrait à l'échelle du micron la microporosité et la connectivité des réseaux microporeux. A plus basse fréquence (celle du mm), la tomographie est également utilisée afin d'étudier le réseau micro- et macroporeux à l'échelle des carottes, à laquelle sont réalisées nombre d'observations et de mesures, notamment pétrophysiques. A l'échelle du puits, les images de paroi permettent d'identifier les zones de dissolution allant des macropores centimétriques aux cavités karstiques plurimétriques. A plus grande échelle, la géophysique de surface permet d'étudier l'extension latérale de ces karsts et hétérogénéités. Enfin, la structure poreuse du site étudié sur cette vaste gamme d'échelles est intégrée à la géologie régionale de la plate-forme de Llucmajor. Ainsi, l'observatoire installé sur le site pour surveiller la propagation de l'eau salée dans l'aquifère a pu être replacé dans son contexte géologique et pétrophysique, afin d'affiner l'interprétation de ses mesures de variations de salinité au cours du temps.
https://tel.archives-ouvertes.fr/tel-00633488

tel-00633488
https://tel.archives-ouvertes.fr/tel-00633488
https://tel.archives-ouvertes.fr/tel-00633488/document
https://tel.archives-ouvertes.fr/tel-00633488/file/TheseHebert.pdf

Éditeur(s) : HAL CCSD
Résumé : International audience
Understand the dynamics of the fresh-salt water interface in aquifers is a key issue to comprehend mixing processand to quantity the discharge of nutrients in to coastal areas. In order to go beyond the current knowledge inthis issue an experimental site has been set up at the alluvial aquifer Riera Argentona (Barcelona – Spain). Thesite comprises 16 shallow piezometers installed between 30 and 90 m from the seashore, with depths rangingbetween 15 and 25 meters. The seawater interface is being monitored using several techniques, the combinationof which will help us to understand the spatial and temporal behaviour of the mixing zone and the geochemicalprocesses occurring there. Specially the deepest piezometers are equipped with electrodes in order to performcross-hole electrical resistivity tomography (CHERT). In addition, all piezometers are also equipped with FiberOptic cable to perform distributed temperature measurements. Two single steel armoured fibre optic cable lines ofaround 600m length were installed in all boreholes. The objective is to use the cable both as passive and activetemperature sensor. The first is being done for the continuous monitoring of temperature whereas; the secondprovides a higher temperature resolution used to monitor field experiments.Periodic CHERT measurements are carried out between the piezometer equipped with electrodes, resultingin parallel and perpendicular vertical cross sections of the site resistivity. The position of the fresh-salt waterinterface can be identified due to the resistivity contrast between the saline and fresh water. Preliminary resultsof periodic distributed temperature measurements will be also be used to monitor the position of the mixing zonethanks to the contrast and seasonal temperature changes. Periodic down-hole EC profiles will be used to validatethe method.AcknowledgementsThis work was funded by the projects CGL2013-48869-C2-1 y CGL2013-48869-C2-2-R of the Spanish Government.We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the ConsellComarcal del Maresme in the construction of the research site
European Geosciences Union General Assembly 2016
Vienne, Austria

insu-01308757
https://hal-insu.archives-ouvertes.fr/insu-01308757

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
To evaluate the performance of downhole and surface geophysical monitoring methods, a series of shallow gas injection-monitoring experiments has been performed in a coastal saline aquifer at Maguelone, France. The recorded data include pressure measurements with a Westbay multilevel completion and CO2 saturation at an observation well derived from electrical resistivity with a modified Waxman-Smits (MWS) model. In this work, the aim is to develop a simulation model capturing the gas transport behavior and consistent with field data. For this purpose, the simulation of the CO2 injection experiment is carried out with two conceptual models, a homogeneous model and a heterogeneous model treated with multiple realization Monte Carlo simulations. Numerical simulator TOUGH2 with the equation of state module EOS7C is used for the simulations. Comparison of the model results with field data suggests that the pressure responses are captured with relatively good accuracy. Similarly, the model also provides an overall reasonable agreement and correct order of magnitude for predicted gas saturation values. However, as the heterogeneity pattern in the field data remains largely unknown, the model predictions can only be used to capture the mean behavior as well as to provide insights into how heterogeneity can influence the system behavior, by means of sensitivity analyses of the influence of heterogeneities on individual realizations.
ISSN: 1750-5836

hal-01467240
https://hal.archives-ouvertes.fr/hal-01467240
DOI : 10.1016/j.ijggc.2016.09.006

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
The New Jersey continental shelf extends 150 km off the shoreline. During IODP Expedition 313, siliciclastic deposits of late Eocene to late Pleistocene age were drilled down to 631, 669 and 755 m below seafloor at sites 27A, 28A and 29A respectively in very shallow waters (33.5 to 36 m depth). Pore water salinities display multilayered brackish-salty-brine units 10 to 170 m thick, where low-salinity water is preferentially stored in fine-grained sediments. The sharp boundaries of these buried aquifers are often marked by cemented layers a few centimetres thick. The mineralogy and scanning electron microscope observations of these layers show two phases of cementation by authigenic minerals: (1) the early carbonate cement is frequently associated with pyrite, and (2) the late silicate cement infills the residual porosity. The isotopic compositions of the carbonate cements vary widely: −2.4 < δ¹⁸O ‰ VPDB < +2.8; −15.1 < δ¹³C ‰ VPDB < +15.6. The δ¹⁸O values indicate that the carbonate cements precipitated with pore waters comprising variable mixtures of seawater and ¹⁸O-depleted fresh water originating from submarine groundwater discharge. The δ¹³C values of the carbonate cements are related to organic matter diagenesis, providing ¹³C-depleted dissolved inorganic carbon during bacterial sulphate reduction and anaerobic oxidation of methane, and ¹³C-rich dissolved inorganic carbon during methanogenesis. The diagenetic cementation processes included chemical weathering of reactive silicate minerals by the CO²-rich pore waters issued from organic matter diagenesis that released bicarbonate, cations and dissolved silica, which were further precipitated as carbonate and silicate cements. The estimated range of temperature (18±4 °C) during carbonate precipitation is consistent with carbonate cementation at moderate burial depths; however, silicate cementation occurred later during diagenesis at deeper burial depths.
ISSN: 0276-0460

hal-01559985
https://hal.archives-ouvertes.fr/hal-01559985
DOI : 10.1007/s00367-017-0506-6

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The main goal of the monitoring stage of Carbon Capture and Storage (CCS) is to obtain an accurate estimation of the subsurface CO2 accumulation and to detect any possible leakage. Laboratory experiments are necessary to investigate the small scale processes governing the CO2–brine–rock interaction. They also provide a means to calibrate the results coming from field scale geophysical methods. In this work we set up an experimental system which is able to perform Electrical Resistivity Tomography (ERT) measurements on centimeter-scale rock samples at various P–T conditions. We present the results of two new experiments related to CO2 monitoring, performed on a cylindrical (4 × 8 cm) Fontainebleau rock sample. In the first one, we have quantified the CO2 saturation at different volume fractions, representing zones from a deep saline aquifer with varying degrees of saturation. In the second one, we have monitored and quantified the effect of CO2 dissolution in the brine at a pressure of 40 bar during eight days, emulating the invasion of CO2 into a shallow aquifer. Results highlight the importance of accounting for the contribution of surface conductivity in highly CO2-saturated regions, even in clay-free rocks, and also for brine conductivity variation due to CO2 dissolution. Ignoring any of these effects will end up in a CO2 saturation underestimation. We present a modified CO2 saturation equation to account for these two influences.
ISSN: 0926-9851

hal-01356323
https://hal.archives-ouvertes.fr/hal-01356323
DOI : 10.1016/j.jappgeo.2016.03.039