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International audience; This work is part of the Aquifer CO2-Leak project and aims to understand, quantify and model the environmental impact of a CO2 leak on water quality in the carbonate freshwater aquifer as well as CO2-water-carbonate interactions. The experiment has been performed within an Oligocene carbonate underground quarry located in Saint-Emilion (France).A water charged with dissolved CO2 was injected in the aquifer through a borehole. Downstream, seven wells were fitted with in-situ probes which automatically measured physicochemical parameters. Periodic water samplings in all wells have been undertaken to determine the elemental concentrations by ion chromatography.The spread of CO2 in the groundwater was monitored as a function of time and was observed to influence the various physicochemical parameters. Five parameters seem to be excellent indicators for monitoring a gas plume during CO2 geological storage in regard to our results: electrical conductivity and pH, and Ca2+, HCO3−, and CO2(aq) concentrations.The interaction between CO2 and limestone is highlighted by a saturation index (SI) calculated with PhreeqC software. It shows (i) a slight trend to dissolution of calcite in the injection well (SI < 0) linked to the reaction process between CO2-H2O-CaCO3 and (ii) a transport process via diffusion for the observation wells with a SI≃0.The evolution of physicogeochemical signatures in the aquifer allows us to understand the reactive and transport processes that occur during the migration of a gasified water plume in the context of leakage from a geological storage reservoir. Our results will make possible to model a leakage in a complex natural reservoir.

International audience; From a natural underground cavity and two locations situated in a well, three CO 2 injections have been designed and performed under controlled conditions in order to study its migration along the carbonate vadose zone and to test geochemical and geophysical techniques. After the understanding of the natural CO 2 dynamics and the establishment of a baseline, some numerical simulations have been performed to help to optimize the monitoring strategy (spatially and temporally). A mixture of CO 2 + noble gases has been injected and the results show that CO 2 subsurface leakage can be anticipated thanks to these inert chemical gases used as tracers (He + Kr).The geochemical and geophysical monitoring approaches are very complementary.

A shallow CO 2 injection experiment was performed on a pilot site near Bordeaux, France, in February 2015. A gas mixture composed of approximatively 90% CO 2 and 5% of each He and Kr as gas tracers was injected through a vertical well, with low injection rate, at 1.80 m depth and into the carbonate vadose zone. A set of geochemical monitoring tools was installed in order to monitor the developing gas plume and to allow comparison between this experiment and a former release experiment which used a higher flow injection was made. Measurement showed a heterogeneous spatial and temporal variation of the gas plume. Difference in time arrival of peak concentration of He, Kr and CO 2 confirmed that diffusion alone did not occur in the porous media. Former studies mentioned advective flow drove through preferential paths. Both advection and diffusion flow may have been observed in the experiment. It also shows that the difference in molecular weights and the heterogeneity of the limestone could lead He to be used as a temporal tracer, and Kr as a spatial tracer as it allows us to estimate the extent of CO 2 plume. The ratios between the various gas components also indicate that a sizeable amount of CO 2 could be consumed before it reaches the subsurface, meaning that the vadose zone could be a buffering zone to CO 2 leakage. The comparison between this experiment and a former one with different injection condition revealed that the presence of advection is dependent of the heterogeneity of the limestone. However, the injection rate induces the amount of gas diffusing or/and advecting. Observations from both, our recent and a previous leakage experiment and future laboratory experiment could improve our understandings of the buffering zone and help to foresee CO 2 leakage for future storage site.

International audience; The DEMO-CO2 project is dedicated to develop an analytical method for CO2 leakage detection within the vadose zone and to understand the behavior of CO2 in the near subsurface carbonate environment during an induced CO2 leakage. A gas mixture of 3 m3 of CO2, He and Kr was released at 3.7 m depth in a borehole located in a carbonate vadose zone. Different detection techniques were used to follow the CO2, and noble gas migration within the vadose zone. A numerical simulation done with COORES™ code was used to predict gas flux at surface. Measurements performed in the field showed heterogeneous gas transfer in the subsurface as a result of contrast of permeability/porosity of the porous media at meter scale. Both advective and diffusive gas transfer could be observed due to the different time frames needed to reach maximum He, Kr and CO2 concentrations, confirming that noble gases can be used as precursor of injected CO2 leakage in case of diffusive transfer prevails. Despite the subsurface heterogeneity, simulations results of CO2 flux at the surface were in relatively good agreement with field measurements. The comparison of CO2 concentration with measurements of ubiquitous gas such as N2, coupled with isotopic 13C analyses could be used to clearly differentiate injected CO2 from natural background CO2 produced by biological activity. Lessons learned through this experiment could help to improve future near-surface gas geochemistry surveys for site assessment, identify CO2 leakage, and leakage monitoring at active carbon capture and storage sites.

The sedimentology and tectonics and their relationship of the Paleozoic series that fill the Ghadames basin and its northern extension consisting of the Jefarah basin in Libya and Tunisia was studied. These formations belong to the Gondwana cycle and are typically interpreted as being deposited in a cratonic basin truncated by the Hercynian unconformity. This study is based on wells. The stratigraphic correlation between the wells is based on the definition of second order cycles. This allowed to produce isopach maps of facies distributions and to reconstruct the paleogeography of the different stratigraphic units. These data allow to address the nature of the deformation and also clarifies the behavior of active high regional areas during the Palaeozoic. The Paleozoic succession in the Ghadames and Jefarah basins can be divided in to five first order sequences, bounded by major tectonic unconformities with sequence duration of 40 - 70 Ma. Within these five sequences eighteen second order sequences (10 - 40 Ma) were differentiated, describing Sequence Boundaries (SB), Maximum Flooding Surfaces (MFS) and sedimentological characteristics. Twenty six wells with its geological well reports and well logs have been used in the study area, i.e. the Ghadames and Jefarah basins (Libya) illustrate how reservoir properties changes laterally and vertically through time (from a proximal to a distal sitting). Two softwares were used, JLog (version 4) petrophysical software for reservoir property analysis and PETREL (Schlumberger software 2014 version) for constructing stratigraphic correlation models. The Paleozoic reservoirs in the study area are spread over a large range of siliciclastic reservoirs with the prospective section extending from Cambrian to Permian. Hydrocarbons within the Ghadames and Jefarah basins originated from two major source rocks: i.e. the Lower Silurian Tanezoft Formation and Middle-Upper Devonian Aouinat Ounine Formation. The nine main Paleozoic reservoirs are respectively the Hassaouna, Haouaz and Memouniat Formations (Lower Paleozoic), Acacus, Tadrart, Ouan Kaza and Tahara Formations (Middle Paleozoic) and M’rar and Asadjefar Formations (Upper Paleozoic). Six stratigraphic cross sections through the area have been reconstructed to illustrate the vertical and lateral reservoir extensions. The dataset demonstrates that the Paleozoic reservoir properties are influenced by both, depositional facies and position within the paleogeographical setting (from proximal to distal part).; L’analyse stratigraphique et tectono-sédimentaire des séries sédimentaires du Paléozoïque qui remplissent le bassin de Ghadamès et son extension au nord, le bassin de Jefarah en Libye et en Tunisie, ont été étudiées. Ces formations appartiennent au cycle de Gondwana et sont, généralement, interprétées comme étant déposées dans un bassin cratonique tronqué par la discordance hercynienne. Cette étude est basée sur l’analyse des électrofaciès et des électroséquences dans plus de 130 puits. Les corrélations stratigraphiques entre puits sont basées sur la définition des cycles de 1er et de 2ème ordre. Cela a permis de comprendre l’évolution des environnements sédimentaires dans les deux bassins, de reconstituer des cartes isopaques de répartition des faciès et de reconstruire la paléogéographie des différentes unités stratigraphiques. Ces données permettent d'aborder la nature de la déformation et clarifient également le comportement des zones actives régionales élevées durant le Paléozoïque. La succession paléozoïque dans les bassins de Ghadamès et de Jefarah peut être divisée en cinq séquences de 1er ordre, délimitées par des discontinuités tectoniques majeurs avec une durée de séquence de 40 à 70 Ma. Dans ces cinq séquences, dix-huit séquences de 2ème ordre (10 à 40 Ma) ont été différenciées, décrivant les limites de séquence (SB), les surfaces d'inondation maximales (MFS) et les caractéristiques sédimentologiques. Vingt-huit puits ont été analysés afin d’identifier et d’évaluer les systèmes réservoirs paléozoïques dans les bassins Ghadamès et de Jefarah illustrant comment les propriétés réservoirs changent latéralement et verticalement dans le temps en fonction des environnements de dépôt. Deux logiciels ont été utilisés, le logiciel pétrophysique JLog pour l'analyse des propriétés réservoirs et PETREL pour la construction de modèles de corrélation stratigraphique. Les réservoirs dans la zone d'étude sont répartis sur une large gamme de réservoirs silicoclastiques s'étendant du Cambrien au Permien. Les hydrocarbures dans les bassins de Ghadamès et de Jefarah proviennent de deux roches mères principales, à savoir la Formation de Tanezuft du Silurien inférieure et la Formation d'Awaynat Wanin du Dévonien moyen-supérieur. Les neuf principaux réservoirs paléozoïques sont respectivement les formations de Hasawnah, de Hawaz et de Mamouniyat (Paléozoïque inférieure), d’Akakus, de Tadrart, d’Ouan Kaza et de Tahara (Paléozoïque moyen) et les Formations de M'rar et d’Asadjefar (Paléozoïque supérieur). Six sections transversales stratigraphiques dans la région ont été reconstruites pour illustrer les extensions verticales et latérales des réservoirs. L'ensemble de données acquises démontre que les propriétés réservoirs des séries paléozoïques sont influencées par les deux, les faciès et les environnements de dépôts.