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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.

The Apulian carbonate margin is widely preserved across the Adriatic domain and has been extensively studied in the south of Italy. In Albania, Oligocene–Pliocene fold-and-thrust tectonics led to widespread exposure of the Apulian Platform and associated Ionian Basin carbonates. However, the portion linking the platform to the basin is missing, preventing a direct reconstruction of the platform margin. Syn-sedimentary folding and faulting are recognized in the uppermost part of both the platform and basinal/slope series. Mass transport deposits (MTDs) occur within the platform succession incorporated into well-bedded intertidal (stromatolites) to shallow-subtidal (rudist packstones) sedimentary sequences. They display significant lateral variability which is accompanied by both rigid and soft deformation structures. Spectacular slumps made up of sediment density flow deposits are recognized in the adjacent Ionian Basin. The lateral extent of basal shear surfaces, syn-sedimentary faults and folds evidenced in the Ionian Basin points toward multiple regional tectonic triggering events affecting the Apulian Platform margin at that time. Bio- and chrono-stratigraphic analyses suggest that the triggers occurred during the late Campanian – early Maastrichtian. Beyond the obvious interest from a stratigraphic point of view, the study of these events recording the dismantling of the Apulian carbonate platform allows for a better understanding of the triggering mechanisms and the sedimentary characteristics of MTDs and slumps at a basinal scale.

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.