Selection, instrumentation and characterization of a pilot site for CO2 leakage experimentation in a superficial aquifer

International audienceCO2 geological storage is one of the options for mitigation of GHG emissions into the atmosphere. Although storage is provided for several millennia, leakage can occur. Therefore the risk that the fugitive CO2 reaches a shallow aquifer cannot be excluded [1]. That is why the consortium launched the research program CIPRES, funded by the French Research Agency, concerning the potential impacts of CO2 leakage on groundwater quality. This program includes the realization of a shallow CO2 release experiment in a carbonated aquifer and its monitoring in the saturated and unsaturated zones, the soil and the soil-atmosphere interface. The experimentation consists in drilling a well into the aquifer, injecting dissolved CO2 in water [2] and tracking its impact downstream. Prior to this experiment, a site had to be selected, instrumented and characterized. Hydrogeological target is the Paris basin chalk which is the largest French aquifer. The selected area is a large parcel, formerly cultivated in intensive monoculture but left fallow for several years, located in Catenoy (Oise) about 50 km north of Paris. The geological log shows the chalk is covered by 6-7 m of sands but water table is located at 12-13 m depth, i.e. integrally in the chalk strata. The site has been equipped with 10 wells to characterize the groundwater flow (hydraulic gradient ~5 10-3 ). 6 wells are aligned in the groundwater flow direction: the PZ2, planned for CO2 injection , the PZ1 located 20 m upstream and 4 monitoring wells located downstream between 10 and 60 m from PZ2. There are also 4 lateral piezometers located near (PZ7, PZ8) and far (PZ9, PZ10) from the injection site: they are dedicated to the lateral plume. These 25 m depth wells are equipped with tubing fully slotted in the chalk. Alongside, 4 wells dedicated to gas monitoring in unsaturated zone have been drilled at 11m depth