A new biogeochemical method for the monitoring of possible seeps in marine CCS fields.

The sub-seabed deep saline aquifer CO2 injection for CCS purposes requires a reliable monitoring of possible CO2 seeps from the bottom sea. A biogeochemical monitoring method of CO2 seeps in marine environments has been set up by the manufacturing of new in situ and laboratory instruments and their use in the marine environment. The new monitoring method consists in the determination, before the CO2 geological storage, of benthic fluxes of dissolved inorganic carbon species (pCO2 and DIC) and of other connected parameters (alkalinity, pH and ?13CDIC), in order to characterise the marine carbonate system, and by investigating the early diagenesis processes, i. e. the processes originating the benthic fluxes. The knowledge of the natural benthic fluxes, before the CO2 injection, will allow the comparison with data obtained after the storage operations so that possible occurrences of unexpected leakages of the CO2 injected in the geological formations under the sea bottom can be verified. This method could also be useful employed as an "early warning" system in case of possible failure of the CO2 storage. For the set up of this biogeochemical monitoring method two new autonomous and automatic benthic flux measurement instruments (the Lander AMERIGO and the Camera Bentica Automatica) and two new laboratory apparatus for laboratory chemical analysis (pCO2 and DIC analyses) has been realized. The biogeochemical monitoring approach is particularly sound in the Northern and Central Adriatic Sea where numerous previous DIC and other carbonate system chemical benthic flux measurements and early diagenesis studies have been carried out, where the new monitoring technology is being applied and where CO2 sub-seabed storages can be carried out. Previous researches allowed the definitions of nine different diagenetic environments in the northern and central Adriatic Sea; each environments is characterized by: 1) homogeneous particulate matter and dissolved nutrient continental inputs; 2) distances from main sediment sources; 3) bottom sediment composition; 4) organic matter; 5) depths); 6) oxygenation of water column. In each diagenetic environment a background value of DIC, alkalinity and pH benthic fluxes has been assigned. These background values can be used as baseline in the CCS to monitoring possible seeps after the injection of CO2 in depth. At present, various researches are in progress in the Adriatic Sea for the acquisition of new data of DIC and other chemical dissolved benthic fluxes by the use of the new technology and for the early diagenesis studies that origin the fluxes.