Tracking and CO2 leakage from deep saline to fresh groundwaters : development of sensitive monitoring techniques
Résumé
The assessment of environmental impacts of carbon dioxide storage in geological repository requires the investigation of the potential CO2 leakage back into fresh groundwater, particularly with respect to protected groundwater reserves. A new 3-year project aims at developing sensitive monitoring techniques in order to detect potential CO2 leaks and their magnitude as well as their geochemical impacts on the groundwater. In a predictive approach goal, a modelling study of the geochemical impact on fresh groundwaters of a CO2 intrusion during geological storage was also performed and serves as a basis for the development of sensitive monitoring techniques (e.g. isotope tracing). Then, isotopic monitoring opportunities are explored. Geochemical processes and perturbations associated with a CO2 gas leak into dilute groundwater are well known: relation between acid gas and solute concentration increase is clearly interrelated involving many different mechanisms as precipitation, oxidation and mineral dissolutions. Some of these minerals may contain hazardous constituents able to release contaminant into groundwater. When CO2 as a gas (supercritical) phase is injected into a brine that fully saturates porous aquifer, hydrogeological and hydrochemical properties changes are more complex than usually observed in groundwaters because of immiscibility between injected and native fluids. Therefore, it is necessary to correctly model the two-phase transport to correctly predict the evolution of a leak in an aquifer. This predictive modelling approach includes a 3D model with (i) storage saline aquifer, (ii) impacted overlying aquifer containing freshwaters and (iii) a leaky abandoned well represented as 1D porous media and corresponding to the interface between rock and cement formations. This model intends to assess the whole path of migration of the supercritical CO2 and the interaction between the fluid and the host rock. A supplementary understanding is the origin and conditions of CO2 migration through the leaking way from the injection storage zone to the shallow aquifer. The illustrated case study is from the Paris basin context and the Dogger formation is considered as the storage geological target. This geological formation has been intensively used for geothermal purpose in the past decades and is now under consideration as a target for the French national program of greenhouse gas emission reduction and CO2 geological storage concept. Above the Dogger aquifer, the Albian aquifer is a major strategic potable water aquifer: the impacts of a CO2 leakage due to a potential integrity failure has therefore to be carefully investigated. The simulations presented in this study aim at helping define guidelines and selection criteria in the field of environmental risks. Therefore, the examination of the effect of the gas intrusion on the quality changes of the local groundwater as well as a better understanding of the water-rock-gas interactions is of primary importance. The main geochemical process simulated is the acidification of groundwaters due to CO2 dissolution inducing Albian minerals dissolution and element releases. The mineralogical composition of the impacted Albian aquifer is therefore crucial to estimate correctly which elements may possibly be released during the arrival of gaseous and dissolved CO2 in freshwaters. Indeed, associated with a decrease of pH due to CO2 migration, hazardous trace elements could be found depending on the final solute concentration .and its control by dissolution/precipitation of a few key minerals. Finally, it is necessary to integrate surface phenomena in order to consider a complete and sensitive monitoring. Another way of consideration is isotope systems that seem to be alternative and persuasive tools to record geochemical modifications induced by CO2 intrusion to predictive model. Isotope systems can track reactions associated with small quantities of CO2 into dilute aquifer which, in case of leak, would not be detected by others methods. Laboratory experimentations on an Albian representative sample will be developed in order to display these tracking tools. The main focus is to point out suitable isotope systems which can characterize water rock interactions, redox conditions evolution, CO2 movement etc. Under very precise protocol, the strontium and carbon isotopes for instance are used to study the effect of carbonate dissolution through its impact on the CO2 concentration. Such isotope systems require the application of different types of methods and apparatus. IRMS (Isotope Ratio Mass Spectrometer), CF-IRMS (Continuous flux- Isotope Ratio Mass Spectrometer), TIMS (Thermo-Ionisation Mass Spectrometer) will be used for a large set of isotope systems. However, other new non classical isotope systems will be investigated using the new generation isotope ratio mass spectrometers MC-ICP-MS that combine the ionisation efficiency of inductively coupled plasma with the advantages of multi-collector data acquisition. With such a technology, isotopes of many different chemical elements can be analysed with higher precision. Thus, isotopic fractionation will be investigated in the frame of this work in the next future: fractionation is interesting since it is mechanism-dependent, e.g. isotopic exchange reactions, physical and chemical processes as kinetic aspect (diffusion etc.) or state changes (adsorption, desorption, dissolution, precipitation etc). Moreover, temporal and spatial variations in water oxygenation are mirrored by distinctive changes in the geochemical and isotopic data (sulphur isotope; selenium isotopic signature to be used as a tracer for sulphur sources). These instrumental advances open news research field and favour the use of multi-isotope approaches in isotopic studies as the main interest of hazardous trace elements transport and geochemical evolution of groundwater due to CO2 intrusion. Experimental interaction between rocks and water under CO2 injections will be run and several isotope systems investigated in the frame of studying the impact on fresh groundwater.