A Geochemical Approach for Monitoring a CO2 Pilot Site: Rousse, France. A Major gases, CO2-Carbon Isotopes and Noble Gases Combined Approach Une méthode géochimique pour la surveillance d’un site pilote de stockage de CO2 : Rousse, France. Approche combinant les gaz majeurs, l’isotopie du carbone du CO2 et les gaz rares

This paper presents the geochemical characterization of various gas end-members involved in a depleted gas field CO2 storage pilot (Rousse, France). In this pilot, CO2 is produced by oxycombustion from natural gas transformed into fuel gas at the Lacq plant, and transported in a pipeline 30 km away to the depleted gas reservoir of Rousse. Gases produced at Rousse before CO2 injection, the Lacq fuel gas and the CO2 resulting from the oxy-fuel combustion were sampled, together with gases from a –45 m monitoring well and from soils in the vicinity of the Rousse structure. For all samples, the bulk gas composition, the carbon isotopic compositions and the abundance and isotopic signatures of the noble gases were determined. The bulk gas compositions of the Rousse natural gas are comparable to the Lacq fuel gas with methane as the main compound with residual C2-C5 and CO2. Soil gases are typical mixtures of air with biogenic CO2 (up to 9-10%), while the monitoring well gases display typical air compositions with no excess CO2 The Rousse gas and the Lacq fuel gas have δ13CCH4 values of –41.0‰ and –43.0‰ respectively. The injected CO2 out of the oxycombustion chamber has a δ13CCO2 of –40.0‰, whereas δ13CCO2 value for soils samples is comprised between –15 and –25‰. The Rousse natural gas and the Lacq fuel gas are both characterized by a high He enrichment, and depletion in Ne, Ar and Kr compared to the air values. The oxyfuel combustion process provides a CO2 with the He enrichment of the Lacq fuel gas, and a Ne, Ar and Kr composition reflecting that of the oxygen produced at the Air Separation Unit (ASU). Indeed, Ne is depleted relatively to the air, while Kr is enriched up to tenfold, which results from the cryogenic separation of the air noble gases within the ASU. Soil samples noble gas compositions are equivalent to that of the air. In the light of these results, the compositions of the various end-members involved in this CO2 storage pilot suggest that noble gas compositions produced by oxyfuel process are sufficiently exotic compared to compositions found in nature (reservoir, aquifer and air) to be directly used as tracers of the injected CO2, and to detect and quantify leaks at soil and aquifer levels. Ce papier presente la caracterisation geochimique des differents gaz, naturels et anthropogeniques, impliques dans un pilote de stockage de CO2 en champ de gaz naturel appauvri (Rousse, France). Dans ce pilote, le CO2 est produit par oxycombustion d’un gaz