A Geochemical Approach for Monitoring a CO2Pilot Site: Rousse, France. A Major gases, CO2-Carbon Isotopes and Noble Gases Combined Approach

Abstract

This paper presents the geochemical characterization of various gas end-members involved in a depleted gas field CO<sub>2<sub/> storage pilot (Rousse, France). In this pilot, CO<sub>2<sub/> 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 CO<sub>2<sub/> injection, the Lacq fuel gas and the CO<sub>2<sub/> 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 C<sub>2<sub/>-C<sub>5<sub/> and CO<sub>2<sub/>. Soil gases are typical mixtures of air with biogenic CO<sub>2<sub/> (up to 9-10%), while the monitoring well gases display typical air compositions with no excess CO<sub>2<sub/> The Rousse gas and the Lacq fuel gas have <i>δ<i/><sup>13<sup/>C<sub>CH<sub>4<sub/><sub/> values of –41.0‰ and –43.0‰ respectively. The injected CO<sub>2<sub/> out of the oxycombustion chamber has a <i>δ<i/><sup>13<sup/>C<sub>CO<sub>2<sub/><sub/> of –40.0‰, whereas <i>δ<i/><sup>13<sup/>C<sub>CO<sub>2<sub/><sub/> 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 CO<sub>2<sub/> 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 CO<sub>2<sub/> 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 CO<sub>2<sub/>, and to detect and quantify leaks at soil and aquifer levels.