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Conference Papers Year : 2011

Comparison of shallow aquifer and soil gas monitoring approaches for detecting CO2 leakage at a natural analogue site in France

Abstract

Natural analogue sites where geologic CO2 is leaking to the surface provide excellent opportunities to test approaches suitable for monitoring for potential CO2 leakage at carbon capture and storage sites. We tested isotope monitoring approaches for CO2 detection in shallow aquifers and the overlying soil zone at a CO2 analogue site near Sainte-Marguerite in the Massif Central (France). The Sainte-Marguerite area is located in the southern part of the Limagne graben (French Massif Central). The basement, composed of highly fractured granite, outcrops toward the west of the study area, notably around the Saladis spring. An intercalated arkosic permeable interval between fractured granite and Oligocene marls and limestones acts as a stratiform drain for fluid migration while the overlying thick Oligocene interval is impermeable and acts as a seal. The Allier river bed is located near the contact between the basement and the sedimentary rocks. Deep CO2-ladden fluids migrate through the arkose interval toward the Sainte-Marguerite area and sustain a number of local springs. The Sainte-Marguerite area is known for the travertine deposits associated with the CO2-rich natural springs. We collected water samples and effervescent gases at the springs as well as soil gases for chemical and isotopic analyses. The analytical parameters included major anions and cations, d13C & d18O of CO2, dD & d18O of H2O and d13C of dissolved inorganic carbon (DIC). Preliminary results revealed that d13C values of CO2 in most groundwater and soil samples were similar. Oxygen isotope measurements revealed equilibrium between CO2 and H2O-oxygen in most samples, but except for a limited number of samples, d18O values of water did not deviate significantly from the local meteoric water line. Our preliminary results suggest that both the groundwater and the soil sampling approaches should be capable of detecting leakage of CO2 provided that the leaking gas has a distinct isotopic composition. The thresholds of leakage detection are currently determined in laboratory column experiments, where CO2 of known isotopic composition is bubbled through a water-saturated sand at different rates and continuously measured for its d13C and d18O isotope composition using laser spectroscopy. The potential for isotope exchange between oxygen from water and oxygen from bubbling CO2 is also evaluated.
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Dates and versions

hal-00631773 , version 1 (13-10-2011)

Identifiers

  • HAL Id : hal-00631773 , version 1

Cite

David Widory, Frédérick Gal, Eric Proust, Bernhard Mayer. Comparison of shallow aquifer and soil gas monitoring approaches for detecting CO2 leakage at a natural analogue site in France. AGU Fall Meeting, Dec 2011, San Francisco, United States. ⟨hal-00631773⟩
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