Chemistry of fluids from a natural analogue for a geological CO2 storage site (Montmiral, France): Lessons for CO2–water–rock interaction assessment and monitoring

Abstract

Abstract Chemical and isotope studies of natural CO 2 accumulations aid in assessing the chemical effects of CO 2 on rock and thus provide a potential for understanding the long-term geochemical processes involved in CO 2 geological storage. Several natural CO 2 accumulations were discovered during gas and oil exploration in France’s carbogaseous peri-Alpine province (south-eastern France) in the 1960s. One of these, the Montmiral accumulation at a depth of more than 2400 m, is currently being exploited. The chemical composition of the water collected at the wellhead has changed in time and the final salinity exceeds 75 g/L. These changes in time can be explained by assuming that the fraction of the reservoir brine in the recovered brine–CO 2 –H 2 O mixture varies, resulting in variable proportions of H 2 O and brine in the sampled water. The proportions can be estimated in selected samples due to the availability of gas and water flowrate data. These data enabled the reconstruction of the chemical and isotope composition of the brine. The proportions of H 2 O and brine can also be estimated from isotope ( δ 2 H, δ 18 O) composition of collected water and δ 18 O of the sulfates or CO 2 . The reconstituted brine has a salinity of more than 85 g/L and, according to its Br − content and isotope ( δ 2 H, δ 18 O, δ 34 S) composition, originates from an evaporated Triassic seawater that underwent dilution by meteoric water. The reconstitution of the brine’s chemical composition enabled an evaluation of the CO 2 –water–rock interactions based on: (1) mineral saturation indices; and (2) comparison with initial evaporated Triassic seawater. Dissolution of K- and SO 4 -containing minerals such as K-feldspar and anhydrite, and precipitation of Ca and Mg containing minerals that are able to trap CO 2 (carbonates) are highlighted. The changes in concentration of these elements in the brine, which are attributed to CO 2 interactions, illustrate the relevance of monitoring the water quality at future industrial CO 2 storage sites.