Drilling and sampling a natural CO2 reservoir: Implications for fluid flow and CO2-fluid–rock reactions during CO2 migration through the overburden

International audienceThis paper presents the initial results of a scientific drilling project to recover core and pressurized fluid samples from a natural CO 2 reservoir, near the town of Green River, Utah. The drilling targeted a stacked sequence of CO 2-charged Jurassic sandstone reservoirs and caprocks, situated adjacent to a CO 2-degassing normal fault. This site has actively leaked CO 2 from deep supercritical CO 2 reservoirs at depth >2km within the basin for over 400,000 years. The project objectives were to gather samples to examine reactive fluid flow in the reservoirs, caprocks and faults, during migration of CO 2 through the geological overburden from the deep supercritical CO 2 reservoirs. Downhole fluid sampling and fluid element and isotope geochemistry show that the shallow reservoirs are being actively fed by inflow of CO 2-saturated brines through the faults. Comparisons of shallow and deep fluid geochemistry suggests that: (i) CO 2 and CO 2-charged brines co-migrated from the deep reservoirs, (ii) the CO 2-saturated brines migrating from depth interact with significant volumes of meteoric groundwater in aquifers in the shallower Permian and Jurassic sandstones, diluting the brine composition, and (iii) that a significant fraction of the CO 2 migrating from depth is dissolved in these brine-meteoric water mixtures, with >99% of the CO 2 in fluids sampled from the shallow reservoirs being derived during fluid migration, after the fluids left their source reservoir. The 87 Sr/ 86 Sr ratio of the brine flowing through the faults is significantly elevated due to the addition of Sr from silicate mineral dissolution during fluid migration. The association of bleached sandstones in the core with CO 2-rich fluids supports interpretations from elsewhere that CO 2-charged brines with CH 4 or H 2 S reductants can dissolve hematite present within the sediment. Analysis of fluid geochemistry an