Advances in Stable Isotope Monitoring of CO₂ Under Elevated Pressures, Temperatures and Salinities: Selected Results from the Project CO₂ISO-LABEL

Abstract

The BMBF project CO₂ISO-LABEL (Carbon and Oxygen ISOtopes under extreme conditions LABoratory EvaLuations for CO₂-storage monitoring) investigated stable isotope methods in laboratory studies for transferral to carbon capture and storage (CCS) field sites including enhanced gas and oil recovery (EGR and EOR). The isotope composition of injected CO₂ and water are useful tracers for migration and water-rock-gas interactions during such operations. However, quantification of carbon and oxygen equilibrium isotope effects at elevated pressures and temperatures are so far scarce. They thus need more investigations under p/T conditions that are characteristic for reservoirs and overlying aquifers. With this, the main objective of the project was to improve stable carbon and oxygen isotope methods for monitoring CO₂ storage sites and their impact of injected CO₂ on reservoir geochemistry under controlled laboratory settings. An important finding was that isotope fractionations of carbon between CO₂ and dissolved inorganic carbon (DIC) were not significantly different from each other in experiments with pure CO₂ and pressures between 59 and 190 bar. Furthermore, influences of rock types (limestone, dolomite and sandstone) and fluid salinities were found to be negligible for carbon isotope fractionation between CO₂ and DIC. Another finding was that water oxygen isotope ratios changed systematically in response to different CO₂/H₂O molar ratios in closed system equilibration experiments. This helps to reconstruct the amounts of CO₂ that equilibrated with formation waters. Results of the project will enable better assessment of geochemical conditions in underground carbon storage sites or other subsurface systems where large amounts of CO₂ interact with water and rocks.