Tracking CO₂ injection, migration and fate at Carbfix2 using stable isotopes

Industrial scale Carbon Capture and Storage (CCS) and Carbon Dioxide Removal (CDR) is required for the world to limit global warming to 1.5-2˚C. Methods that verify CO₂ storage will be necessary for legal accounting and public reassurance. Stable isotopes are inherent tracers readily used throughout the natural sciences as indicators of subsurface reactions and temperatures. The aim of this study is to demonstrate the capabilities of these tracers at the Carbfix2 CO₂ mineral storage site in Iceland. We find that oxygen isotope ratios (δ¹⁸O) of water samples are higher than meteoric water due to water-rock interaction in the geothermal reservoir. Water isotope ratios vary across the dataset due to differing steam-water ratios controlled by hydrothermal phase separation. Carbfix2 monitoring wells are depleted in ¹⁸O and ²H relative to other geothermal production wells and calculated unreacted monitoring well fluids. This is likely because monitoring wells receive background fluids that are isotopically distinct from other geothermal production wells. This work demonstrates the capabilities of stable isotope measurements as tracers of active reactions and processes in CCS reservoirs. Measurements can be used as a direct tracer of injected fluids, while also identifying other important reservoir characteristics such as fluid source and water-rock interaction.