Quantifying CO2 Dissolution and Mineralisation Using Inherent Isotope Ratios at the CarbFix2 Project, Iceland

Injection of CO2 into reactive rocks for permanent fixation into new minerals guarantees storage security1. The leading demonstration of this technology is the CarbFix2 project, in SW Iceland, which captures CO2 and H2S gases emitted from the Hellisheiði geothermal field via dissolution into water and then injects this mix into the subsurface. The captured CO2 and H2S then reacts to form stable minerals within the basaltic rocks2.Traditional verification of subsurface CO2 sequestration has predominantly relied on artificial or indirect geochemical tracers. Here, we demonstrate the use of inherent isotopic ratios of noble gases and stable isotopes of C, D and O within CO2 and H2O samples obtained from the CarbFix2 project, in order to monitor and quantify surface CO2 capture via dissolution and subsequent subsurface mineralisation.Initially, shifts in the inlet and outlet CO2/3He and C isotope ratios (δ13C) from the Carbfix2 CO2 capture tower are used to determine that 50% (±4%) of the CO2 was removed from the inlet gas stream via dissolution in the water wash. This estimate of the portion of CO2 captured correlates with independent measurements from existing methods used by CarbFix.It is then calculated that the dissolved CO2 has a CO2/3He of 9.6 x 109 (± 8.9 x 108) and a δ13CCO2 isotope ratio of -5.0‰ (± 0.2‰) V-PDB. Comparison of these values to those measured in CarbFix2 monitoring wells shows that lower CO2/3He and higher δ13CCO2 than expected are observed in the monitoring wells, compared with a baseline scenario where no mineralisation occurs. This indicates that a significant portion of CO2 has been removed from the injected fluids.Through integration of these findings with monitoring well data, mineralisation and mixing dynamics at a reservoir temperature of 265°C were explored. A critical role for oxygen isotope ratios of water (δ18O) in distinguishing remaining injectate from background reservoir CO2 was identified, which aids in the interpretation of the CO2/3He and δ13C data.The results align with previously documented estimates of the proportion of CO2 mineralised obtained from other independent methods2,3. However, further sample collection and analysis is required to affirm these promising initial mineralisation estimates acquired from inherent tracers.References1Snæbjörnsdóttir et al., 2020, Nature Reviews Earth & Env. 1, 90–102. DOI:10.1038/s43017-019-0011-82Clark et al., 2020, GCA, 279, 45-66, DOI:10.1016/j.gca.2020.03.0393Ratouis et al., 2022, IJGGC, 114, 103586, DOI:10.1016/j.ijggc.2022.10358