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 δ¹³C_CO2 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 δ¹³C_CO2 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 methods^2,3. However, further sample collection and analysis is required to affirm these promising initial mineralisation estimates acquired from inherent tracers.

References

¹Snæbjörnsdóttir et al., 2020, Nature Reviews Earth & Env. 1, 90–102. DOI:10.1038/s43017-019-0011-8

²Clark et al., 2020, GCA, 279, 45-66, DOI:10.1016/j.gca.2020.03.039

³Ratouis et al., 2022, IJGGC, 114, 103586, DOI:10.1016/j.ijggc.2022.103586