Monitoring CO2 mineralisation and dissolution at CarbFix2 using inherent isotopes of CO2, H2O and noble gases

• First use of inherent isotopes to monitor and verify in-situ CO 2 mineralisation. • Shifts in CO 2 / 3 He and δ 13 C CO2 indicate ∼50 % CO 2 dissolution during gas capture. • Shifts in CO 2 / 3 He and δ 13 C CO2 indicate CO 2 mineralisation in storage reservoir. • Water isotopes are key for distinguishing injectate and background fluids. • Promising results for wider use of isotope tracers in geological CO 2 storage. Mineralising CO 2 in mafic and ultramafic geological reservoirs is a rapid and durable means of CO 2 storage. To date, verification of mineralisation storage has primarily relied upon externally added or indirect geochemical tracers. Here, we use changes in inherent CO 2, H 2 O and noble gas isotope ratios to monitor CO 2 dissolution and mineralisation at the CarbFix2 carbon capture and storage (CCS) project in Iceland. Reductions in CO 2 / 3 He ratios between gas inlet and outlet samples of the CarbFix2 scrubbing tower indicate 49 ± 4 % CO 2 dissolution in water. We calculate that dissolved CO 2 has a CO 2 / 3 He of 1.0 ± 0.1 × 10 10 and a carbon isotope ratio (δ 13 C) of -5.0 ± 0.2 ‰ VPDB. CarbFix2 monitoring well isotope data record lower CO 2 / 3 He and higher δ 13 C CO2 than predicted by a baseline scenario where no mineralisation occurs. We model the evolution of injectate CO 2 / 3 He and δ 13 C CO2 for different chronologies of mineralisation and mixing at the reservoir temperature of 265 °C. Oxygen isotope ratios of H 2 O (δ 18 O) are key for distinguishing between the remaining injectate and background CO 2 in monitoring wells because the combined effects of fractionation and CO 2 loss make injectate and background CO 2 / 3 He and δ 13 C indistinguishable. Monitoring well data intersect modelled mineralisation scenarios at similar extents of mineralisation previously recorded. Uncertainties regarding background reservoir fluids and temporal variation of isotope data necessitate additional sampling to further validate this mineralisation verification at CarbFix2. Nevertheless, these initial results are promising for the wider application of inherent isotope tracers to monitor and verify in-situ CO 2 mineralisation.