Open versus closed system injection: A key critical geochemical control on in-situ CO2 mineralisation in basalts

Over a decade ago, CarbFix1 in Iceland (Matter et al., 2016) and the Wallula Basalt Project in Washington state (McGrail et al., 2017) showed in short succession that carbon mineralisation in the field at shallow depth (≤1km) and low T (c. 35 °C) can be achieved in less than two years, orders of magnitude faster than previously anticipated. Since then, no other field site has yet reproduced this carbon storage success -other than CarbFix2, which has been injecting CO 2 since 2014, albeit into a superhot basaltic reservoir at more than 250 °C (Clark et al., 2020). This contribution looks again at the low TP precursor sites, especially CarbFix1, to identify the reasons that drove the mineralisation of CO 2 . It seeks to disentangle the confounding factors that affected carbonatisation. Based on geochemical modelling, it is reasoned that CarbFix1 (and Wallula) benefited primarily from the fact that injection was not continuous over a long period of time, only five and three weeks, respectively. This finite time frame allowed the attainment of closed reaction-controlled conditions along the flow path, which aided in the formation of secondary phases, including carbonates. It is reasoned that future CO 2 storage efforts into basalts will run on a considerably longer and, crucially, continuous time frame. Under such open injection conditions, the geochemistry of the reservoir will change; our models indicate that the groundwater will be buffered at pH ≤∼6.5. This pH buffer coincides with the ‘sweet spot’ for mineralisation of CO 2 in basalts at low temperatures (20–50 °C) that (Snæbjörnsdóttir et al., 2018) postulate would allow (Ca)-Mg-Fe carbonate formation whilst keeping zeolite and smectite formation at bay. Open conditions will thus not run counter to, or become detrimental to, the carbon mineralisation efforts in basaltic terrains; however, the secondary mineralogy will very likely differ from that observed at CarbFix1. • Sensitivity analysis reveals the system status (open vs. closed) as a key condition • The carbon storage trials CarbFix1 and Wallula were short closed system injections • Long-term carbon injection will maintain open conditions affecting secondary mineralogy. • Buffer capacity of an open CO 2 injection system is much larger than for a closed system • Open system pH conditions will be around pH 6.5 and affect the carbonate mineralogy