On-site dissolved-gas analysis and electric-resistivity tomography as new tools to trace CO₂ mineral sequestration in...

Mineral sequestration of CO₂ in basalt aquifers is a promising pathway for permanent carbon storage. Within the DemoUpStorage R&D project, CO₂ was dissolved in saline groundwater and co-injected with He as an inert tracer into a deep basalt aquifer at a pilot injection site near Helguvik, Iceland. We tested a combination of two novel geochemical and geophysical tools to monitor the subsurface CO₂ dynamics. Real-time, on-site monitoring of dissolved gases was conducted over one year using a gas-equilibrium membrane-inlet mass spectrometer (GE-MIMS). Electrical resistivity tomography (ERT) was used to track changes in aquifer resistivity before and after injection. GE-MIMS measurements revealed a clear increase in He concentrations downstream of the injection point, confirming fluid transport. However, no corresponding CO₂ increase was observed, indicating substantial CO₂ retardation relative to the fluid transport. No anomalies in He or CO₂ were detected in the overlying freshwater aquifer, indicating minimal upward migration. ERT data showed localized resistivity decreases suggesting basalt dissolution near the injection borehole. Together, GE-MIMS and ERT provided complementary, resource-efficient insights into CO₂ transport, reaction, and storage processes in the basalt aquifer. These tools enhance monitoring and assessment capabilities, supporting the development of safe and effective geological CO₂ storage.