VISUALIZING BASALT-CO2 REACTIVITY DURING FLOW-THROUGH EXPERIMENTS (Invited Presentation)

Predicting the timing and magnitude of CO₂ storage in basaltic rocks relies partly on quantifying the dependence of reactivity on flow path and mineral distribution. Flow-through experiments that use intact cores, as opposed to crushed material, are advantageous because the spatial heterogeneity of pore space and reactive phases is preserved. Combining aqueous geochemical analyses and petrologic characterization with non-destructive imaging techniques (e.g. micro-computed tomography) places constraints on the relationship between irreversible reactions, pore connectivity and accessible surface area. Our work enhances these capabilities by dynamically imaging flow through vesicular basalts with Positron Emission Tomography (PET) scanning. PET highlights the path a fluid takes by detecting photons produced during radioactive decay of an injected radiotracer (FDG). We have performed single-phase, CO₂-saturated flow-through experiments with basaltic core from Iceland at CO₂ sequestration conditions (50 °C; ~76-90 bar total pressure). Constant flow rate and continuous pressure measurements at the inlet and outlet of the core constrain permeability. We monitor geochemical evolution through cation and anion analysis of outlet fluid sampled periodically. Before and after reaction, we perform PET scans and characterize the core using micro-CT. The PET scans indicate a discrete, localized flow path that appears to be a micro-crack connecting vesicles, suggesting that vesicle-lining minerals are immediately accessible and important reactants. Rapid increases in aqueous cation concentration, pH and HCO₃^- indicate that the rock reacts nearly immediately after CO₂ injection. High cation: silica ratios suggest incongruent volcanic glass dissolution and ion exchange from vesicle-lining zeolites may be important reaction mechanisms. After ~24 hours the solute release decreases, which may reflect a transition from more to less reactive minerals, a decrease in available reactive surface area or precipitation. Thermodynamic calculations indicate that the fluid remains slightly undersaturated with calcite, and the fluid is saturated with respect to secondary silica-rich phases. Our experiments show how imaging techniques are helpful in interpreting path-dependent processes in open systems.