Geochemical Trapping of CO₂ in Basaltic Aquifers: Implications from CO₂ -Water-Rock Interaction Experiments

Abstract

Quite recently, a basaltic aquifer receives particular attention as a suitable candidate for CO₂ aquifer storage because basaltic rocks contain high concentrations of Ca, Mg, and Fe that can enhance CO₂ geochemical trapping via acid neutralization and carbonate mineral formation. Despite the increasing interest in basaltic aquifer CO₂ storage, there are few experimental researches on CO₂-water-basalt interaction. In addition, the influence of rock alteration on the geochemical trapping processes has not been considered. In this study, therefore, we conducted CO₂-water-basalt interaction experiments using three types of fresh to altered basaltic rocks.
The fresh basalt (non-altered basalt) is composed of olivine, plagioclase, and basaltic glass. On the other hand, the basic schist (high-T altered basalt) completely consists of secondary greenschist minerals (albite, epidote, chlorite, actinolite, quartz) , and the seamount basalt (low-T altered basalt) is mainly composed of secondary clay minerals (celadonite, smectite, Fe-oxyhydroxide) . In our experiments, the cations eluted from fresh basalt and basic schist are mostly Mg and Na, while the fluids reacting with seamount basalt contain large amount of Na and K with noticeably higher pH. This suggests that Na and K eluted from clay minerals play an important role in acid neutralization. Our experiments further demonstrate that, regardless of alteration types, a Na-dominant condition emerges at an early stage of the CO₂ storage in a basaltic aquifer, leading to dawsonite (NaAlCO₃ (OH) ₂) precipitation. The present findings lead us to propose a new perspective on carbonate formation sequence of dawsonite → siderite → calcite/dolomite in basaltic aquifers.