Controls on mineral carbonation reactions in basalt fracture networks

Basalts could provide secure long-term CO2 sequestration through mineral carbonation. Here, a series of flow-through experiments was conducted to investigate the effects of temperature, brine chemistry, and transport limitations on carbonation reactions in basalt microfractures. Post-reaction sample characterization revealed that carbonates preferentially formed in diffusion-limited vesicles and dead-end fractures, while injection paths were altered to clay minerals. The extent of secondary precipitation increased with increasing influent [NaHCO3] (from 6.3 to 640 mM) and temperature (from 100 to 150°C). Elevated [NaHCO3] also enhanced Mg- and Fe-uptake into carbonate minerals while carbonation at low [NaHCO3] was limited to Ca-carbonates that were strongly localized on Ca-rich pyroxene grains, consistent with predictions from complimentary 2D reactive transport models. The results highlight how complex coupling between fluid chemistry and transport conditions govern the extent and location of mineral carbonation reactions in natural basalts, which can inform selection of favorable storage sites and CO2 injection schemes with respect to long-term carbonation efficiency.