Simulation of uranium mobilization potential in a deep aquifer under geological carbon storage conditions

Geological carbon storage is considered as a promising strategy to reduce greenhouse gas emissions. For those subsurface systems with uranium-bearing minerals in rock formations, there is an uncharacterized possibility for injected CO2 to cause uranium mobilization due to the coupled chemical and physical interactions at mineral--water interfaces. We developed a TOUGHREACT model to assess the uranium mobilization potential from uraninite (UO2) dissolution induced by CO2 injection into a hypothetical CO2 storage reservoir in a time scale of 30 years of CO2 injection and 100 years after. Numerical simulation results show that the concentration of HCO3− increased after CO2 injection, and CO2 was able to migrate toward the shallow aquifer through existing leakage pathways. HCO3−-assisted UO2 oxidative dissolution was the dominant mechanism that led to the increase of dissolved uranium concentration in a form of uranyl carbonate complex and the mechanism of H+-assisted UO2 dissolution could be ignored. Sensitivity tests suggested that the increase of UO2 content and O2 level in the CO2 storage reservoir resulted in the most significant increase of dissolved uranium concentration, but no sign of uranium intrusion was predicted up to the shallow aquifer. In summary, uranium release induced by CO2 injection is possible if the target CO2 storage reservoir contains U-bearing minerals and is exposed to oxidizing environment, but our simulation assured that significant upward migration of uranium from deep CO2 storage reservoir driven by CO2 injection is unlikely even at the worst scenario.