Noble gas as a proxy to understand the evolutionary path of migrated CO2 in a shallow aquifer system

Abstract To provide confidence in the safety of a carbon capture and storage (CCS) project, researchers have focused on developing monitoring techniques to trace the unlikely, but potentially possible, migration of CO2 from a deep reservoir. Among the various techniques, noble gas tracing is a beneficial approach, owing to the unique noble gas fingerprints present in injection fluids, the deep reservoir, and the shallow aquifer above the storage area. However, the value of this approach has been limited to demonstrations in a natural analogue CO2-rich reservoir and an artificial injection test site. Therefore, further efforts are required to link those valuable observations to an actual CCS site. In this study, we outline how to use these tracers for actual monitoring work in a shallow aquifer system. First, two artificial injection tests were performed using He, Ar, Kr, and SF6 to understand the behavior of the leaked plume in the shallow aquifer system. In both tests, the noble gas ratio remarkably changed with the solubility-controlled process and the mixing process. To extend and link the valuable findings from the artificial injection tests to an actual CO2 leakage event, we performed a leakage simulation using data from a real CO2 injection site, i.e., the Weyburn–Midale site. This simulation suggested that combinations of 4He with other heavier noble gases can be used to monitor CO2 leakage, as they allow us to separate and explain the major interactions governing the migration of the leaked plume in the shallow aquifer system. Additionally, although the high CO2 density of a dissolved plume is known to add uncertainty in quantitative approaches, the influence of those effects was negligible when compared to the errors arising from the wide variation in the noble gas fingerprints in the leaked CO2. This study, therefore, provides insight into the evolutionary path of the migrated CO2 plume in the shallow aquifer system and to the results can be used to inform the tracing of a leakage source within a shallow aquifer despite various mechanisms complicating the plume distribution.