The application of inherent noble gases and stable isotopes in tracing the migration and fate of injected CO₂

Inherent fingerprints, composed of the noble gas and stable carbon and oxygen isotopic composition of captured CO2 streams, are potentially powerful tracers for use in Carbon Capture and Storage technology that avoids the expense and complication of adding chemical tracers to the injected CO2. We will present a synthesis of the recent work undertaken in this area by our group and the key implications of our results for monitoring future CO2 storage projects using inherent tracers. We will highlight the high quality systematic measurements of the carbon and oxygen isotopic and noble gas fingerprints recently measured in anthropogenic CO2 captured from combustion power stations and fertiliser plants, using amine capture, oxyfuel and gasification processes, and derived from coal, biomass and natural gas feedstocks. We find that δ¹³C values are primarily controlled by the δ¹³C of the feedstock while δ¹⁸O values are predominantly similar to atmospheric O2. Noble gases are of low concentration and exhibit relative element abundances different to expected reservoir baselines and air, with isotopic compositions that are similar to air or fractionated air. The use of inherent tracers for monitoring and verification was assessed by analysing CO2 samples produced from two field storage sites after CO2 injection. These experiments at Otway, Australia, and Aquistore, Canada, highlight the need for robust baseline data. Noble gas data indicates noble gas stripping of the formation water and entrainment of Kr and Xe from an earlier injection experiment at Otway, and inheritance of a distinctive crustal radiogenic noble gas fingerprint at Aquistore. This fingerprint can be used to identify unplanned migration of the CO2 to the shallow subsurface or surface.