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Chemical tracers are proposed as an effective means of detecting, attributing and quantifying any CO2 leaks to surface from geological CO2 storage sites, a key component of Carbon Capture and Storage (CCS) technology. A significant proportion of global CO2 storage capacity is located offshore, with some regions of the world having no onshore stores. To assure regulatory bodies and the public of CO2 storage integrity it is important to demonstrate that robust offshore monitoring systems are in place. A range of chemical tracers for leakage have been tested at onshore pilot CCS projects worldwide, but to date they have not been trialled at injection projects or CO2 release experiments located offshore. Here, for the first time, we critically review the current issues surrounding commercial scale use of tracers for offshore CCS projects, and examine the constraints and cost implications posed by the marine environment. These constraints include the logistics of sampling for tracers offshore, the fate of tracers in marine environments, tracer background levels, marine toxicity and legislative barriers – with particular focus on the Europe and the UK. It is clear that chemicals that form a natural component of the CO2 stream are preferable tracers for ease of permitting and avoiding cost and risks of procuring and artificially adding a tracer. However, added tracers offer more reliability in terms of their unique composition and the ability to control and regulate concentrations. We identify helium and xenon isotopes (particularly 124,129Xe), and artificial tracers such as PFCs and deuterated methane as the most suitable added tracers. This is due to their conservative behaviour, low environmental impact and relative inexpense. Importantly, we also find that SF6 and C14 are not viable tracers for CCS due to environmental concerns, and many other potential tracers can be ruled out on the basis of cost. Further, we identify key challenges that are unique to using tracers for offshore monitoring, and highlight critical uncertainties that future work should address. These include possible adsorption or dispersion of tracer compounds during ascent through the overburden, longevity of tracers over the timeframes relevant for CCS monitoring, the permissible environmental effects of tracer leakage, and tracer behaviour in seabed CO2 bubble streams and in dissolved CO2. These uncertainties directly affect the selection of appropriate tracers, the injection programme and concentrations necessary for their reliable detection, and appropriate sampling approaches. Hence offshore tracer selection and associated expense are currently poorly constrained. Further, there is limited experience of sampling for tracers in the marine environment; current approaches are expensive and must be streamlined to enable affordable monitoring strategies. Further work is necessary to address these unknowns so as to evaluate the performance of potential tracers for CO2 leak quantitation and provide more accurate costings for effective offshore tracer monitoring programmes.

The authors would like to acknowledge the financial support of the UK CCS Research Centre (www.ukccsrc.ac.uk) in carrying out this work. Author Kremer is supported by NERC grant NE/N015908/1. The UKCCSRC is funded by the EPSRC as part of the RCUK Energy Programme. Midland Valley Exploration are thanked for an academic license for Move. Porosity and permeability analysis were undertaken in the University of Aberdeen Petrophysics laboratory with the aid of Sophie Harland. The South African National Energy Development Institute (SANEDI) Stakeholder Engagement team under the South African Centre for Carbon Capture & Storage (SACCCS) is thanked for making the scientific work possible. The National, Provincial and Local Government structures including Traditional Authorities, Municipalities, landowners and local residents are thanked for granting permission to conduct the monitoring in the areas of interest. CGS staff are thanked for their assistance and support in the field. We thank two anonymous reviewers for their comments, which helped to improve the manuscript. ; Peer reviewed ; Postprint

The work was supported by funding from the UK CCS Research Centre (UKCCSRC). The UKCCSRC is funded by the EPSRC as part of the RCUK Energy Programme. The South African National Energy Development Institute (SANEDI) Stakeholder Engagement team under the South African Centre for Carbon Capture & Storage (SACCCS) is thanked for making the scientific work possible. The National, Provincial and Local Government structures including Traditional Authorities, Municipalities, landowners and local residents are thanked for granting permission to conduct the monitoring in the areas of interest. Council for Geoscience staff are thanked for their assistance and support in the field. ; Peer reviewed ; Publisher PDF