Monitoring of near surface gas seepage from a shallow injection experiment at the CO2 Field Lab, Norway

•Near surface gas measurements were used to monitor surface CO2 leakage from a shallow test injection.•Several small areas of surface leakage were detected with high CO2 concentrations and fluxes.•These areas were not as predicted by modelling prior to the experiment.•Isotopic measurements suggest low level leakage closer to the injection point.•Implications for monitoring at CO2 storage sites are considered. Near surface gas measurements are presented from a shallow (20m depth) CO2 injection experiment at the CO2 Field Lab site in Svelvik, Norway, which was designed to test a variety of monitoring tools. Small areas of surface seepage of CO2 were detected during the experiment and these spread as the injection rate was increased. These features only accounted for a small fraction of the injected gas. Isotopic measurements revealed traces of injected CO2 at 50cm depth nearer the injection point. The spatial extent of this is unknown but it is not likely to imply a significant amount of CO2 seepage. The locations of the gas escape were not as anticipated by prior modelling and highlight the difficulty of predicting where leakage may occur and, hence, where to deploy monitoring equipment. This unpredictability and the limited size of the seeps implies that monitoring will have to be flexible, preferably mobile and capable of detecting small features in large areas if successful leakage detection at surface is to be achieved. Low level seepage, such as that suggested isotopically here, could be significant for carbon auditing if it occurs over wide areas. This could be tested in areas of natural CO2 seepage.