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

Near surface gas measurements are presented from a shallow (20 m depth) CO2 injection experimentat 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 50 cm 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 capableof 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 CO2seepage.