Learnings Arising from Modelling of Single Well Tracer Tests to Characterise CO2 Injection

Single well tracer tests have been widely used for oil industry and environmental applications; however, they have only occasionally been used during CCS projects to estimate residual supercritical CO2 saturation during the appraisal phase of site investigation. Two tracer experiments are designed and numerically modelled as single-well push-pull operations to evaluate residual CO2 trapping; this being one of the key trapping mechanisms in CO2 storage. This study assesses the feasibility of using a gas partitioning tracer (Xenon) in the presence/absence of a residually trapped CO2 zone to calculate residual gas saturation. Numerical modelling of the tracer propagation and partitioning is used to calculate the difference in tracer breakthrough times during water production from the two tests. A 1D homogeneous radial model is used. Sensitivity calculations were performed to analyse which parameters influence the calculation of the residual gas saturation. The residual gas saturations calculated reflect the input values, but the calculations are very sensitive to the tracer arrival times, which are strongly affected by numerical controls and the volume of water injected after CO2 injection to generate the residually trapped zone. The numerical models may be used to optimise and interpret the design of the field tests.