The paper presents a modeling strategy as well as simulation results of a designed conservative tracer test in the depleted Altensalzwedel natural gas reservoir in order to know the tracer concentration and breakthrough time at the production wells. Krypton is considered as a suitable tracer. The production wells are located in several hundred meters to a few kilometers away from the injection well. The numerical simulation has been performed by using the newly implemented compositional gas flow module of OpenGeoSys (OGS), a scientific open-source simulator for calculation of coupled geohydraulic (single and multiphase flow), transport (heat and mass transfer), geomechanical and geochemical processes. The tracer breakthrough curves at the production wells show that the breakthrough times vary between 2 and 3 years. The model verification is presented by comparing the OGS results with a one-dimensional analytical solution. The numerical results have been verified by code comparison, additionally.