The conversion of a depleted gas reservoir into under ground storage is currently considered to substitute liquefied liquefied natural gas tanks. This study investigates the validity of the operation scenario of the storage with a compositional simulation model and analyzes potential compositional changes of the gas for the purpose of predicting and managing the gas quality in the future. According to the simulation results, the overall operation conforms to the design concepts and operation scenario, confirming the validity of the pre-storage operation scenario. Moreover, the compositional variation of gas in the reservoir was closely examined, clarifying the changes in each component.