Underground Gas Storage (UGS) is a means of installing peak shaving capacity during high-demand seasons worldwide. In this work, studies of the UGS were performed on a partially depleted gas reservoir through compositional simulation. Prediction of reservoir fluid phase behavior and history match was done by utilizing detailed reservoir information. The performance of UGS with different scenarios of reservoir depletion, gas injection, and aquifer strength was analyzed. The injection capacity and deliverability of reservoir was set to 350 MMSCF/D (6 months) and 420 MMSCF/D (5 months), respectively. Based on different scenarios and the anticipated target rate, the optimum pressure for converting this reservoir to UGS was found to be about 1600 psia. Also, it was found that if the reservoir is depleted to a lower pressure, it contains insufficient base gas reserve and may not meet the target withdrawal rate. It was found that this problem can be overcome by injecting higher volume of gas in the first cycle. Furthermore, simulation studies showed that an active aquifer can lead to irreversible reservoir shrinkage, increase in water-gas ratio, and pressure rise in reservoir. Another source of pressure rise during the UGS operations is the difference between z-factors of injected and reservoir fluids. It was found that injecting lean gas with high z-factor into a reservoir containing fluid of lower z-factor results in pressure rise at the end of each cycle. At successive cycles, composition of reservoir fluid approaches that of the injected gas because of continual mixing. Theoretically, composition of reservoir fluid will approach to the injected fluid after infinite cycles, provided complete mixing occurs in reservoir. Under these conditions, difference between z-factors of injected and reservoir fluids become smaller, and reservoir pressure stabilizes.