Underground gas storage (UGS) involves storage of large quantities of natural gas in a storage horizon to support the natural gas demand in domestic, commercial, and industrial areas. The main objectives of this book are to investigate the possibility of (UGS) development and feasibility of underground sour gas storage in a depleted fractured gas reservoir. Compositional simulation was employed to build a dynamic reservoir model, develop the history-matching phase of the reservoir, and construct injection/withdrawal (I/W) cycles. A systematic procedure was employed to determine optimum well pattern (well number, location, horizontal, vertical). Simulation study specified that the use of horizontal well is superior to vertical well because of less water production during storage cycles. Water cut, productivity and injectivity indexes of individual wells were considered as prominent factors to find the optimum well pattern. The results of the simulation showed that the presence of H2S and CO2 in the injected gas stream improved condensate production. It was found that UGS reservoir meet lower pressure at the end of I/W cycles in the case of underground sour gas storage.