It is possible to predict the behavior of fluids in permeable and porous medium under different operating conditions by using reservoir models. Since geological data and reservoir properties can be defined most accurately by reservoir models, it has been accepted as a reliable prediction tool among reservoir engineers. In this study, a gas reservoir has been modeled with IMEX Module of CMG Reservoir Simulator. Rock properties, gas composition and certain production data were entered to the model as input data and the measured field data were matched with simulated ones. After the 5 year depletion of the reservoir by vertical wells, the average reservoir pressure dropped from an original reservoir pressure of 2150 psi to 1200 psi. This depleted reservoir was planned to be used for gas storage purposes. The remaining gas was used as cushion gas during the conversion of this reservoir to an underground gas storage field. Afterwards, horizontal wells were defined in the model and certain production/injection scenarios were simulated for the gas storage operation.