The cushion gas, providing the pressure energy necessary for withdrawal of working gas, makes up the largest part of the investment in underground gas storage projects. The suggested method of reducing this cost is the replacement of some part of the cushion gas with less expensive inert gas, such as nitrogen. In the replacement, there might be some problems due to mixing between natural gas and inert gas. Turkey has sharply increasing demand for natural gas. The constant imported gas and varying demand throughout the year due to the distinct seasons in the region are the reasons for severe need for gas storage in Turkey. There are no underground storage units in Turkey but two gas reservoirs in the Thrace basin grant the potential for gas storage. From this point of view, the gas mixing problem is investigated for a typical gas reservoir by coupled use of a gas reservoir simulator and a transport model. A two-dimensional (2-D), single-phase numerical gas reservoir simulator is developed to obtain the pressure distribution during production and injection cycles. The velocity distribution is found by using these pressures. Then the two-dimensional transport model is used to calculate nitrogen concentrations around the injection wells. Both models are used effectively for controlling the mixing problem in an underground gas storage reservoir.