Underground natural gas storage is a process which effectively balances a variable demand market with a nearly constant supply of energy provided by the pipeline system. Cushion gas in storage reservoirs provide the necessary pressure for the withdrawal of working gas and it makes up for the largest part of investments in underground storage reservoirs. Usually, a part of the cushion gas is replaced by an inert gas such as Nitrogen in order to reduce the investment costs. Due to this replacement there might be problems caused by the mixing of inert cushion and natural gas, which is a cause of concern as it affects the quality of the natural gas.;In this study, the inert gas mixing problem is investigated in storage reservoirs by using a reservoir simulator based on the equation of state. The degree of mixing between the two dissimilar gases was found by the amount of inert cushion that is produced along with the withdrawn gas. Reservoir parameters like Volume of inert cushion, porosity, permeability, reservoir pressure and reservoir temperature were varied to study their effect on mixing.;It has been found that reservoir parameters porosity, permeability and temperature do not impact the degree of mixing to a great extent. However, reservoir pressure has a slight impact on the mixing between these two dissimilar gases. It is evident from these simulations that the degree of mixing is purely a function of withdrawal rate and also the percentage of inert cushion in the storage reservoir to a great extent.
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