In this study, a real low quality gas reservoir (which consists of 85% nitrogen) is considered for natural gas storage (UGS) operation. The native reservoir gas is used as cushion and this will considerably reduce the cost of storage plan; but the main concern during this technique is the possible mixing between nitrogen and natural injected gas which may degrade the quality of withdrawal stream.This paper presents a sensitivity analysis on some relevant parameters (such as gas production and injection rate, skin factor, and perforation length) which may affect mixing of inert cushion and natural gas during natural gas storage in a nitrogen reservoir. The extent of mixing between nitrogen and natural injecting gas is evaluated by recording the amount of inert cushion gas, which is produced along with natural gas. Compositional simulator is applied to construct dynamic reservoir model and define injection/withdrawal cycles. Simulation results revealed that mixing is strongly affected by the rate changes and it can be effectively controlled by choosing a proper storage strategy.


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