Advantages and disadvantages of in situ N₂ presence in underground gas storage (UGS) process: Technical and economical evaluation of a case study

Underground gas storage (UGS) is an industrial process used for compensating the supply and demand chain for natural gas in different seasons of the year. In this process, the excess natural gas produced in summer is injected into an underground reservoir to be harvested in winter. This porous medium also might contain an in situ gas which can be used as the cushion gas. The presence of a non-hydrocarbon gas (e.g. nitrogen) in the reservoir prior to the UGS process could have advantages and disadvantages during the process. If natural gas is injected as the cushion gas, the operational costs will be high. Besides, a significant advantage of nitrogen as the cushion gas is the reduction of preparation time defined as the initial injection period. The presence of nitrogen in the formation also increases the reservoir pressure, which results in a higher rate and pressure during production periods. However, a higher initial reservoir pressure causes lower gas injectability in the storage process; hence, during injection periods, the volume of injected gas is reduced. In addition, an unexpected mixing of cushion gas with the injected gas reduces the quality and heating value of gas. Therefore, a separation unit is required and more operational costs are expected. In this work, a reservoir located in Middle East with great nitrogen volume is selected. Based on the physical properties of the reservoir, a simulation by ECLIPSE 300 is made and the advantages and disadvantages of nitrogen existence in the process of UGS is analyzed, from technical and economical point of view. The presence of nitrogen saves millions of dollars due to reduction of preparation time and costs. Besides, an unwanted mixing of nitrogen with natural gas, causes millions of dollars loss of gas value due to reduction in gas market value and nitrogen separation.