Investigation of Underground Sour Gas Storage in a Depleted Gas Reservoir

Underground Gas Storage (UGS) involves storage of large quantities of natural gas to support the natural gas demand in domestic, commercial and industrial areas. Storage of sour gas can be advantageous from economic standpoint, as it reduces treatment costs and increases the potential of production from shared reservoirs. This paper investigates feasibility of UGS in one of Iranian depleted fractured gas condensate reservoirs. Compositional simulation was employed to build dynamic reservoir model, develop the history matching phase of the reservoir and construct Injection/Withdrawal (I/W) cycles. One sweet gas stream and three sour gas streams with different compositions were tested for storage into reservoir during summer season. Results of simulation showed that presence of H2S and CO2 in the injected gas stream improves condensate production. Condensate Production Enhancement (CPE), defined as the percentage of condensate recovery increase due to sour gas injection relative to the sweet gas injection, was calculated for different compositions of storage gas. Also, Condensate Holding Ratio (CHR), defined as the ratio of condensate in the withdrawn sour gas to that in the withdrawn CH4, was estimated for different storage gas compositions. Results showed that CPE has a higher rate in earlier cycles and declines at later cycles. CHR is higher for sour gas storage compared to sweet gas. Furthermore, heating value of produced gas was calculated in all I/W cycles and compared with heating value of injected gas. It was indicated that difference between heating value of produced and injected gas increases with increase of H2S and CO2 content of the injected gas. Also, it was found that the reservoir has lower pressure rise at the end of I/W cycles in the case of underground sour gas storage compared to sweet gas storage. The presence of acid gas components decreases the z-factor of injected gas stream resulting in smaller difference between z-factors of injected gas and reservoir fluid.