The economic viability of underground natural gas storage in depleted oil reservoirs was examined with reservoirs Z-16T and Z-16RS located onshore, South-East, Nigeria. The geologic and engineering information on the reservoirs were gathered with which the costs analyses were conducted. The storage costs and storage capacities of the reservoirs were used for profitability analyses through the expected revenues. It was shown that reservoir Z-16T has advantage over reservoir Z-16RS and is preferred for use as underground gas storage vessel having higher Internal Rate of Return (IRR) of 26.8% compared to 10.5% for reservoir Z-16RS and shorter pay-out period of 3.19 years and break-even point of $59.1 million as against 5.25 years and break-even point of $46.7 million for reservoir Z-16RS.
