Effect of water encroachment into an underground gas storage reservoir on its storage capacity (verification of inventory) is presented. Inventory verification is to determine the amount of natural gas to be injected into a particular underground reservoir. Reservoir and production history of a depleted oil reservoir, Z-16T, located in the Niger Delta region were obtained for the analyses. A volumetric equation was used to estimate the storage capacity of the reservoir in two cases (We = 0 and We ≠ 0, for fifteen different pressures). Microsoft Visual Basic computer model developed based on the same volumetric equation was applied for the same purpose. It was confirmed that the amount of gas to be injected at any given pressure is higher when there is no water influx into the storage reservoir. This has shown that water influx reduces the storage space of an underground storage reservoir.
Key words: storage, capacity, water influx, inventory, encroachment, gas, underground.