Due to the complexity of fluid composition and the dynamic characteristics of a gas reservoir with an underlying aquifer and an oil rim, estimating the storage capacity of such a reservoir is a complex problem. In this work, the method of numerical simulation is used to calculate the dynamic characteristics of such reservoirs. The obtained data on the distribution of the three phases in the reservoir show the expansion of the gas cap and the penetration of the underlying water into the oil rim, resulting in a three-phase transition zone which increases seepage resistance. The capacity of the reservoir is calculated by the method of numerical simulation taking into account the seepage resistance of the three phases, and the material balance method without taking into account the seepage resistance. The results of calculations show that the storage capacity calculated by the first method is 74% of the value calculated by the second method. Thus, we can conclude that the three-phase transition zone is the main parameter affecting the calculated reservoir capacity, and the effect of the transition zone cannot be neglected when estimating the storage capacity of the reservoir.