This paper characterizes the influence of water drive influx on the type of the dependence of reduced average reservoir pressure on the cumulative gas production. It is assumed that the insignificant influence of the reservoir aquifer does not affect the type of the dependence of the reduced average reservoir pressure on the cumulated gas production. The type of the dependence fully characterizes the gas depletion of reservoir development. The use of the material balance method in such cases leads to the overestimation of the initial gas: while some volume of water is intruded into the productive gas-saturated part of the reservoir, the decrease in reservoir pressure is partially compensated due to gas off-take. Thus, not knowing the true amount of the original gas in place, it is not possible to make up a proper plan of the field development strategy and provide recommendations for its improvement, which requires significant investment. To prove this assumption, the authors have built a synthetic 3D ongoing geologicaltechnological model of gas reservoir. The 3D model includes the volume of water below the gas-water contact, which is limited by the size of the model. To carry out a complex analysis, analytical Fetkovich aquifer with water volume of 5 million m³ and 25 million m³ respectively and with the same productivity index is used. Aquifers are connected to the model boundary from the south-east. According to the modelling results, the graph-analytic estimation of drained the reserves is performed. The authors fully confirm the assumption that the insignificant effect of the water drive influx does not affect the type of the dependence of the reduced average reservoir pressure on the cumulative gas production. On the basis of the study results the authors estimate the possible error that is allowed when using the material balance method. According to the simulation results, the error can be up to 19.47% of the initial gas in place in the model. This error significantly exceeds the limits and can lead to wrong decisions during the field development planning