The article presents a methodology for analysing historical gas production data and determining the gas reserves and the petrophysical parameters of a reservoir-aquifer system. These parameters are obtained from a fitting algorithm using production data sets. A forecast of the future field gas production can be created on the calibrated mathematical model basis. The developed method is based on the material balance assumptions and the widely used Fetkovich and van Everdingen-Hurst equations for calculating water influx. To conduct the calculations and analyse production data, the computer application was developed using Python programming language. A user-friendly graphical interface makes the proposed application convenient and intuitive to use. The software was calibrated based on the literature data from the gas field of known parameters and then validated using five case studies of the actual gas fields in the Polish Carpathian Foredeep. From the tests, very high compatibility between the computed and the real field values were obtained. An additional comparison with the commercial program MatBal confirmed the proper functioning of the application.