The Haag field in the Molasse Basin in Upper Austria is a depleted dry gas reservoir, that has been converted to underground gas storage. Three horizontal wells have been drilled to implement a gas storage facility. During their development many areas are subject to uncertainties. This work investigates the overall range of uncertainty in well performance prediction. Well performance is calculated by semi-analytical and numerical approaches. For the semi-analytical approach a software package of Petroleum Experts, PROSPER, is used. The inflow performance of a well is calculated based on a specified well model. The three models applied are Kuchuk and Goode, Goode and Wilkinson, and Babu and Odeh. Further work is then based on Kuchuk and Goode model. The numerical approach is based on the finite difference method, and the simulation software of Schlumberger, ECLIPSE. The simulation model used is the history matched model of the gas storage facility. Inflow performance curves for different scenarios were determined by these two approaches and their results are then compared. The inflow performance curves calculated semi-analytically are considered to be more realistic, because they are based on models specifically developed for horizontal wells. Therefore, these inflow performance curves are used as reference curves. The calculations indicate that the inflow performance calculated numerically is too optimistic as compared with the semi-analytical approach. Thus, the inflow performance of horizontal wells, calculated numerically, needs to be corrected for future simulation forecasts. However, it is not possible to determine a general correction factor. Performance must be evaluated for each well individually. For this task it is recommended to construct semi-analytically inflow performance models.