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Increasing renewable energy production can result in energy storage requirements on variousscales given the fluctuating availability of renewable sources. Power-to-Gas in combinationwith underground gas storage in porous media is a viable option for mitigating offsetsbetween energy demand and availability. Leakage of stored gas along fault zones couldhinder or prohibit storage development, requiring a quantification of potential leakagescenarios. This study investigates the sensitivity of fault zone geometry, petrophysicalparameters and capillary pressures on leakage rates during a gas storage operation usingnumerical scenario simulations. A structural model of an anticline structure in the North German Basin is used for thisanalysis, which spans from Permian up to Quaternary horizons and includes several faultsystems. For the sensitivity analysis, a 2D-slice perpendicular to the main N-Sstriking fault system is selected. In this model, the fault zone is represented as twoindependent units, namely a fault core and a damage zone. Literature analysis indicate thatthe highly fractured damage zone has high absolute permeability and thickness,while the fault core has a low permeability in is reasonably tight. East of the faultzone the storage reservoir is saturated with methane, with reservoir pressures setaccording to a gradient of 15 MPa/km, translating to an absolute pressure at thefault interface of around 86 bars. The remaining formations are set to hydrostaticpressure conditions. Petrophysical parameters, such as permeability and porosity areassumed homogeneous and isotropic. The leakage rates through the fault zone aredetermined for variable fault zone thicknesses (100; 50; 10; 1 m), damage zonepermeabilities (10; 5; 1; 0.01 mD) and capillary entry pressures (varying from 0 to 9.6bars). Results of the simulation with varying fault zone parameters show a strong influence onthe reservoir pressure decline and gas leakage rates. In the worst-case scenario, i.e. assuminga highly permeable damage zone, the gas leakage rate upwards through the damage zonequickly reaches 9000 sm3/d and remains constant afterwards. After about 1 year, gasbreakthrough is observed at the fault core, with leakage rates reaching 500 sm3/d after 2years. Leakage gas volume is 7879678 sm3 after 2 years, which corresponds to 2.26 % oftotal gas in place. At a fault zone with thickness of 100 m, reservoir pressure declines by 14bars compared to the initial condition. In comparison, reservoir pressure declines by around 5bars due to pressure redistribution within the storage formation if the fault zone is assumed tobe completely tight. Cases with different capillary entry pressure do not show a cleartrend, but methane leakage volume can reach up to 6837049 sm3 when capillaryentry pressure is 3.2 bars. The results indicate that significant leakage rates canoccur, showing the sensitivity of storage model to the fault zone parametrization. [ABSTRACT FROM AUTHOR]