Overcoming the intermittency problem of renewable energy is an issue that has to be addressed in order to achieve an efficient energy system. Hydrogen has been gaining interests, as green energy carrier, which is included in the energy transition plans of not only the Netherlands, but worldwide. Due to its physical characteristics, large scale storage of hydrogen requires volumes that can only be provided by porous media in the subsurface. Underground Hydrogen Storage (UHS) in depleted gas fields is a large scale storage possibility that is generating attention and increased research. However, it is currently in a Low Technology Readiness Level, meaning that more research, and especially field scale projects are necessary. There is very few literature that tries to use alternative gases as a cushion gas for a UHS. Therefore, this investigation is relevant for the future development of UHS. This investigation will use CMG GEM, a commercial compositional reservoir simulator to model the fluid interactions in the subsurface when hydrogen is stored in depleted gas fields. This will be done by means of a sensitivity analysis, where the hydrodynamic behaviour between hydrogen and possible alternative cushion gases, such as methane, carbon dioxide and nitrogen are studied. Apart from the technical analysis, a simplified economic evaluation is used to calculate a levelized cost to store hydrogen, which will allow for an economic optimized selection of cushion gases in an UHS. The two main constraints for an UHS system are the purity of the extracted hydrogen and the rate at which it is extracted from the subsurface. The results of this investigation show that the mixing of hydrogen with an alternative cushion gas will change drastically based on the degree of the reservoir heterogeneity and the location of the perforated interval. This ever-increasing mixing will have an effect on the capability of the system of delivering pure hydrogen for the expected time.