In recent years, the demand for large-scale energy storage coming from renewable resources has been increased. Solar and wind plants contribute to generating gigawatts of electricity, which often have a considerable mismatch in grid power demand. Hydrogen gas is an effective and flexible energy carrier that can be generated by the conversion of renewable energy. There are conventional means of storing hydrogen, such as batteries; however, they provide limited capacity for storage. As an alternative, depleted natural gas reservoirs can serve as underground hydrogen storage (UHS) facilities with a huge storage capacity. In this work, two major topics related to UHS in porous media were tackled and studied. The first topic investigates and addresses potential risks associated with hydrogen loss due to diffusion processes and reactions with minerals and brine. This part has been completed in the framework of an interdisciplinary project, “Underground Sun Storage” which aimed to store hydrogen in a depleted gas reservoir in Molasse Basin, Upper Austria. Based on field data, a geological and a transport model for this field were realized. An analytical diffusion model was employed to estimate the hydrogen loss into the caprock and the neighboring layers in a storage cycle based on hydrogen plume propagation. The diffusive loss depends on the diffusion coefficient and the exposed surface area to the hydrogen gas. Due to the scarcity of data and uncertainties of different nature, proper assumptions and simplifications were considered. Lastly, a geochemical modeling workflow was presented to study and quantify potential geochemical processes that can lead to hydrogen loss in the storage site. The geochemical reactions were assessed by equilibrium and kinetic batch models at the reservoir pressure and temperature to investigate the short- and long-term impacts of hydrogen on the formation-water and minerals. The second major topic in this dissertation studies another aspect of such storage. It turns out in-situ bacteria in the reservoir, called Archaea, can metabolize hydrogen and CO2 and produce natural gas. The fact that hydrogen is served as a substrate for these microorganisms results in the bacterial growth and induces hydrogen degradation. The question of how bacteria grow and transport in porous media is not known. In the same way, there is a concern about biofilm formation, which leads to injectivity issues. Finally, the conversion rate of hydrogen degradation to methane production is of much interest in energy conversion projects. The principal insights into bacterial growth and transport in pore space under saturated flow conditions were achieved via experimental and numerical analysis; in addition, based on experimental outcomes, recommendations for future research were made. Finally, through the flow simulation on the experimental segmented images, the influence of the bacterial growth on the hydraulic properties of the porous medium is quantified, and changes in the flow field and the relationship between porosity and permeability modeling. This dissertation attempts to contribute to knowledge by combining various aspects that are relevant for hydrogen storage and conversion assessment, both numerically and experimentally. The primary objective of this research was to create and introduce new insights and concepts that combine diverse disciplines that were related to hydrogen storage and conversion, deliver effective methodologies to identify the potentials and risks in such projects and provide the groundwork for future experimentations and numerical studies.