The intermittent production of the renewable energy imposes the necessity to temporarily store it. Large amounts of exceeding electricity can be stored in geological strata in the form of hydrogen. The conversion of hydrogen to electricity and vice versa can be performed in electrolyzers and fuel elements by chemical methods. The nowadays technical solution accepted by the European industry consists of injecting small concentrations of hydrogen in the existing storages of natural gas. The progressive development of this technology will finally lead to the creation of underground storages of pure hydrogen. Due to the low viscosity and low density of hydrogen, it is expected that the problem of an unstable displacement, including viscous fingering and gravity overriding, will be more pronounced. Additionally, the injection of hydrogen in geological strata could encounter chemical reactivity induced by various species of microorganisms that consume hydrogen for their metabolism. One of the products of such reactions is methane, produced from Sabatier reaction between H 2 and CO 2 . Other hydrogenotrophic reactions could be caused by acetogenic archaea, sulfate-reducing bacteria and iron-reducing bacteria. In the present paper, a mathematical model is presented which is capable to reflect DuMuX was used to model the evolution of a hypothetical underground storage of hydrogen. We have revealed that the behavior of an underground hydrogen storage is different than that of a natural gas storage. Both, the hydrodynamic and the bio-chemical effects, contribute to the different characteristics.

Within the framework of energy transition, hydrogen has a great potential as a clean energy carrier. The conversion of electricity into hydrogen for storage and transport is an efficient technological solution, capable of significantly reducing the problem of energy shortage. Underground hydrogen storage (UHS) is the best solution to store the large amount of excess electrical energy arising from the excessive over-production of electricity, with the objective of balancing the irregular and intermittent energy production, typical of renewable sources such as windmills or solar. Earlier studies have demonstrated that UHS should be qualitatively identical to the underground storage of natural gas. Much later, however, it was revealed that UHS is bound to incur peculiar difficulties, as the stored hydrogen is likely to be used by the microorganisms present in the rocks for their metabolism, which may cause significant losses of hydrogen. This paper demonstrates that besides microbial activities, the hydrodynamic behavior of UHS is very unique and different from that of a natural gas storage.