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Abstract

The large scale storage of energy is a great challenge arising from the planned transition from nuclear and CO2-emitting power generation to renewable energy production, by e.g. wind, solar, and biomass in Germany. The most promising option for storing large volumes of excess energy produced by such renewable sources is the usage of underground porous rock formations as energy reservoirs. Some new technologies are able to convert large amounts of electrical energy into a chemical form, for example into hydrogen by means of water electrolysis. Porous formations can potentially provide very high hydrogen storage capacities. Several methods have to be studied including high hydrogen diffusivity, the potential reactions of injected hydrogen, formation fluids, rock composition, and the storage complex. Therefore, in August 2012 the collaborative project H2STORE ("hydrogen to store") started to investigate the feasibility of using burial clastic sediments of depleted gas reservoirs as well as recently used gas storage sites as potential hydrogen storage media. In Germany, such geological structures occur at various geographic sites and different geological strata. These deposits are characterized by different geological-tectonic evolution and mineralogical composition, mainly depending on palaeogeographic position and diagenetic burial evolution. Resulting specific sedimentary structures and mineral parageneses will strongly control formation fluid pathways and associated fluid-rock/mineral reactions. Accordingly, H2STORE will analyze sedimentological, petrophysical, mineralogical/geochemical, hydrochemical, and microbiological features of the different geological strata and the German locations to evaluate potential fluid-rock reactions induced by hydrogen injection. Such potential reactions will be experimentally induced in laboratory runs, as analogues for naturally occurring processes in deep seated reservoirs. Finally, rock data determined before and after these experiments will be used as major input parameters for numerical modelling of mineralogical and microbiological reactions. Such reactions are expected to have a strong affect on rock porosity-permeability evolution and therefore the characteristics of flow processes in reservoir and the barrier properties of sealing rocks. The special topic of this study will be the modelling of hydrogen propagation in the subsurface reservoir formation supplemented by its mixing with the residual gases as well as the simulation of coupled bio-dynamic processes and of reactive transport in porous media. These numerical simulations will enable the transfer of experimental results from the laboratory runs to the field-scale and the formulation of the requirements for hydrogen storage in converted gas fields. Thus, the major objectives of H2STORE are to obtain fundamental data on the behaviour of clastic sediments in the presence of formation fluids and injected hydrogen, its impact on petrophysical features and the development of the most realistic modelling for proposed and experimentally induced rock alteration as well as complex gas mixing processes in potential geological hydrogen reservoirs. Moreover these results will be used when discussing the possibility of "green" eco-methane generation by hydrogen and carbon dioxide interaction in the geological underground.