One of the key challenges in the implementation of the ambitious energy and environmental policy goals of the German government in the course of the planned exit from carbon dioxide emitting and nuclear-powered electricity production to renewable resources is the provision of sufficiently high, constant and secure energy supplies for the German population and industry. Important sources of regenerative energy in Germany are wind and solar, which both depend on the prevailing weather conditions and partly strong fluctuations in energy productions. These fluctuations require a considerable expansion of the storage capacity for the required energy supply. A promising solution to this problem could be the electrolytic conversion of electrical to chemical energy, combined with storage in the geological underground. The basic idea is to obtain hydrogen by this process, which can then be stored. Geological structures in the underground, e.g. depleted natural gas reservoirs, saline aquifers and salt caverns offer the possibility of storing hydrogen-containing natural gas or even gas mixtures of hydrogen and carbon dioxide as well as pure hydrogen. The injection of hydrogen and carbon dioxide into such structures could also promote methane formation (analogous to the "power-to-gas-P2G" technology), as has already been demonstrated in some hydrogen-rich city gas storage facilities. However, the safety requirements must be taken into account in such underground storage options. This is also required by the German Institute for Standardization in the submission of "underground storage of gas" and in the "Data sheet for the evaluation of underground gas storage". They state that a reservoir must be designed to ensure the long-term inclusion of the stored products. The reservoir must therefore have a technical and also a geological seal. The research project "HyINTEGER" is a valuable contribution to the technical use of the underground storage of hydrogen and hydrogen-gas mixtures. Therefore the lowest possible reactivity of the reservoir components (e.g. minerals, formation fluids, microorganisms, etc.) and a high impermeability (e.g. sufficiently powerful, largely impermeable outer layers)of the caprock as well as the special requirements for the use of subterranean large structures are to be defined for a energy storage. The absence of tectonic disturbances, corrosion-resistant technical installations (all drill rigs and piping) are necessary for safe storage. The corrosion resistance of the materials used is an essential prerequisite for a safe usage of the reservoir and necessary between the reservoir and the surface ("Hole integrity"). For an effective storage of chemical energy, it must also be taken into account that geological reservoirs are a complex system of natural and technical components / materials which can interact with each other. Therefore, it is planned to investigate such interactions in detail in the "HyINTEGER" collaborative project and to contribute to the assessment of the benefits as well as the risks associated with an industrial application of this technology.
The aim of the working packages planned in "HyINTEGER" is to investigate the interactions between the technical facilities of a hydrogen storage tank and the natural (underground) components in a highly corrosive, highly saline environment using materials, engineering and geoscientific methods. Both the chemical-mineralogical, microbiological and petrophysical-geohydraulic-geomechanical properties of the storage and cover rocks will be investigated, as well as the stability and tightness of the technical facilities. This research is based on previous and ongoing work that has been conducted regarding gas storage at various national and international research networks. The great importance of this topic and the necessity for this kind of research is highly relevant and has been highlighted in two studies on geological hydrogen storage (Reitenbach et al., 2014, Hyunder, 2014). In HyINTEGER this will be done in close cooperation between application-oriented, scientific research and the practical requirements of the industry. The planned investigations are based on experimental autoclave tests with H2- and / or CO2. The reactions induced by these gasses will be evaluated by observations and analysis and then be integrated into numerical simulations. These experiments are intended to consider reactions between the components of the well completions (e.g. tubing’s, cementations) and the characteristic components of the reservoir (e.g. mineral composition, formation fluid, hydrocarbon compounds and microorganisms) under specific storage conditions. Such reactions will not only influence the storage capacity and density of the reservoirs (e.g. porosity, permeability, injectivity and leaking risk and tightness of the cap rock), but also the physicochemical behavior (e.g. corrosion resistance, brittle behavior, tensile and shearing properties) of the underground reservoir and therefore the risk of leakage along the drilling holes. In order to investigate such reactions and their effects on the storage properties and the technical inventory, especially (high-resolution sub-) microscopic, micro tomographic (and synchrotron), petrophysical-geohydraulic, mechanical and bio-, geo-, hydro- and isotopic-chemical analytical methods shall be used. These data/samples/fluids will be collected before and after the planned autoclave experiments so that the effects of a sample exposure are examined against two main gas phases: (1) with hydrogen (partly with the addition of hydrogen sulfide H2S, acetic acid C2H4O2) and (2) previous or later H2 addition so that the effects of these two gases on aging processes and methane generation reactions can be evaluated. In addition, the influence of microbial metabolic processes on the integrity of the borehole as well as the corrosion on the installations will be investigated. The materials used for this purpose can serve as potential sources of energy or nutrients for microorganisms and thus enhance the formation of (permeability-reducing) biofilms. The consideration of potential organic and inorganic reactions in the storage and the drilling components as well as mutual influence are an important part of the work in HyINTEGER. Particularly in the case of an availability of electron acceptors such as sulfate or carbonate / carbon dioxide, which are the main constituents of important mineral components (e.g. anhydrite, calcite, dolomite, etc.) in the storage rocks and the drilling cements. Additionally borehole pipes or even storage gases such as methane or hydrogen can be oxidized due to microbial activity. In such processes H2S or CH4 can be produced from the biomass, depending on the metabolic reactions. The formation of H2S is particularly important for the integrity of drill holes and materials used because of its high corrosiveness. Complementary to the laboratory experiments the examination of samples of the technical materials as well as rocks, formation fluids, and gases from long-term hydrogen-exposed drillings will be used. Corresponding field trials are planned for this during 2016 by the Austrian research group "Underground Sun.Storage" and the Argentina Company "Hychico". From both projects a cooperation with HyINTEGER has been arranged. This allows the project to compare induced changes in aging with those from natural hydrogen storage sites and to check the results of the laboratory results. All data collected in these studies are intended to be used in numerical simulations. Thus, to characterize the horizontal (in the reservoir) and vertical (along the borehole piping) propagation of hydrogen in and the (geological or technical) tightness of a reservoir. Leakage scenarios and the effects of multiple, cyclic inputs and outputs of hydrogen on the various components of the reservoir (rock, formation fluid, biocenoses, possibly natural gas / oil) and the drilling system are also included in the research. It is intended to evaluate the possible changes in borehole injectivity and gas composition over time in the reservoir. The simulations on the μm-scale are also included, since these have a great influence on the pore space or fluid movements in the entire reservoir due to alteration and other geochemical reactions such as precipitation on a small scale. Since all of these investigations and modeling are particularly relevant to the pore space structures of the reservoir rocks and the saturation and flow capacity of the formation fluids (in the presence of hydrogen), new fluid flow and transport models will be developed and also being adapted for the μm-scale within the project.