The geological subsurface may have a significant role to play on the transition to a less carbon intensive energy production of the world. This concerns both the exploitation of energy resources, such as (tight) gas and geothermal energy, as well as the use of geological formations as storage reservoirs for compressed air, (synthetic) methane or hydrogen. Geo-reservoirs thus may play an important role for future energy supply; however, they have not been adequately characterized for these new types of usage. This manuscript aims at highlighting some of the identified knowledge gaps and identifies the mayor fields of research. Structured research is required for characterizing and quantifying the governing coupled processes, both experimentally as well for numerical simulation. Geo-reservoir usage may be manifold, so a number of uses as mentioned above and their respective implications are highlighted. For actually applying the scientific knowledge for at sites, a work flow is required to bring the basic process understanding to the large scale.
Energy consumption in the world is still depending to a large extent on conventional fossil resources – the main reason for global warming and climate change (Metz et al. 2005, UN 2012). The new energy policy paradigm in Germany (Energiewende: from fossil fuels to renewable energy) foresees the utilization of renewable resources to produce 50% of the German electrical power demand by 2030 and 80% by 2050. In order to face this huge challenge, clean/cleaner energy resources in the underground have to be considered in the near future for producing geothermal energy (Huenges 2010) and unconventional gas (Horsfield and Schulz 2012). Geo-reservoirs may also serve as large storage systems for fluctuating renewable energy transferred into e.g. compressed air, synthetic methane, hydrogen or heat. Furthermore, geo-reservoirs may be used for dampening effects of fossil energy by new technologies, e.g. for the switch from carbon intensive fossil fuels to natural gas via CO2 capture and utilization (CCU) (May 2012, Bauer et al. 2012, Kühn et al. 2012) or CO2 capture, utilization and storage (CCUS) or as bio-chemical reactors to convert hydrogen and carbon dioxide into methane under the favorable high pressure and high temperature conditions of the deep underground. Georeservoirs may thus be used for production, storage as well as conversion purposes in the context of the "Energiewende".
