The temperature of the Earth’s subsurface increases with increasing depth and is therefore an almost inexhaustible repository of energy. Geothermal energy utilizes this energy stored as heat in the Earth’s subsurface and therefore can be viewed as a renewable energy source. Geothermal energy has the potential to replace fossil fuels in the current energy mix and therefore can significantly contribute to the reduction of CO2-emissions to the Earth’s atmosphere. In addition, geothermal energy is independent of day/night fluctuations, weather conditions or seasons. It is available 24 hours a day, 7 days a week, all year-long. Today, geothermal energy is already used for electricity production, heat supply and even cooling. Shallow geothermal energy taps the heat stored in the upper 400 m of the subsurface and is mostly used for private and local heating/cooling applications. Deep geothermal energy targets the energy stored in depths > 400 m and can be used e.g. for district heating or electricity generation of several 1000s households.
Current deep geothermal energy applications either directly produce existing hot water from a deep geothermal reservoir, utilize the heat stored in the produced water and reinject the cooled water back into the reservoir, or “mine” the energy by circulating water through existing pathways (fractured reservoir) in the hot subsurface. In order to produce energy from a geothermal reservoir, good knowledge about the Earth’s subsurface (where is the reservoir = exploration) and drilling (accessing the reservoir) are required. Each step (exploration, drilling and production) comes with certain risks and uncertainties, which mostly depend on the structure and composition of the Earth’s subsurface.