Ecology & risks
Before any technology is used, possible positive and negative effects on the environment must be determined and weighed up. The parties involved must agree on which risks (impact x probability of occurrence) are considered acceptable and to what extent. Based on this, suitable measures must be developed to reduce or completely exclude all unacceptable risks and to utilize the positive effects for a future climate-neutral society. In the case of geothermal plants, induced seismicity and groundwater pollution are often the main potential negative impacts. However, other impacts on the environment must also be taken into account in order to protect people and nature.
For most of the challenges mentioned, the risk can at least be minimized through technical solutions. For example, mineral deposits in parts of the plant can be prevented using so-called inhibitors. Hydrocarbons and gases can be separated and returned to the layers.
Induced seismicity
In principle, the Upper Rhine Graben is one of Germany's naturally seismically active zones. This is due to the geological situation. Human activities in mining or the injection of fluids into the deep underground can lead to anthropogenic seismic events. This so-called induced seismicity is triggered by deformations or distortions in the subsurface when the rock reacts brittlely to the corresponding processes. The magnitude can be used to infer the size of the fracture surface in the subsurface. Maximum magnitudes that have so far been attributed to the injection of water in geothermal projects in the Upper Rhine Graben are 2.1 in Insheim, 2.7 in Landau, 2.9 in Soultz and 3.4 in Basel. So far, no noticeable seismic events have been recorded in the Upper Rhine Graben during the drilling process. Geothermal plants in which the thermal water circulates during operation at moderate flow rates of up to 25 liters per second have not generated any induced seismicity for more than 30 years in some cases. Examples of this are Riehen in Switzerland and Bruchsal in Baden-Württemberg.
The potential risk of induced seismic activity must be carefully assessed and avoided at the planning stage of a geothermal project. In addition, seismic monitoring is the rule during the construction and operation of a plant. For this purpose, seismometers are installed in the immediate vicinity of the boreholes. The design of new deep geothermal plants in the Upper Rhine Graben can draw on many years of experience with comparable regional projects. In the Soultz-sous-Forêts project in France, correlations were established between the volume or pressure of the cooled thermal water returned and the seismic activity. The reinjection pressures were therefore adjusted over several years so that there is no longer any noticeable induced seismicity.
Contamination of the groundwater
When drilling and operating the plant, it must therefore be ensured that the thermal water does not enter the aquifers with drinking water quality. An appropriate construction of the drilling site makes it possible to collect, control, treat and dispose of any liquids that occur so that the seepage water on the surface is not impaired. When drilling through the relevant aquifers, fresh water can be used as a drilling fluid or - with an appropriate drilling rig - drilling can even be carried out using the air-lift method without a special drilling fluid. In both cases, a corresponding steel standpipe is cemented in immediately afterwards. This effectively prevents the groundwater levels close to the surface and the seepage water from being impaired by the subsequent drilling work.
During the rest of the drilling process, at least one further inner steel pipe is cemented in from the surface, creating at least a fourfold barrier. Subsequently, further pipes with a smaller diameter are successively installed and cemented in deep boreholes. With this installation and the cementing of further casings, the deeper aquifers, which are generally not used due to their mineralization, are also protected. In this way, the aquifers are effectively separated from each other during the drilling of the borehole. This ensures that there is no vertical connection between the groundwater levels. The quality of the installations and the cementation of the casings is checked. In the event of insufficient sealing, the required groundwater protection can be achieved through subsequent remedial measures, for example through post-cementation. These are standard methods that can and should be used at any time if necessary.
Groundwater monitoring is used to monitor groundwater protection: a network of near-surface groundwater measuring points enables changes in the groundwater to be detected at an early stage by monitoring the water level, electrical conductivity and temperature at regular and short intervals. This allows operators to react quickly.
Economic risks
The operator of a geothermal project can dispose of the energy that it extracts from the subsurface in the form of hot water free of charge. However, the conditions of deep geothermal energy entail considerable risks for profitability. These conditions include, above all, geological and geotechnical conditions, but also the local energy situation, existing supply systems, economic conditions and the requirements of the approval process. As far as investment costs are concerned, drilling costs account for more than half and therefore the largest share.
When developing a geothermal reservoir, there is a risk that thermal water cannot be extracted in sufficient quantity or quality due to incorrect forecasts regarding the required depth of the borehole. This risk is known as the exploration risk. The quantity results, among other things, from the possible extraction rate and the temperature difference between the extracted and reinjected thermal water. As far as the quality of the thermal water is concerned, the chemical composition, for example the proportion of gas it contains, can cause difficulties during extraction. The geothermal industry now offers exploration insurance to manage the exploration risk.
Even if the exploration potential is fulfilled, a geothermal project can prove to be uneconomical. This is because the operation of a geothermal plant also harbors risks for profitability. For example, processes may occur during operation that require the plant to be technically upgraded or additional maintenance work to be carried out.
Authorization
The construction of a geothermal plant is subject to the Federal MiningAct (BbergG ). The "Deep Geothermal Energy" guidelines published by the State Research Center for Geothermal Energy(LFZG) currently provide a good overview of the project steps and the approval procedure in Baden-Württemberg. Deep geothermal projects begin with preliminary studies on the underground properties as part of an exploration permit. This exploration permit includes a detailed work program on which regular reports must be submitted to the State Office for Geology, Raw Materials and Mining(LGRB ). In Baden-Württemberg, early public participation is already provided for with the application for the exploration permit, i.e. citizens are given the opportunity to comment and discuss the matter. Interventions in the subsurface to explore or develop the reservoir are always carried out via detailed operating plans, on which the relevant specialist authorities comment. The LGRB is responsible for coordinating the procedure. This authority also decides after a preliminary environmental impact assessment whether and how an environmental impact assessment must be carried out during the approval process.
In Baden-Württemberg, early public participation for projects of particular environmental significance is regulated by the state's Environmental Administration Act(UVwG). This law emphasizes the right to environmental information as the basis for citizen participation in political decision-making processes. In addition, the rights of the public are strengthened through statutory environmental mediation in the event of a dispute and through the participation rights of recognized environmental and nature conservation associations. The project managers therefore have the task of providing citizens with detailed information at an early stage.