In the exploration phase, various investigations are carried out in order to be able to build up a digital model of the subsurface that is as detailed as possible. The model is used to determine suitable drilling targets for energy extraction and to estimate the economic viability of the potential plant by predicting water volumes and temperatures.
Several methods are used to explore the subsurface:
- Interpolation between existing geothermal boreholes in the vicinity of a new project: For this purpose, the existing data of previous boreholes are taken as a basis and analyzed. This allows to derive a first general prognosis on the depth as well as on the hydrogeological properties and temperatures of the thermal water conducting layer. In the Bavarian Molasse Basin this is the Malm-Aquifer.
- Seismic Surveys: A Seismic survey is a geophysical exploration method, which allows an indirect image of the subsurface at the position of the planned geothermal plant. Today, the most suitable method for this purpose is reflection seismics. With this method, sound waves are generated that travel through the subsurface and are reflected at layer boundaries. The sound waves are usually generated with so-called Vibro-trucks. Measuring instruments at the surface, usually geophones, record the reflected sound waves. From the travel times of the sound waves from emission to recording, an image of the different rock layers with their properties and structures can be created. By integrating drilling data, e.g. from hydrocarbon exploration, the image can be further refined.
Schematic representation of the interpolation method for estimating the drilling depth. This method is commonly used in the preliminary study
Seismic exploration of the subsurface: Vibro trucks send sound waves into the ground, which are reflected at layer boundaries. The reflected sound waves are recorded by geophones. The depth of the layer boundaries can be derived by softwares from the signals.
Vibro-trucks conduct sound waves into the subsurface during a seismic survey (source: SWM).
Geophones pick up the reflected sound waves (source: tz)
Representation of the interpreted layer boundaries along several 2D measurement lines (source: Wintershall, Geothermie Trebur).
Based on the seismic data and other geological information, a reservoir model is finally created. This is a thermal-hydraulic model of the subsurface, which can be used to predict how much thermal energy can be extracted at the planned geothermal site. The model indicates at what depth the thermal water conducting layer can be found, which then allows to set the drilling targets. Particular attention is paid to structures that have a higher probability of carrying increased amounts of water. The depth of the reservoir also allows to draw conclusions about the temperature, as this usually increases by about 3°C per kilometer of depth. Again, wells from oil and gas exploration help to make accurate predictions.
The model is the basis of the economic feasibility study for the project as well as the design of the components of the future plant. Together with the seismic data, a risk analysis for the well is also prepared in order to be prepared for strata that are difficult to drill or possible gas inclusions and to counteract these by taking appropriate measures.