Measures to Control Induced Seismicity

Various measures are applied with a view to monitoring and controlling induced seismicity in the context of deep geothermal energy projects in the best possible way.

A central element of the counteractive measures is to ensure the best possible knowledge of the subsoil. This knowledge is generally not available in advance and has to be acquired for a specific area, with the level of knowledge subsequently increasing over the course of a project. Seismic 3-D modeling of an area, for example, provides valuable information about a potential location. This can only be used to portray the larger structures, however, whereas the smaller structures, the stress levels of faults and the permeability of the rock cannot be portrayed reliably using these geophysical methods. A more accurate picture therefore only emerges once the first drilling is carried out and feedback is received from the subsoil following interventions.

Seismic risk assessments carried out in advance of geothermal energy projects should highlight the following points:

The naturally occurring seismicity in a given region is an essential element of a risk assessment. In principle, the seismic risk is lower in geothermal energy projects in regions with lower natural seismicity over long periods.

Earthquakes cannot be predicted. Historical data, however, enable a range of scenarios to be developed that portray various future seismicity rates.

The impact of an earthquake depends not only on its magnitude but also on the depth of focus, the distance to the epicenter and the local subsoil. Microzonation records the local geological and geotechnical properties of the subsoil in order to assess whether it weakens or strengthens the vibrations caused by an earthquake. The same information should also be taken into account in connection with risk assessments for geothermal energy projects.

Due to varying local conditions and specific project designs, every geothermal energy project needs its own probabilistic risk and hazard analysis.

The precise way in which the traffic light system should work must be defined in advance, and responsibilities allocated among those involved. In addition to these preliminary clarifications and the ongoing analysis of new information about the subsoil, a “traffic light system” is generally used as a measure to control induced seismicity.

Traffic light systems aim to avoid the negative effects of induced seismicity, or to control them as far as possible. The traditional traffic light system is based on the close monitoring of induced seismicity and involves various measures depending on the seismic activity registered. A traditional system of this kind was applied in Basel in 2006 and subsequently found to be unsatisfactory, while an extended, adaptive traffic light system was used for the geothermal energy project in St. Gallen in 2013 based on the experience gained. The observed seismicity serves as an indicator of the necessary measures, and predictions are also made as to its possible course. This is based on the anticipated impact of planned pumping procedures, the permeability of the rock and other factors. The example in St. Gallen, however, also highlighted the limits of this adaptive system. Although a yellow alarm was triggered, which would have required immediate termination of pumping work, the work had to be continued due to rising gas.

A great deal of research and development will be required in this area over the coming years, and the findings will need to be tested in future demonstration and pilot systems. Based on the current state of science and technology, it is difficult to assess the extent to which we will be in a position to prevent (or control) induced earthquakes.

Project GEOEST2020+

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