It is well known that deep geothermal energy can cause earthquakes. These are usually mild earthquakes that can only be recorded using sensitive measurement equipment. The injection of cooled water appears to be a significant cause of these. By determining the location of the earthquakes, we are able to establish how the substrate responds to differences in pressure in connection with the injection. This allows us to become more familiar with the substrate.

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Seismometer network

The injection of water into the deep substrate causes small movements that give rise to earthquakes. Artificially induced earthquakes are a focal point in the development of every deep geothermal energy project and are an important topic of research. It is for this reason that VITO has further developed a seismometer network. The network allows us to pinpoint the soil movements to within several tens of metres. By comparing the locations of consecutive earthquakes, we can calculate the direction in which the pressure effects of the injection continue. We are therefore able to monitor whether they occur in proximity to major fracture systems. By monitoring the injection (pressure and temperature), VITO is hoping to guarantee the safe operation of the geothermal energy plant.

 

Traffic light system

We use a traffic light system in the geothermal power plant, which is based on four parameters:

  • The calculated magnitude of an earthquake
  • The epicentre of the quake
  • The number of quakes per unit of time
  • The maximum ground acceleration measured by the seismometers in the monitoring network

Limit values have been defined for each parameter, which define a green, an orange and a red zone. If any one of the parameters is in the orange zone, the parameters in the geothermal power plant such as flow rate and injection pressures are gradually reduced. If any one of the parameters is in the red zone, the plant is temporarily shut down. The parameters can be followed below.

The local intensity is shown for each earthquake, and we calculate this value using the seismometers in the monitoring network around Balmatt. More powerful quakes are also picked up by seismometers located further away. For these quakes, the Belgian Royal Observatory calculates the local magnitude on the Richter scale (ML). The local intensities calculated by VITO are systematically higher than the magnitude of the Belgian Royal Observatory. For the traffic light system, we convert the local intensity into ML. The converted or adjusted values are also displayed. The time of the events is shown in local time (Belgian time).

Yellow blocks: seismometers, blue block: geothermal energy plant

What are microearthquakes?

Seismic activity is the result of the reinjection of partly cooled water from the Balmatt geothermal energy plant. The first earthquakes occurred during the injection tests carried out in September 2016. This was also the case at the time of the launch of the geothermal energy plant in December 2018. All movements occurred close to the injection zone of well 2, which is located beneath the municipality of Dessel.

How do these occur?

Small movements occur along the cracks and crevices in the limestone, which then open up further as a result of the enforced flow and overpressures.

To summarise

Seismicity is inherent to the injection of water in the deep substrate. With our seismometer network, we can accurately locate the soil movements. By comparing the locations of consecutive events, we can determine the direction in which the pressure effects of the injection continue. We are therefore able to monitor whether they occur in the region of major fracture systems. As a result, we are able to redirect the injection in a timely fashion, if necessary, and thus ensure that the intensity of the earthquakes remains sufficiently low.