For the VITO geothermal heat project, 2020 was a year with the brakes on. Since the tremor measuring 2.1 on the Richter scale on 23 June 2019, the plant has been shut down, but the project has not. The pilot project has always been based on the premise that geothermal heat extraction would not (and should not) cause vibrations that can be felt on the surface. The limit for perceptible vibrations had been set at 2.5 on the Richter scale. In order to be able to measure this very accurately and to leave absolutely nothing to chance, VITO had installed a network of 7 seismometers locally and the measurements recorded by that network were monitored continuously, in real time. The most characteristic parameters relating to movements of the earth's layers in the deep subsurface served as the basis for a safety system that took the form of a traffic light system. If the safety margin for the parameters were to be reached, this would cause the plant to operate at a lower intensity, or even bring it to a stop.

A wealth of information

In 2020, in collaboration with international experts in induced seismicity (vibrations caused by human activity), the researchers began to study in detail all the vibrations recorded since the plant's trial run up to the present date. This involved a total of 266 vibrations, from – 1 to 2.1 on the Richter scale. The research focuses on determining the mechanisms in the deep subsurface that are responsible for the vibrations and on correctly assessing the risk of induced seismicity if the plant were to operate at full capacity.  

When they mapped the vibrations, it was found that these occurred in clusters. The parameters of the measured vibrations were also compared to the standards, with which buildings in Flanders must comply, and also to the standards for nuclear reactors, in view of the fact that the Belgian Nuclear Research Centre is located in the immediate vicinity. Even the most severe vibrations were found to be well below those standards. (Factor 100 to 1000) 

After analysing the characteristics of the vibrations, the experts concluded that the cause was attributable to the planes of motion, which were sliding horizontally in relation to each other. That does not correspond to the pattern of an existing fracture in the subsurface, however. The hypothesis that the cooled water was injected into a fracture therefore seems unlikely. Since the force of the vibration is dependent on the underground motion, investigating the mechanism of the underground motion is high on the agenda. 

The parameters of the vibrations were also compared with the operational parameters that had been recorded in the plant. This showed that injection pressure plays a crucial role in causing the vibrations. Injecting the cooled water under pressure causes a change in the forces acting on the rock. The system adapts to this disturbance until a new equilibrium is reached. During the extended operating period of the plant, the injection pressure stabilised, indicating that the system had reached a dynamic equilibrium. The abrupt failure of the pump due to a power outage a few days before 23 June 2019 led to sudden pressure changes in the subsurface, resulting in vibrations.  

The cooling also has an effect on the state of tension in the subsurface and can also lead to vibrations. Unlike the pressure, the cooling systematically spreads outwards. A new equilibrium is only reached when the natural heat flow and the heat extraction are in balance, but such a balance is often not achieved.

Modification to the plant and the seismometer network

The results of the research have led to a number of modifications to the geothermal power plant on the Balmatt site and to the seismometer network. The initial aim of the modifications is to enable even more precise investigations to be carried out into the cause of the induced vibrations and to assess as accurately as possible the risks of vibrations that are perceptible at the surface.  

First and foremost, the plant is now equipped with a voltage safety device to deal with the consequences of a power failure. The pump will continue to operate in the event of a momentary dip in the voltage of the mains supply. In addition, a pressure maintenance system was also installed on the injection well, so even if the pump should suddenly fail, the pressure in the well will decrease only gradually. Slowly reducing the injection pressure prevents sudden pressure fluctuations in the deep subsurface, thereby reducing the risk of vibrations. 

The release of gas from the pumped water can also lead to pressure fluctuations. The VITO researchers have reduced the chance of degassing occurring in the injection well by increasing the operating pressure and extending the injection tubing. At the operating pressure used (40 bar), it was found that some of the dissolved gases were being released at certain points in the installations. Now that a number of elements in the plant have been modified, the operating pressure can now be increased (to 55 bar). At that pressure, the gases remain in solution. 

Due to the fact that a higher pump flow was found to have caused an increased injection pressure, the pump will now be replaced by a type that can deliver lower flows than the current one. Of course, this will have an impact on the power that the plant can deliver, but the heat production is of minor importance in this research phase. First of all, it is a case of unravelling the underlying mechanisms of the vibrations. 

An extremely important modification involves the refinement of the seismometer network. The researchers can now call upon 11 seismometers, positioned at different locations and depths, ranging from 30 m to 2 km deep. The measurements recorded by this more dense network will provide insights into where the vibrations are originating. And also the seismometers will allow the risks of vibrations (frequency and force) to be extrapolated more reliably, should the pump flow rate be driven upwards again. This result will determine the operational margins within which the plant can operate safely.

Towards a safe sustainable energy source

In summary, the following are the objectives of the next phase of the VITO's geothermal heat project:

  • to reliably estimate the frequency of vibrations 
  • to reliably locate the point at which the vibrations occur 
  • a more effective understanding of the mechanism at the point of origin, of the maximum power of the vibrations and of their evolution over time 
  • to identify the orientation and dimensions of fracture planes 
  • a more effective understanding of velocity distribution and damping in the shallow subsurface and therefore of the potential damage caused by the vibrations 
  • to determine the relationship between heat production and seismic activity, as that will provide an answer to the question as to the feasibility of geothermal heat production in the Mol-Dessel region.
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