Balmatt energy plant

In February 2021, the Board of Directors of VITO decided to restart the deep geothermal power plant on the Balmatt site in Mol for a one-year research programme. The main objective is to better understand the hydraulic and seismological characteristics of the subsurface reservoir.

Balmatt deep geothermal power plant 2.0

Preliminary research has led to a number of adjustments to the geothermal power plant on the Balmatt site and to the seismometer network. The main aim of the modifications is to enable more accurate research into the cause of the induced vibrations and to assess the risks of surface vibrations as accurately as possible. 

First and foremost, the plant is now equipped with a voltage protection device to deal with the consequences of a power failure. In addition, a pressure maintenance system has been installed at the injection well, which is currently being fine-tuned. Because 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. Based on the observation that a higher pump flow caused an increased injection pressure, the pump was replaced with a type that can deliver lower flows than the original pump.

Refined seismometer network

An extremely important modification was the refinement of the seismometer network. The researchers can now rely on 16 seismometers, positioned at different locations and depths, ranging from the ground surface (close to the centre of Dessel) and from 30 m to 2 km deep.

The extension was necessary in order to:

• detect more low magnitude vibrations (from M_{L corr} 0.0);
• determine the location of the vibrations more accurately;
• gain a better understanding of the source mechanisms;
• obtain information on the strength of the vibrations at the surface;
• to find out how often vibrations of a certain magnitude occur (magnitude-frequency relationship).

The more detailed information obtained in this way should provide a better understanding of the seismological behaviour of the deep subsurface beneath Mol and Dessel, the causes of the microseismicity, and the relationship between the activities at the geothermal site and the vibrations, and ultimately allow a better assessment of the seismic risk.

Towards a safe sustainable energy source

Between February and April 2021, final preparations were made for the restart. The first tests took place on 20 April. Initially, the tests were intended to check that the installations and components were still functioning properly. Gradually, the system pressure of the installations was increased to more than 50 bar in order to prevent the release of gases from the pumped brine.

Step by step, the test phases became longer and the flow rate increased. The maximum flow rate at which the brine was produced and injected was 25 m³/hr. This resulted in a maximum injection pressure of approximately 90 bar. The last and so far longest test phase started at the end of November 2021 and is still ongoing in mid-February 2022. The longer test phases allow more reliable estimates to be made of the flow characteristics of the reservoir. In addition, the researchers evaluate the adjustments already made to the plant.

From November 2021 to November 2022, four test phases passed, including a few lasting more than four months. In the process, the plant pumped 206 000 m³ of water and supplied 7 935 MWh of heat to the heat network of VITO, SCK-CEN and Belgoprocess. This brings the total of all test phases to a saving 912 500 m³ of natural gas, equivalent to the average annual consumption of 466 households.

Update 16/11/2022

The extensive seismometer network detects more and smaller vibrations. Of 254 vibrations, location and magnitude could be determined. Typically, the magnitude is between ML corr -0.7 and 0.1. In 7 vibrations, the ML was 1.0 or more. One of these occurred during the start-up of test phase 11. Then the seismic monitoring network at and around the geothermal power plant detected a tremor with a magnitude of ML of 2.1 on 16 November 2022 around 10am (08:59:04 UTC). This tremor was felt in the immediate area.

Based on the magnitude and peak ground acceleration (PGA), the tremor occurred in the red range of the stoplight system of the geothermal power plant. This means the power plant was shut down in a controlled manner. Further steps will be determined after additional investigation of the vibrations that occurred in cooperation with external experts and in consultation with the responsible authorities.

At the request of VITO, the residents of Mol and Dessel have reported what they felt from this vibration. This will allow researchers to accurately map the above-ground effects of the vibration.