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Induced Seismicity at the Balmatt Geothermal Project

Fri, 30 September, 2022

GeoSmart partner, VITO produced this article about dealing with the consequences of induced seismicity at the Balmatt geothermal project.

One Sunday eve at the end of June 2019, a cool breeze is tempering the first summer heat. The Graspop metal meeting is on its last legs. Mol-Donk and Dessel will soon be the quiet villages they used to be. At the Balmatt geothermal plant we are prepared for Monday to come. Friday morning, circulation was stopped abruptly due to an external power cut. We had decided to wait with the restart until after an inspection of the installation at the beginning of the following week. Then, at 2019-06-23 17:30 UT, the early summer peace is suddenly disturbed. A tremor of ML = 2.2 rocks the area. Locally, the event is hardly noticed. The people who do feel the tremble, do not pay much attention to it. But the event is picked up by the local and national seismometer networks and traced back to the reservoir from which the Balmatt geothermal plant extracts the heat. Suddenly, more people are alarmed. Phones start ringing. Emails are exchanged. People have questions, are concerned. What next?

The earthquake not only triggered a red-light condition under the seismic traffic light system (TLS) that was in place, it also sparked a sequence of events that determined the course of the project. The events led to a common understanding of the importance of additional testing to determine the risks and benefits of geothermal energy production at Balmatt. In the end, these tests will be crucial for the future of VITO’s geothermal plant and will provide valuable data for research into induced seismicity.

In this blog, I tell the story of what happened just before and after the 23rd of June the way I perceived it myself, a researcher at VITO who was closely involved in the project from the start. I tried to stick to verifiable facts as much as possible. However, my view on the facts is limited, and the story becomes subjective when discussing how these facts influenced the course of the project. In that sense, it may be told in a different way, depending on the perspective of the storyteller and his relationship to the project. Nonetheless, I hope that the story contains elements that may help to define ways to deal with concerns about induced seismicity in geothermal projects.

What happened before

The installations of the Balmatt geothermal plant were tested from December 2018 until April 2019. During the first tests, water was circulated for 1 to 3 days at a rate of 40 to 75 m3/h. Wellhead pressure in the injector rose to almost 100 bar. A second sequence of testes involved three 2-day circulation phases at flow rates between 50 and 65 m3/h conducted in monthly intervals. During these tests maximum well-head pressures in the injector again reached around 100 bar. After the tests it was concluded that the surface installations worked properly, and the plant was commissioned. However, the tests also proved that the injectivity was lower than anticipated; based on the injection tests that were performed in the fall of 2016, an injection pressure of about 80 bar was expected for a flow rate of 130 m3/h.

On the 15th of May, a first attempt was made to start production. It was decided to increase flow rate to the anticipated 130 m3/h as fast as possible and then see how the injection pressure would evolve. It proved difficult to obtain a stable production. This was attributed to the high injection pressure and degassing of the brine causing hammering effects and temporary two-phase flow in the surface installations and the injection well. After three 2 to 3-day long circulation phases, a fourth attempt was made at flow rate of 60 – 65 m3/h. This time the plant ran without major difficulties for 10 days. On day 9, the injection pressure had levelled-off and started to fall, indicating that the circulation was reaching a stable condition. Then, on the morning of the 21st of June, the circulation was stopped abruptly due to a power outage on the grid.

When testing the surface installations, dozens of seismic events were recorded by the seismometer network that we laid out around the Balmatt geothermal plant. The events occurred both during circulation and after shut-in. The strongest event occurred on the 18th of January 2019 at 08:24 UTC with a magnitude of ML = 1.8. Seismic activity continued during start-up. During the short circulation phases, the number of recorded seismic events was comparable to what we had seen during testing in December 2018 – April 2019, without the occurrences of any event ML > 1. More than a hundred seismic events, including six events of ML > 1, were recorded during long circulation phase.

About 3 days after circulation had stopped, the ML = 2.2 seismic event occurred. That was felt in Mol and Dessel and caused a red-light status of the TLS. This meant that a possible restart was postponed until the cause of the earthquake was known, and the seismic risk of further geothermal operations had been evaluated.

The first reactions

The questions and concerns that were uttered after the earthquake differed depending on the person’s distance to the site, and their relationship to the project. Representatives of the local authorities and local citizens mainly had questions about the possible impact of the geothermal operations on the environment, possible damage to property, and the compatibility of a deep geothermal plant with the nuclear activities in Mol and Dessel. They also wanted to know what the setback meant with respect to the plans to roll-out geothermal district heating in the area. The neighbouring nuclear companies raised similar concerns with respect to operation of the plant in the vicinity of nuclear installations. They pointed to the consequences of a damaging earthquake and the liability of VITO in that respect. Moreover, they argued that VITO’s project communication was not transparent and had fallen short on the risks of the project. The question of whether it is justified to run a geothermal plant next to nuclear installations was also raised by the responsible authorities on federal level. They were asking for an in-depth evaluation of the seismic activity that was observed during testing and start-up and an analysis of the hazards arising from it. Finally, they wanted to know what measures VITO could take, if any, to minimise the risk of a damaging earthquake. At a higher level, the question was asked whether VITO had managed the project in a professional way. Experts in the fields of geology and seismology contested the previous analyses of the seismic risk by VITO and questioned the efficacy of the TLS. They also objected to the way VITO was communicating with its stakeholders of the geothermal project.

The first actions

In response to these questions and concerns, VITO’s board of directors took on the project. They took steps to screen the project and to improve communication. An external agency was asked to evaluate the way the project had been managed so far. Besides, an expert study was ordered to identify the cause of the observed earthquakes and to analyse the seismic risk and possible hazards of geothermal energy production at Balmatt.

The analysis revealed that strike-slip (to oblique) faulting along either NN-E-SSW or W-E trending planes was the cause of the induced earthquakes. It provided information about the ground motion that was caused by the 2 strongest events and of the potential impact of higher magnitude events. The study also included an evaluation of the TLS. This resulted in recommendations to revise the parameters and threshold values used in the TLS. Further, the experts concluded that detection and location performance of the seismic monitoring network was poor. They recommended to extend the network to improve the robustness of parameters used for the TLS. This would make it possible to add further parameters that may help to avoid nucleation of large magnitude events. Finally, the study highlighted the principal areas of uncertainty and made recommendations for future research.

In terms of communication, various initiatives were launched in function of the target group. Three public sessions were organised to inform the local communities and interested people about the geothermal project, the associated seismicity, and the next steps. The first session took place on the 18th of July 2019. It was meant to capture the concerns of the local population. Towards the end of the year, two information evenings were organised to explain the progress of the research on the causes and risks of the observed seismic activity. As the communication immediately following the earthquake had given rise to mistrust among experts, authorities and the neighbouring companies, a communications agency was hired to advise VITO and to improve communication with the public and the neighbour companies.

Parallel to the public information actions, a mediator was appointed to explore the sensitivities and sentiment with respect to the Balmatt project of the neighbouring companies, the local, regional and federal authorities. Furthermore, the expert group that had been appointed in the context of the environmental permit was called together several times. Discussion within the expert group focused on the progress and the results of the expert study, communication, and VITO’s responsibility and liability.

To get a broader picture of the support in society for geothermal energy, VITO ordered a public survey. 1500 people living in three different regions of Belgium were interviewed. The survey did reveal an overall positive sentiment towards geothermal; both in the region around Balmatt and in the 2 other regions, 54 – 59 % of the interviewees showed a positive attitude towards the development of a deep geothermal plant in the immediate vicinity. 14 – 19 % were negative. In Flanders, almost 2 out of 3 interviewees would consider the connection to a geothermal district heating network in case it would be available. This conclusion was supported by the many questions about the viability of VITO’s initial plans to roll-out a geothermal district heating in Mol and Dessel asked during the information sessions. The survey also revealed that the knowledge about geothermal was overall limited. Geothermal was not top-of-mind when people were asked for applications in the subsurface, but as soon as there was concrete talk about how to extract heat from the subsurface, geothermal energy generated a lot of interest and sympathy from a large audience. A striking conclusion of the survey was the observation that familiarity, openness and support were not more present in the Campine region, where VITO had actively informed about geothermal energy for a long time and has developed the Balmatt geothermal plant, than in the other regions.

Preparing for a restart

The feedback from the communication actions showed that there was a support base to further explore the geothermal potential of Flanders. Based on this observation the board confirmed that it remains VITO's intention to use the Balmatt project to demonstrate that geothermal energy can make a real contribution to the Flemish climate objectives. The experience gained in Mol should make it possible to refine the knowledge of the subsurface and of the effects of the geothermal energy production, so that the risks of the technology can be properly identified and the preconditions can be determined to operate geothermal plants in a socially responsible manner. In addition to building up knowledge and demonstrating the practical feasibility of geothermal energy production, it is also the explicit intention of VITO to meet its energy needs and any other parties in a sustainable manner with a functioning geothermal plant. In line with these objectives, VITO started to prepare for further testing at Balmatt.

In terms of research, a dedicated project was then defined to investigate the outstanding issues with respect to source location, source characteristics and the relationship between seismicity and geothermal operations at Balmatt. The research should allow VITO to answer the question whether the Lower Carboniferous Limestone Group (LCLG) near Mol is suitable as a hydrothermal reservoir from a seismologic point of view. The proposed research bears on gathering data on the relationship between seismic activity and operational conditions. Hence, to carry it out, the geothermal plant must be running. In preparation of the research, the installations were adapted to improve controllability of the operations, to optimise production and to produce at very low flow rates. In addition, the seismometer network was extended to reach a lower detection limit and magnitude of completeness, and to allow better positioning of the seismic events.

In terms of communication, VITO launched a new communication initiative to strengthen the ties with the authorities, the local municipalities and residents, and the neighbouring companies. This initiative had the objective of entering an open dialogue with each of them to reach a common understanding about the set-up and goals of the planned research, and the risks and benefits of geothermal energy production at Balmatt. The initiative was positively welcomed. It proved essential to get support for the seismological research. In parallel, a new website ( was launched to explain to the general public how geothermal works. VITO’s project-website was extended to provide information about the research going on at Balmatt.

In terms of responsibility and liability, VITO made an estimate of possible damage due to the planned production tests. VITO then checked whether it could meet its legal liabilities.

On the 23rd of February 2021, the board gave its approval to start the production tests at Balmatt. In contrast to the first time, production would be started at a low flow rate. Then, depending on the observed seismic activity, the flow rate would be increased in a stepwise manner. In this way we want to determine the operational window within which safe geothermal production at Balmatt is possible.

In line with the promise made during the earlier public sessions, a fourth information evening was organised on the 31st of March 2021. During this event, VITO gave an overview of the conclusions of the expert studies that had been conducted and of the next steps that VITO planned to evaluate the operational window and feasibility of geothermal energy production in Mol-Donk. After this session, we could focus on the final technical preparations. Late April 2021, the pumps were started again.


The GeoSmart project has received funding from the European Union's Horizon 2020 research and innovation programme grant agreement 818576

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