Shallow geothermal heat production Roer Valley Graben (Roerdalslenk)

Shallow geothermal heat production Roer Valley Graben (Roerdalslenk)

Commissioned by EBN, TNO Geological Survey of the Netherlands (TNO-GDN) conducted exploratory research into the properties of shallow rock layers of the North Sea Groups in the Roer Valley Graben (Roerdalslenk). The research also included a first order assessment of whether geothermal cooling could possibly lead to the reactivation of pre-existing faults.

The aim of the research is to provide an initial assessment of the extent to which these shallow, highly porous rock layers behave differently compared to deeper, more compact rocks. Additionally, the research examined whether the likelihood of vibrations that may occur, due to reactivation of an existing fault in these shallow rock layers, is different from deeper rock layers.

Increasing interest in shallow geothermal heat

The motivation for this assignment is the increased interest in the Netherlands in geothermal heat production at depths between 500 and 1500 meters. This is also known as low-temperature geothermal heat production, or shallow geothermal heat. The Roer Valley Graben (Roerdalslenk) is a geological structure in the southeast of the Netherlands, that may be suitable.

In this area, the North Sea Groups consist of thick, sandy rock layers that are very permeable. Furthermore, these rock layers are deep enough so that the temperature of the geothermal water from these rocks is suitable for, among other things, low-temperature heating networks. However, because natural earthquakes occur in the Roer Valley Graben, it is considered a potential risk area for geothermal heat production.

Difference between shallow and deep rock layers

The North Sea Groups in general, and thus also in the Roer Valley Graben, consist of the youngest sedimentary rocks in the Netherlands. These sediments mostly consist of sands and clays. Because these sediments were generally not buried very deeply, they retained a high porosity. The sand grains are not always cemented together, which is referred to as unconsolidated. Deeper rocks have experienced more pressure and temperature and are therefore often less porous and more consolidated. Shallow rocks are expected to behave mechanically differently due to, for example, pressure and temperature changes that occur with geothermal heat production.

It is expected that shallow rocks will react more plastically (deformable without breaking), which could reduce the likelihood of generating seismic waves. However, in the current standard analysis concerning the reactivation of faults due to geothermal heat production, this plastic behaviour has not yet been considered. EBN’s assignment to TNO-GDN is therefore to investigate what we know about these young sedimentary rocks, and how the expected difference in mechanical behaviour influences the potential reactivation of existing faults in the subsurface.

Conclusions

The exploratory research by TNO-GDN shows that the amount of data and its quality of the North Sea Groups in the Roer Valley Graben is still limited. Therefore, collecting additional data is one of the recommendations. An inventory of natural earthquakes in this area shows that these seismic events most likely originate in much deeper rocks than the North Sea Groups. Furthermore, preliminary exploratory simulation models indicate that including the plastic behaviour of shallow rocks leads to a lower likelihood of reactivating faults. It is therefore important to further investigate this behaviour and include it into future models.

Want to know more?

The report (English only) is available online and can be downloaded via NLOG. You can also view and download a Dutch management summary here.