Update on the deep borehole in Bülach

Since April 2019, Nagra has been drilling a deep borehole in the Herrenwis area. Now is an appropriate time to review what has happened so far and where we stand today.

Radioactive waste will one day be disposed of deep underground in geological repositories. The Federal Government has the lead in the site selection process in Switzerland. Nagra is drilling deep boreholes in all three potential siting regions to complete the overall picture of the underground geological environment. The investigations include studies of the thickness, permeability and composition of the Opalinus Clay host rock in which the repository will eventually be constructed. The rock samples obtained from the Bülach borehole are suitable for determining these properties.

We have spoken to different project managers of the deep borehole campaign.

What insights have been gained?

<em>Michael Gysi is “Project Manager Drill Site Geology”, and his responsibilities include the rock samples (“drill cores”) obtained during drilling.</em>‘ style=’width:100%’><figcaption></figcaption></figure><p>We have gathered information on the thickness, permeability and composition of the Opalinus Clay. We have already drilled through the Opalinus Clay rock layer and have reached a depth of over <a href="http://www.nagra.ch/tiefbohrung_buelach.htm" target="_blank" rel="noreferrer noopener">1080 metres</a> (Link in German) where the rock is 234 million years old. Additional analyses will allow us to accurately determine the thickness of the Opalinus Clay layer. However, initial data indicate a thickness of well over 100 metres in the depth range from a good 850 to almost 1000 metres.</p><p>Despite some drilling difficulties, we were able to bring high-quality rock samples up to the surface. These will allow us to investigate the rock layers in detail in the laboratory and thus to gain a better understanding. We also successfully conducted numerous measurements in the borehole and now have large volumes of data, which we will analyse in detail over the coming months. This will provide us with a better understanding of the geological conditions in the Nördlich Lägern region.</p><p>When analysing the data from the 3D seismic measurements conducted beforehand, we identified a different rock body in the layer above the Opalinus Clay. We now know from the deep borehole that this layer is a fossilised coral reef. The idea that a coral reef existed here over 160 million years ago is fascinating. Whether this fossilised reef will have any impact on the siting region is being evaluated at present.</p><h2 class="wp-block-custom-heading">What challenges did the drilling procedure involve?</h2><figure class=

Matthias Ammen is head of safety at the drill sites as well as “Drilling Operations Manager”.

First of all, I am very pleased that we have already reached a depth of more than 1080 metres of the planned 1350 metres in Bülach without any incidents worth mentioning. We first had to coordinate the work of over 50 subcontractors at the drill site. In geological terms, a deep borehole is challenging and surprises can lie in store. For the boreholes, we have to take precautions to make sure they stay open for the various investigations and do not break out at the sides. In the lower section of the Opalinus Clay in the Bülach borehole, we were not entirely successful in this respect, but we learned several useful lessons that will help us make improvements when drilling the next boreholes. For example, depending on the rock type, we will use different core bits and adjust the composition of the drilling fluid.

What took us by surprise?

Lukas Oesch is “Project Manager Regional Participation” and the liaison between the Nördlich Lägern siting region and Nagra.

Over 2000 people have come to look over our shoulder here at the drill site in Bülach, and many of them also visited the information pavilion. We were surprised at the huge interest generated by the deep boreholes, and we were able to conduct interesting conversations with the visitors and respond to their questions. In this way, we also learned a lot about the concerns of the local population in connection with the boreholes. We were always in close contact with the representatives of the local communities and the authorities, as well as with the residents living near the drill site. The collaboration with everyone concerned went well and together we were able to find solutions to their concerns.

We also appreciate the keen interest shown by the regional and national media. In a contribution to the TV programme “Schweiz aktuell” (Switzerland Today), the process from recovering a drill core to analysing it was documented in detail.

Outlook

Maurus Alig is “Coordinator Major Project Sectoral Plan for Deep Geological Repositories Stage 3”.

Many visitors to the drill site ask us whether the knowledge gained from this borehole shows that Bülach is the safest site. We are not yet able to answer this question. The significance of the knowledge we gain here must always be evaluated in comparison with that gained in the other siting regions.

The deep borehole in Bülach has provided us with valuable information. We will continue with our deep borehole campaign and remain open for further results and will then carefully evaluate all the information as a whole. We believe that, together with the information gained from the 3D seismic campaign, the deep boreholes will allow us to identify the differences between the three siting regions that are relevant for the safety of a deep geological repository. We will be able to reliably evaluate the siting regions in two to three years.

Our second borehole Trüllikon 1 is proceeding well, and we will soon reach the Opalinus Clay. The preparations for the upcoming boreholes are at an advanced stage, and drill sites are currently being prepared in the communities of Bözberg and Marthalen. We will continue to provide information on any new insights gained.