In-Situ Stress Field in the Siting Regions Jura Ost, Nördlich Lägern and Zürich Nordost

  pdf NAB 24-19 In-Situ Stress Field in the Siting Regions Jura Ost, Nördlich Lägern and Zürich Nordost(35.31 MB)

The stress field is a relevant state variable for the design of underground structures, the evaluation of the geodynamic stability, and the long-term safety of a deep geological repository.

During the deep borehole drilling (TBO) campaign between 2019 and 2022 a large number of  new stress data was acquired from 9 boreholes in the siting regionsJO, NL and ZNO. Especially the availability of stress magnitude data was improved substantially with a total of 139 new Micro-Hydraulic Fracturing (MHF) tests^[1]. The minimum horizontal stress magnitude (Shmin) has been successfully measured from 112 , 28 of them in Opalinus Clay (with at least 9 successful tests at each site). In addition, the maximum horizontal stress magnitude (SHmax) has been derived with more than 60 sleeve reopening tests. The orientation of SHmax was constrained from borehole failure analysis (breakouts and drilling-induced tensile fractures) and from the MHF-induced fractures. The information on the lateral stress magnitudes complement the estimates of the overburden stress (Sv) from integrated rock density.

The prevailing SHmax orientation is NNW-SSE (160 ± 21⁰), in agreement with the regional and large-scale trend and the results prior to the new exploration campaign (Heidbach and Reinecker, 2013). Importantly, the SHmax orientation shows no variation with depth beyond the standard deviation within the Mesozoic sedimentary pile, especially across Opalinus Clay and its confining units. The only clear exceptions are two significant rotations (30 to 40⁰) of the dominant SHmax direction in the borehole TRU-1-1 within the Malm, and across the evaporites of the Zeglingen Fm. (Muschelkalk). The rotation in the Malm is interpreted as a local phenomenon associated with a fault zone. The rotation across the Zeglingen Fm. has previously also been observed in the borehole Herdern-1 (20 km to the southeast), and in the boreholes GTB Sonnengarten-1 and Schafisheim, south of the Folded Jura.

Whereas the new stress data essentially confirms the dominant orientation of SHmax, important new insights are gained for the stress magnitudes and the stress regime at the different sites. This is due to mainly two factors: 1) a very high density of MHF results acquired in the new campaign, and 2) improved testing equipment and testing protocols. For the Opalinus Clay, a fairly consistent trend with depth (gradient) is observed for both Shmin and SHmax, irrespective of site. The dominant stress regime in Opalinus Clay at the borehole locations is normal faulting (Sv≥SHmax≥Shmin) in all three sites, and not strike slip faulting (SHmax≥Sv≥Shmin) as anticipated before the TBO campaign.

The wealth of new stress magnitude data, as well as rock mechanical properties from core testing and petrophysical logging, allowed for a more robust calibration of site-specific, geomechanical, numerical 3D «stress models». These models were built for all three siting regionsand describe the spatial distribution of the entire stress tensor from surface to a depth of 2000 m. The models are used to assess the and of major fault zones on the local stress field, and to predict the stress magnitudes away from the boreholes. The models are also used to address questions related to future scenarios (e.g. fault reactivation potential), but this is documented in a separate report (Neotectonics NAB 24-013).

1. Mechanical layering across the Mesozoic sediments is the key control on the variability of stress magnitudes from one lithology to the next.
2. The impact of the identified major faults on the stress state is rather local and not detectable beyond 500 m distance from the fault. This result is independent of the fault friction coefficient which only controls the stress change in the near field < 100 m.
3. The stress regime at the boreholes in the Opalinus Clay is indeed normal faulting, but beyond the boreholes the model predicts that in JO and to lesser extend in ZNO the stress regime can also change to strike-slip.
4. Most importantly, the stress field in Opalinus Clay in the various sites appears to be relatively insensitive to future external loading (tectonics and/or erosion

^[1] In comparison to only 15 MHF tests directly from the candidate sites before 2019 (Nagra, 2014, Dossier IV).