Stress field. During the HDR experiments the magnitudes and orientations of the stress field have been identified from near surface to a depth of 2500 m. The maximum horizontal stress was determined to be approx. NW-SE directed with a gradient of SH ≈ 18MPa + 33.3MPa/km. The minimum horizontal stress is directed normal to SH and has a gradient of Sh ≈ 8MPa+14.3MPa/km. The vertical stress forms the intermediate stress with a gradient of SV ≈ 25.8MPa/km.
Stress field. In Xxxxx & Backers (2015) the stress field for the Xxxx Formation was modeled to apply the relevant stress states in a simulation. A transpressive regime of SV ≈ SH was assumed from literature. Based on the geologic situation a regime of S1 ≈ S2 >> S3 seems likely. The vertical stress SV was estimated from the overburden and calculated using rock densities, for which typical values for the present lithologies were taken. For this location SV gradients of 25 to 25.5 MPa/km were calculated. At the depth of the Xxxx Formation SV is 98 MPa. Lithological properties as well as Sh/SV ratios of 0.55 to 0.9 were determined from borehole stress measurements in the nearby xxxxx Schlattingen SLA-1 and Xxxxxx. The horizontal stress ratio KH = SH/Sh is given as 1.3 to 2.0. In St. Gallen, an injection test at 3810 m TVD in 2013 yielded a minimum horizontal stress gradient of 12.2 MPa/km. For the Xxxx Formation at 3882 m depth Sh amounts to 49 to 62 MPa. A pore pressure gradient of 10 MPa/km (close to hydrostatic conditions) was measured at 3785 m TVD. As this is the only measurement, this gradient was assumed valid for the whole model presented in this deliverable. Maximum horizontal stress (SH) is the most difficult and least accurate to calculate. Using an average friction coefficient µ of 1.0, a stress polygon was calculated to constrain the possible values of SH. Because of seismic activity in the region, the stress state was assumed to be near critical. Based on the assumption SH ≈ SV and limited by the stress polygon, SH in the Xxxx Formation is estimated to 98 ± 10 MPa. The maximum horizontal stress orientation was determined from literature as well as from borehole breakouts. As shown by the World Stress Map Project data (Figure 4), the general orientation in the molasse basin is N-S in the Eastern Alps to WNW-ESE in the Western Alps, perpendicular to the overthrust front of the Alps (e.g. Xxxxxxxxx et al., 2010; Kastrup et al., 2004; Xxxxxxxx et al., 2008). At the St. Gallen location the direction determined from borehole breakouts varies with depth as determined by evaluating the orientation of borehole breakouts by Xxxxxxxxx (2018). The results of his analysis, which has been performed within S4CE are given in Table 1. The nearby xxxxx Schlattingen and Xxxxxx also yielded orientations around NNW. Other sources state orientations of 165° to 169°. At a depth of 4,074 to 4,335 m MD the orientation deviates from NNW to NNE (Xxxxx & Backers, 2015). Focal mechanism ...
Stress field. The stress field has been implemented as shown in Figure 14. The PCT is over-pressurized by 15 MPa above hydrostatic conditions. Hence, the effective stresses are significantly reduced. The Muschelkalk acts as a seal. The Xxxx Formation, Dogger, Muschelkalk and PCT have a reduced minimum horizontal stress according to our stress field determination stated in Section 1.1.8. <.. image(Ein Bild, das Text, Karte enthält. Automatisch generierte Beschreibung) removed ..>
