| Abstract |
The in-situ state of stress plays a major role in determining the response of the rock mass to hydraulic stimulation injections used to develop heat-exchangers in low-permeability EGS reservoirs. As such, stress and its heterogeneity must be specified in any geomechanical model of the stimulation process. This paper presents the results of an evaluation of stress magnitudes in the granitic EGS reservoir in Basel, Switzerland. The profile of minimum principal horizontal stress, Shmin, is constrained by hydraulic tests, but the magnitude of the maximum horizontal principal stress, SHmax is uncertain. Here we derive estimates for SHmax by analysing breakout width data from an acoustic televiewer log run the 5 km deep borehole BS-1. Some 81% of the borehole below the granite top at 2.42 km is affected by breakouts, which is favourable for examining the depth trends of the estimates. A primary objective of the analysis was to evaluate the impact of four different failure criteria on the SHmax magnitude estimates. The criteria where Rankine, Mohr-Coulomb, Mogi-Coulomb, and Hoek-Brown 3D. All were parametrized using strength data from a single multi-stage triaxial compressive test on a core plug taken from near the well bottom. A numerical approach was employed to derive SHmax magnitude from the estimated breakout widths, taking into account all stress components at the borehole wall including the remnant thermal stress arising from the cooling of the borehole wall by the drilling. Previous studies of breakout width have shown that large, small-scale fluctuations are associated with fractures, which reflect variations in strength or stress, or both. At larger scales, breakout width tends to decrease with depth. Assuming there is no significant systematic change in the strength characteristics of the rock along the length of the hole, for which there is no evidence, the large-scale trend has the consequence of implying a small gradient of the SHmax profile. This result is independent of the failure criterion, and also of the profile of Shmin used in the analysis. The absolute values of SHmax depend upon the failure criterion used. Criteria that consider the strengthening effect of the intermediate stress (Mogi-Coulomb and Hoek-Brown 3D) yield profiles that violate frictional limits on the strength of the crust above 4 km, whereas the profiles of the Mohr-Coulomb and Rankine criteria do not (the latter two are essentially identical for the case where pore pressure and wellbore pressure are equal and in the range of Shmin and SHmax relevant for our analyses). The Mohr-Coulomb/Rankine criteria profiles indicate a trend in SHmax from favoring strike-slip faulting above 4200 m to strike-slip/normal faulting below. This is reasonably consistent with focal mechanisms recorded during the reservoir stimulation which show a mix of strike-slip and normal faulting throughout the depth range considered. |