http://hera.wdcb.ru/tols/tecton/method/biblio/magn/Zoback_03.pdf

Determination of stress orientation and magnitude in deep wells

Reservoir Geomechanics: Frontmatter

A fluid injection-induced seismicity experiment was conducted in the German Continental Deep Drilling Program (KTB) main borehole at 91 km depth (in situ temperature of 260°C) to extend knowledge about stress magnitudes and brittle faulting to depths and temperatures approaching the brittle-ductile transition Almost 400 microearthquakes were induced at an average depth of 88 km by injection of KBr/KCl brine into a ∼70 m open hole section near the bottom of the borehole Although most focal plane mechanisms were poorly constrained due to the very small size of the induced earthquakes, several different clusters of microearthquakes with distinct mechanisms were defined Most of the microearthquakes for which focal plane mechanisms were determined were strike-slip events with a NNW trending P axis, essentially parallel to the direction of maximum horizontal compression observed in the borehole The largest induced earthquake, M 12, occurred 18 hours after injection was started This event was a strike-slip/reverse faulting event which also had a NNW trending P axis Utilization of a precise relative location technique indicates that many of microearthquakes occurred relatively far (50–100 m) from the well bore Modeling of the pore pressure disturbance caused by injection suggests that many of the earthquakes were induced by extremely small pore pressure perturbations (<1 MPa) less than 1% greater than the ambient, approximately hydrostatic pore pressure at depth Thus it is apparent that there are critically stressed, permeable fault zones in the crust, even at great depth and temperature A frictional analysis of the focal plane mechanisms of the induced microearthquakes indicates that fault slip is consistent with the stress magnitudes and orientations determined in situ at depths to 77 km in the borehole and relatively high coefficients of friction (∼06–07) reported by Brudy et al [this issue] This and the observation that very small pore pressure perturbations were able to trigger seismicity appear to confirm the hypothesis that “Byerlee's law” (ie, that differential stresses in situ are limited by the frictional strength of well-oriented, preexisting faults) is valid to great crustal depth and that the crust is in brittle failure equilibrium at depths and temperatures approaching the brittle-ductile transition, even in this relatively stable intraplate area

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JB02814

Injection-induced earthquakes and crustal stress at 9 km depth at the KTB deep drilling site, Germany

For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A continuous profile of the magnitudes and orientations of the three principal stresses has been estimated to depths of 7.7 km and 8.6 km in the German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1–3 km), estimates of the magnitude of the least horizontal principal stress provided by modified hydraulic fracturing experiments at 6 km and 9 km depths, and analysis of compressional (breakouts) and tensile (drilling-induced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160°±10°E, which is consistent with the average orientation found throughout western Europe. The only significant change in stress orientation was observed directly below a major fault zone crosscutting the borehole. The profile of stress magnitudes we have obtained demonstrates that to a depth of 8 km, the state of stress in the brittle crust in southern Germany is in frictional equilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well-oriented to the in situ stress field is comparable to their frictional strength based on predictions of Coulomb faulting theory for a coefficient of friction of about 0.7 and near-hydrostatic pore pressure.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JB02942

Estimation of the complete stress tensor to 8 km depth in the KTB scientific drill holes: Implications for crustal strength

Rheology and strength of the lithosphere

IT has been suggested1–6that in many cases the average strength of the continental crust is quite low (tens of megapascals), so that the crust has little effect on the large-scale deformation of the lithosphere. But laboratory friction studies7,8, combined with simple faulting theory9,10 (as well as extrapolation ofin situ stress measurements from the upper 3 km of the crust11), imply that if pore pressure is approximately hydrostatic at mid-crustal depth, crustal strength is appreciable (hundreds of megapascals) and would markedly constrain the nature of lithospheric deformation12–15. Here we report estimates of the magnitude of in situstresses to 6 km depth in the KTB borehole in southern Germany. Our results indicate a high-strength upper crust, in which the state of stress is in equilibrium with its frictional strength. We suggest that plate-driving forces in the continental lithosphere in this part of western Europe are transmitted principally through the upper crust, and that this may also be the case in other continental areas of moderate to elevated heat flow.

/pdf/upper-crustal-strength-inferred-from-stress-measurements-to-3kxchitjt8.pdf

Upper-crustal strength inferred from stress measurements to 6 km depth in the KTB borehole

Scientific rationale for establishment of an International Program of Continental Scientific Drilling : report of the International Meeting on Continental Scientific Drilling : Potsdam, Germany, Aug. 30 - Sept. 1, 1993

Geologists, geophysicists, and science administrators from over 25 countries gathered in Potsdam, Germany, from August 30 to Septmeber 1, 1993, to review the scientific rationale for establishing an International Continental Scientific Drilling Program (ICDP). Such a program is intended to complement the highly successful Ocean Drilling Program and would operate on a similar scale.

The meeting was convened under the auspices of the Coordinating Committee for Continental Drilling (CC-4) of the International Lithosphere Program, a joint program of the International Union of Geological Sciences and the International Union of Geodesy and Geophysics. The conference was hosted by the recently established GeoForschungsZentrum Potsdam (GFZ), a major new Earth science research institution in Potsdam, founded by the German Ministry for Research and Technology.

Rolf Emmermann

Papers

Upper-crustal strength inferred from stress measurements to 6 km depth in the KTB borehole

Scientific rationale for establishment of an International Program of Continental Scientific Drilling : report of the International Meeting on Continental Scientific Drilling : Potsdam, Germany, Aug. 30 - Sept. 1, 1993

Toward establishing an international continental scientific drilling program