Stress information from deep boreholes

As alluded to in the Introduction, the need to measure in situ stresses in civil, mining and petroleum engineering, as well as in geology and geophysics, has led to the development of a large number of stress measurement techniques. In this section I will only discuss a few of these techniques, concentrating on the methods used in stress measurements in deep boreholes. For a comprehensive review of current rock stress measurement techniques the interested reader is referred to Amadei and Stephansson [1997].

While the methodology for determining the orientation and relative magnitude of in situ stresses is now well-established [e.g. Zoback and Zoback, 1989, 1991, 1980] and has been utilized at literally thousands of sites around the world [e.g. Zoback, 1992], there have been extremely few sites where in situ stress magnitude has been measured at depths greater than 2-3 km, see review in Brudy et al. [1997]. This is in part due to the very few deep boreholes drilled but also because conventional stress magnitude measurement techniques, e.g. overcoring and hydraulic fracturing, are technically extremely difficult in deep boreholes. Recent developments [Brudy and Zoback, 1999, 1993; Peška and Zoback, 1995] in the interpretation and analysis of drilling-induced compressive and tensile failures in wellbore image data have, however, made stress orientation and magnitude estimation considerably easier and also allow for a continuous stress profile along the borehole. The use of an ``integrated stress measurement strategy'' (ISMS) for continuous stress orientation and magnitude estimation was first reported from the Cajon Pass borehole [Zoback and Healy, 1992] and was further extended [Brudy et al., 1997] in the stress analysis of the KTB deep borehole. The ISMS involved a combination of direct measurement of the least principal stress, using hydraulic fracturing, and detailed analysis of extensive drilling-induced wellbore failure. For later reference I have included, see Figure 12, the stress profiles from the Cajon Pass borehole [Vernik and Zoback, 1992] and from the KTB main borehole [Brudy et al., 1997]. An ISMS is utilized in Paper I of this thesis and I will briefly review the new wellbore image data methods here, starting however with the well-established but very important hydraulic fracturing method.

Figure: State of stress inferred from the Cajon Pass and KTB scientific boreholes, both drilled into crystalline rock. Left) Stress profile from the Cajon Pass borehole using the hydrofracturing data of Zoback and Healy [1992] for $S_h$ and the borehole breakout analysis of Vernik and Zoback [1992] for $S_H$, assuming nonporous rock ( $\alpha = 0$). Figure modified from Vernik and Zoback [1992]. Right) Stress profile from the KTB boreholes. Stress magnitudes from hydraulic fracturing in the pilot hole is shown as solid, $S_h$, and open, $S_H$, squares. Between 3 km and 6.8 km, the results from the combined analysis of borehole breakouts and induced tensile fractures have been merged into a solid, $S_h$, and an open, $S_H$, polygon. Below 6.8 km the $S_H$ magnitude is only calculated from tensile fractures. At 7, 7.7 and 9 km km only the estimation for the least possible $S_h$ magnitude is shown. Figure modified from Brudy et al. [1997].