The borehole stability of the KTB-Hauptbohrung in Windischeschenbach, Bavaria, Germany, with its target depth of 10 km is described down to the depth of 8,715 m. The well is drilled from surface in crystalline rock, mostly gneisses and amphibolites. The types of wellbore instabilities observed so far (breakouts, washout zones, undergauge sections) as well as their causes (lithology, tectonics, in situ-stresses, miid properties) are discussed. The operational impact of the instabilities (changes in mud system, increase in mud weight, vertical drilling strategy, sidetracks) and the countermeasures taken are elaborated.

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