In the KTB pilot hole in crystalline rock, mechanical instabilities were governed predominantly by structural anisotropies of foliated and graphitic gneisses, and to a lesser extent by anisotropies of the in-situ-stress field. Breakouts were observed mainly in steeply dipping gneiss sections of the upper 2500m, whereas technically induced hydraulic fractures appeared nearly everywhere down to 4000m. Core disking occurred first and abruptly at 3575m, with a change of rock from strongly layered gneiss to weakly texturized amphibolites.
The Continental Deep Drilling Program (KTB) is the largest and most challenging geoscientific venture of the Federal Republic of Germany. It is directed towards fundamental research of the physical and chemical conditions and processes in the deep continental crust with the aim to enhance understanding of the structure, dynamics, and evolution of intracontinental crustal regions. A major objective of the KTB pilot hole was to prove the feasibility of the scientific goals of the Continental Deep Drilling Program. Herein, borehole stability appeared as a central problem for drilling technique as well as for conducting measurement and testing programs.
About 70% of the sections drilled in the KTB pilot hole were seen to consist of paragneisses, and the remaining 30% consist of amphibolites and metagabbros. The paragneisses were characterized by steeply dipping foliation in the upper parts, whereas the amphibolites appeared as competent and only sectionally texturized.
Structural anisotropies were observed mainly in samples of paragneiss and to a lesser extent in the amphibolites, whereas the fine grained and non-texturized lamprophyrs turned out to be nearly isotropical elastic. Failure of gneiss specimens generally took place by shear fracturing along the foliation (Fig. 1). Low compressive strengths were observed on cores, where the joints were covered with chlorite or biotite. The metagabbros, on the other hand, are characterized by relatively high uniaxial compressive strength of more than 100 MPa. The tensile strength of the KTB gneiss rocks is controlled by the direction of loading with respect to foliation. Failure takes place mainly along joints or foliation covered with graphite, chlorite, biotite, or laumontite.
Laboratory investigations conducted by Althaus et aI. (1989) on granitoid specimens under ambient temperatures of up to 300°C in a heated triaxial high-pressure cell have shown that the amount of fluids in intergranular pore space controls the fracture behaviour of the rock. Formation fluids were encountered in the pilot hole first at 3447m as concentrated salinar solutions containing methane and helium gases. At and below that level, the hole has turned out to be remarkably stable.
Main factors for instabilities of the KTB pilot hole were the structural anisotropy of the rock mass related to foliation, layering, cataclasis, and to a lesser extent, the anisotropy of the regional stress field. Cataclastic zones were encountered at 1650m and between 1950m and 2000m, causing jamming problems to the drilling tools.
