Abstract
Abstract The Continental Deep Drilling Program of the Federal Republic of Germany (KTB) is a non-commercial, geoscientific research project that is part of the international lithosphere research.The goals are to investigate the physical and chemical conditions and processes of the earth's deep continental crust. Two wells, the pilot hole and the ultra deep hole, were drilled in the program. The ultra deep borehole reached a depth of 9101 m on October 10, 1994. Drilling conditions are characterised by crystalline rock, borehole temperature exceeding 250 °C and severe borehole instability. The paper describes drilling strategy, equipment and the experience which was made during the operation. Special consideration will be put on borehole stability that became the number one problem with increasing depth.
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