To enable access to unconventional reservoirs of oil and (shale-) gas, geothermal energy and minerals, complex curved boreholes need to be drilled in the Earth's crust. Directional drilling techniques, incorporating down-hole robotic actuation systems called rotary steerable systems, are used to generate these curved boreholes. In practice, however, boreholes drilled with such systems often show instability-induced borehole spiraling, which negatively affects the borehole quality and increases

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... rag losses while drilling. As a basis for controller synthesis, we present a directional drilling model in terms of delay differential equations. Next, the problem of curved well-bore generation is formulated as a tracking problem and a model-based robust control strategy is developed, solving this tracking problem while guaranteeing the prevention of borehole spiraling. The effectiveness of the proposed approach is illustrated by representative case studies for the generation of curved boreholes.