In the drilling industry the efficiency of the operations depends 'on a considerable number of interacting processes in which the drilling fluid plays a central role'. This was stated by Swanson [1] already more than 20 years ago in the context of conventional drilling methods used for vertical shallow well-bores. Today, due to the increasing requirements in drilling depth and directional drilling, the situation is even more challenging. This is due to high pressure/high temperature (HPHT) conditions, such as in deep geothermal drilling beyond depths of 6000 m. At these conditions, the rheological properties of the drilling fluids change considerably and in consequence, the hole cleaning process is affected. In directional drilling the main challenge is the lack of generalized and reliable models for the transport characteristic of drill cuttings. As a result, cutting beds may form and potentially increase the risk of clogging. Besides this drilling is a dynamic process. For larger time scales this is due to the passage through different geological formations with varying rates of penetration and due to intermitting operations. For small time scales, this is due to the orbit eccentricity of the rotating drill string or geometric entities causing unsteadiness of the flow. Merely due to the transport of drill cuttings additional flow instabilities may occur. In particular in geothermal drilling, these issues have to be assessed in view of the huge financial risk of the entire drilling proj ect. In order to support decision making, simulations based on suitable models are extremely valuable.
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