The drilling mud weight and the mud composition are two key operational parameters that can be adjusted to prevent hole-instability problems. This paper discusses the methodology and the borehole-stability model employed for optimising the drilling mud weight for the drilling of deviated wells in the northern North Sea. In the course of the project, a shale coring programme was implemented so that the rock strength could be characterised, and leak-off-test data were analysed to estimate the in-situ stress. A comparison is made with field observations

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