Wellbore stability analysis has become a standard activity in well planning in our industry. Pragmatic methodologies have been developed that makes the wellbore stability analysis accurate with few input variables and simplistic models. This approach requires careful calibration of models to offset well data (both drilling data and log data). The typical sequence in field developments is that one often starts out with the low-risk wells first, typically the wells with low wellbore inclination (in a normal faulting stress regime as typically found on the Norwegian Continental Shelf (NCS)). Then as the field matures one typically starts to drill more complex wells with higher and higher inclination and longer and longer hole sections. In these high angle wells the mud weight requirement to provide a stable hole increase. This again results in a narrower safe operational mud pressure window between the collapse pressure of the well and the fracture gradient and the minimum horizontal stress. In addition to the narrower safe operational mud pressure window in the high angle wells, the hole cleaning, the transport of drill cuttings, becomes more challenging. In addition, in high angle wellbores one reduces the attack angle between the shale bedding and the wellbore axis and therefore one may have to take this effect into account, which will require more advanced wellbore stability calculations with more input data. In addition, in high angle wells gravity will be more effective on liberating in-situ fractured material into the wellbore. In this paper we investigate a well section which resulted in severe unexpected drilling challenges as the well tested higher inclinations in the field than tested before. This paper does only address the rock mechanical analysis of the data in a larger cross-functional root cause analysis. In this case we selected to go for a more conservative approach using a weak plane model, although the root cause of the problems does not seem to be rock mechanics related.

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