Wellbore stability analysis can save millions of dollars by reducing non-productive time associated with borehole failures during drilling. We have developed an integrated approach to evaluate wellbore stability along any arbitrary well trajectory in the presence of weak bedding planes. We present a case study from the Monterey Formation in California, wherein a weak layer overlays the target reservoir and the buildup-section of horizontal wells started in this weak layer. The developed approach is used to show that the weak layer slippage is the main cause of severe drilling problems encountered in the field. The predictions of our wellbore stability modeling are consistent with field observations that modest deviations from a vertical well in the weak layer can result in severe drilling instabilities and also, horizontal wells drilled to the South-West in the field are more prone to borehole instabilities than those drilled to the North-East. Finally, we recommend well trajectories to mitigate borehole instabilities caused by the weak layer slippage.

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