The drilling of extended-reach wells is an increasingly common practice for improving the recovery from mature fields, and for producing distant oil and gas resources using existing infrastructure. From a geomechanical point of view, the drilling and completion of extended-reach wells may become technically very challenging, as these wells often have long sections drilled at high inclinations that can be prone to borehole instability problems such as pack-offs or collapse of the wellbore wall.

The case of an extended-reach well drilled in the Ekofisk field in the North Sea where borehole stability issues were observed and eventually resulted in the loss of the well is presented. A wellbore stability assessment is performed with well-specific stress and formation strength data that explores the possible failures that may have resulted in the loss of the well. In particular, a plane of weakness model is used to model possible shear failure along the bedding of the overburden shale formations. The uncertainty in the rock matrix strength is accounted for, as well as the cohesion, friction factor and orientation of the bedding plane, on the mud window using a Monte Carlo approach. This paper focuses in particular on how the properties of the bedding plane affect the minimum required mud weight, and compare to the actual mud weight used in operation.

The generated mud window acknowledging failure along the weakness planes suggests that this type of failure was a relevant failure mechanism over the 13 1/2-in section of the well, as the mud weight employed was not high enough to avoid it. Accounting for uncertainty and the failure along the weakness planes in extended-reach wells to be drilled in the Ekofisk area may generate safer mud windows that in turn may reduce the occurrence of wellbore instability in the field.

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