This paper presents the method and analysis of borehole stability prior to drilling the first horizontal well located in Block K/18 of the Dutch Sector of the North Sea. Field data from eight previous wells drilled in the area were analyzed and used to construct a geomechanics model for numerical simulations of potential behavior of the horizontal well under design. The challenge was to drill a high-angle hole through the highly reactive and potentially overpressured Vlieland Shale, then into the partially depleted (6.8 ppg equivalent mud weight) underlying Vlieland Sandstone. Borehole stability analysis provided useful information for well plan and drilling considerations.

The horizontal well was a great success technically and economically. The excellent results can be attributed to prewell planning and good communications between operating personnel and associated service companies. In addition, drilling practices, adopted as a result of the understanding of borehole mechanics, helped overcome problems in borehole instability. Details of drilling and completion operations are included in this paper.

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