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Technical Paper: Innovative Whipstock Technology/Procedures Successfully Complete Challenging Low-Side, Uncemented Casing Exits: UK North Sea

Society: SPE
Paper Number: 149625
Presentation Date: 2012
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Abstract

Maximizing profitability in mature fields is dependent on reducing drilling and operational expenses to maintain optimized hydrocarbon production.  As Forties field UK North Sea matures, drilling challenges are becoming increasingly more difficult and complex.  Wellbore instability caused by the loss of reservoir pressure and anisotropic properties of overburden shale is a major issue as targets are pushed further away from the platform. 

To continue successful development of Forties field, the operator is required to drill high-inclination deviated wellbores sidetracked from existing boreholes. The unstable shale outside of the sidetrack window requires a low-side casing exit. To optimize operations, the sidetrack must be completed on the first attempt.  When a first sidetrack fails, a second is often initiated approximately 10m (or interval thereof) further up in the wellbore with a higher mud weight. Unable to get more than a few feet away from the original wellbore within such a short distance, the new sidetrack can frequently re-enter the zone already damaged by the previous attempt and again runs into trouble. This broken formation becomes even more destabilized with increased mud weight. 

To solve the operational/economic challenges, a unique wellbore departure system was developed to deliver fast, high-quality windows and sidetracks tailored specifically to meet the operator’s low-side application objectives without compromising performance. The low-side exit requires a unique set of pre-job equipment modifications which is performed in the service provider’s workshop prior to shipping equipment to the well-site. The modification allows an upward force to be exerted at the tip of the whip face on setting the permanent packer/anchor, thereby overcoming the natural gravitational forces. This upward force does not come into effect until the packer is energized, thus ensuring the whipstock assembly remains flexible enough to mitigate wellbore tortuosity encountered whilst running in the hole. 

The system was successfully applied initially on three challenging uncemented whipstock sidetracks with single-trip window success (up to 77° inclination/180° orientation). On all three jobs the anchoring and milling technology worked flawlessly with no issues when subsequently tripping directional BHA or liners through the window. Application engineers performed pre-/post-job briefings with the service provider’s rig site/offshore supervisors to ensure specific low-side exit guidelines were followed, and that lessons learned or suggestions for improvement were captured and documented for prosperity. 

The authors will present Forties field case studies that document procedural repeatability and how the tools and techniques could be used for any challenging low-side uncemented casing exits.

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