The HPHT well, A-16A, was planned to test a certain part of the Kvitebjørn field in the North Sea for hydrocarbons in order to prove sufficient reserves to justify a field development. Drilling fluid selection and optimization in the planning phase was considered to be one of the key factors to be able to drill the pilot slim hole section through the Deco/Brent and Cook formations which brought challenges above the standard Kvitebjørn wells due to the risk of high depletion combined with high initial pressure.

The well was planned with a pilot hole, A-16, in order to test the drillability of the overlying strata and prove the absence of highly depleted sand formations. It was important to penetrate all possible depleted zones in the pilot well to verify that sidetrack can be drilled to produce from the Statfjord target. Oil based drilling fluid weighted with treated micronized barite (OB TMB) together with a wellbore strengthening approach was successfully implemented to achieve the pilot well goals.

Managed pressure drilling (MPD) and rig assist snubbing (RAS) equipment were rigged up and the 5 ¾-in pilot section was drilled through a 7-in temporary upper completion (TTRD) set in 9 7/8-in casing. MPD and RAS technologies were used in order to control the bottomhole pressure accurately and to ensure that the additional two barriers were maintained in the well at all times, even if the primary fluid barrier was lost due to crossflow.

The paper provides detail about the drilling fluid planning and execution for the pilot slim hole section; this includes the fluid design and work performed to select and optimize wellbore strengthening materials (WSM) package. The WSM package was optimized by sophisticated, formation fractured, laboratory tests based on the fracture size estimated by proprietary software. Initial formation losses observed while drilling were cured with the particles kept continuously in the fluid, which eliminated the use of extra lost circulation material (LCM) pills or cement slurries. The section was then drilled with more than 500 bar (7252 psi) hydrostatic overbalance (including MPD back pressure), with no further formation losses and without differential sticking incidents while taking pressure points in the extremely depleted zones.

The interval was drilled through extremely depleted formations with the highest overbalance drilling in the operator's experience. Superior integrity of the WSM prevented losses and minimized fluid treatments; this reduced the overall costs with minimal logistics. Low density (~1.5 specific gravity) WSM prevented particle settling in the well or in the drillstring and within the surface equipment, which proved the reliability of the design for MPD sections.

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