In many cases the casing exit point of a CTD sidetrack may have to be established below the cap rock to avoid high pore pressures and thus excessive mud weights. If the target is attic oil updip from the mother well, it may become necessary to drill a tangent section with very high inclination.
In the case of A-19A on Gullfaks field a tangent section with 125 degrees inclination had to be drilled with the TD ten meters shallower than the casing window. The challenging wellpath was successfully drilled and is likely to increase the number of achievable targets for CTD in the Gullfaks field.
Focus in the planning phase was centered on friction and forces available and thorough simulations were performed by the contracting company. The conclusion was that the well could be drilled if the friction factor in open hole was no more than 0,4. The experience from first CTD sidetrack in this field was that the friction factor in open hole was less than 0,4 when tripping. However, differential sticking seriously impeded weight transfer while drilling.
A solids free drilling fluid was regarded as the best choice for improving weight transfer. No problems were observed due to differential sticking. Priority was also aimed at investing the necessary time for frequent wiper trips to enhance hole cleaning.
The planning and observations of the operational challenges encompassed during this CTD operation are discussed further in the contents of this paper.