Tosi, Gianbattista (Eni) | Stensland, Dag (Eni) | Winterton, David (Eni) | Zappalorto, Luigi (Eni) | Wang, Haifeng (Schlumberger) | Dupuis, Christophe (Schlumberger) | Denichou, Jean-Michel (Schlumberger) | Horstmann, Mathias (Schlumberger)
Goliat was the first oil discovery in the Barents Sea and holds 174 million barrels of recoverable oil. It is operated by Eni Norge (65% share, with Statoil holding the rest of the equity). Finding oil is important, but safely and efficiently developing it is critical. The remoteness of these frontier projects amplifies the consequences of any delays or issues. Risk prevention and mitigation is the object of particular attention in this new frontier area. Landing the first producer was a particularly sensitive operation, requiring a safe stop a few meters above the top of the reservoir to reduce the risk of landing the section within the reservoir gas cap bearing sandstone.
A geometrical landing would be limited by a depth uncertainty of more than 10 m true vertical depth (TVD): relying on surface seismic information is subject to seismic time-to-depth conversion and the inherent limited resolution of seismic data. Taking these uncertainties into account normally requires setting the casing long in advance of the reservoir top, giving way to a much longer portion of the reservoir section exposed to the overburden shales, with consequent 8 ½’’well bore instability issues and risk of plugging the completion screens.
For the Goliat well, the operator adopted a different approach to land the objective while preventing the risks of setting casing too soon or too late. A new ambitious objective was set: stopping and casing as close as possible to the reservoir, but no closer than 5 m TVD. This could only be done by using the latest generation of deep directional resistivity (DDR) logging-while-drilling tools in the 12 ¼" section, increasing both the precision and the accuracy of the landing by relying on a direct detection of the reservoir top before drilling into it.
The DDR real-time automatic inversion of the subsurface layering revealed the top of the reservoir from 19 m TVD below the bottom hole assembly, a new record. By tracking the top boundary, even at a steep inclination near 70°, the operator confidently stopped drilling when the bit was 6 m TVD above the top of the reservoir, as planned, safely minimizing the distance to be drilled in shales before intersecting the reservoir. The distance to the reservoir was verified in the next section drilled.
The use of DDR for landing wells accurately either above or just below a top reservoir is now a proven powerful option for drilling programs in which risk prevention is required at the top of the reservoir.