The Valhall field, located in the Norwegian Sector of the North Sea, is a high porosity chalk field characterized by its weak reservoir rock. Global field behavior during depletion includes extensive compaction and associated subsidence.

One highly successful completion alternative, horizontal wells, was, in its early application to Valhall, plagued by the premature failure of the production tubulars associated with reservoir draw down/compaction. Numerical modeling, with subsequent field implementation, has shown, however, that casing with sufficiently low diameter:thicknesses can withstand the rigors of draw down and associated formation loads. Unfortunately, the use of thick-walled casing presents its own difficulties in terms of installing the string (torque/drag) and the subsequent limitations on tool diameter (smart well completions, for example) associated with the stout cross section. Further, a one-size-fits-all doctrine is not optimal, in that the porosity, and therefore strength, of the reservoir rock varies laterally, thus penalizing well designs on the flanks of the reservoir where the chalk porosity is lower and chalk strength is, therefore, higher.

The current paper addresses the above issues by modeling the behavior of the wellbore and surrounding region. Starting with the virgin reservoir, the model considers (sometimes extensive) pre-wellbore depletion, drilling the wellbore, installation of casing and cement, and subsequent draw down. Significant variations to the model include the cross section of the production casing, the quality and quantity of the cement sheath and the perforation pattern. Behavior of these various configurations during subsequent draw down permits them to be ranked according to the life expectancy of the resulting completion. The discussion is enhanced by field results, as several of the completion alternatives have been installed in Valhall wellbores.

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