Analysis of Horizontal Casing Integrity in the Valhall Field

摘要

The Valhall field, located in the Norwegian Sector of thernNorth Sea, is a high porosity chalk field characterized by itsrnweak reservoir rock. Global field behavior during depletionrnincludes extensive compaction and associated subsidence.rnOne highly successful completion alternative, horizontalrnwells, was, in its early application to Valhall, plagued by thernpremature failure of the production tubulars associated withrnreservoir draw down/compaction. Numerical modeling, withrnsubsequent field implementation, has shown, however, thatrncasing with sufficiently low diameter:thicknesses canrnwithstand the rigors of draw down and associated formationrnloads. Unfortunately, the use of thick-walled casing presentsrnits own difficulties in terms of installing the stringrn(torque/drag) and the subsequent limitations on tool diameterrn(smart well completions, for example) associated with thernstout cross section. Further, a one-size-fits-all doctrine is notrnoptimal, in that the porosity, and therefore strength, of thernreservoir rock varies laterally, thus penalizing well designs onrnthe flanks of the reservoir where the chalk porosity is lowerrnand chalk strength is, therefore, higher.rnThe current paper addresses the above issues by modelingrnthe behavior of the wellbore and surrounding region. Startingrnwith the virgin reservoir, the model considers (sometimesrnextensive) pre-wellbore depletion, drilling the wellbore,rninstallation of casing and cement, and subsequent draw down.rnSignificant variations to the model include the cross section ofrnthe production casing, the quality and quantity of the cementrnsheath and the perforation pattern. Behavior of these variousrnconfigurations during subsequent draw down permits them tornbe ranked according to the life expectancy of the resultingrncompletion. The discussion is enhanced by field results, asrnseveral of the completion alternatives have been installed inrnValhall wellbores.