Bespoke Casing Design to Reduce ECD and Enable MPD for HPHT Culzean Project in the North Sea

Abstract

The Culzean field combines both UHPHT reservoir conditions and an unusually narrow drilling window, at the top of the main reservoir, where reservoir pressure lies on a regional ‘broken seal’ rock strength line. This drove a need to find an improved well architecture, to allow production wells to be drilled close enough to the reservoir crest to maximize gas production volumes. The solution: to develop heavy, ultra-high strength, sour service tubulars and use these in a well layout more typical of Deep Water designs than North Sea HPHT wells. Instead of setting a full production casing string, before drilling the target reservoir, a short production casing liner is hung from the fully rated sour service intermediate string, and tied back after the reservoir section has been drilled and a production liner run. This greatly reduces drilling circulating pressure losses in the reservoir section, allowing crestal targets, whilst providing very robust intermediate and production casing strings for long term well integrity. The reduction in ECD combined with the use of MPD technology, enables the drilling of this complex reservoir, in a safe and efficient manner.