Summary

The Northern Underwater Gas Gathering, Export, and Treatment System (NUGGETS) subsea development in the northern North Sea consists of five gas wells and a 40- to 70-km tieback to the Alwyn platform, with first gas in 2001 and peak gas production of 6 million std m3/d in 2004. Project life was expected to be 10 years, with the main constraints being methanol (MeOH) requirements for hydrate management and sealine minimum turndown.

Because of increasing water production, the wells were shut in one after another, and the field was scheduled to be decommissioned in 2010. At that time, the minimum recommended MeOH concentration was approximately 28% (wt/wt; MeOH/water), which allowed for a maximum water production of 40 std m3/d. Because of a concerted effort to keep gas rates at targets that respected all constraints and to reduce MeOH use to zero, an additional 4.0 million BOE has been produced from NUGGETS. This represents an incremental recovery of approximately 3%. In addition, the field life has been extended, with the possibility of further prospects being tied into the existing facilities.

With MeOH constraints removed, the new issues became subsea-system-life longevity and reservoir management. Current field-operations philosophy is optimized to respect the minimum gas rate per well with or without water production. It is also aimed to manage water coning in the reservoir. The reservoir has very high permeability with kv/kh ≈ 1 and strong aquifer influx. Moreover, numerical and analytical methods were used to investigate the coning.

This paper provides a critical assessment of the methods used from the flow-assurance, well-performance, and reservoir-management points of view. It concludes with a set of observations and recommendations for operators of dry-gas fields with strong aquifers and long subsea tiebacks.

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