NUGGETS subsea development in the Northern North Sea consists of 5 gas wells with 40 - 70 km tie-back to the Alwyn platform with1st gas in 2001 and a peak production of 6 MMSCM/D of gas in 2004. Project life was expected to be 10 years with main constraints being methanol requirements for hydrate management and sealine minimum turndown.
Due to increasing water production, the wells were shut-in one after the other and the field was due to be decommissioned in 2010. At that time, minimum recommended MeOH concentration was ˜ 28% (wt/wt) which allowed a maximum water production of 40 Sm3/d. Thanks to a concerted effort to keep gas rates at targets which respected all constraints and reduce methanol use to zero, an additional 2.8 MBOE has been produced from NUGGETS. This represents an incremental recovery of about 2.0%. In addition, the field life has been extended with the possibility of further prospects being tied-in to the existing facilities.
With methanol constraints removed, new issues are subsea system life longevity and reservoir management. Current field operations philosophy is optimised to respect minimum turndown of the subsea line and the minimum gas rate per well with/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 as well as analytical methods were used to investigate the coning.
This paper provides a critical assessment of the methods employed from a flow assurance, a well performance and a reservoir management point of view. It concludes with a set of observations and recommendations for operators of dry gas fields with strong aquifers and long subsea tie-backs.
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