The Ness Field Subsea Development: A Case History
Authors: Starkey M.G. , Dodds Alex 
Published: October 2008

This paper describes the experience gained from operating the Ness field from the start of production in 1987 up to the present It reviews production difficulties due to the waxing and corrosive Properties of the fluid and discusses the performance of downhole and subsea equipment and the horizontal well.

INTRODUCTION

The Ness field is an under saturated oil producing reservoir located in the UK North Seablock9/13b (Fig 1). The Ness subsea facilities are situated in a water depth of 380 ft approximately 3.9 miles west-Northwest of the Beryl, ‘A’ platform and 4.5 miles Southwest of the Beryl ‘B’ platform to which it produces. The initial development was based on the reserves found in the Beryl formation sandstone by the exploration well 9/13b-S28A in May 1986. This well became one of two development wells brought on stream 15 months later on 11 August 1987. In February 1988 the third Beryl formation development well 9/13b-S38Zalso discovered oil in the Heather formation sandstone which overlays the Beryl. The Heather formation started production when well9/13b-S45Z was completed in this zone in October 1989.

The initial Beryl formation development plan called for two producers with a peak production of 20,000 BOPD and water injection to maximise recovery by providing pressure support. Early water production was expected and so gas lift facilities were included. The 1989 material balance results indicated additional untapped Beryl formation reserves warranting a further production well. In 1989 additional flowlines were added to the facilities and S45Z was drilled. S45Z did not find untapped Beryl formation reserves and was completed in the Heather formation. In June 1990 a separate two slot template structure was added to cater for a Heather water inject or and a horizontal Beryl formation producer. Potential now exists to use this existing infrastructure to develop another satellite field. Waxing and corrosive properties of the reservoir fluid have caused significant operational problems. This paper covers the experience to date of producing the Ness field and in particular the Beryl formation. It also provides are view of the performance of some of the equipment used. Finally the lessons learnt from Ness are discussed.

BACKGROUND

Reservoir and Fluid

The maximum oil column at the structural crest of the Ness Beryl formation is 212 ft at 9900 ft TVDSS (Fig 2). It is a homogeneous reservoir with permeability ranging from 200 to 1400 md. The Heather formation consists of a sequence of thin sands and the permeability is of the order of 100 md.

Initially water injection into the Beryl formation was deep in the aquifer. However in 1993 water injection was redirected into the oil leg to provide a waterflood sweep. To date only one of the production wells has seen some seawater breakthrough and sulphate scaling has not been a problem. As expected water coning and water production has been seen. Watercuts started to rise after 3 months and are now above 80%. The aquifer is extremely active and the OWC has moved up to 1O,070ft TVDSS from the original 10,112 ft TVDSS.