Abstract

The Otter field in the Northern North Sea has been developed using three subsea production wells each equipped with dual electric submersible pumps (ESP). Dual ESP were selected as they will maximise well availability and minimise operating costs associated with workovers. The field is located 21 kilometres from the host platform, making this development the longest subsea tie-back with ESP completed to date.

The equipment selected for the ESP system is described, including downhole, subsea and the topsides equipment. A rigorous testing programme was performed to prove that over such a distance from the variable frequency drives ESP could be successfully operated, controlled and data received.

The experience gained during the development will serve as a guide to future long step-out or deep-water subsea developments by identifying critical components and the operational philosophy developed for the Otter field. Factors limiting the development of fields at greater step-out are discussed.

Conclusions can be drawn on the steps required to implement a successful development and some of the pit falls to be avoided.

Introduction

The Otter field lies in the Northern area of the Northern North Sea on the edge of the Viking Graben area (see Figure 1). The field is operated by TotalFinaElf exploration UK plc on behalf of the joint venture partners Dana Petroleum (E & P) Ltd, Esso Exploration and Production UK Ltd and Shell UK Ltd.

The field was discovered in 1978 by Phillips Petroleum, however the field remained undeveloped until 2002 due to the limited size of the discovery and technical challenges. Operatorship of the field was acquired by Fina Exploration in 1997 and at this time interest in developing the field was renewed. To prove reserves and well deliverability a delineation well, 210/15a-5, was drilled and tested in 1998. The results of this well were encouraging and a development screening study was launched. A further delineation well was drilled in 2000 to confirm a field extension to the North prior to the launch of the development project.

The hydrocarbon reserves in the Otter field are found in faulted compartments of the Middle Jurassic Brent sequence, with the top reservoir at around 2000m below sea-level. The reservoir has good porosity and excellent permeability, with horizontal wells able to produce in the range 15-20,000 bbl/d. The reservoir fluid was under saturated at initial conditions with a gas/oil ratio of 450 scf/bbl, and contains low levels (<0.3%mol) of carbon dioxide and traces of hydrogen sulphide. The reservoir was initially normally pressured, approximately 62 bar above the bubble point. Wells flow during initial stage of production, but require artificial lift once water cut increases or the pressure depletion occurs.

Development Strategy

The development consists of three production wells each equipped with dual ESP and two water injection wells to provide pressure support. A full description of the Otter development can be found in Ref 1.

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