Banff - A High Angled Fractured Chalk Development For the New Era
Authors: Sykes G. , Abdelmalek N. , Evans N. 
Published: December 2007

Banff is a fractured chalk reservoir in the Central North Sea, UK. The reservoir consists of steeply dipping beds overlying a salt dome piercement with net vertical closure in excess of 3000 ft. Seismic imaging difficulties and appraisal well uncertainty have led the field partnership to embark on a phased development which has been designed to address the key uncertainties associated with full field development. Phase 1 production will commence from two ambitiously designed production/appraisal wells in August of this year via an Early Production System (EPS).

Introduction

The Banff Field consists of fractured chalk and sandstones of Palaeocene to Cretaceous age in a steeply dipping raft adjacent to a large salt diapir in the UK Central North Sea. The field is operated by Conoco (UK) Ltd. and the partnership includes Ranger, Enterprise, Hardy, Amoco and Santos.

Banff was discovered in 1991 and subsequently appraised by a further three wells. These wells have proved that the reservoir holds significant productive potential but reservoir complexities indicate that well positioning, identification of productive zones within the chalks and completion techniques are critical to project success.

The reservoir itself lies at an angle of up to 50 degrees and can be thought of as a trap door allowing the salt diapir to push up from below. This has resulted in an oil column in excess of 3000 ft with the majority of the oil-in-place (OIP) volumes located in the Cretaceous Tor Chalks. The main technical uncertainties include:Imaging of reservoir structure through 3D seismic has proved difficult with major impact on the planned drilling program.Connected OIP volumes.Recovery mechanism.Long term productivity.

Given the need to understand and quantity these critical uncertainties field partners have elected to divide the development into two stages. Phase 1 will consist of a six month Early Production System comprising of a converted mobile offshore drilling unit and a dynamic position tanker. It is intended to produce up to 5.5 MMSTB over the test period through two ambitiously designed wells. Each well has a primary objective of maintaining a long (c.3500 ft) stratigraphically aligned section within the Tor and Ekofisk chalks with secondary objectives designed to improve understanding of the key reservoir uncertainties. Completion and stimulation techniques have been studied and extensive data acquisition and well intervention plans are in place to evaluate and conclude on the above uncertainties. Plans for the Phase 2 full field development will be simultaneously evaluated and optimised in parallel with the Phase 1 activities as data become available.

Geological Summary

The Banff Field comprises a steeply dipping raft of sandstone, fractured chalk and carbonate cap rock reservoirs adjacent to a large salt diapir on the western margin of the Central Graben (Figure 1). Significant volumes of hydrocarbons have been discovered by the 29/2a-6mst, 29/2a-7 and 29/2a-10 wells (Figure 2). These wells have produced oil from the Andrew and Maureen Sandstones, Ekofisk Chalks, Ekofisk Sandstones, Tor Chalks, and carbonate cap rock of the salt diapir. The wells have proven an oil column over 3000 ft long, with the free water level estimated at -7610 ft TvDSS (Figure 3).

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