Abstract

The need to re-develop one of Shell's mature fields located in the offshore area of the Niger Delta of Nigeria was necessitated by it's drastically declining production performance over the years. The Bonny field which commenced production in 1973, had a peak production oil rate of 40 Mbpd in 1976. This however decreased over the years to its present rate of 5Mbpd. Unearthing the reason/reasons for this decline in production and developing a blue print for resuscitating production in this field is the main thrust of this paper.

Introduction

The acquisition of 3-D seismic data between 1994-1995 in this field having a total of 24 wells and its subsequent interpretation between 1996-1997 necessitated the commencement of an integrated petroleum engineering subsurface review, which was carried out between 1997-1999. Results of this review identified the root causes of the decline in production, and also showed significant increases in the field's hydrocarbon initially in place (HCIIP) volumes/ultimate recovery (UR). The large undeveloped reserves in the field triggered the subsequent Development Concept Selection (DCS) and Field Development Plan (FDP).

A phased development strategy throughout the fields life cycle has been adopted to restore the field to it's optimum production level, enhance utilisation of facilities and maximise the value of additional data gathering exercise, thereby optimising profitability whilst minimising risk. To achieve this, a two-phase development strategy backed by an optimum economic option has been put in place.

The phase one development strategy involves the drilling of 5 new horizontal oil wells and 4 new vertical/deviated gas wells all targeted at only 3 of the 25 hydrocarbon bearing reservoirs in the field. With this, it is estimated that about 95.5 MMstb (about 65%) out of 146.6 MMstb of undeveloped reserves in the field would be developed. The phase 1 development plan further integrates the development of 1388 Bscf of gas reserves in one of the 3 reservoirs. This takes place before 2010.

The phase two development which is less significant will focus on drilling 13 horizontal wells and recompleting 5 existing wells targeted at 11 reservoirs. This will only take place after a comprehensive review of the phase 1 field development programme subsequently.

Field Location and History

The Bonny field is located in OML-11 (Figure 1) approximately 6km northeast of Bonny Crude Oil Terminal (BCOT), in the southern fringes of the Niger Delta, within the Eastern Swamp/offshore Asset Area of Shell Nigeria (SPDC). The field was discovered in 1959 by BONN-2 and to date, 24 wells have been drilled. Twenty-five hydrocarbon-bearing sands have been penetrated in the field between the depths of 5500 ftss and 11050 ftss.

The first 4 wells (1958-1962) were used to explore and appraise the field before a development phase involving 16 wells was embarked upon between 1970 and 1974. Of the 24 wells drilled so far, 22 were completed as oil wells while the last 2 (BONN-23 and BONN-24) were completed as gas wells to provide fuel gas to the Bonny Crude Oil terminal (BCOT).

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