High Pressure/High Temperature Production: Completing the Process Efficiently
Authors: Hull T.L. 
Published: October 2008

The Elgin and Franklin reservoirs contain large quantities of sour gas condensates under abnormally high pressures and at high temperature. This paper describes the handling, offshore treatment and transportation of the production from the fields through a common offshore production facility. The selected development concept incorporates substantial offshore processing which delivers gas of a quality to meet a demanding commercial gas specification. This extensive process combined with the large flow rates available from the two fields, peak production rates of 14.6MM sm3/day gas and 175,000 bpd oil, results in a very large, complex offshore production facility. A new build jack-up production platform provides the support for the required facilities.

After describing the development concept, the rationale for selection of the field development architecture, the offshore process and jack-up platform in particular, is discussed.

Introduction

The Elgin and Franklin fields are located in close proximity (only a mile or so apart at the closest extremeties) in the Central Graben area of the UK North Sea, about 100 miles offshore and due east of Aberdeen. Water depth is modest; about 90 metres. Both reservoirs are deep, about 5500 metres, and are thus at a high temperature, about 200°C, although at a normal depth gradient. The reservoirs are, however, under abnormally high pressures of about 1100 bar.

As discussed in associated papers the combined reserves, gas condensates, are large - about 750MM BOE. Sales gas reserves are about 50 BCM and liquid reserves (including oil and NGL products) are about 430MM bbl.

Reservoir fluid compositions are indicated in Table 1. Both fields are rich in liquids, but are sour. The Elgin field is richest, containing about 11 bbl/MCM. Franklin contains about 6.6 bbl/MCM.

The substantial size of the Elgin/Franklin reserves combined with the properties of the reservoir fluids dictated that a strategic perspective be undertaken in considering development alternatives. Consequently a holistic approach considering production, processing, transportation to end users and marketing strategies was taken. This overarching perspective has led to a field development architecture that fully processes the produced fluids offshore with gas being exported at commercial sales quality.

The nearby Shearwater field, Shell operated, was also being considered for development contemporaneously. Joint development options with Shearwater were evaluated and common gas and oil export sustems were implemented.

Description

The location of Elgin/Franklin fields and their pipeline export systems are shown in Figure 1. The offshore field architecture is illustrated in Figure 2. Wellhead platforms in each field are connected to a common production, utilities and quarters (PUQ) platform. Well flowlines are manifolded on their respective platforms. The commingled production from each is piped to the PUQ for treatment. Both wellhead platforms are classified as not normally manned. The Elgin wellhead platform is connected to the PUQ with a 100-metre long bridge. The Franklin wellhead platform is connected to it through a subsea system consisting of a 42" pipeline bundle (containing 2-12" lines) and a control/power umbilical.