A Case Study of Integrated Gas Field System Modelling in the North Sea Environment
Authors: Deutman Robert , van Rijen Mike 
Published: December 2007

Abstract
The recent deregulation of the UK gas market has complicated the reservoir management of North Sea gas fields. In addition to the traditional long-term, field-specific contracts with British Gas, producers may now choose to enter into multiple short- to medium-term contracts with multiple buyers. These new contracts are no longer necessarily linked to specific fields. Instead, sellers may choose to produce from a portfolio of gas fields.
The Southern North Sea is an established gas province containing extensive infrastructure linking mature gas fields to onshore processing plants. Small prospects and Near Field Potential need to be evacuated using the existing facilities in order to be economic. Possible back-out effects on existing fields need to be modelled rigorously. Integrated subsurface and surface modelling is considered essential to manage a cluster of fields in the rapidly changing gas market.
In the Greater Sole Pit basin, a number of gas fields share capacity in common production facilities. A case study is presented in which four full field subsurface models and a tank model were linked to a detailed model of the Sole Pit production facilities. The resulting integrated model uses Shell's Gas Field Planning Tool simulator to provide accurate production profiles for all Greater Sole Pit fields under a range of future scenarios.
The paper concludes with examples of business applications for the integrated Greater Sole Pit model. These include optimisation of compression facilities and the screening of new field development options.

Introduction
The UK gas market has gradually been deregulated, leading to a competitive environment in which various sellers are seeking to increase their market share. Traditionally, North Sea gas was sold to British Gas, the monopoly buyer, through long-term contracts linked to specific fields. Currently, a growing proportion of gas is sold to multiple new buyers through short- to medium-term contracts which are not necessarily linked to a specified single field. The deregulated market poses significant changes and challenges to the reservoir management and planning of fields in the high-cost, high technology North Sea environment.
In the Greater Sole Pit area, Southern North Sea, several Shell operated gas fields share common production facilities. A number of small prospects and Near Field Potential have been identified in the area. In order to produce these economically, they will also need to make use of the existing facilities. The modelling of possible back-out effects on existing fields, and of remaining capacity in the facilities becomes increasingly important. Therefore, Shell is moving away from its established integrated gas field modelling tools to a new generation of gas system modelling software, having more sophisticated and more flexible modelling capabilities.

The Greater Sole Pit Area
Shell U.K. Exploration and Production acts on behalf of Shell and Esso and other coventurers as operator for a group of gas fields, collectively known as the Greater Sole Pit fields, located in the Sole Pit Basin in the Southern North Sea (see figure 1). The first discovery in the area was made in 1966, but development did not start until 1990 when the Barque and Clipper fields came on stream. Currently there are four producing fields in the area: Barque, Barque South, Clipper and Galleon, with a combined GIIP of some 8 Tscf. Development of three more fields (Skiff, Ensign and Clipper South) is under consideration.
All fields produce dry gas from the hydrostatically pressured Rotliegend formation, a thick package of tight Aeolian sandstones with gas columns of up to 1000 ft. The recovery process is natural depletion.
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