Subsurface staff have tried to include uncertainties in their predictions for some years (e.g. Smith, Hendry and Crowther1). Unfortunately, these are not normally carried through to the development design because of the time required to engineer all the possible solutions. This paper describes a systematic process for incorporating the uncertainty, using the optimisation tool described by MacGowan and Goodwin2.

In this method, optimum facilities are designed for several hundred reservoir configurations. These can be analysed to show which of the sub-surface uncertainties are responsible for major changes to the development. In turn, this information can be used to shape any proposed appraisal programme.

The sub-surface representation adopted by MacGowan and Goodwin is described in more detail. It is shown that the representation produces profiles which are in excellent agreement with profiles generated through full simulation. Given the level of reserve uncertainties, it should be possible to derive an adequate sub-surface representation for any other reservoir process, not just depletion of dry gas.

These ideas have been applied to the proposed District 3 development in the Algerian Sahara. The results have influenced the exploration and appraisal programme. In addition, they have provided tremendous insight into how uncertainty will affect the development.

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