Reducing costs, quantifying uncertainty and assessing risk with minimum data are important issues in the appraisal of discoveries. Here there is a trade-off between the benefits of appraisal drilling to reduce reservoir uncertainty and the consequent increase in costs.

The principal technology issues include:

• reducing uncertainty with respect to trap boundaries and volumes in advance of development project sanction (see also Section 5.2);

• identification and location by remote sensing of fluid phases over the trap volume (Section 5.2);

• improved integration/optimisation of available reservoir data, together with that from appropriate analogues, to permit quantitative handling of within-reservoir uncertainty by geoscientists and production engineers in their assessment of development economics and production system design (see Section 5.2);

• reduced costs/increased effectiveness of drilling programmes when direct intervention in the reservoir is required (see also Section 5.3).

Until recently, prospect appraisal was considered by E&P companies as a distinct business process intervening between the drilling of a successful exploration well and the sanctioning of a field development project. However, several factors have changed the status of appraisal. The first is the increasing use of 3D seismic in advance of exploration drilling. The second is the option of extended well tests which provide both information to reduce uncertainty in reservoir performance and early cash flow from the hydrocarbons produced under test. The third factor is the option now offered by sub-sea technology of staged development, in which early production can be achieved within the context of a flexible, progressive development strategy. Many traditionally distinct, sequential activities conducted between discovery and first production are now often conducted concurrently.