Planning of reservoir sections of wells in faulted and stratified reservoirs is often complicated by the large uncertainties involved. Incomplete knowledge of geological factors and the performance of equipment and operators gives rise to uncertainty with respect to operational outcomes throughout the construction, installation and operation phases, which in turn influence the project cash flows and eventually the overall profitability of the well project. The aim for high production rates promotes technologically ambitious well designs, i.e. penetration of a high number of reservoir zones or an otherwise challenging section trajectory. Efforts for obtaining smooth and safe operations, on the other hand, motivate simpler concepts. Complicated causal relations and lack of methods for quantifying uncertainty makes risk-based optimisation in the well planning process difficult. This paper describes a modelling approach aiming at improving the basis for decision-making by analysis of the uncertainty involved. Risks identified in a review of experiences from a field on the Norwegian continental shelf are presented. Objectives for a risk-based decision tool are discussed and a modelling framework outlined. Examples of detailed modelling for analysis of selected phenomena, depending on proper pressure control in the well construction phase, are described.

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