Ensemble-based Methods for Well Drilling Sequence and Time Optimization under Uncertainty

One of the main activities in the (re-)development of large hydrocarbon fields is the drilling of new wells. The drilling process may involve a large number of wells and take several years to complete. Challenges include the selection of a limited set of wells from many possible well paths, and the scheduling of the drilling itself. Complications may arise from constraints imposed by platform availability and processing capacity, and from differences in the complexity of well paths that influence cost, time and risk. Several strategies are investigated to find an optimal drilling sequence in the presence of uncertainty. An ensemble-based robust optimization method is employed to maximize the expected value of an economical objective. Parameterizations of the drilling sequence in terms of both priority and time controls are considered. While these controls are continuous, the associated objective function is piece-wise constant, which poses challenges for gradient-based optimization schemes, including those based on perturbation approaches. The impact of optimizer, perturbation sampling scheme, and of control type and mapping are determined. Results show that approaches based on priority controls can deliver significant improvements of the well drilling sequence. Perturbation schemes leading to initially severe but reduced re-ordering later on are recommended.