Modelling complex multi-branch wells requires a detailed treatment of flow within the well and an accurate description of the flow in the near-well region of the reservoir. Modern geostatistical packages provide high levels of detail – ideally this should be maintained in the near-well region whilst keeping the remainder of the reservoir in a less detailed sense to provide boundary conditions.

We describe a model in which a model of the wellbore flow in terms of pressure, velocity and saturation is solved simultaneously with the reservoir flow equations. Constraints at branches, items of well equipment and well target conditions are included in the formulation, as well as the effects of friction, gravity and fluid acceleration. The resulting couplings are carried through into the main linear solution algorithm for the full system. A simpler well model is used as a predictor for the Newtonian solution of the detailed well model.

Intelligent grid coarsening is used in reservoir regions away from the well of interest. Finer detail near the well may be retained from geological modelling. By running a number of geological realizations costing possible outcomes the benefits and risks involved in various well designs may be assessed. An example is given of the evaluation of cost and risk in the choice of well design.

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