Drilling engineering and drilling operation are two steps of the well construction process that are treated sequentially and separately. The interface between those two steps is the drilling program, i.e., a unique solution that has been singled out and analyzed in detail during well planning. However, when an unexpected situation occurs during the drilling operation, the drilling program is of little use to take informed decisions as it does not contain any information about acceptable tolerances.

An alternate approach for passing information between the planning stage and the operational phase is to capture the constraints and margins that should be respected during the well construction. These constraints can be combined with the foreseen uncertainties that are embedded at every stage of the design and engineering process. Such uncertainties are associated with expected available measurements during the drilling operation, but also uncertainties that exist on the modelling side as well as the natural deviations that will take place during the drilling operation. From this point of view, the drilling program turns to be an ensemble of possible solutions, each of them being associated with potential risk levels.

To illustrate how such a method can be applied in practice, the paper explains how the surveying program can be automatically generated to respect anticollision criteria with neighboring wells and acceptable driller's target dimensions. To achieve this, it is necessary to have a very generic well path calculator. Then a well path generator can instantiate hundreds of well path realizations respecting geometrical constraints related to the subsurface environment, e.g., angle of incidence to cross faults, angle of penetration of hard stringers, but also other constraints such as those related to geo-mechanical margins. Wellbore position uncertainties can be evaluated based on a surveying program, and well paths can be nudged to respect anti-collision criteria with neighboring wells. The dimension of the driller's target can also be estimated as a function of the wellbore position uncertainty and its geological boundaries. By investigating multiple combinations of survey instruments for the surveying program, it is then possible to obtain the set of surveying programs that provide sufficient margins for anti-collision and acceptable dimensions for the driller’s target.

The result of the analysis is automatically generated surveying programs and their envelopes of well paths that respect a set of subsurface and directional surveying constraints. One of those possibilities can be chosen as well as an associated well path that gives a large clearance to the constraints. At the operational stage, when unexpected situation occurs, acceptable margins are readily available allowing for deviating from plan in an informed way and without recourse to qualitative judgement.

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