The process of designing a 3D wellbore trajectory, in general, takes into consideration only the expected initial and final coordinates, some “drillable” curvatures that can be delivered by the current technologies, and any of the various 3D planning models, each with a particular number of defining parameters. Some of the questions usually disregarded are: which model to take to design the trajectory, which values to take for the defining parameters, what to measure in the process of design, and what should be optimized. The objective of this work is to provide a systematic approach to 3D trajectory design based on torque and drag performance. For this purpose, it is unquestionable that the torque and the drag caused by the trajectory curvature, which are, considering all other variables the same, determined by the trajectory model and its parameters, dominate the discussion. Other things like trajectory length, borehole diameter, tubular good geometry, although influential, are irrelevant in the decision process because if they affect one model, they affect all others. Therefore, the use of an efficient, accurate, and general T&D model is of fundamental importance, and then we are left with the duty of measuring the adequate cost (or loss, or objective) function and optimizing this function. In this endeavor, a fast and accurate 3D stiff analytical T&D solution that allows analyzing a large number and wide range of parameters is of fundamental importance. Armed with such tool, and with the guidelines resulting from this work, the well designer can quickly determine the best trajectory and parameters that optimize the borehole construction and yet reaching the fundamental purpose of the well to be designed. Instead of requiring an extensive experience and/or creative (non-replicating) capacity of the well designer, the process delivers a systematic approach to trajectory design, based on the relevant objective parameters (e. g., minimum T&D, minimum equipment wear, reduced casing, cementing, hole cleaning and pipe sticking difficulties, and so far). To reach this goal a reasonable, but not thorough, understanding of the causes and effects of torque and drag is necessary in order to effectively play with the trajectory parameters. The causes and consequences of wellbore tortuosity is particularly discussed. In the process, several types of trajectory common in the industry are used and compared. To effectively compare the various models, it is assumed that all trajectories (based on the same initial and final conditions) have the same length (measured depth). This is not a necessary condition because different trajectory construction may require different amount of curvature control, which affects its cost. The T&D model itself is not covered because it has been discussed in another publication; however, a brief discussion is presented in the Annex 2. Any appropriate model serves the purpose although the requirement of being fast, accurate, 3D, and using a stiff model is necessary. Flexibility and efficiency on how the data are entered and changed are also important to a successful, efficient analysis.

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