A design criterion is developed to select the optimum bottomhole rotary assembly configuration (drill collar size and stabilizer positions). This allows maximum correction in the hole inclination by changing the weight on the bit while drilling a section of a hole. A computer algorithm is written in FORTRAN code to optimize the bottomhole assembly configuration. This algorithm can be used with any BHA model that calculates forces at the bit under static or dynamic conditions. A case study is presented to explain the design algorithm.


Directional wells are usually kicked off from vertical by some type of bent sub/bent housing and downhole motor. When the hole inclination becomes sufficiently high, drilling may resume with a steerable system or a rotating system.

In conventional rotary systems a packed-hole assembly uses a sufficient number of stabilizers so that no significant change in hole angle occurs. Likewise, proper stabilizer positioning can increase the hole angle via the fulcrum principle, or it can decrease via the pendulum principle (1985). Conventional rotary systems are normally designed to drill holes with a constant rate of change of inclination. However, discrepancies between prediction and field results frequently occur. Therefore, experience and knowledge are necessary tools in the selection of such a drilling system.

Steerable systems are widely used in drilling horizontal and extended reach wells. This is mainly because, drilling direction can be changed in a more controllable way to follow a predetermined trajectory without the need of changing the assembly. The bottomhole configuration of steerable systems, however, creates major torque and drag problems while drilling highly deviated wells in the sliding mode. It becomes difficult to have even torque distribution on the bit which is essential to control tool-face. High drag can limits the ability of maintenance of a constant weight on the bit. This reduces penetration rate and makes it hard to control tool-face as the reaction torque at the bit varies. Furthermore, drag can limit the total horizontal displacement.

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