The dynamic deviation control mechanism of the prebent pendulum BHA in air drilling

The deviation control of vertical well is one of the most challenging problems in the development of air drilling technology. Exploring a new hole deviation control method for air drilling is very necessary due to there is no suitable auto vertical system can be used in air drilling. In this paper, we designed a prebent pendulum BHA (Bottom hole assembly) with a special bend sub and two stabilizers, and used it to control hole deviation in air drilling. Firstly, the dynamic mode for the prebent pendulum BHA was established, in which the nonlinear stiffness of the perbent BHA and the contact features between BHA and wellbore were considered, and the dynamic lateral force on bit was calculated. Then the deviation control mechanism was interpreted based on the motion stability and the dynamic lateral force, the influence of these two aspects on hole deviation control were analyzed. Thirdly, a prebent pendulum BHA with a stable motion and larger dynamic lateral deviation reducing force on bit has been designed, and used successfully in air drilling in Tarim oil field. This means that the deviation control method with the prebent pendulum BHA proposed in this paper is a useful hole deviation control method in air drilling, and the added costs are almost negligible compared to conventional methods.