Dynamics characteristics of axial–torsional–lateral drill string system under wellbore constraints

The combination of air and liquid drilling technologies is usually used for increasing rate of penetration (ROP) and protecting reservoir in drilling engineering. Meanwhile, dynamics characteristics of drill string system are closely relevant to drilling safety and well trajectory quality in the practical drilling engineering. Hence, in this paper, dynamics characteristics of axial–torsional–lateral coupled drill string system under wellbore constraints are emphatically concerned in the air and liquid drilling process. Firstly, a rotor dynamics model for revealing the dynamics characteristics of bottom hole assembly (BHA) with single stabilizer is established by the lumped parameter method. Moreover, two novel contact models for describing real-time interactions of BHA-sidewall and bit-formation are established through Hertzian contact theory and discontinuous support model, respectively. Then normal, stick–slip and bit-bounce modes of BHA are discussed on basis of coupling vibrations of drilling string in lateral, axial, torsional and whirling motion. Meanwhile, the dynamics responses of the BHA in the processes of air and liquid drilling are conducted by contrast analysis. Subsequently, dynamics behaviors of the BHA at different hardness formations are further revealed through numerical computations. The results indicate that the lateral, axial and torsional vibrations of BHA in the air drilling process are obviously stronger than the liquid drilling process in the same operation conditions. When drill bit suddenly encounters hard formation, especially in air drilling process, the dynamics characteristics of drill string system are easily disturbed, and the coexistence motion of stick–slip and bit-bounce is appeared in BHA, thereby leading to severe whirling and lateral impact of drilling string. Besides, when the BHA is out of contact with the sidewall, only the forward whirling of BHA is implemented; when the sidewall is frequently impacted by the BHA, forward and backward whirling of drilling string are easily happened; when the BHA is contact with the sidewall, the backward whirling of drilling string is easily appeared. Therefore, the proposed model in this paper may offer a comprehensive understanding for dynamics of BHA, and is expected to further develop dynamics studies of the BHA with double or multiple stabilizers.