The integrated drill string dynamic model based on the lumped mass method and discrete element method

The dynamic model of the drill string is very useful for studying its motion to improve the drilling speed. The rock breaking efficiency of the drill bit is a crucial factor influencing overall drilling efficiency. Traditional dynamic models face a bottleneck in the development of drill string dynamics, as they cannot simultaneously consider the rock-breaking process of the drill bit and motion of drill pipes spanning hundreds to thousands of meters. Hence, this paper develops an integrated model including the drill string, drill bit, formation, and wellbore. The drill string model is established with the lumped mass method (LMM) and the other parts of the integrated model are established using the discrete element method (DEM). A simulation of the motion of the bottom hole assembly (BHA) in a vertical well is conducted, yielding results that closely aligns with laboratory experiments. This alignment confirms the reliability of the established drill string model. The drilling simulation results conducted with the integrated model under different ground rotational speeds (GRSs) and weight-on-bits (WOBs) demonstrate that the developed model can be effectively used to investigate rock-breaking of drill bit and drill string motion simultaneously. And the proposed model establishes the relationship between drill string motion and drill bit rock breaking.