Numerical investigation and field verification of the torque and bending moment of drill string for different lateral motion states in the vertical well

Abstract

Severe vibrations during the drilling process are a primary cause of drill string failure and reduced drilling efficiency. However, from a strength theory perspective, material failure results from excessive loads. Among various vibration types, lateral vibration is considered the most detrimental. Therefore, we investigate the force characteristics of the drill string under different lateral motion states using numerical simulations and validate the results against field data. In an ideal vertical well, axial loads generally do not cause drill string failure; thus, this study focuses on two parameters: torque and bending moment. The results reveal that in addition to the commonly recognized forward whirl (FW), backward whirl (BW), and irregular motion (IM) states, there also exist non-revolution (NR) motion and forward whirl without contact with the wellbore (NFW). The torque values increase across these motion states in the following order: NR, NFW, FW, BW, and IM. Unlike torque, the bending moment during NFW is higher than that of FW. BW and IM present the highest risks of drill string failure. While torque increases with rotational speed, the relationship between bending moment and rotational speed depends on the specific motion state of the drill string. Time-domain characteristics of torque and bending moment can provide preliminary indications of the drill string’s motion state. In near-ideal vertical wells, the location of drill string failure can indicate the type of load causing the damage. Comparisons with field data validate the reliability of the simulation results and conclusions presented in this study.