Analysis of Drillstring Dynamics Based on Discrete Element Method: New Insights and Model

Summary The modeling and prediction of drillstring dynamics have always been an essential portion of drilling engineering. As an emerging numerical simulation technique in recent years, the discrete element method (DEM) has been widely applied to wellbore stability evaluation and reservoir fracture reconstruction. In this paper, we propose a new framework for analyzing the dynamics of drillstring based on DEM and the theory of Timoshenko beam. First, a 3D Timoshenko beam bond model (TBBM) is derived in the framework of DEM, on the basis of which the drillstring-particle circular hypothesis and Rayleigh damping are integrated to formulate the dynamical coupling scheme. Bit-rock interaction, eccentricity, gravity, drillstring-wellbore contact, and joints are defined within this framework. The numerical accuracy of the TBBM is fully discussed and verified in five different static/dynamic tests, in which the computational error of the TBBM can even be minimized to 0.00116% in some specific scenarios. Simulations under various combinations of weight on bit (WOB) and rotational speed (RS) are conducted for horizontal gas well drilling configurations and the corresponding drilling experiment bench to analyze the kinetic behavior of the drillstring under practical conditions. The deformation, frequency, and stress distribution characteristics of the drillstring under transverse vibration are emphasized. In addition, unexpected stick/slip vibration modes of the bit are observed. The results illustrate that the DEM-based framework for drillstring dynamics provides new insights into vibration detection and potential drilling failures that may be further utilized for drilling tool optimization and real-time drilling control.