Research on friction characteristics of drill string in whole well section of gas drilling based on finite element method

During gas drilling, the drill string friction is directly related to the safety of drilling and tripping. When the drill string reaches the horizontal section, the friction problem is prominent, which greatly increases the risk and difficulty of trajectory control in the horizontal section. The purpose of this paper is to study the frictional characteristics between drill string and wellbore wall. Firstly, the dynamic mathematical model of drill string in gas drilling is established, a new boundary conditions between wellhead and bottom hole is proposed. Secondly, the governing equation of drill string system is established by using Lagrange equation. Thirdly, the improved Generalized-α method is used to solve the dynamic equation of drill string system. Finally, the effects of weight on bit (WOB) and rotational speed on friction torque of drill string system are analyzed, as well as the effects of different borehole curvature and friction coefficient on friction characteristics of drill string. The findings indicate that as the WOB and rotational speed increase, the lateral motion range and friction torque of the drill string gradually rise; With an increase of borehole curvature and friction coefficient, the friction resistance of the drill string increases obviously. Additionally, it is observed that the average friction resistance of the drill string is greater in horizontal sections compared to vertical and deflecting sections. The average lifting friction on the drill string is less than the lowering friction. Theoretical research plays a crucial role in guiding the optimization of drilling parameters and the implementation of friction reduction.