Optimal Design Method of Oscillator Installation Position in Sliding Drilling

Abstract

ABSTRACT: In the process of sliding drilling, installing oscillators on drill strings can decrease axial friction and increase wellbore extension effectively in sliding drilling. Traditionally, the positions of oscillators are most determined by drilling experience, which constraints the drag reduction efficiency of oscillators. To overcome this shortcoming, a new optimization design method of oscillator installation position is presented. Firstly, the initial position of the drill bit is determined according to the well structure data, drilling parameters and drilling time. Then, the drill string in the borehole is divided into key nodes according to the joint position. Based on the data of screw induced torque, the action length of screw induced torque is calculated, and the action length of static friction zone is determined. Next, based on the initial conditions and boundary conditions, establishes a dynamic model of drill string by considering the friction nonlinearity and uses finite difference method to solve it, the friction distribution of drill pipe is analyzed, and the friction of key nodes in each distribution area is calculated. Finally, the initial position of the drill bit and the action length of the static friction zone are re-determined, and the friction of key nodes is accumulated. By analogy, the maximum accumulated friction in the static friction area is determined. Based on the oscillator’s propagation distance model and the minimum drag reduction efficiency, the recommended best installation position is calculated, and sensitivity analysis is carried out to evaluate the drag reduction effects of various parameters. The results show that, compared with the calculation results of determining the installation position of oscillator by drilling experience in the past, the model in this paper has a more significant effect on reducing friction and the installation position is more reasonable. The fluctuation of the displacement of drill string and the hook load are more obvious when the oscillators are installed in the preferred position, and the effect of reducing the average friction is significant, which is more conducive to the transfer of axial force. When other parameters are constant, using the recommended positions of oscillators or increasing the number of oscillators can not only beneficial to the transmission of axial force, and can reduce the average friction of the drill string, but also improve the drag reduction efficiency of the drill string, increasing the rate of penetration and increasing the borehole extension distance. The research results are of significant guiding significance for optimal design and safety control of drag reduction oscillators in sliding drilling.