Nonlinear Dynamic Model and Characterization of Coiled Tubing Drilling System Based on Drilling Robot

Abstract

Due to the uncontrollable dragging force and velocity of the drilling robot, the weight on bit (WOB) and rate of penetration (ROP) can not be controlled. So, there is not an application of in the drilling engineering even through downhole robots have been developed for many years. Dynamic characteristics are the theoretical basis for controlling WOB and ROP. In this paper, the power source of the driving force of the drilling robot is found, which is the pressure difference between inside and outside of the drilling robot. The calculation model of the dragging force of the drilling robot is obtained. On the basis, a fluid–solid coupling nonlinear dynamic model (CT-drilling robot-hydraulic motor-bit-rock-drilling fluid) is established. The nonlinear dynamic characterization is simulated by Runge–Kutta method. The nonlinear dynamic model of the coiled tubing drilling system introduces the characteristics of drilling fluid, rock mechanics and ROP for the first time. The effects of robot position, friction coefficient, displacement fluctuation of drilling fluid and WOB fluctuation on the dynamic characteristics of drilling system are studied. It is found that the displacement of drilling fluid, position of drilling robot and friction coefficient are the main factors affecting the dynamics of the CT drilling system based on drilling robot. This paper finds out power source of the drilling robot, which is the pressure difference between the internal and external of the drilling robot. The acquisition of dynamic characteristics will provide a theoretical basis for the control method of downhole robot, rock breaking mechanism based on drilling robot, bit selection and design of bottom hole assembly, etc. It will also promote the development of downhole robots and solve the bottleneck problem of coiled tubing “buckling” and “locking”.