Summary
The nonlinear contact between the bottomhole assembly (BHA) and the borehole can cause backward whirl. During an actual drilling process, the backward whirl will increase wear of the downhole drilling tool and lead to bending or even bulking failure. Therefore, analyzing the drillstring whirl characteristics considering multidirectional coupling effects, combined with field conditions, is a key issue and research hotspot. In this paper, considering the relationships between axial, torsional, and lateral vibrations of the drillstring system, the whirl model is established with finite element method. Research results indicate that with higher driving speed and larger friction coefficient between drillstring and borehole wall, the drillstring lateral vibration changes from forward to backward whirl. On the contrary, the increasing of drilling fluid density can effectively suppress the occurrence of backward whirl. As drilling fluid density increases, forward and backward whirl coexist, and only the forward whirl occurs at last. The research results can provide a basis for formulating whirl control strategy, and promoting the efficient and safe operations of drilling engineering.
Aiming at the current development of drilling technology and the deepening of oil and gas exploration, we focus on better studying the nonlinear dynamic characteristics of the drill string under complex working conditions and knowing the real movement of the drill string during drilling. This paper firstly combines the actual situation of the well to establish the dynamic model of the horizontal drill string, and analyzes the dynamic characteristics, giving the expression of the force of each part of the model. Secondly, it introduces the piecewise constant method (simply known as PT method), and gives the solution equation. Then according to the basic parameters, the axial vibration displacement and vibration velocity at the test points are solved by the PT method and the Runge–Kutta method, respectively, and the phase diagram, the Poincare map, and the spectrogram are obtained. The results obtained by the two methods are compared and analyzed. Finally, the relevant experimental tests are carried out. It shows that the results of the dynamic model of the horizontal drill string are basically consistent with the results obtained by the actual test, which verifies the validity of the dynamic model and the correctness of the calculated results. When solving the drill string nonlinear dynamics, the results of the PT method is closer to the theoretical solution than that of the Runge–Kutta method with the same order and time step. And the PT method is better than the Runge–Kutta method with the same order in smoothness and continuity in solving the drill string nonlinear dynamics.
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