Abstract
Considering the limitations of the static buckling theory on the eccentric wear of sucker rod and tubing, a new dynamic analysis method for the transverse vibration of sucker rod in the tubing is proposed. Taking the axial distribution load at the rod body and the dynamic load at the bottom into account, the dynamic model of transverse vibration is established based on the space buckling configuration of rod string which regarded as the deformation excitation during the down stroke. To solve the mathematical equations, the finite difference method is used to discretize the well depth, and the Newmark-beta method is used to discretize the time. Meanwhile, a restitution coefficient is introduced to depict the change of velocity and the momentum after the collision. The result shows the phenomenon of rod-tubing collision occurs mainly in the down stroke after the rod string post buckling; the collision force from the wellhead to the bottom increases gradually, of which distributed almost along the entire well depth; and the high frequency collision occurs below the neutral point where the collision force is also the biggest. Further, the collision frequency and the collision force decrease successively from the neutral point to the wellhead direction. But during the up stroke, few collisions occur, and the collision force is also very small. The simulation model is suitable for the eccentric wear analysis of rod-tubing, and provides a new theoretical basis for the optimal allocation of the centralizer.
Highlights
The field statistics show that the rod-tubing eccentric wear phenomenon in the tubing is the main cause of rod-tubing failure and the pump checking in vertical wells
The eccentric wear phenomenon at the bottom or below neutral point has been improved well after centralizers installed, but disappointingly, the position points above the neutral point become the new weakness points
The simulation results about the rod-tubing eccentric wear in the last column in Table 2 is in fully consistent with the experimental results of last third column, the results obtained by the static method in last second column are partly consistent with the experimental results
Summary
The field statistics show that the rod-tubing eccentric wear phenomenon in the tubing is the main cause of rod-tubing failure and the pump checking in vertical wells. When the pressure load of rod at the bottom exceeds the first static buckling critical load, the sucker rod will produce buckling and cause the eccentric wear with the tubing inner wall. Based on the above static buckling theory, the centralizer optimal design method anti-eccentric wear was established and used extensively [8, 9], namely, to install the centralizers below the neutral point of sucker rod string. Considering the radial clearance between the centralizer and the inner wall of the tubing, the buckling configuration of the sucker rod string has been changed after centralizers allocated. In the author’s opinion, the transverse buckling deformation of the sucker rod stimulates the sucker rod to produce the transverse vibration in the tubing and cause the collision friction with the tubing inner wall. The method provides a new theoretical basis for the eccentric wear analysis of rod-tubing and optimal configuration of the centralizer
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