According to literature, most failures of coiled tubing are classified into four kinds of causes, mechanical damage, corrosion, string manufacturing, and human error. Among these, the failures due to mechanical damages take 29% of all failures. Although there are various kinds of mechanical damages, longitudinal plowing marks (LPM) are the major mechanical damages, account for 46%. Understanding the mechanism of LPM damage on coiled tubing surface will help to reduce this kind of damage.

A conjecture is proposed that the running of the CT-gripper block-chain system will produce a period excitation which arouses CT string system resonance, and the vibration of CT string may make slippage between the gripper blocks and CT string at the wave peak and stop slippage at the wave trough, which may form fish scale damage. Because of the mismatch of the diameters between CT with grown diameter and gripper blocks, the first principle stress on the surface of CT will be large enough so that rock debris can carve CT easily when a gripper block is clamping it. FEM is employed to calculate the stress of CT when it is in this situation. The results verify the guess.

To determine the CT string system resonance, we tried to find which gives the system a period excitation and how the system resonates. First a mechanical model of CT-gripper-chain is build, and excitation frequency is found to be related with the velocity of CT string running and length of gripper block. After idealization, a dynamic model for CT string in vertical well is built, and the wave equation is established, then frequency equation is derived and natural frequencies are found. After that, finite difference method is used to conduct numerical calculation. The results show that when system resonates, it must take minutes to reach large stress amplitude of the CT string enough to make damage on it. The field recorded data indicate that there maybe exists resonance actually.

After synthesizing the information above, an image of the process how LPM is formed has been constructed. Finally, the measures to reduce LPM damage are recommended.

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