In the process of coiled tubing running in, due to the influence of the gravity of the string itself and the friction between the string and the well wall, the string changes from the initial approximate linear state to the curve state when it is under pressure, which is the buckling of the string.
In the process of coiled tubing running in the well, it may produce longitudinal bending deformation and damage. When the coiled tubing enters the well, in order to overcome the resistance, it is necessary to apply axial pressure on the surface of the tubing. When both ends of the coiled tubing bear pressure load, the condition is a slender rod without transverse support. When the pressure exceeds the critical load, it will cause the longitudinal bending of the tubing. The coiled tubing first becomes a sine wave with unequal liquid distance in a single plane, and finally becomes a spiral shape with the increase of axial pressure. The coiled tubing is bent into a spiral shape, which causes additional radial contact force and increases the friction between the tubing and the wellbore. The larger the axial force is, the greater the friction force is. A vicious circle is formed at this point, and any additional force added will be lost due to the friction at this point. The locking of coiled tubing in the well is called screw locking.
Coiled tubing bends can occur in any well section (vertical, horizontal, or deviated). However, the critical compression load is different in different intervals.
Due to the influence of many factors on the bending deformation of the string in the well bore, it is necessary to simplify and assume the stress of the string and the string when the project allows.