Buckling behavior and axial load transfer assessment of coiled tubing with initial curvature in constant-curvature wellbores

Abstract

This paper presents a mechanical equilibrium equation for coiled tubing (CT) with an initial curvature for use in a constant-curvature wellbore, based on the beam–column method. The critical sinusoidal and helical buckling loads, the rules for the contact between the CT and wellbore, and the transmission of the axial force along the length are determined. The proposed model shows that the initial curvature of the CT has a greater influence on the critical sinusoidal buckling than on the helical buckling. In addition, both critical buckling loads increase with the initial angular radian value, and the buckling loads in constant-curvature wellbores are greater than those in vertical wells. The critical buckling loads decrease exponentially with an increase in the borehole curvature radius, which changes slightly when the curvature radius of the well increases to a certain extent. When the CT is undergoing sinusoidal buckling, the contact force between the CT and well is sensitive to the initial angular radian value, axial force, and curvature radius of the well. However, when the CT experiences helical buckling, the contact force is primarily affected by the axial force. When the CT has a small amount of residual buckling, its buckled shape is similar to that of a straight pipe. Thus, the distribution and magnitude of the contact force are similar. Meanwhile, the transmission of an axial force along the length is affected more significantly by the friction coefficient and outer diameter of the CT than by the initial angular radian value and curvature radius of the well.