Theoretical and numerical study on post-buckling behavior of tubing string in ultra-deep wells

Tubing string will buckle under radial constraint and axial loading, and then contact with the constraint pipe (casing string). The contact between the strings may occur in any position, in any direction, with random and nonlinear characteristics. To analyze tubing string post-buckling behavior, differential equations for helical buckling of tubing string are established, and the relationship between axial force and helical pitch is revealed. A nonlinear buckling mechanical model of tubing string is established considering inner and outer pressure, axial force and tubing-casing contact. Finite element results indicate that tubing string is operating in a very demanding environment. In the case, tubing string from the neutralization point to the packer is in nonuniform or noncomplete sinusoidal or helical buckling. The maximum and average values of bending moment and contact force distributed on the tubing string increase with the increase of axial compression load. The work presented in this paper can provide a technological basis for tubing string post-buckling behavior analysis in ultra-deep wells.