This paper studies helical buckling, and also critical (sinusoidal) buckling, of pipes (tubing and drillstring) in horizontal wells. The large frictional drag of helically buckled pipes is also studied.

Helical buckling is developed from critical (sinusoidal) buckling as the axial load keeps increasing. But fully developed helical buckling of pipes will not occur in horizontal wellbores until the axial load becomes very large, about 1.8 times the critical buckling load that predicts the onset of sinusoidal buckling, and about 1.3 times the so-called helical buckling load that appears in the current literature. The so-called helical buckling load in the current literature is actually the average axial load in the helical buckling development process. This means larger bit weight or tubing packer load may be applied to increase the drilling rate or to ensure a proper seal, before helical buckling of the pipes can occur.

However, once fully developed helical buckling occurs, the frictional drag may become much larger than it was before the onset of helical buckling. The pipe could even become "locked-up" so that the bit weight or packer setting load can not be increased any more by slacking off weight at the surface.

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