Abstract: The bending failure stability accrued with the increase of the running load, when the small size casing, which was run into the wellbore under the deep well, encountered the resis-tance. The casing contacted the wellface and the friction load was increased between the casing and the wellface, because of
the casing bends, counteracted some forces, which caused the running loads to decrease at the contact position. According to the typical drilling pipe bend theory and relative experimental results, the calculating model of the running casing encountered the resistance was established, the model is reasonable and reli-able by the analysis and example verification, it provided the theoretical supports for the maximum load.

Key words: running casing, drilling pipe bending, friction resistance