Abstract：Casing failures are often caused by the rheological load generated by the soft rocks in China，which give rise to large economical loss. Based on the theory of the viscoelastic rheological model，the coupled mechanical model of the soft rocks，the concrete sheath and the casing is developed. Hence，the equations to compute the rheological load of casing and concrete sheath under the uniform in-situ stress are derived according to the analysis of constitutive law，geometric deformation compatibility，and static equilibrium. The software is developed；and the correlation between the casing load and the rheological parameters of the soft rocks，initial creep of the wellbore，mechanical properties of the concrete sheath is obtained. The results indicate that the rheological mechanical properties of the soft rocks，initial creep of the wellbore，mechanical properties of the concrete sheath have large effects on the casing load；and initial creep of the wellbore can reduce the rheological load and decrease the casing stress；however，no concrete sheath in the annulus can undergo the casing service life because the concrete sheath can increase the casing stress. The achieved results are coincident with the field data，which are already used for cementing and casing design in the Jianghan Oilfield，China.
