Shale is identified as one of the main factors strongly impacting the distribution of stress in the casing and cement sheath system. The risk of radial cracking induced by thermal stress should not be overemphasized, and it highly depends on fluid temperature in the wellbore and fluid circulation time. Long-term casing and cement integrity is fundamental to the successful and economical development of high-pressure, high-temperature (HPHT) hydrocarbon reservoirs. Casing failure and cement debonding as a result of extreme downhole conditions have been recorded worldwide. Research on casing and cement sheath failure in the past few years has been conducted assuming either plain strain or axisymmetric strain in a homogeneous formation. It usually underestimates casing deformation and cement sheath failure caused by the effect of a nonhomogeneous formation.

We present a three-dimensional (3D) finite element model consisting of five formation layers to simulate casing and cement sheath mechanical response in interbedded, nonhomogeneous formations. The radial stress of cement sheath is found to be highly variable and affected by the contrast in Young's moduli in the different formation layers. Maximum stress concentration is predicted in the casing-cement sheath confined by sandstone. Application of this research is useful for casing design, as well as evaluation of downhole failures.

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