Creep, the time-dependent deformation of rock, will increase the pressure applied on the interface between the cement and formation. The objective of this paper is to study the influence of the formation creeping effect on the cement sheath integrity and zonal isolation. It focuses on the failure behavior of the cement sheath in the long period after drilling. The paper also investigates the changing of mechanical properties of cement to avoid loss of zonal isolation.

The interface pressure between the cement and formation cannot be measured directly in the field, so it will be valuable to predict this pressure through alternative methods. A Casing-Cement -Formation System (CCFS) analytical model based on linear-elasticity and Cam-Clay plasticity model was built. The CCFS model includes four layers, casing layer-cement layer- plastic creeping layer and the formation layer. This plastic- transition layer is formed because of formation creeping. The axial stress and tangential stress distribution of the cement sheath were calculated by the CCFS model. The contact pressure between the cement sheath and formation was calculated. Mohr-Coulomb yielding criterion was applied to estimate failure behavior for the cement sheath.

Two case studies were performed with the new CCFS model and previous CCFS model that do not consider the formation creeping effect. The comparison between two models showed that without considering the formation creeping effect, we might underestimate failure of the cement sheath. The simulation result by our CCFS analytical model indicated that the creeping effect would make the interface between the casing and cement vulnerable to shear failure. We changed the Young's modulus and Poisson's ratio for the failed case to investigate the influence of mechanical properties of the cement material. The result showed that a lower Young's modulus and higher Poisson's ratio were preferred for improving zonal isolation.

Instead of pursuing how creeping happens, this paper accepts formation creeping as a fact in the whole life of the well. The geomechanical impacts of the plastic-creeping formation, although undetectable from the surface observations, may cause detrimental consequences to cement integrity.

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