Effect of Poroelasticity on the Integrity of Cement Sheath in Heavy Oil Wells

摘要

Integrity of cement sheath is critical for safe and effective operation of heavy oil wells. The cement sheath has been generally treated as elastic solid in existing models for integrity analysis. However, it has been evidenced by some recent research findings that cement is essentially a porous solid, and thus poroelasticity effect may be remarkable. In this paper, we established a transient poro-thermomechanical coupling finite element model and investigated the pore pressure and stress evolution within the cement sheath during the heat injection process of heavy oil wells, with the emphasis on clarifying the effect of poroelasticity of cement and formation on the integrity of cement sheath. It is found that temperature rise during heat injection results in considerable pore pressure increase and thus greatly affects the stress distribution within the cement sheath. For the cement sheath between the casing and the rock formation, the pore pressure within the cement sheath decreases gradually with time due to fluid diffusion into the formation. In contrast, the high pore pressure remains almost constant within the cement sheath between two casings, resulting in tensile effective stress within the cement sheath and raise a high risk of failure, which cannot be predicted by existing elasticity model.