Evaluation of cement sheath integrity subject to enhanced pressure

Well-cementing (cementation) is an influential stage of a wellbore completion, as the cement sheath is responsible for providing a complete zonal isolation. Therefore, it is of utmost importance to understand the cement mechanical failure mechanisms since well cement failure and interfacial debonding between the cement and casing and cement and rock formations can lead to a barrier failure. During the wellbore lifetime, a cement sheath is subjected to pressure loading variations. This paper demonstrates the results of an experimental-numerical study to investigate the cement sheath integrity after being subjected an enhanced pressure. A constitutive model specifically formulated for the modelling of quasi-brittle materials is applied to the investigation of cement sheath integrity, incorporating both compression and tensile damage mechanisms. Laboratory experiments are carried out to obtain strength properties of cement class G followed by calibration of the model parameters based on the obtained experimental results. A three-dimensional finite element framework employing the constitutive model for cement sheath and a surface-based cohesive behaviour for the interfaces is developed for integrity investigations. The effects of different orientations of in-situ stresses, different stiffness's of surrounding rock, and the eccentricity of the casing within the wellbore on the integrity of the cement and interfaces are investigated. The significance of cement sheath centralisation and elevated risk of cement mechanical failure caused by wellbore operations in anisotropic fields with soft rocks formation were highlighted. Furthermore, the relatively high magnitude of tensile damage (cracking index) within the cement sheath confirms the importance of tensile properties to be incorporated into the constitutive modelling.