Matching analysis and experimental study of mechanical properties of cement sheath interface

The cement sheath interface is an important component of the wellbore barrier system. A mismatch between the interfaces of the cement sheath may result in leakage. This paper presents a method for assessing cement sheath interface adaptability based on the Dundrus composite parameter method, and the corresponding test flow for evaluating cement sheath integrity is provided for verification. According to the calculation results, stress concentrations are more common at the first interface of the cement sheath. The second interface is relatively unaffected by stress concentration under the same working conditions as the first interface. Stress concentration at the interface can be effectively alleviated by increasing the elastic modulus of the cement sheath or decreasing the Poisson's ratio. However, if the elastic modulus of the cement sheath increases, the cement sheath will be more susceptible to plastic deformation, and the strength coefficient of the singular stress field at the interface will decrease by approximately 10%. As a result, reducing the Poisson's ratio of the cement sheath effectively reduces stress concentration at the interface end. Especially in sandstone formations, the strength coefficient of the singular stress field at the interface can be reduced by 63% at most, resulting in a good interface adaptability. As a result of the above calculations, three groups of cement slurries with different mechanical parameters were selected and tested. As expected, the test results confirmed the applicability of the method. In light of the above results, cement slurry engineers should factor in the stress singularity effect when designing cement slurry systems. To prevent plastic deformation, high-strength and low-elastic cement should be employed, and the Poisson's ratio should be appropriately reduced in order to prevent interface stress concentration, thereby ensuring the integrity of the cement sheath in an increasingly harsh downhole service environment.