Research on Cement Sheath Zonal Isolation: Experimental Evaluation, Theoretical Prediction, and Improvement Measures

Effective zonal isolation of the cement sheath is essential for maintaining wellbore integrity and ensuring the safe and efficient development of oil and gas reservoirs. As exploration in China extends to deeper and more challenging formations, the cement sheath must endure high temperature, high pressure, and complex stress conditions. Failure of zonal isolation can cause serious safety and operational risks. This study investigates the mechanical behavior, integrity, and interface sealing of the wellbore cement sheath through both experiments and theoretical analysis. To better simulate actual wellbore conditions, a hollow-cylinder cement specimen was designed. Compared with standard solid specimens, the hollow type showed about 20% higher Young’s modulus and higher deviatoric stress under pressure. The twin-shear unified strength theory accurately described the stress–strain relationship. Results showed that under high pressure, the cement sheath can enter a plastic state, leading to microannulus formation and interface debonding due to temperature differences. To prevent mechanical failure, the study recommends optimizing construction pressure, using cement with low Young’s modulus and high Poisson’s ratio, minimizing wellbore cooling, and selecting materials with low thermal expansion coefficients. An interface sealing evaluation model was established based on the stress intensity factor method. Field data and microseismic monitoring verified the model’s accuracy. The analysis revealed that fracturing can cause interface and zigzag cracks, whose growth depends on cement properties and operational parameters. Increasing the interface bonding strength and using cement with higher Young’s modulus and Poisson’s ratio can reduce crack length and prevent deflection. When E+106.496v−54.36635≤0 E+106.496v−54.36635≤0, zigzag crack deflection into the sheath can be avoided. Finally, failure evaluation methods for cement sheath zonal isolation were developed, covering tensile failure, strength failure, plastic deformation, and crack propagation. Application to high-pressure gas and shale oil wells produced mechanical property charts that guide the design of cement sheaths to prevent yielding and crack-induced isolation loss. Overall, this research provides a theoretical foundation for evaluating and improving cement sheath zonal isolation, optimizing mechanical properties, and guiding wellbore construction design.