Research on the Damage Evolution Mechanism of the Cement Sheath under Hydraulic Fracturing Cyclic Loading Conditions

During multistage fracturing, cyclic loading can easily lead to cement sheath failure and annular pressure leakage. To clarify the evolution of stress states and their impact on annular sealing integrity, this study applies shakedown theory. The cement sheath is treated as an ideal elastoplastic material, and the Mohr-Coulomb criterion is used to derive an analytical solution for the shakedown limit load. The effects of in situ stress, wellbore geometry, material parameters, and fracturing operations on the cement sheath are systematically analyzed. Results show that the shakedown limit load of the cement sheath is influenced by in situ stress, wellbore geometry, and cement properties. The limit load increases with higher in situ stress but decreases as the diameter ratio increases. Both the cohesion and internal friction angle of the cement significantly enhance its load-bearing capacity. In Well L119, the bottom-hole pressure during fracturing exceeded the elastic limit. When the inner wall pressure reached 123.10 MPa, the cement sheath yielded plastically. The plastic zone nonlinearly propagated from the inner wall to a radius of 242.5 mm. Therefore, cement sheaths in deeper sections exhibit better structural stability, while the sealing integrity of shallow sections should be carefully monitored during multistage fracturing. Optimizing the well design with a diameter ratio of 0.64 and using cement with a higher cohesion and internal friction angle can effectively improve the resistance of the cement to damage and its load-bearing limit. This study offers a quantitative basis for optimizing fracturing parameters and provides practical guidance for wellbore integrity management.