Abstract: To investigate the failure process and damage form of cement sheath integrity under alternating pressure, the experimental results of the stress-strain ontological relationship and mechanical parameters of cement stone under uni/triaxial and cyclic loading are used to establish a three-dimensional mechanical model of the“production casing-cement sheath-technical casing” under alternating pressure. The mechanical response under alternating pressure was analyzed and verified by using full-size cement sheath integrity failure experiments. The CDP and interfacial separation criterion were used to simulate and analyse the cement sheath body integrity and interfacial integrity respectively. The results show that the degree of damage to the cement body increases with both peak pressure and the number of cycles, and the growth rate of tensile damage is higher. After the tensile damage value reaches its maximum value, the compressive damage value continues to increase as the number of cycles increases, and the cracks continue to extend. As the peak of pressure increases, the number of cycles required is gradually decreases for interfacial integrity damage to sprout and its evolution to form a micro-annular gap. The increase in the peak of pressure and the cycle times can both lead to damage to the integrity of the cement sheath, but the impact of the peak of pressure is more significant, directly damaging its integrity, while the cycle times further exacerbate the integrity damage and evolve into failure. The brittle characteristics of cementite and differences in tensile and compressive strengths are key factors in the differences in the rate of growth of damage.

Key words: alternating pressure, cement sheath integrity, XFEM, Cohesive element, bilinear traction-separation criterion