With the development of offshore oil fields into the middle and late stages, more and more oil wells are facing the problem of disposal. The reliability of the plugging between the cement plug and the wellbore is directly related to the safety of offshore operations and the problem of marine ecological damage caused by oil and gas leakage. Starting from the cement failure mechanism between the cement plug and the casing, consider the heterogeneity of the formation and the magnitude of the ground stress. The finite element software is used to simulate the interaction between the cement plug and the casing with different characteristics, and then judge the risk of sealing failure. Furthermore, the influence of cement plug properties on casing stress and cementation strength is obtained. Study show that the use of high strength, low modulus of elasticity, low Poisson's ratio cement is conducive to prevent cement plug tensile failure. When the local layer is hard, the method of reducing the elastic modulus of the cement plug can be adopted; and when the local layer is soft, the method of appropriately increasing the elastic modulus of the cement plug should be adopted. It has certain guidance and reference significance for the safe disposal of offshore wellbore.

In deep water drilling, the existence of boost line makes the wellbore temperature change violently, and the thermal stress caused by it has a great influence on the strength of the riser. The paper considers the variable mass flow caused by the boost line fluid entering the wellbore during deepwater drilling. Based on the principle of conservation of mass and energy, the paper establishes a mathematical model for transient heat transfer in different regions of the wellbore and formation, analyzes the effect of the displacement of boost line on the temperature field of the wellbore, and calculates the transient stress of the casing under thermal effect. The results show that with the increase of cycle time, the temperature of the inner wall of the riser above the critical well depth first decreases and then increases. The thermal stress of the inner wall of the riser first increases and then decreases to zero, and then gradually increases, and the final thermal stress remains unchanged. With the increase of cycle time, the thermal stress of the inner wall of the riser increases with the increase of the circulating temperature of the inner wall of the riser below the critical well depth, and the rate of increase decreases gradually, and the final thermal stress remains unchanged. As the displacement of the riser increases, the circulating temperature of the inner wall of the riser increases, and the thermal stress on the inner wall of the riser increases. The research results can provide reference for the analysis of the factors affecting the riser stress in deep water drilling.