Among the casing failures in 46 ultradeep wells in a particular block, 11 ultradeep wells had the same casing failure forms occur during the drilling of cement plugs. The casing failure forms are local bending deformation and local internal pressure damage, which eventually led to a local burst failure of the casing in the 11 typical ultradeep wells. Therefore, a casing strength checking model is established to investigate the primary reasons for the local burst failure of the casing and the local burst failure mechanism of the casing. A case study is performed on casing in typical ultradeep wells with force conditions to evaluate the safety state of the casing and make necessary recommendations to improve its safety. The obtained results indicate that an abnormally high dogleg (AHD), a decrease in the temperature at the bottom of the casing, and an annulus trap space are the characteristics of typical ultradeep wells, and the local burst failure of the casing is caused by the coupled superposition impact of a decrease in the annulus pressure buildup (APB), the casing temperature effect, and the magnification effect of the casing bending stress. The safety factor of the casing can be increased by changing the operation parameters and the material properties of the casing. To address the casing failure problems during the drilling of cement plugs in typical ultradeep wells, the protection and monitoring of the seal quality of the cement sheath should be improved to avoid expansion of the well diameter and formation of the annulus trap space. The casing strength and the injection temperature of the drilling fluid can be increased, and the drilling fluid displacement can be decreased.

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