Failure Analysis and Quality Inspection Experimental Study on Special Thread Casing

In petroleum engineering, casing installation is a crucial step in well cementing, primarily involving the threading and lowering of the casing into the wellbore. If the casing detaches after being set in the well, it directly impacts subsequent operations and can even lead to well abandonment, resulting in significant financial losses. However, incidents of casing disconnection frequently occur during actual operations. To investigate the causes and mitigate future risks, a failure analysis of the disconnected casing is required. Taking a specific disconnection incident as a case study, this paper establishes an analytical investigation method for casing disconnection incidents, examining both casing quality and construction process factors. It also conducts experimental research on casing failure. The experimental results show that the material and physicochemical properties of the casing product, including non-destructive testing, tensile and impact performance, hardness, geometric dimensions, metallographic structure, thread parameters, make-up and break-out tests, and physical performance (tensile resistance) all meet specific technical conditions and relevant API Spec 5CT standards. The cause of the thread slippage failure is attributed to misalignment between the external thread of the casing and the coupling during make-up at the construction site, leading to severe deformation from compression on the guide surface. Additionally, the external thread end was at least 10mm from the shoulder face, indicating incomplete thread engagement, ultimately causing the casing to disconnect and fall into the well.