Buckling failure mechanism of testing tubing string induced by expansion joint piston force in ultra-deep wells: experimental, numerical and design optimization approaches

The complex mechanical environment during ultra-deep well testing operations easily induces buckling failure of the tubing string near the expansion joint, posing significant risks to operational safety. To address this issue, this study established a coupled wellbore temperature–pressure field and tubing string mechanical behavior analysis model, revealing the generation mechanism of piston force loads under the partially stretched state of the expansion joint and their dominant role in tubing string buckling. A case study of an ultra-deep well was used to quantify the influence patterns of parameters such as wellhead pressure, pump pressure, and displacement on tubing string buckling risk. Additionally, a new expansion joint design was proposed. The results indicate that when the expansion joint is in a partially stretched state, an additional piston force load of up to 368.94 kN is generated on the inner and outer cylinders, which is the primary cause of tubing string buckling failure. To mitigate plastic deformation of the tubing string, increasing wellhead annular pressure, reducing pump pressure, enhancing displacement, and decreasing the installation depth of the expansion joint are effective measures. To resolve the issue of excessive piston force in conventional expansion joints, a dual-pressure chamber expansion joint was designed based on force equilibrium principles. Experimental verification demonstrated that this design reduces piston force loads by approximately 50%. This study provides theoretical foundations and technical solutions for the safety design and optimization of testing tubing string with expansion joint, offering critical insights for the development of ultra-deep oil and gas resources.