Evaluation Method for Tubing String Failure Risk in Ultra-Deep High-Temperature and High-Pressure Gas Wells

Under the extreme conditions of ultra-high temperature and high pressure (HPHT) at the bottom of ultra-deep wells, the inherent performance of tubing strings deteriorates, and the loads they endure become increasingly complex. These factors pose significant challenges to maintaining tubing integrity. To address the risk of tubing string failure in ultra-deep HPHT gas wells, this study proposes a quantitative evaluation method based on reliability theory. The approach systematically incorporates uncertainties in actual tubing performance parameters, the effects of elevated temperatures and corrosion-induced strength degradation, and the complexity of in-situ loading conditions. Evaluation results reveal that the combined influence of high temperature and corrosion substantially weakens tubing strength, thereby elevating the risk of failure. Key factors affecting tubing failure risk include temperature, corrosion rate, steel grade, and manufacturing tolerances. The implementation of corrosion mitigation strategies, enhanced manufacturing precision, and the use of higher-grade steels can significantly reduce this risk. Case study results show strong alignment with field observations, validating the method's effectiveness in accurately assessing tubing failure risks and providing valuable guidance for field integrity management.