Analysis and control measures on the air tightness detecting pressure of tubing in ultra-deep gas wells

In order to prevent the initial damage to the tubing threads of ultra-deep gas wells caused by the improper pressure during air tightness detection, the finite element mechanical model of air tightness detection for three types of tubing was established by means of the finite element method which is used to analyze contact problems. Then, the stress during the air tightness detection on the tubing threads of a certain ultra-deep gas well in Tarim Oilfield was studied quantitatively and systematically. It is indicated that when the gravity of the tubing with suspending weight in the lower part exceeds a certain value, air tightness detection will lead to plastic deformation of the last three connection roots at the big end of tubing thread, and the plastic deformation area increases with the increase of the lifting force. In the same operational mode, the stress concentration at the last three connection roots of the big end cannot be improved significantly by increasing the wall thickness of tubing. The service safety factor of tubing thread can be improved significantly by modifying the air tightness detecting pressure and increasing the steel grade of tubing thread. The model established in this paper can provide the theoretical basis for quantitative study on the distribution laws of air tightness detecting stress of tubing thread.