Abstract: During casing running and fracturing, it is easy to cause fatigue failure to casing threads under multi-axial alternate loads, affecting the construction progress. To address this problem, a finite element model of long round thread of the casing was established by Abaqus software with finite element method, and an analysis was conducted on the stresses and strains of casing thread under multi-axial alternate loads such as tension, compression, and internal pressure after buckling, and the life of casing threads under multi-axial alternate load was calculated by the life assessment method based on the Mises yield criterion. It was found in the study that under the action of multi-axial alternate loads such as tension, compression, and internal pressure, the maximum stress on the casing thread acted at the bottom of the 5 teeth at the big end of the male buckle; under the action of alternate loads such as axial tension and compression, the fatigue life of the lower thread is only 1 298 times. When the internal pressure was 46, 66, and 100 MPa, the fatigue life of the thread was decreased with the increase of the internal pressure, 1 094, 1 050, and 901 times, respectively. The study results provide a reference for the analysis of casing thread failure during fracturing in tight oil and unconventional oilfields.

Key words: casing thread, stress, strain, multi-axial alternate load, plastic deformation, fatigue life