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According to the tubing string failure statistics in the oilfield, fatigue crack of tubing joint thread in high temperature high pressure (HTHP) ultradeep gas wells remains a problem because it can cause tubing strength degradation, tubing fracture failure, well workover, and even well abandonment. In order to obtain a better understanding of tubing thread fatigue and quantify the fatigue life of tubing joint thread in HTHP ultradeep wells, experimental study, elastic–plastic mechanical simulation, and multi-axial fatigue calculation are carried out in this paper. Based on the similarity theory, the vibration mechanical testing device of the tubing string is designed, and the multi-axial load of the tubing joint thread in the actual working condition is obtained. Meanwhile, tubing joint BX1 thread model is established with ansys software, and the stress distribution of tubing joint thread is analyzed on the boundary condition acquired from experiments of vibration test of tubing string. Finally, according to the multi-axial fatigue theory, the fatigue life prediction value of the tubing thread made of Super 13Cr110 and Super 15Cr125 is compared and analyzed. The work presented in this paper can provide theoretical method and technological basis for the study on the frequent failure mechanism of tubing joint thread in HTHP ultradeep gas wells.

With the development of drilling technology and reinforced exploration and exploitation of unconventional reservoirs, there has been a great increase of complex wells. Meanwhile, however, consequent casing wear is and will continue to be a serious problem that causes enormous economic losses and many safety issues. The purpose of this paper is to find out the mechanism of casing wear and establish casing wear prediction model. Casing wear experiment was carried out to study the effect of contact force, rotation speed, and casing grade on wear depth. Meanwhile, wear coefficients under different working conditions were obtained through the normalizing of data. With the extensive research of downhole drag and torque calculation method, a contact force calculation model was established. Through the combination of crescent-shaped model and wear-efficiency model, the past complicated casing wear prediction models and confusing empirical formulae were greatly simplified. Therefore, the wear volume and depth of the casing string can be accurately predicted. Finally, a prediction software was developed to predict downhole casing wear of oil and gas wells. Comparison with the field data confirmed that the established model and software had enough accuracy to help predict and analyze casing wear at field.