Fluid-Structure Interaction Modeling and Characteristics of a Tubing String System for High-Pressure High-Production Ultra-deep Gas Wells

To study the fluid-structure interaction (FSI) of a tubing string system in a high-pressure high-production ultra-deep gas well, the water hammer equation of natural gas fluid and the motion equation of the tubing string were derived in detail by the microelement method. Furthermore, in addition to the compressibility and dynamic friction effects of the natural gas fluid, the influence of annulus fluid and the curved well section were also considered in the modeling process. Then the FSI model of the tubing string system was solved by the improved method of characteristics and the developed computer codes. Next, it can be seen that the FSI modeling method in this paper is effective and reasonable. Results of the present paper also can better reflect actual conditions. Finally, the model was applied to the actual FSI calculation for different shut-in times of a gas well. The results indicate that the peak value of pressure is much larger than that of non-FSI and steady-state pressure. The yield limit of the tubing string is exceeded, so it can be damaged easily. The vibration velocity of the tubing string is also very high and changes alternately, resulting in fatigue failure risk. Meanwhile, the more instantaneous the shut-in time is, the greater these peak values become, and the earlier their arrival times, the more severe the impact on the integrity and safety of the tubing string and the wellbore. Therefore, a more reasonable shut-in time in high-pressure high-production ultra-deep gas wells is needed.