Analysis Of Fluid-structure Interaction Vibration Characteristics Of Carbon Dioxide Flooding Injection String During Start-up And Stop Pump Process

The CO2 drive injection technology has improved the recovery rate of crude oil while at the same time sequestering greenhouse gases,providing a strong guarantee for the "double carbon" target and achieving a win-win situation for both economic and social benefits.The CO2 drive injection tubing column is an important channel for transporting CO2 to the bottom of the well,and the effect of fluid flow rate and pressure will induce vibration in the column,which in turn affects the fluid movement,forming a fluid-solid coupling phenomenon,and the vibration will lead to damage to the column,especially the transient load when starting and stopping the pump is stronger and more harmful.Therefore,in this paper,the following studies are carried out to investigate the fluid-solid coupled vibration characteristics of the CO2 drive injection column:Based on the investigation of the CO2 drive injection process,the phase change and physical parameters of the CO2 fluid were analysed and the column temperature and pressure distribution functions and patterns with well depth were obtained using the recursive method;Considering the compressibility of CO2 fluids,a 4-equation model for the fluid-solid coupled axial and lateral vibrations of CO2 driven injection columns was developed,and the system of equations was solved using the characteristic line method for the axial vibrations of the columns.Then,MATLAB software is applied to program the theoretical equations obtained to analyze the changes in fluid flow velocity,pressure and axial vibration velocity of the tube column and axial additional stress in the process of starting and stopping the pump and at different flow rates and pressures;Finally,finite element software was applied to build a three-dimensional model of the tube column to analyse the inherent frequency and deformation of the tube column with and without fluid-solid coupling,unidirectional and bidirectional coupling,wall thickness and axial force,etc.Compared with the effect of no fluid-solid coupling,the inherent frequency of liquid CO2 coupled with the tube column was reduced by 10.3%,the inherent frequency of supercritical CO2 coupled with the tube column was reduced by 6.5%,the inherent frequency of bidirectional fluid-solid coupling was smaller than that of unidirectional fluid-solid coupling.Thicker columns reduce vibration damage,with the best effect at a wall thickness of 7.34 mm,and axial forces have a weaker effect on the low-order region of the mode and a stronger effect on the high-order region.Research shows that: Best oil drive when supercritical CO2 is integrated with crude oil;With the increase of injection volume and pressure,the value of each parameter in the pipe column increases,when the pump is started after 12 s,the multi-cylinder pump set is fully started,smooth operation and little damage to the pipe column,it is recommended to use a small displacement and long cycle to protect the pipe column during the pumping process;Consideration of fluid-structure coupling reduces the intrinsic frequency of the column,and bi-directional fluid-structure coupling takes into account the interaction between the fluid and the column,making the results more realistic.The results of the study provide a reference for the optimal design and safety assessment of the pipe column structure during the production of CO2 oil and gas injection.