Study On Sustained Casing Pressure Of Gas Well Considering Phase Change And Migration Of CO₂

The annular pressure of gas well not only threatens the daily production on site,but also brings huge economic burden to economic exploitation.In the process of gas production,constructors often face acid gas wells with high carbon dioxide content.Due to the active and easy phase change of CO₂ physical properties,once the cementing cement leaks,the CO₂flowing from the bottom of the well to the wellhead will show two or even three phase states.It is of great significance to establish the calculation model of annular pressure applicable to CO₂ gas wells and realize the prediction of wellhead annular pressure for on-site safety production.In this paper,firstly,the migration law of the channeling fluid in the annulus protection fluid is studied,focusing on the change of the gas migration speed in the annulus protection fluid under different Reynolds numbers.Based on the percolation theory,a model of fluid migration in annulus considering the Reynolds number is established,and the law of fluid migration in annulus is clarified.At the same time,the model of gas migration is applied to the prediction of annulus pressure in conventional wells.The sensitivity analysis of annulus pressure parameters is realized by case well calculation,and the law of the influence of Reynolds number on gas rising speed is revealed.After the velocity change rule of channeling gas in annulus protection fluid is clarified,the more active carbon dioxide in acid gas well is studied.Considering that the physical parameters will change obviously before and after the phase transformation of carbon dioxide,in order to ensure the accuracy of the physical parameters of carbon dioxide involved,the first step is to optimize the calculation model of physical parameters of carbon dioxide,and obtain the change rule of carbon dioxide compression factor,density and viscosity with temperature and pressure and the distribution rule of each parameter in the wellbore.Then,based on the established gas migration model in the annulus,considering that the phase state of carbon dioxide will change under different temperature and pressure conditions,combined with the optimized calculation method of physical parameters of carbon dioxide,the prediction model of annular pressure of acid gas wells with phase change is proposed.Finally,the model of annular pressure is simulated,and the recovery rule of wellhead pressure and the distribution of carbon dioxide phase state in the annulus are obtained.