Prediction And Mechanism Of Hydrate Deposition And Blockage In Deep-water Gas Well Test

Hydrate prediction and prevention is an important safety and security content in deep-water gas well test.At present,research on hydrate prediction mainly focuses on formation condition judgment and formation region analysis.The process of hydrate deposition to blockage is unclear,and there is a problem of excessive prevention.It is of great theoretical significance and practical engineering significance to analyze and predict the hydrate deposition and blockage process in gas well test,which can guide the precise prevention measures of hydrate,ensure the safety of test flow and reduce the influence of large amount of inhibitor on the test result.In this paper,the wellbore fluid flow in deep-water gas well test is mainly annular mist flow pattern.The droplet distribution in the center of the pipe and the liquid film near the pipe wall are theoretical analyzed.The effect of different gas-liquid flux,temperature and pressure on the droplet distribution and liquid film are also discussed.On this basis,the process of formation,migration and deposition of hydrate particles is obtained.The migration model of hydrate particle formed from the liquid film and gas center to the pipe wall,and deposition criteria of hydrate particle are established.There are different immersion states between hydrate particles transported to the wall and the liquid film.Considering the effect of non-spherical on the forces exerted by hydrate particle,the force analysis of hydrate particle,mainly including the solid-solid adhesion,drag force and additional couple,radial lift force and net gravity,is established to apply to the annular-mist flow.Based on the force analysis,the concept of critical deposition diameter and deposition criterion in gas well test process are put forward.The influence of gas-liquid flow flux,temperature and pressure,pipe diameter and other factors on critical deposition diameter are researched in gas well test process.Combined the critical deposition diameter with particle size distribution of hydrate particle,the hydrate deposition ratio was determined,and the model of hydrate deposition-plug and hydrate deposition layer growth rate suitable for gas well test are established.In order to simulate the vertical section of the test column,a simulation experiment device of small visualized hydrate deposition is set up to verify the model.The result shows that the hydrate deposition process includes hydrate formation,localized deposition,and thickening until it is completely plugged.Comparing the experimental value with the theoretical value,the average error is less than 5%,and the reliability of the model is rather high.On the basis of of hydrate deposition layer growth rate model,a deep-water test well is used to analyze the evolution of hydrate deposition and blockage.The result shows that,within the hydrate formation region,the evolution of deposition and blockage at the same location can be divided into four stages,namely the initial hydrate deposition stage,the critical deposition stage,the metastable growth stage and the the rapid growth stage.According to the temperature and pressure data of the location in the formation region,the evolution of hydrate deposition at different position and time is obtained and the effect of different factors,such as gas flow flux,gas composition,methanol concentration and salt concentration on the hydrate deposition and blockage are analyzed.It provides a theoretical basis for the prediction of hydrate deposition process in deep-water gas well test and the optimal design of preventive scheme.