Phase Analysis In CO₂ Injection Well

In recent years,global climate change caused by greenhouse gas excessive emissions has attracted more and more attention from the international community.CCUS technology has been recognized as an effective means to reduce CO₂ emissions by the international community.CO₂ drilling,CO₂ fracturing and CO₂ displacement/storage technology have been widely used in the petroleum industry.CO₂ injection well serves as a channel connecting the surface and the reservoirs predicting the temperature and pressure of the injection well which in order to study the phase transition of CO₂ is not only conducive to the optimization of the wellhead injection parameters,but also a theoretical basis for studying the CO₂ flow in reservoirs.The phase transition of carbon dioxide depends mainly on temperature and pressure,many previous models have been used to predict the temperature and pressure in the well,however,because of the unique physical properties of CO₂ these models cannot accurately describe the temperature and pressure of the injection wells.Therefore,considering CO₂ physical properties vary with temperature and pressure and the phase change in the storage injection well,this paper establishes a unsteady temperature and pressure coupled model in the injection well to study the phase transition law in the injection well,which lays a theoretical foundation for the follow-up study of carbon dioxide drilling,carbon dioxide fracturing and carbon dioxide displacement/storage technology.In this paper,the physical parameters of carbon dioxide in NIST Refprop 9.0 software of American National Institute of Standards and Technology are invoked by Matlab to solve the problem that physical parameters calculation model is only suitable for calculating specific physical parameters at certain conditions.At the same time,the variation law of physical parameters,flow and heat transfer characteristics,phase change behavior law are analyzed.The unsteady temperature and pressure coupled model was established and discretized by full implicit difference.The distribution of temperature and pressure in well is obtained by means of double iteration of temperature and velocity using numerical solution tool MATLAB as programming tool.Based on the actual injection data of CO₂ fracturing and CO₂displacement/storage,the model calculation was carried out and the model sensitivity analysis was carried out with the parameters of injection temperature,injection pressure,injection volume,geothermal gradient and tubing wall roughness.The results show that in the initial stage of supercritical storage injection,the CO₂ in the wellbore undergoes the process of changing from supercritical state to liquid state,and then from liquid state to supercritical state.With the progress of injection construction,the well will be filled with supercritical CO₂at the end of supercritical injection.In the process of fracturing liquid injection,the CO₂ in the well will change from liquid state to supercritical state.With the progress of injection construction,the supercritical transition depth will gradually move to the bottom of the well.the supercritical transition depth changes rapidly at the initial stage of injection,and becomes stable gradually with injection.Since the well pressure is much higher than the critical pressure of CO2,the phase transition of the well is mainly determined by the temperature of the well.Injection temperature and geothermal gradient,which have a great influence on the temperature distribution in the wellbore,has a great influence on the phase transition of CO₂.