Study On Gas-liquid Separation Of CO₂ Enhanced Produced Fluid In Oil Field

CO₂ enhanced oil recovery is considered to be an important technology to achieve large-scale CO₂ emission reduction and increase oil production,and several pilot projects have been carried out in the world.Fluid produced by CO₂ flooding contains oil and associated gas（including alkane and CO₂）and water.Compared with degassed crude oil,dissolved gas crude oil’s physical properties and rheological properties are different.In the process of gathering and transportation,it is easy to appear slug flow and other gas-liquid unstable flow conditions.All these have brought great challenges to the oil and gas gathering,transportation,and treatment process system.Aiming at the problem of poor gas-liquid separation effect in the gathering and transportation system on the ground of CO₂ flooding,the foaming characteristics of the CO₂crude oil contained in the gas-liquid separator,the influence of the inlet and outlet conditions of the separator on the separation effect,and the flow characteristics inside the separator were studied.A pressure kettle depressurization experiment analyzed the foam behavior of crude oil containing CO₂.The influences of viscosity,depressurization rate,initial pressure,temperature,liquid volume,and mechanical stirring on foaming behavior and defoaming rate of crude oil containing CO₂ are studied.The results show that a lower depressurization rate,an appropriate increase of inlet and outlet pressure,and liquid volume reduction are beneficial to reduce fluid foaming.Besides,40°C is the weakest foaming temperature of crude oil.Stirring under the separator’s continuous operation will accelerate the formation of foam,so it is not recommended.The CO₂-oil-water mixed transport process was built,and a horizontal separator with a GLCC at the inlet was designed for gas-liquid separation.After the fluid enters the separator,the foam behavior is analyzed,and the separation efficiency is evaluated under various working conditions.The results show that under the condition of a high gas-liquid ratio,the pressure fluctuation of fluid entering the separator can be significantly reduced by using-27°tangential inlet GLCC.When the liquid flow rate is constant,the foam extinction rate decreases with the gas flow rate increase.When the gas flow rate is constant,more foam will be produced,and the foam extinction rate will be faster with the rise of the liquid flow rate.Also,increasing the gas phase or the liquid phase’s flow rate will increase the percentage of liquid in the gas at the gas phase outlet.A horizontal separator with a GLCC at the inlet was designed in CFD software,which was the same as the experiment.Analyzing the flow field and phase distribution in the separator under the different gas-liquid ratios and throttle liquid outlet valve conditions.The results show that GLCC can significantly reduce the turbulence intensity after the fluid enters the separator.There was no clear droplet at the gas phase outlet under different conditions.A higher or lower gas-liquid ratio is not conducive to the pre-separation of GLCC.Moderately increasing the gas-liquid ratio and throttling the liquid outlet valve helps reduce the mixing degree of phases and increase the horizontal separator’s gas-liquid separation efficiency.The artificial neural network has a significant advantage when dealing with nonfunctional relations.By using BP neural network,the throttling degree at the liquid outlet of the separator is controlled according to the inlet conditions and constraint conditions so that the gas content in the liquid outlet of the separator meets the transport requirements specification of the separator.It provides guidance and suggestions for the control and prediction of the valve opening at the gas-liquid separator’s liquid outlet in the field.