Study On The Direct Numerical Simulation Of Droplet Motion In Oil-water Two Phase Flow

Oil and water two phase flow is an inevitable problem in oil and gas storage and transportation industry.Water content of produced liquid in onshore oilfields in China nowadays is usually higher than before,and the petroleum exploitation in extended potential regions of the ocean area also lack wellhead oil and water separation due to cost reasons,which leads to wide exist of oil and water two phase flow in oil gathering and transferring device.However,the presence of oil and water two phase flow in storage and transporting facilities may make chemical and electrochemical corrosion intensifies worse and bring a series of risk of oil resource loss,environmental pollution and public safety endanger,as well as the loss in economy and safety in crude oil trading and refining.Therefore,it is necessary to carry out oil and water separation in the process of storage and transportation,and the analysis and research of oil and water two phase flow is the basis of high efficient oil and water separation.Currently,the research method based on flow pattern is deficient in fuzzy,quantitative and nonuniform definition of pattern,which requires the study on direct numerical simulation of oil and water two phase flow.Firstly,through the analysis of the pros and cons of simulation methods in microscopic,mesoscopic and macroscopic scale,the LBM method in mesoscopic scale and Cahn-Hilliard equation in macroscopic scale are selected as the simulation method in this paper.Detailed derivation of the LBM color ladder model and the Stokes-Cahn-Hilliard simultaneous model are provided and a fast solution method for the solution of partial differential equations in macro model,Shift-Matrix method,is introduced and established in details.As a pioneering and basic research,program codes corresponding to the key implementation steps of each method are also provided in this paper for further reference.Under the condition that the oil flow is stationary and the gravity effect is neglected,the two models are verified by the fusion of two water drops.Difference intime-consuming of the two methods are compared,and the result shows that the C-H-S model with Shift-Matrix algorithm performs faster calculation speed in MATLAB programming.By checking the error convergence process and combining with the characteristics of MATLAB language,the calculation speed difference is analyzed.The superior C-H-S model is then applied to simulate the process of fusion of three water droplets,which proves that this method is correct and generic.Afterwards,the influence of gravity is introduced,and the migration and fusion of droplets is simulated in this paper.The result shows that the common used Stokes formula applied in the design of gravity separator in current oil and gas storage and transportation industry is not enough to accurately describe the droplet velocity.It's also found that water droplets also shows a deformation process,especially the deformation is irregular and asymmetric in fluid systems influenced by horizontal force,while water droplets are usually treated as regular and symmetrical in the current research on oil-water separation.The effects of droplet initial position,droplet size,the density difference between oil and water,and the force in horizontal directions in dropping process are also studied in this paper.It can be concluded that larger droplets and oil-water density difference may cause the migration velocity larger;the small droplet and distance form droplet initial position the computational domain may cause a long migration time and more forms of droplet movement;the horizontal direction force may change the droplet morphology the contact area at the bottom of the position.Besides,the difference in arrangement of droplets may cause the difference in the migration direction,speed and deformation.These results will have a certain impact on the selection and dosage of oil water separation method in actual production.The reliability and validity of the numerical model established in this paper are verified through various examples of droplets migration and fusion in oil and water two phase flow.The key codes in algorithms are provided,which is helpful for the future study on more complicated oil and water two phase flow(more liquid drops and more conditions combination)and can be referred in actual oil and water separation process.