Study On Wax Diffusion Characteristics In Water-in-Oil Emulsion

In recent years,with the rapid development of offshore oil and gas exploitation,the distance of subsea pipelines has been increasing substantially.Due to the adoption of multiphase transportation technology,water-in-oil emulsion is one of the flow patterns formed in subsea production and gathering system.Because of the complexity of water-in-oil emulsion flow and cold surroundings,subsea pipelines have to face a greater risk of wax deposition.In this paper,the wax diffusion characteristics in water-in-oil emulsion are studied systematically,mainly focusing on the influence of dispersed water droplets.The first part is about the near-infrared spectroscopy(NIRS)test method of wax appearance temperature(WAT).A new WAT test method with kinetics mode of NIRS is developed in this study.The supercooling generated in continuous cooling process is able to be eliminated by extrapolation.The influences of wavelength and cooling rate on test result are analyzed.This study also improves the WAT test method with spectrum mode of NIRS.The baseline tilt of opaque oil sample is solved by density correction.Compared with cross polarized microscopy and differential scanning calorimetry from various aspects,NIRS is a reliable and practical WAT test method with high sensitivity,repeatability and many other advantages.Furthermore,the variations of droplet size distribution with water content and stirring speed are investigated by microscopy observation and image processing method.A set of characterization parameters is summarized to describe the basic characteristics of the droplet size distribution,and the relation of these parameters and emulsification conditions is discussed.The Rosin-Rammler distribution model is adopted to describe the cumulative distribution of droplet size with a good goodness of fit and a remarkable regression effect.In addition,the wax diffusion experiments of water-in-oil emulsion are carried out on a cold finger apparatus.Via the property analysis of wax deposits,the variation of the average wax diffusion rate with water content,temperature and time are obtained under static condition.A method to calculate the equivalent thermal conductivity of wax deposits is proposed according to emulsion properties and deposits composition.Based on this,the radial temperature distributions inside crude oil and emulsion after different diffusion time are simulated.The simulation results verify that the existence of water phase will influence wax diffusion by altering the radial temperature distribution under static condition.Also,the simulation results reveal the internal relation between wax diffusion and heat transfer process.Finally,the characteristics of each stage in wax molecular diffusion process are analyzed.Based on Fick's diffusion law,a method to calculate the wax diffusion coefficient in water-in-oil emulsion is put forward according to the results of wax diffusion experiments.With geometrical analysis method,the tortuosity of wax diffusion path in water-in-oil emulsion is derived.Based on this,a formula for the blockage coefficient of dispersed phase is proposed and validated.This coefficient is able to quantify the influences of water content and droplet size distribution on the blockage of wax diffusion path and help to deepen the understanding of wax diffusion mechanism in water-in-oil emulsion.