| During multiphase transportation in deep-water pipelines,solid phase deposition problem representing by scaling,wax,asphaltines and hydrates easily occurs,which may result in pipeline plugging,threatening the safety of pipelines.When precipitation and deposition of wax as well as formation and plugging of hydrate coexist,they will synergistically escalate precipitation/formation of each other,promoting the possibility of pipeline blockage.However,investigations on the coupling problem are not enough yet.A knowledge base is urgently required,e.g.,of coupling thermodynamics and kinetics,as well as of the flow properties and tendency of pipeline plugging.This is the key and base to face the challenges raised by the complicate composition of products and complex production environment in the future subsea exploitation.This is also the fundamental of achieving flow assurance.In this thesis,the effects of wax crystals on hydrate nucleation,growth,agglomeration,flow properties as well as deposition characteristics are studied.This study is summarized as follows:1.A high pressure flowloop was used to study the influence of wax on hydrate nucleation under different initial pressures in W/O systems.It was shown that although wax-containing systems even had the same intrinsic nucleation driving force with wax-free systems,wax prolonged the hydrate induction time.Wax was found to inhibit hydrate nucleation through two significant effects: one was the macroscopic heat-transfer effect produced by wax deposition with insulating effect,and the other was the microscopic mass-transfer resistance originating from the wax crystals that were adsorbed at or adjacent to the oil-water interface.Initial pressure had a dual effect on hydrate nucleation: one was as intrinsic nucleation driving force,the other was by affecting wax precipitation.Based on the heat-transfer theory of a flow system and the hydrate nucleation theory,an improved model for hydrate induction time prediction was developed by coupling calculation of hydraulics and heat-transfer and by introducing mass-transfer effect of adsorption of wax crystals.2.Based on hydrate formation experiments,combining the microscopic observation through PVM(particle video measurement)probe and macroscopic physical parameters,such as pressure and temperature,partial hydrate nucleation and initial fast growth were observed.In the hydrate fast growing stage,the pores on hydrate shells were changed by wax crystals,and consequently the mass transfer of hydrate forming molecules was influenced,promoting hydrate formation.However,in the following growth stage,wax crystals were found to inhibit hydrate formation.Because the wax-containing systems were prone to form larger hydrate aggregates,decreasing the hydrate reaction surface area.An area occupying parameter of wax crystals as well as a fractal dimension parameter of pores on hydrate shell were introduced to consider the effect of wax.And the inward and outward hydrate growth shell model was modified and updated,which then could describe the hydrate growth kinetics of a wax-containing W/O system.3.Flow experiments in the high pressure flowloop as well as cyclopentane hydrate formation and viscosity measurement experiments in the rheometer were conducted.Results showed that there was additional viscosity increment in flow systems with was crystals and hydrates.It was found that wax crystal,water droplets and hydrate particles had interactions between each other,and wax-hydrate coupling aggregates formed in wax-containing systems.With the help of the macroscopic viscosity characteristics and microscopic information provided by PVM probe,formation mechanisms of coupling agglomerates were proposed: wax crystals adsorbed on the oil-water interface could enhance the water permeation through hydrate shell,increasing the cohesion force between hydrate particles,and weakening the function of AA(anti-agglomerates).Combining the effective medium theory of suspension systems and bimodal model,a viscosity model was presented to predict the viscosity of wax-containing systems with good accuracy.4.Flow and deposition experiments of wax-containing system with different wax contents,flow rates and environment temperature were conducted.It was shown that wax crystals would change the agglomeration,bedding and adhesion characteristics of hydrates.Compared with the wax-free systems,even with the same concentration of AA addition,wax-containing systems could not form stable hydrate slurry flow.According to the flow properties and high pressure visual window observation,a method to determine the deposition level was presented.Four different types of plugging scenarios with different flow properties and deposition level were summarized,which included rapid plugging,transition plugging,gradual plugging-I and gradual plugging-II.Then the plugging mechanisms of different wax content systems were proposed.For low to medium wax content systems,the synergistic effect between wax and hydrate would promote the formation of coupling aggregates,increasing the fluid viscosity,resulting in coupling deposition.For high wax content systems,due to the insulation effect of wax deposition layer,there was low possibility for hydrate formation,while wax deposition in W/O system mainly controlled the plugging mechanism. |
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