Study On Hydrate Growth And Desalination Effects In Water/Oil Dispersion

In the oil and gas industry,the transported fluids are often multiphase mixtures of oil,gas and water which could easily form hydrates causing plugging of pipelines under low temperature and high pressure conditions.With the exploration and production of oil and gas moving to deep sea and cold regions,this problem is becoming more and more significant.Initially,the research on the issue of hydrate flow assurance mainly focused on thermodynamic hydrate inhibitors and later attention was drawn to the method of kinetic hydrate inhibitors.On the other hand,to transport hydrate particles in the form of slurry without preventing formation of hydrates represents another solution to the problem of operational safety for pipeline transportation.Though hydrates present itself as a nuisance for the petroleum industry,they also have wide applications in different fields including cool storage,desalination,gas separation and storage.The application of slurry flow technology for safe transportation and the evaluation of effectiveness of methods of hydrate based desalination,gas separation and storage depend on the key issue of hydrate formation kinetics.In terms of all above,this work investigated formation mechanism of hydrates in water/oil emulsions,viscosities of water/oil emulsions and hydrate slurries and hydrate based desalination by water-oil/water-oil-gas dispersion systems.This study is summarized as follows:1.The formation process of methane hydrates was investigated with a high-pressure stirring batch reactor equipped with Particle Video Microscope?PVM?and Focused Beam Reflectance Measurement?FBRM?probes.The experiments implied that nucleation of hydrate was followed by rapid growth of particles when increase in particle size was accompanied by depletion of free water.Subsequently the growth rate decreased when the distribution of different phases and particle size gradually stabilized.Some water remained encapsulated inside hydrate layers after formation.2.A novel hydrate growth model was proposed based on experimental observations and the characterization of chord length distributions by the FBRM probe.Hydrate layers were assumed to constitute mass transfer barriers for water and gas molecules in this model thus water in the system was divided into free water outside hydrate particles and encapsulated water inside hydrate layers.It was hypothesized that gas molecules diffuse from bulk oil to the interface of water-hydrate particle and then react with water on the surface of particles.The depletion of free water on particle surface was supposed to cause decrease in reaction area limiting further reaction.The model was regressed to obtain apparent kinetic constant and fraction of free water.The calculated results showed well agreement with experimental data.It suggested that apparent kinetic constant increases with rpm and fraction of free water mainly relies on initial water content.Both rpm and water content could affect the size of hydrate particles and reaction area.3.The viscosities of water-in-oil emulsions and hydrates slurries were measured using a high-pressure online viscometer and the variation of viscosity of hydrate slurry during hydrate formation was analyzed.The effects of temperature,type of anti-agglomerant and water content on the viscosity of stabilized hydrate slurry were investigated.The results indicated that the viscosity of slurry increased and fluctuated significantly during hydrate formation and then tended to stabilize.The sensitivities of viscosities of water/oil emulsions and hydrate slurries to water content and temperature depend on the type of anti-agglomerant.4.Desalination experiments using brine in cyclopentane dispersion systems were conducted under atmospheric pressure and the influences of different conditions on yield of dissociated water and removal efficiency were studied.Compared with commonly-adopted high water cut systems,excess cyclopentane addition could significantly increase yield of dissociated water while removal efficiency stays around80%,indicating limited encapsulation of residual brine.Increase in rpm or decrease in temperature/initial salinity could promote hydrate formation but would adversely affect removal efficiency.Removal efficiency was notably affected by the ratio of washing water/dissociated water and yield of dissociated water exhibited inverse correlation with removal efficiency.5.Desalination experiments of CO₂+cyclopentane hydrates were performed in a high-pressure magnetic stirring reactor and the effects of water cut,temperature,initial pressure,formation time were investigated.It was demonstrated that increase in pressure in a certain range for high water cut systems could effectively raise yield of dissociated water while removal efficiency dropped a little.This is beneficial for the development of desalination process with both high yield and high removal efficiency.Yield of dissociated water and removal efficiency showed smaller volatility when water cut was around 60%.Yield of dissociated water was influenced by water cut,rpm,temperature and pressure while variation in removal efficiency was closely related to water cut and change in subcooling.The addition of CO₂ could promote rapid formation of large amount of hydrates but would cause decrease in removal efficiency probably limited by means of separation.