Study Of Hydrate Induction And Growth Process In Oil-Gas-Water-Wax Multiphase System

With the prosperous development of offshore oil and gas resources,the multiphase transportation has been widely used because of its economical efficiency.However,the hydrate forms easily in the high pressure and low temperature conditions at seabed.When the hydrate and wax deposition occur at the same time,both of the two formation affect each other,and leads to a great challenge to the safe operation of the pipeline.Thus,the study of hydrate formation in crude oil with wax is crucial to the safety guarantee of multiphase transportation.Hydrate induction and growth process in oil-gas-water-wax multiphase were specially investigated in this thesis and the influence mechanism of wax crystal on hydrate formation was explored.First of all,the formation of hydrate in emulsion was studied by using an autoclave with on-line viscometer in this thesis.A new judgment method for induction time of hydrate formation was given by analyzing the fluid viscosity various versus time in autoclave.The effects of water cut,emulsibility,shear disturbance,history effect on the induction time and growth rate of hydrate were explored.The prioritized nucleation mechanism of hydrate in the oil-water interface of emulsion was revealed.It was discovered that the residual gas of historical effect made the oil-water interface easily reach the critical gas concentration required by the hydrate nucleation,and then accelerated the nucleation rate.Secondly,the hydrate formation in emulsion with wax was experimentally studied.By the investigation of hydrate formation under different wax content,water cut and stirring intensity,the inhibiting effect of wax on hydrate formation was verified.And the inhibition behaviors were explored from the aspects of mass transfer,adsorption effect,cross nucleation.When the wax crystal precipitate out,the bulk viscosity of emulsion increases,and then the diffusion coefficient of molecules is decreased.Meanwhile,the wax absorbing on the interface of water drop occupies the space of hydrate formation,which reduced the probability of nucleation and prevented hydrate formation.The induction time of hydrate formation in emulsion with a wax content of 8% is relatively extended for about 60 min,compared to the system without wax at the condition of 2.2MPa and 3.5?.Thirdly,the nucleation mechanism of hydrate in emulsion was further studied by molecular dynamics simulation.A simulation system of emulsion was established and the nucleation process of hydrate in emulsion was studied by the simulation.Number of hydrate cages evolution,F_4? order parameter and density distribution of H₂O and H₂S molecular were investigated.The hydrate nucleation rate,growth rate,and growth location in the growth process were analyzed using VMD visualization software.The hydrate in the emulsion is easy to be nucleated in the water droplet interface within a thickness of 1nm,and the formation rate of hydrate is accelerated with the increase of water content and water droplet interface area in the simulation system.Finally,the wax model was added into the simulated emulsion system and the migration path of water molecules was quantitively studied.The result showed that water droplets adsorbed wax crystal easily,and thus hindered the diffusion of gas molecules from water droplet interface to its inside,which slowed down the hydrate formation rate,and its initial nucleation position was in the center of water droplet.