Experimental Study On CO₂ Hydrates Growth Kinetics And Slurries Flow Characteristics In High Pressure Flow Systems

The formation and accumulation of hydrates in oil and gas pipelines can easily cause blockage of pipelines and obstacles to oil and gas production equipment,which seriously threatens the development and gathering of oil and gas resources.When using hydrate risk control technology to prevent and control hydrates,the key is to clarify the growth kinetics and slurries flow characteristics of hydrates under high-pressure pipeline transportation system.Therefore,in this paper,with the help of high-pressure visual hydrate experiment loop,selecting pure water,industrial white oil,CO₂ gas and various types of compound surfactants as the experimental mediums,CO₂ hydrate formation and slurries flow experiments in different systems were carried out to explore the effects of pressure,pipeline loading,flow rate,water content,and surfactant HLB value on the kinetics of hydrate growth.Further,the empirical calculation formula for the induction time of hydrate in the flow system was established,and the fluid properties of the hydrate slurry under each system were clarified.First of all,the control variable method was used to design and carry out experiments on the formation of CO₂ hydrate in pure water systems,focusing on the effects of initial pressure,pipeline loading and flow rate on CO₂ hydrate induction time and hydrate volume fraction.The experimental results show that the induction time is inversely proportional to the initial pressure and flow rate,with the increase of the loop loading,it shows a nonlinear phenomenon of shortening and then extending;The volume fraction of hydrate increases with the increase of initial pressure,and decreases with the increase of pipeline loading and flow.The sensitivity analysis results of the multi-factor method shows that the initial flow rate is the main factor that affects the induction time under the piped pure water systems,and reducing the flow rate can effectively extend the induction period of the hydrate.In addition,hydrates produced in the pure water system are in the form of flocs or flakes,which tend to accumulate on the upper wall of the pipeline and form an aggregation layer,which is the main mechanism that causes hydrate blockage.Secondly,considering the problem of increasing water content in the later stages of oil and gas field exploitation,several groups of high-water-containing systems(50-90%)hydrate formation experiments were conducted to explore the water content and oil phase type on the hydrate growth kinetic parameters and apparent viscosity of slurrys The experimental results show that the induction time is gradually shortened with the increase of water content,but not obvious with the change of the initial oil phase;the gas consumption and the volume fraction of solid hydrate both increase with the increase of water content,and the apparent viscosity and friction of the slurries also increases with the increase of water content.Therefore,in order to avoid or suppress the formation of hydrates and ensure the safety of fluid transportation in multiphase mixed pipelines,the total amount of water phase should be reduced.At the same time,it should be noted that the oil phase will affect the apparent viscosity of the slurry in the stable flow phase of the hydrate in the oil-in-water emulsion system.Furthermore,9 sets of experiments were designed and carried out using orthogonal experiments,the primary and secondary effects of oil phase type,water content,emulsifier addition and water-oil ratio on the viscosity of the slurries are determined.It is considered that the addition of emulsifier in the oil-water system is the main factor affecting the viscosity of the hydrate slurries.At the same time,the stratification of the oil-water two-phase and the significant enhancement of the viscosity of the hydrate slurries are the root causes of the pipe blockage in the oil-water emulsion system.In addition,restarting the circulation pump after clogging is difficult to break through the clogged sections.So,in order to assure that the pipeline under the system does not block,the amount of emulsifier should be reasonably controlled.The viscosity reduction can be achieved by adding 1%of the emulsifier in the aqueous phase under high water conditions.Finally,Tween-80,Span-80,OP-10 and other surfactants were used in proportion to obtain a mixture of surfactants with different HLB values,and 12 groups of CO₂ hydrate experiments were carried out based on the experimental loop and found to exist.The experiment concluded that there is a critical HLB value,when the HLB value of the compound surfactant is 4.3?9.2,it shows an inhibitory effect on the hydrate induction period,and when the HLB value is greater than 9.2,it shows a promotion effect.The volume fraction of hydrate shows a gradual increasing trend with the increase of HLB value,which mainly depends on the emulsification of the surfactant and the type of oil-water emulsion formed by the hydrophilic/hydrophobic surfactant under the experimental conditions.Compared with the blank group experiment,different types of surfactants have a certain effect of anti-polymerization and dispersion,which can effectively improve the flow of hydrate slurries.In addition,based on the Herschel-Bulkley rheological model,the fluid properties of the hydrate slurries were analyzed and the results show that the hydrate slurries was pseudoplastic fluid which has shear dilution,and the functional relationship between the rheological index and the solid phase hydrate volume fraction is obtained by fitting.