Study On The Microstructure Characteristics And Formation Kinetics Of Methane Dominated Hydrate

In the process of oil and gas exploitation,the formation of natural gas hydrate is easy to lead to the blockage of pipelines and equipment,which brings huge security risks to the oil and gas transportation process.At the same time,hydrate can also be used as a medium to store and transport gas.Hydrate also has high application value in other industrial fields,such as carbon dioxide capture,refrigeration storage,seawater desalination and so on.Considering the influence of hydrate in different aspects,it is necessary to adopt corresponding methods to inhibit or promote the formation of hydrate,so the study of hydrate formation kinetics is very important.The kinetics of hydrate formation is the change of different behaviors over time in the process of hydrate formation,including pressure,temperature,gas consumption,generation speed,etc.In this paper,experimental research and theoretical analysis are used to study the changes of different behaviors with time in the process of hydrate formation in different systems.The typical type I methane hydrate,type II tetrahydrofuran(THF)hydrate and methane-THF mixed hydrate were selected as the research objects.The differences of hydrate microscopic morphology were obtained.Meanwhile,the variation of different particle morphology and size with time was analyzed,and the dominant factors for the growth and agglomeration of hydrate particles were determined.Based on the analysis of gas consumption rate in different systems,the macroscopic hydrate formation kinetics model in different systems was established by taking the fugacity difference of gas phase and gas-liquid-hydrate three-phase equilibrium as the driving force of hydrate formation,and considering the mass transfer resistance between gas-liquid two-phase.In terms of particle morphology,the microscopic morphology of crystal particles in the liquid phase in different systems was obtained.The hydrate crystal particles formed by the dissolved methane gas in the liquid phase appear as hexagonal flat plates and agglomerates formed by hexagonal flat plates.The change process of the particle morphology can be divided into the leading phase of formation and agglomeration,the leading phase of crushing,and the stable phase.Meanwhile,hydrate is formed on the surface of bubbles in the liquid phase,and the whole bubble is spherical particles.The morphology of the THF hydrate crystal particles is octahedral single crystal and hexagonal flat plate,and the particle shape is relatively regular.Agglomeration is mainly achieved by forming THF hydrate between the particles,connecting the two particles together.The crystal morphology of methane-THF hydrate is affected by pressure and THF concentration.When the concentration of THF is high,the crystal morphology of THF hydrate is similar to that of THF hydrate.When the concentration of THF is low and the pressure is high,the particle morphology is more flat and dendritic,which is closer to that of natural gas hydrate.The different morphologies of the hydrate crystal particles are related to the filling rate of the guest molecules in the pores.The difference of hydrate formation rate in different systems is mainly due to the different contact area between gas and liquid,which affects the mass transfer between gas and liquid,which is the main limiting step of hydrate formation process.In different hydrate formation systems,the hydrate accumulation layer at the gas-liquid interface and the hydrate particles floating in the liquid phase will affect the mass transfer between the gas and liquid phases.Based on the kinetic model of hydrate formation with the fugacity difference as the driving force,the mass transfer between the gas and liquid phases is fully considered.The driving force of hydrate formation is redefined as the fugacity difference of gas when the gas phase is in equilibrium with the gas-liquid-hydrate three-phase equilibrium.The effect of hydrate on gas dissolution in liquid phase and the effect of hydrate layer on mass transfer at gas-liquid interface are combined.The mass transfer resistance of gas-liquid phase produced by hydrate layer is introduced to describe the gas consumption in the formation process.The hydrate formation rate parameter in each system is a constant value,which is not affected by pressure.The higher the pressure is,the easier the gas molecules pass through the hydrate layer and dissolve in the liquid phase.The change of hydrate layer thickness has a greater impact on the mass transfer resistance.Enhancing or weakening the mass transfer between gas and liquid is an important step to promote or inhibit hydrate formation.