The Study On CO₂ Hydrate Formation And Dissociation Process Using MRI

Global warming caused by CO₂ emissions from human activities is becoming more and more serious,so CO₂ emission reduction is imperative.Since the method of storing and recovering CO₂ by gas hydrate technology has been widely concerned,systematic research on the formation and decomposition characteristics of CO₂ gas hydrate is of great significance for the development of CO₂ hydrate storage technology.In this thesis,a self-designed hydrate formation and dissociation magnetic resonance imaging?MRI?experimental system was used to systematically study the formation and decomposition of CO₂ hydrate films at gas-water interface and CO₂ hydrate in porous media.The high-field MRI system was used to observe the growth and decomposition process of hydrate films at the CO₂/water interface in situ and the real-time thickness of the hydrate film was measured.The experimental results show that CO₂ gas molecules diffuse into the water through the hydrate film and react with water to achieve the thickening of the hydrate film.Slow depressurization can enhance the mass transfer of dissolved CO₂ to the hydrate interface and accelerate the growth of hydrate films.The memory effect can accelerate the growth rate of hydrate film in the initial stage of a reformation process,and the memory effect is gradually weakened with the elapse of the dissociation water placed time at atmospheric pressure.The growth of hydrated dendrites in water depends on the amount of dissolved CO₂.The hydrate film absorbs latent heat from the aqueous phase and decomposes from the bottom to the top from the interface,and the decomposition of the hydrate film will cause the water phase to be transiently subcooled to produce ice of metastable structure.The low-field MRI system was used to obtain the one-dimensional saturation profiles and T₂ spectrum distributions of hydrate formation and decomposition process in different particle size sand packs.The experimental results show that under the same temperature and pressure conditions,the hydrate growth rate increases with the decrease of the particle size.When the particle size is the same,the growth rate increases with the decrease of initial water saturation.The growth rate in the larger pores is faster than that in the small pores,and the large pores gradually transform into small pores with the formation of hydrates,resulting in a gradual decrease in the overall hydrate growth rate.The hydrate dissociation rate increases with the decrease of the particle size,and the initial hydrate saturation has little effect on the dissociation rate.The dissociation rate in large pores is relatively stable,so the overall hydrate dissociation rate in the sand pack is relatively stable.The NMR T₂ spectrum was used to calculate the fractal dimension corresponding to the pore structure of the sand pack in the hydrate formation decomposition process.The results show that with the growth of hydrates,the fractal dimension gradually increases until the end of hydrate formation tends to a fixed value.When the initial water saturation is large enough,a fractal growth stage will occur during the formation process,and the greater the initial water saturation,the longer the fractal growth stage.Conversely,the fractal dimension gradually decreases toward the intrinsic fractal dimension of the pore structure as the dissociation of the hydrate,and since the hydrate dissociation rate is much faster than the hydrate growth rate,there is no fractal stage during dissociation.The above research results provide important experimental data for the study of hydrate growth and decomposition kinetics,and provide theoretical support for the further application of hydrate technology.