| Natural gas hydrate(also known as combustible ice),is an ice-like substance with cage structure formed by natural gas and water under high pressure and low temperature conditions.Due to its high energy density,wide distribution,huge reserves and cleanness,it is recognized as the new green energy that is most likely to replace conventional energy sources such as coal and petroleum in the 21st century,and is the strategic high point of global energy development in the future.Gas hydrate application technology has also become a new technology for benefiting humanity.Natural gas hydrate and the related application technology involve complex phase transition and gas-water transfer processes,including the decomposition and formation of hydrate,the formation and dissolution of methane nanobubbles,etc.To explore the kinetic process of natural gas hydrate phase transition at molecular scale,clarify the evolution rule of nanobubbles and their interaction mechanism with hydrate phase transition,and analyze the microscopic mechanism of nanobubbles affecting hydrate decomposition,nucleation and growth are of important theoretical and practical significance for clarifying the key factors affecting the phase transition of natural gas hydrate,developing efficient hydrate mining technology and improving hydrate application technology.The molecular dynamics simulation method was used to explore the above scientific issues in the paper.Because the molecular force field has important effect on the accuracy of the simulation results,and both CH4 and H2O fluids are involved during the natural gas hydrate phase transition.In this paper,we compared the common molecular force fields of CH4 and H2O,and it was found that the self-diffusion coefficient and density of the methane gas can be predicted well by UA force field,the interface tension of gas-water system can be predicted well by UA-TIP4P/ice and AA-TIP4P/ice force field.Based on the most accurate prediction of the methane hydrate phase diagram by UA-TIP4P/ice reported by previous study,the UA-TIP4P/ice force fields are finally selected to conduct the subsequent study in this paper.We also proposed methods of identifying water molecules and methane phases,nanobubbles,and hydrate cage structures to lay the analysis foundation of gas and water state transition during hydrate phase transformation.In order to explore the hydrate decomposition and nanobubble evolution of the free system,the hydrate-water and hydrate-water-gas systems were established,the micro-characteristics of hydrate decomposition at different temperatures,the nanobubble evolution characteristics and formation mechanism,and the impact of nanobubbles on subsequent decomposition were analyzed.It was found that the decomposition mode of the hydrate in the free system was layer-by-layer,and the methane gas reservoir accelerated hydrate decomposition by reducing liquid phase methane concentration.The evolution process of nanobubbles during hydrate decomposition is related to the decomposition driving force,the stronger the decomposition drive force,the earlier the initial bubble formation,the closer to the decomposition interface,the more small bubbles formed,and the more complex the pathway of bubbles merged.It was also found that the nanobubbles tend to be formed in a region having a larger methane concentration and a larger methane molecule diffusion coefficient,thereby proposed a nanobubble evolution model based on the synergic control of gas supersaturation and diffusion.It is further found that the formation of nanobubbles during the hydrate decomposition reduces the methane concentration in the liquid phase and promotes the subsequent decomposition of the hydrate.In order to explore the influence mechanism of pore wall wettability on hydrate decomposition,the pore-hydrate systems with hydrophilic/hydrophobic wall were established,the characteristics of hydrate decomposition,nanobubble evolution and gas-water migration in different pores were explored.It was found that the pore wall surface accelerates the decomposition of hydrate,and strong hydrophilic hydroxylated silica pore walls have a stronger promoting effect on hydrate decomposition and are more significant at low temperatures.The hydrate near the pore wall decomposes rapidly,and due to the stronger ability of absorbing water molecules on the hydrophilic wall,the hydrophilic wall is more effective to promote the decomposition of the hydrate near wall at the beginning.For the graphene pore-hydrate system,the surface cap bubbles and bulk spherical bubbles were formed at relatively high temperatures,while only surface cap bubbles were formed at lower temperatures.For the hydroxylated silica pore-hydrate system,the bulk spherical bubbles were formed in the temperature range of the study.The higher the temperature,the stronger the diffusion ability of the methane and the water molecules,and the diffusion depth of the methane molecules parallel to the wall is stronger than that in the direction perpendicular to the wall.At low temperatures,the hydrate in the graphene pore is decomposed by gradually contracting outward and inside,and the hydrate decomposition is relatively uniform;while the formation of bulk spherical bubbles in the hydroxylated silica pore leads to the uneven hydrate decomposition,which further leads to local differences in the decomposition rate of hydrate crystals,up to 4.3 times in different regions.In order to explore the molecular mechanism of hydrate nucleation and growth,different sizes of methane nanobubble-water systems were established,and the micro-characteristics of nanobubble dissolution and hydrate formation were analyzed.It was found that as the nanobubbles gradually dissolved into water,the bubble size decreased,the total potential energy of systems decreased,the F3 order parameters of water molecule decreased,and the number of cages increased,revealing the strong correlation between hydrate nucleation and growth and the dissolution of the nanobubbles.The smaller the nanobubble size,the faster the dissolution rate,the shorter the induction time for hydrate nucleation,therefore the smaller bubbles are more conducive to hydrate nucleation.The growth rate of the hydrate depends largely on the increase rate of the methane concentration:the faster the methane concentration increases,the faster the hydrate grows.The F3 order parameter of the water molecules in the nanobubble interface has a gradient change,providing a better water environment for the hydrate nucleation.In order to explore the nucleation pathway of the hydrate,the formation path of the first cage was tracked,and it was found that in the gas molecules forming the cage structure,the directed methane molecules fluctuate around the central methane molecule,and the directional distance is 0.67nm,the directional angle is 60°,the directed methane molecules and the central methane molecule gathered in the "three-body structure" pattern.Three methane molecules aggregated in the three-body structure can improve the stability of the surrounding hydrogen bond structure,and the free energy between them is the lowest,thereby the hydrate formation control mechanism based on the object molecules three-body aggregation was proposed.It is further found that with the growth of hydrate,the number of five-membered water rings and cages is highly correlated with the number of three-body aggregation structures.The introduction of three-body aggregation structure can control the hydrate nucleation sites and increase the hydrate growth rate,and the more the number of three-body aggregation structures,the higher the control probability of the hydrate nucleation’sites,the faster the rate of hydrate growth.This paper explored the micro-mechanism and kinetic properties of the hydrate phase transformation process,revealed the effects of nanobubbles on hydrate decomposition and nucleation and the aggregation pathway of the object molecules during nucleation.It provides certain theoretical guidance for the safe and efficient mining of natural gas hydrate and the application of natural gas hydrate technology.In order to further increase the understanding of gas hydrates and their application technology,more in-depth studies are needed.Explore the phase transition mechanism of other types of hydrate such as half-cage hydrate,find more effective additives for hydrate mining and gas production and explain the effect of the combined action of additive molecules and object molecule three-body aggregation structure on hydrate nucleation are the feasible directions of future research,which is of great practical significance to improve the application technology of gas hydrate. |
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