Study On Phase Transition Micro-characteristic And Gas Replacement Mechanism Of Natural Gas Hydrate

Natural gas hydrate,as a new kind of clean energy,has attracted great attention from various countries all over the world in recent years.The microscopic characteristics of natural gas hydrate formation and dissociation decide its spatial occurrence and pore structure,and the complicated phase transition associate with internal deterioration of heat and mass transfer as well as weakened strength of reservoir cementation,which are key points to the safe and high-efficient exploitation of natural gas hydrate.Thus,in this study,in-situ visualization systemes of natural gas hydrate were developed to analyze the characteristic and occurrence of hydrate formation in different reservoir conditions,and illuminated the characteristics and enhanced apporaches of hydrate dissociation with the synergistic effect of multiple time-varying factors.Furthermore,the mechanism and optimized methonds of gas replacement in natural gas hydrate were investigated.These researches in this study are supposed to lay the foundation of the safe and high-efficient exploitation of natural gas hydrate.For the existing defects of the hydrate visualization investigation such as non-in-situ,insufficient spatial and temporal resolution as well as difficult to distinguish components,etc.,a 3D X-ray CT visualization system and the Gaussian resolution equation were developed to investigate the microscopic characteristics of gas hydrate dissociation as well as the spatial occurrence of gas-water-solid phases.Also,a NMR visualization system,the pore network models,and a PRF thermometry method based on the proton chemical shift in liquid water were developed to analyze the effect of the microscopic formation characteristic on the macroscopic physical properties,which overcame the limitations of distributed thermometry monitored with thermocouples and infrared thermal imaging.Additionaly,a MRI visualization system and the phase resolution method based on the correction of transverse relaxation time were develop to illuminate the microscopic characteristics of hydrate dissociation with the synergistic effect of multiple time-varying factors.Moreover,a simulative low-temperature and high-pressure experimental system was used to discuss the mechanism and optimized methonds of gas replacement in natural gas hydrate with multiple phase zone.First of all,the X-ray CT and MRI visualization system were used to investigate the microscopic characteristics and spatial occurrences of gas hydrate formation.The results showed the heterogeneous nucleation and growth of excess-gas hydrate in pore spaces,which further expounded the "floating" morphology in lower hydrate saturation as well as the "pore-filling" and the "pore-cementing" morphology in higher hydrate saturation.When analyzing the stability of the trapped gases in hydrates,the depressurized porous medium generated the concentration of micro-bubbes under the gas-water interface,and then enhanced the secondary carbon dioxide capture and storage with hydrate further growth.These results provid a new idea of carbon dioxide capture and storage by the hydrate-based technology.Then the effects of saline ions,temperature,fine particle,and saturation approach on excess-water hydrate formation were studied.The results clarified the control mechanism of phase dissolution,chemical potential difference and percolation capacity on brine saturation process,which suggested the optimal preparation method for laboratory formation of excess-water gas hydrate in different reservoir conditions.The MRI visualization system was used to observe the microscopic characteristics of hydrate dissociation under different pressure,temperature and phase states.The effect of the phase transformation(dissociation,ice generation,and secondary hydrate formation)on water production rates and methane recovery rates were conducted.The results demonstrated that the characteristic of "annular dissociation" was controlled by the sensible heat of reservoirs,and the characteristic of "uniform dissociation" was controlled by the heat transfer from environments,which expounded the main limitions of tradional technology for exploiting hydrate resources.As a result,the phase state regulation method was presented to mitigate ice generation and secondary hydrate formation.The combinations of depressurization and thermal stimulation was proposed to enhance hydrate dissociation rate and aviud the heat dissipation.The nitrogen injection method was suggetted to prevent the inhibited gas production form large amount of water production.Based on the above research results of the microscopic characteristics and the enhanced methods of hydrate dissociation,the depressurization,thermal stimulation,and N2 injection were used to promote gas replacement in natural gas hydrate.The replacement characteristics in different phase states were analyzed,including gaseous carbon dioxide,liquid carbon dioxide,and non-equilibrium state of methane hydrate.The Avrami and Shrinking Core model were used to discuss the carbon dioxide replacement process,which could be divide into two stages:(1)a fast surface exchange reaction followed by(2)much slower diffusion-limited transport as a result of the formation of mixed hydrate layers in the first step.