| Natural gas hydrate is considered to be one of the most potential alternative energy sources in the world because of its huge resources,high energy density,clean and efficient combustion.At present,the South China Sea has huge hydrate resources,and its exploration and development research is entering a critical breakthrough stage.To realize the safe and efficient utilization of gas hydrate resources will be of great significance to alleviating the energy shortage,optimize the energy structure and guarantee the energy security.The exploitation of actual hydrate resources is essentially a complex process of multi-field and multi-factor coupling of heat and mass transfer with phase change and gas-water seepage in porous media.It has proved the mass transfer control mechanism of gas hydrate decomposition process at the micro pore scale.Analyzing the influence mechanism of the microstructure evolution of the hydrate phase transition process on the actual reservoir core permeability and gas-water migration will have important theoretical guiding significance for the efficient development and utilization of hydrate resources.In this paper,focusing on the above scientific issues,in order to explore the microscopic control mechanism of hydrate phase change,the Brownian motion process of nanobubbles in the hydrate decomposition liquid was captured and tracked for the first time based on the liquid in-situ transmission electron microscopy and atomic force microscope technology at the micro-nano scale.It has been proved that the nanobubbles from hydrate phase transition process control the mass transfer mechanism.The study found that the average particle size of the nanobubbles in the hydrate decomposition liquid is 100-150nm,and they have a log-normal distribution law.The degree of gas supersaturation in the solution is the main controlling factor affecting the evolution of bubble size.Based on the dynamic light scattering technology and citing the bubble interface electric double layer theory,the law of the potential change of the nanobubble system is clarified.The decrease of the bubble diameter leads to an increase in the surface charge density and an increase in the Zeta potential.Through the calculation of the internal pressure and density of the nanobubble and the experiment,it is found that the nanobubble shortens the induction time of hydrate secondary formation,which further reveals that the internal high pressure and high density of the nanobubble are the key factors to induce hydrate secondary nucleation,namely the memory effect.Based on the in-situ visualization of hydrate formation and decomposition process,the experiment results show that hydrate preferentially nucleates on the surface of micro-nano bubbles,and then enter the stage of crystal growth and film spreading.The micro-nano bubbles control the hydrate growth morphology,and the dendritic growth morphology preferentially tends to grow along the bubble direction,and controls the hydrate growth path,which enhances the gas mass transfer rate in the hydrate growth process.With the help of MRI technology,it was discovered that the pressure disturbance and temperature drop caused by the stepwise pressure drop above the phase equilibrium caused a large amount of dissolved gas in the liquid phase to precipitate to form a micro-nano bubble enriched liquid,which promoted the secondary formation of hydrate at the phase interface.This series of experimental systems clarified the mechanism of micro-nano bubbles to enhance mass transfer and promote phase-to-nucleation during the microscopic formation of hydrates.Subsequently,based on the microscopic morphological evolution of hydrates in pore phase transitions,a low-field nuclear magnetic resonance NMR in-situ measurement of core permeability technology during hydrate phase transitions was developed.Through the free water/bound water in the micro-pore space during the hydrate phase transition-hydration,the real-time saturation distribution of the hydrate and the evolution of the pore structure have established that the critical hydrate saturation for the hydrate transition from a pore-filled type to a cemented form is about 35%.A core permeability model with different hydrate occurrence saturations has been established.We further develop the core phase change gas-water migration production numerical model under different permeability,and clarify that the higher pressure difference inside the low-permeability core is the main driving force for gas-water production,and the radial heat transfer and reservoir sensible heat are the main energy source for hydrate decomposition.The basic physical property analysis was carried out around the core sampling of natural gas hydrate reservoirs in the South China Sea,and the three-dimensional core structure,particle characteristics and hydrate occurrence form were systematically extracted.The permeability of reservoir cores was measured by developing low-field nuclear magnetic resonance NMR.Based on the establishment of the physical model of reservoir cores with physical property characteristics in the South China Sea,this paper simulates and analyzes the core gas-water production rules and mining efficiency of three mining technologies,including depressurization,heat injection and combination.The reservoir permeability is a direct control of the hydrate phase change.The key to the decomposition of gas-water multiphase seepage;reservoir cores with lower permeability are beneficial to increase gas production efficiency under heat injection conditions,but are not conducive to the initial gas-water flow,and gas production fluctuates greatly.For the lower permeability reservoirs in the South China Sea,the pressure-heat joint modulation method can greatly increase the gas production rate of a single depressurization method.In the actual hydrate mining process,timely drainage is an effective means to improve the gas production efficiency of the reservoir. |
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