Natural gas hydrate(also called flammable ice)is becoming an alternative form of energy with huge potential,due to its enormous reserves,cleaness,as well as high energy density.Secure and efficient utilization of gas hydrate resource is of crucial significance in terms of remission of energy crisis,optimization of energy structure,and energy security.In the nature,the exploitation of gas hydrate is a complex process in porous media,which couples multi-fields and multi-factors like heat transfer and permeation of gas and water.To illuminate the pore scale occurrence of gas hydrate,evolution of microstructures during phase transition,as well as effects of microstructure on macro property,is of great importance to the efficient exploitation and utilization of gas hydrate resources.Therefore,in this work,the first time-resolved evolutions of gas hydrate microstructures during in-situ formation and decomposition are reported at a hundreds-nanometer-scale,based on microfocus X-ray CT and synchrotron CT,as well as a "stop-and-go" method.The full process of hydrate reaction at the phase interface is captured and followed,indicating the diffusion-limiting mechanism of hydrate phase transition in pore spaces.The initial formation of gas hydrate is found to occur at the gas-water interface,with the following growth proceeding toward water phase.There exists a water layer between hydrates and solid matrices,which could be as thin as around 1 μm.Moreover,the in-situ hydrate decomposition is initiated at the gas-hydrate interface,with the hydrates close to the sand surface largely unchanged.Free water from decomposition gradually accumulates at the decomposing hydrate surface,isolating the hydrates from the gas phase.The very low solubility and diffusivity of gas molecules in water strongly limit the out-diffusion of released gas into the gas phase,resulting in a declining decomposition rate as the water layer thickens.A gradient of gas concentration in this water layer is directly observed,indicating a metastable enrichment of gas in water.The results illustrate a diffusion-limiting mechanism of in-situ gas hydrate decomposition.Moreover,the effects of micro structures on the macro property involving thermal property,permeability,and mechanical property are studied by use of microscale observation,pore network model as well as point-heat-source thermal conductivity measurements.The influencing mechanism of different component ratio,constituent,as well as its thermal property on the effective thermal conductivity is investigated.A hybrid model for the prediction of effective thermal conductivity of methane hydrate-bearing porous media is proposed,with a good agreement with the reference data.The absolute and relative permeabilities of hydrate-bearing sediments during in-situ decomposition are obtained with consideration of the hydrate microstructures.The wedge-shaped hydrate patches persisting throughout hydrate decomposition are considered to constrain the rolling and shear motion of the porous matrices.The work above is to provide a theoretical basis to the micro-mechanism of natural gas hydrate phase transition at pore scale as well as its effect on the macro physical property,guiding the future resource exploitation and its efficient utilization.