Study On Seepage Characteristics Of Hydrate-Bearing Porous Media By Experiment And Modeling

In order to cope with the reduction of conventional fossil fuel reserves,the deterioration of the ecological environment and the national energy security,natural gas hydrate（NGH）,as a new clean mineral with great application prospects,has been attracting widespread attention from energy workers in many countries.Many countries have been increasing the research on the exploitation of NGHs.For the in situ exploitation methods of NGHs,the seepage characteristics of sediments is one of the important characteristics for estimating gas productivity.It determines the flow process of fluid and has an important influence on heat transfer and material transfer in hydrate-bearing sediments.The seepage characteristics of sediments are constantly changing with the hydrate dissociation during the exploitation of NGHs.However,due to the instability and the heterogeneous distribution of hydrates,many technical challenges have been posed for the in-depth understanding of the seepage characteristics of hydrate-bearing sediments.Therefore,it is important for accelerating the research and exploitation of NGHs and promoting the development of natural gas industry in China to study the seepage characteristics of hydrate-bearing porous media,obtain the effective research methods of permeability and gas-water relative permeability,and explore the effect mechanism of hydrate on seepage characteristics.Firstly,based on the microfluidic technology and the low field NMR technology,the methane hydrate formation and dissociation in porous media were studied at pore and core scale.Secondly,considering the effect of methane gas on T₂ distribution at high pressure,the NMR measurement method of the spatial distribution of hydrate saturation was established.Considering the heterogeneous distribution of hydrates,a permeability model correction method was further established based on the seepage equivalent principle.The influence of hydrate distribution on the permeability of porous media was studied.Then,a micro-simulation method of the permeability variation of hydrate-bearing porous media was established based on the single-phase lattice Boltzmann model.The influence mechanism of hydrates on permeability was revealed.A new permeability model of hydrate-bearing porous media was established.Finally,a micro-simulation method of the relative permeability of gas-water flow in hydrate-bearing porous media was established based on the color-gradient lattice Boltzmann model.The quantitative relationship between the gas-water relative permeability and the microscopic distribution of fluid was analyzed.Combined with the microfluidic technology,the microscopic measurement method for seepage characteristics of hydrate-bearing porous media was established.Changes in permeability and gas-water relative permeability was studied in the micromodel during methane hydrate dissociation.From the studies on the hayrate formation and dissociation in porous media,the following results are obtained.During the process of methane hydrate formation in the micromodel with excess water in the presence of gas phase,the hydrate nucleation in porous media is stochastic.Hydrate formation only occurs at the gas-liquid interface.Hydrates grow towards the gas phase.Hydrate reformation occurs in the aqueous solution in the original hydrate area,while it does not occur at the gas-water interface where hydrates are formed with no previous hydrate history.During the process of methane hydrate formation in the core sample using the excess-gas method,methane hydrates tend to form in the pore center of sandstone.The dissolved gas in the small pores can directly participate in forming hydrates in the absence of free gas.The rate of hydrate formation is smaller in the small pores in that the supply of methane molecules in the small pores is slower than that in the large pores.The initial distribution of water affects the heterogeneous distribution of methane hydrates.During the process of methane hydrate dissociation by the depressurization method,the water content gradually increases in the small pores.The water content decreases firstly and then increases in the large pores.The water content of the core sample decreases firstly and then increased.From the studies on the permeability variation of hydrate-bearing porous media,the following results are obtained.The temporal and spatial variation of permeability happen in the process of hydrate dissociation.The heterogeneity of hydrate distribution in the core sample severely affects the seepage characteristics of porous media.Using permeability model correction method considering the heterogeneity of hydrate distribution,the determination coefficient can be improved by up to10.4%between the calculated permeability and the measured permeability.The coefficients of relative permeability models are related to the pore characteristic of hydrate-free porous media.The rock particles arrangement has no effect on permeability variation.The permeability variation is sensitive to the microscopic distribution of hydrates in the pores.The control seepage channel controls the change characteristics of permeability.Permeability variation of hydrate-bearing porous media can be quantitatively characterized by the relative control seepage channel size and the relative control seepage channel tortuosity.Within the scope of the microfluidic study,the amplitude of permeability variation caused by the change in hydrate distribution（1.4%）is smaller than that caused by the change in hydrate saturation（2.9%）in hydrate reformation.From the studies on the gas-water relative permeability variation of hydrate-bearing porous media,the following results are obtained.The gas-water relative permeability in hydrate-bearing porous media is affected by hydrate（including hydrate saturation and hydrate growth-habits）,rock wettability and capillary number.The changes of gas-water relative permeability caused by the changes of influencing factors depend on the three parameters representing the microscopic distribution of fluid:The fluid saturation,the relative specific interfacial length of the fluid and the fluid relative tortuosity in the flow direction.Changes of influencing factors will change the effect of driving force,viscous force and capillary force on the seepage characteristics of hydrate-bearing porous media.Within the scope of the microfluidic study,the effective permeability of gas and water flow in porous media has an obvious increasing trend,while the relative permeability of gas and water has no obvious change during the hydrate dissociation.