Study On Seepage Characteristics Of Methane Hydrate Sediments Under Effective Stress

As a widely distributed clean energy source,gas hydrate has been widely concerned at home and abroad.The pilot exploitation of marine gas hydrate carried out by Japan and China has preliminarily verified the technical feasibility,but the daily gas output and the continuous mining time still can not meet the large-scale and commercial exploitation,among which the permeability of reservoir is one of the important factors.Sand and silty clay are the main deposits of hydrate in the South China Sea,whose particle size distribution and swelling and water sensitivity of soil particles seriously affect permeability.At the same time,the pore structure changes under hydrate decomposition and formation stress also lead to the uncertainty of gas production.The main research work is as follows:Firstly,the gas permeability of sediment containing methane hydrate under different effective stresses was studied.The steady-state gas-phase seepage experiment platform of high pressure hydrate was set up to determine the temperature and pressure conditions of gas hydrate reservoir in Shenhu sea area of South China Sea.The critical saturation which affects the law of permeability change was found.Above the critical saturation,with the increase of hydrate saturation,the soil agglomeration structure produced by water loss in the sediment becomes the soil supporting skeleton,which makes the permeability of hydrate sediment greater than that of pure soil sediment.The concept of stress sensitivity was put forward,and the influence of effective stress on gas permeability of silt sediment under different hydrate saturation was analyzed.Secondly,the gas-phase permeability measurement platform of methane hydrate sediments was improved,and the gas-phase permeability of silt sediments was studied during gas hydrate decomposition and reformation.By using Fick's law,the attenuation characteristics CH4 gas permeability coefficient and diffusion coefficient in silty clay samples were analyzed.The damage degree caused by hydrate decomposition to gas permeability of three kinds of silt were summarized.Ives and Pienvichitr models and Tien models were used to predict the decay percentage of the porosity of the samples after hydrate decomposition.And the influence mechanism of hydrate reformation on gas permeability was found.Thirdly,the characteristics of water-phase percolation in saturated silty clay of water-bearing compounds were studied by using a self-built water-phase permeability measuring experiment.The water phase percolation morphology of non-darcy flow and darcy flow in hydrate sediments was obtained by data fitting,and the existence of minimum pressure gradient threshold during water phase percolation was found.The permeability coefficient and permeability under different percolation patterns of water phase were calculated.The minimum pressure gradient threshold,water phase permeability coefficient and water phase permeability with hydrate saturation were summarized.The relationship between minimum pressure gradient threshold and permeability was clarified.Finally,the variation of permeability with sediment compaction and hydrate saturation in sandy sediments was investigated.The expression used to estimate the transient variation of effective permeability during hydrate step-down decomposition was proposed by combining Masuda model and Ren model with porosity and effective stress.In addition,the parameters of sediment porosity,effective stress,decomposition pressure and permeability were used in the coupled thermo-hydro-chemo-mechanical theory,the methane hydrate production model was established,and the effects of the compressibility and saturation of hydrate deposits on gas production and gas production rate were simulated and analyzed.