Study On Mechanical Properties Of Low Permeability Hydrate-bearing Sediments In The South China Sea

Natural gas hydrate is considered as the most promising new strategic energy source due to its high energy density,wide distribution range.and abundant reserves.The successful trial production of natural gas hydrates in the South China Sea shows that the depressurization method has the most commercial potential.The hydrate reservoirs in the South China Sea are mainly composed of fine-grained sediments with low permeability.and therefore the reservoirs tend to experience an increase in excess pore water pressure under pressure.This will cause the dissipation of excess pore water pressure during the deformation process of the reservoirs,showing obvious consolidation properties.On the other hand,the increase in excess pore water pressure within the hydrate reservoirs leads to a decrease in effective stress and shear strength.showing significant undrained shear properties.In addition,the decomposition of natural gas hydrates can cause reservoir deformation and strength reduction,which may trigger geological disasters such as submarine landslides,wellbore instability,and submarine surface collapse.Therefore,it is necessary to study the mechanical characteristics of low-permeability sediments in the hydrate reservoirs of the Shenhu Sea area in the South China Sea,as well as the impact of changes in hydrate saturation on the mechanical properties of low-permeability hydrate containing sediments.This is of great significance for predicting and evaluating the stability of hydrate reservoirs during the commercial exploitation of hydrates in the South China Sea.Therefore,this study uses the method of combining theory with experiment and develops a prediction model of undrained shear strength of low permeability hydrate-bearing sediments based on the critical state theory.Remolded specimens are made using in-situ samples obtained from the Shenhu area in the South China Sea.One-dimensional consolidation tests and undrained triaxial shear tests are carried out to explore the effects of hydrate saturation changes on the consolidation properties and undrained shear strength of hydrate-bearing sediments.According to the experimental results,the undrained shear strength prediction model is verified and revised.Based on the proposed strength theory model,stability of submarine slope is analyzed.The main research contents and conclusions are as follows:(1)According to the published consolidated undrained triaxial shear test results of hydrate-bearing coarse and fine sediments,the pore water pressure coefficient A and stress ratio η of two types of hydrate-bearing sediments are calculated.The effects of hydrate saturation and effective confining pressure on A values and η values are analyzed.The mechanism of hydrates on the undrained shear properties of hydratebearing fine sediments is explored using critical state soil mechanics.A prediction model of undrained shear strength of low permeability hydrate-bearing sediments is developed based on the critical state theory.(2)Indicator tests are conducted on in-suit sediment samples to obtain basic physical indicators,such as specific gravity,particle size distribution.and liquid plastic limit.A hydrate-bearing sediments consolidation test system is self-developed.Onedimensional consolidation tests are conducted on remolded specimens of South China Sea sediments with different hydrate saturations to explore the effect and mechanism of hydrate on the compression and consolidation properties.The relationship curves between the average consolidation degree and time are calculated by the experimental results,the effects of hydrate saturation on the average consolidation degree are analyzed,and an empirical model for calculating the average consolidation degree considering hydrate saturation is developed.(3)The consolidated undrained triaxial shear tests are carried out to study the effects of effective consolidation pressures and OCR on undrained shear strength and pore water pressure response of sediments in the South China Sea.According to the experimental results,the isotropic compression index λ and swelling index κ are 0.175 and 0.029,respectively.The undrained shear strength of normally consolidated sediments increases linearly with the increase of effective consolidation pressures.The undrained shear strength of overconsolidated sediment increases nonlinearly with the increase of OCRs.The relationship between the normalized undrained shear strength of normally consolidated sediments and the effective shear strength index can be approximately summarized as a linear relationship.A prediction model for pore pressure coefficient at failure considering OCRs is developped.(4)The existing method of consolidated undrained triaxial shear test on hydratebearing sediments is optimized.The consolidated undrained triaxial shear tests are carried out to explore the effects of effective consolidation pressure and hydrate saturation on undrained shear strength of low permeability hydrate-bearing sediments in the South China Sea.The applicability of the prediction model of undrained shear strength of hydrate-bearing sediments is verified according to the test results,and the model is reasonably revised.The revised prediction results are in good agreement with the test results,indicating that the undrained shear strength of the low permeability hydrate-bearing sediment is a function related to two parameter of effective hydrate saturation and void ratio.The effects of hydrate decomposition on undrained shear strength of low-permeability hydrate-bearing sediments was analyzed in detail using the prediction model.(5)Taking the northern slope of the South China Sea as the research background.in which the target area of hydrate production is located.The safety factors are calculated using the limit equilibrium method.The results of different analysis methods are compared,and the comparison results showed that "φ=0" analysis method can obtain a reasonable safety factor.The numerical model of submarine slope is established.The safety factors are calculated through the strength reduction method to explore the effects of slope angle and hydrate decomposition on the stability of submarine slope.The mechanism of instability of submarine slopes of low permeability hydrate-bearing sediments is analyzed and different instability modes are summarized.