Study On Phase Equilibrium And Mechanical Properties Of Hydrate-Bearing Sediments

Natural gas hydrate is widely distributed in marine sediments and permafrost worldwide.As a clean energy with abundant reserves,it is expected to solve the current energy shortage problem.Unsuitable exploitation of hydrate and climate change will cause hydrate decomposition of hydrate-bearing reservoirs,induce large-scale subsidence of the seabed,submarine landslides and other geological disasters,and threaten the safety of submarine pipelines,drilling platforms and other structures.Therefore,the stability evaluation of hydrate-bearing reservoirs is crucial.The phase equilibrium and mechanical properties of hydrated soil are important factors affecting the stability of hydrated reservoir.The structure,capillary effect,permeability effect,physical and chemical effect of sediments determine the phase equilibrium and mechanical properties of sediments containing natural gas hydrate.In order to determine the phase equilibrium and mechanical properties of natural gas hydrates and reveal the influence mechanism of fabric,capillary effect,permeability effect and physical and chemical effect on the phase equilibrium and mechanical properties of natural gas hydrates,a series of micro-macro experiments were carried out using artificially configured sediment samples containing clay particles.A phase equilibrium model and a shear strength model of hydrate-bearing sediments were established,which can consider the capillary effect of sediments and the surface effect of soil particles.The following important conclusions were drawn:（1）In order to explore the water retention capacity and thermal conductivity of hydrated sediments,the SWCC and thermal conductivity of hydrated sediments were tested.The results show that the higher the clay content,the stronger the water retention capacity of the soil.The permeability of the gas and liquid phases of the soil is analyzed based on the BC model.The thermal conductivity of the sample increases with the increase of water content,and the increase of clay content in the overall trend will reduce the thermal conductivity of the soil.（2）The monitoring system of hydrate formation and decomposition process was developed by using low field nuclear magnetic resonance instrument.By analyzing the results of nuclear magnetic resonance test,the migration and evolution law of pore water in the process of hydrate formation and decomposition was obtained.The results show that:T₂ nuclear magnetic map shows that hydrates first generate in large pores and then to small pores,and the opposite is true in decomposition.In the process of hydrate formation,the hydration number gradually decreases and tends to be stable,and the hydration number basically remains stable in the process of hydrate decomposition.The lower the moisture content is,the larger the hydration number is.The larger the initial formation pressure is,the smaller the hydration number is.The increase of clay content will increase the hydration number of hydrate.The apparent effective activation energy?E and the two-dimensional spectrum T₁/T₂ characterizing the water activity state were introduced for analysis and discussion,which quantitatively explained the influence of the surface water state of soil particles on the synthesis of hydrates,especially the influence of clay particles on the water state.The greater the apparent effective activation energy is,the lower the water activity is,and the more stringent the temperature and pressure conditions for hydrate synthesis are.（3）The effects of initial formation pressure,initial moisture content,clay type and content of pore hydrate on the phase equilibrium state were studied by isothermal stepwise heating method.The experimental results show that:The lower the initial moisture content and the higher the initial formation pressure,the greater the degree of phase equilibrium curve deviating from the pure water phase equilibrium curve.Under the same temperature conditions,the higher the clay content is,the greater the stability pressure of hydrate in the sediment sample with stronger plasticity of clay component is.Based on the above experimental results,the pore hydrate phase equilibrium model considering the capillary effect of sediments,the surface effect of soil particles,and the permeability effect was established within the framework of thermodynamic chemical potential equilibrium.The model was correlated with the water holding characteristics of unsaturated soil,and the P-T-w relationship model with macro parameters and easy to determine was established,and the?T-P-w relationship model was expanded.（4）The effects of initial moisture content,axial stress,clay type and content on the shear strength of hydrate-bearing sediments were studied by using the self-developed hydrate direct shear apparatus,and the direct shear strength test under the condition of hydrate decomposition was carried out.The test results show that:With the increase of hydrate saturation,the shear strength increases continuously,the friction angle increases linearly slowly,and the cohesion increases exponentially.Under the same initial moisture content,the shear strength of hydrate-containing samples tends to be lower when the clay content is higher and the plasticity of clay component is stronger.Under the framework of unsaturated effective stress,the strength model considering both capillary effect and adsorption effect is established.The physical concept of the model is clear,and it is a strength model for the four-phase system of hydrate-liquid water-soil particles-gas.It is suitable for hydrate decomposition samples and unsaturated sediment samples with clay particles.The shear strength data under different hydrate saturations are well simulated.（5）In order to better describe the stress-strain relationship of hydrate-bearing sediments,a two-sided interface model of GHBS is established.The elliptical yield surface and a ray are used as two boundary surfaces,and the radial and biased mapping rules are used respectively.The effect of virtual peak stress ratio on reaction overconsolidation and strain softening characteristics of soil are introduced.The established model takes into account the plastic deformation of the specimen in the boundary surface,and can simulate the plastic deformation generated during the loading process after isotropic loading-unloading.The parameter calibration method is introduced based on triaxial test.The hydrate saturation is connected with the state parameters,and the relationship between plastic modulus and dilatation is corrected.A simple evolution law of cementation strength is obtained,which can reflect the changes of yield surface and mapping center in the shear process.Compared with the experimental data,the model can accurately reflect the mechanical behavior of hydrate-bearing sediments with different hydrate saturation,confining pressure,hydrate occurrence conditions and clay sediments.