Study On Mechanical Behaviors Of Silty Sediments During Natual Gas Hydrate Recovery

Natural gas hydrate,based on its abundant reserves and wide distributions in the permafrost and deep-ocean reservoirs,is recognized as the most promising new energy for commercial development in the 21st century.The hydrate recovery would lead to the cementation reduction of reservoirs and attenuation of sediments strength,which can result into the sedimentation and deformation of reservoirs,and even induce the engineering geological disasters like the reservoirs instability and structural failure.The host sediments of hydrate reservoirs in the South China Sea are mainly composed of unstratified silty sediments.The comprehensive evaluation of mechanical behaviors of hydrate reservoirs is an important task for the safety of CH4 hydrate recovery.Under this background,in this paper,analysis on the modulus,strength,deformation behaviors and their influence factors of CH4 hydrate-bearing sediments were conducted,the influence rules of CH4-CO2 displacement,depressurization and thermal decomposition on the mechanical behaviors of CH4 hydrate-bearing silty sediments were verified,and a production simulation study by depressurization of the low-permeability hydrate reservoirs at GMGS3-W19 station of Shenhu area located in the South China Sea was carried out using a hydrate recovery fully coupled model combined with the strength and deformation data of remolded CH4 hydrate-bearing sediments,as to provide analysis basis and methods for the safety of CH4 hydrate recovery in our country.The main work is as follows:Probe,shear and dissociation experimental studies on different kinds of CH4 hydrate-bearing sediments ranging from sand to silt were conducted,the influence rules of host particle,confining pressure and temperature on the strength behaviors of CH4 hydrate-bearing sediments were analyzed,and that the greater particle size can enhance the strength of CH4 hydrate-bearing sediments was discovered.This paper also studied the small-strain modulus and strength evolution of host sediments during hydrate formation,discovered that the small-strain Poisson's ratio gradually reduces during hydrate formation,verified that the hydrate cementation strength depends on the specific surface area of host particle,and discovered that silty sediments show the less loss of small-strain modulus and strength than sandy sediments during depressurization.Isotropic consolidation,shear and dissociation experimental studies were carried out on CH4 hydrate-bearing silty sediments prepared according to the particle gradation curves of hydrate cores in the South China Sea,the strength change and deformation behaviors of CH4 hydrate-bearing silty sediments during CH4-CO2 displacement,depressurization and thermal decomposition were analyzed.This paper discovered that the thermodynamic stability difference of hydrate is the main reason for the mechanical behaviors change of silty sediments during CH4-CO2 displacement,the effective stress and effective stress ratio dominate the deformation range and the initial hydrate saturation affects the deformation path of sediments during depressurization and thermal decomposition,analyzed the respective sediments deformation proportions induced by pore pressure reduction and hydrate dissociation,and verified the cohesion reduction induced by hydrate dissociation is the main reason for the decrease of silty sediments strength.CH4 hydrate-bearing sediments were remolded using the hydrate cores obtained from Shenhu area located in the South China Sea,the influence rules of hydrate saturation and confining pressure on the mechanical behaviors of remolded CH4 hydrate-bearing sediments were analyzed,the linear relationship between the hydrate saturation and strength parameters of sediments were established.The deformation behaviors of remolded CH4 hydrate-bearing sediments during depressurization were also studied,the failure of sediments during depressurization would not occur when the axial load is lower than the 80%strength of hydrate-bearing sediments.A hydrate recovery fully coupled model was built and a production simulation study by depressurization of the low-permeability hydrate reservoirs at GMGS3-W19 station of Shenhu area located in the South China Sea were conducted combined with the mechanical parameters like the elastic modulus,cohesion and internal friction angle of remolded CH4 hydrate-bearing sediments.It's found that the largest strain of reservoirs equals 1.4%,indicating the slope is stable during depressurization.However,the sediments around the production wellbore move approximately 0.16 m which have exceeds the radius of wellbore,0.15 m,the production wellbore may be bent and further affect the safety of CH4 hydrate recovery.