Study On Physical And Mechanical Characteristics Of Multi-type Natural Gas Hydrate-bearing Sediments In The South China Sea

With its large abundance and high energy density,natural gas hydrate(NGH),as an emerging clean energy,has attracted attention from all over the world.However,due to the low strength of NGH-bearing formations and dissociation probability,exploitation of NGH possesses high risks.Hence the risk evaluation is of utmost importance to the NGH drilling and production,to which the solid understanding of the physical and mechanical characteristics of NGH-bearing sediments is essential.This study measured the physical and mechanical properties of NGH-bearing sediments via the rock triaxial test using laboratory-made NGH-bearing core samples which are specially made to simulate the potential NGH reservoirs in the South China Sea.Then,based on the quantitative analysis on the acquired data,the failure criterion and constitutive model were proposed.At last,the wellbore stability on a well encountering natural gas hydrates in the shallow marine sediments was accomplished,based on the proposed models.It is shown that the peak strength generally grows with the increase in the NGH saturation and effective confining pressure.However,the increase ratio caused by growth of the effective confining pressure decreases eventually.The Young's modulus rises with the increasing effective pressure,and there is no clear relationship between the Poisson's ratio and the variables.The response feature of NGH-bearing sediments with different cementation level varies.With the equal porosity,NGH-bearing sediments with relatively poor cementation are more heavily influenced by the variation in the NGH saturation and effective pressure,which means being prone to instability caused by NGH dissociation.The modified Mohr-Coulomb criterion for all three types of sediments and Duncan-Chang constitutive model for the sandy shale have been established,which can accurately predict the shear strength of the NGH-bearing sediments under different conditions.Moreover,based on the mechanical characteristics of marine shallow sediments in deepwater areas and their in-situ stress state,the wellbore stability model adjusted to the deepwater marine shallow sediments has been set up.The wellbore stability analysis on a deepwater well in the South China Sea has then been accomplished.The results shows from top to bottom,the collapse tendency increases and the fracture risk narrows.The safety window of the drilling fluid density in deepwater shallow sediments is generally narrow and the margin widens as the depth increases.The formation with NGH inside has a significantly low collapse pressure gradient,with the increase in the rock strength,and is hence suitable for seawater drilling.However,the fracture pressure in this formation decreases because of the change in in-sit stresses.This study quantitatively analyzed and modelled the physical and mechanical characteristics of NGH-bearing sediments in the South China Sea and conducted a case study of wellbore stability.The findings of this research provide theoretical basis and reference for the design and optimization of the future drilling and production engineering of NGH in the South China Sea.