Experimental And Numerical Study On The Effect Of Clay Minerals On The Mechanical Properties Of Clayey Silt Hydrate-bearing Sediments

Natural gas hydrate has high energy density,wide distribution,clean and pollution-free,abundant reserves,and is considered to be the new strategic energy with the most development potential in the 21st century.Unlike standard oil and gas wells,the decomposition of natural gas hydrate during extraction will easily causes the reservoir's deformation and strengh to decrease,resulting in the instability of output well system and engineering geological disasters.Clayey silt hydrate reservoirs dominate the South China Sea hydrate reservoirs in our region.Quartz and clay minerals containing montmorillonite and illite make up the majority of the mineral composition.The hydrate reservoirs are highly heterogeneous,mineral composition and its content will change with the stations,and especially changes in clay minerals',will cause greater differences in mechanical properties.Studying the effect of clay minerals on the mechanical properties of clayey silt hydrate sediments is very important for mining hydrate safely.So,the mechanical properties of two kinds of clayey silt hydrate-bearing sediments each containing illite and montmorillonite were studied.The effects of clay mineral types and contents on the strength,modulus,and deformation characteristics of clayey silt hydrate-bearing sediments were examined,as well as the changes in mechanical properties of clayey silt hydrate-bearing sediments during depressurization decomposition.Simultaneously,a computational model of clayey silt hydrate-bearing sediment was developed and optimized based on mechanical parameters obtained from experiments.Then the mesoscopic characteristics of contact force chains and displacement field during the shearing were analyzed to reveal the mechanical evolution mechanism of clayey silt hydrate-bearing sediment.Firstly,based on SHWSZ-3 low temperature and high pressure hydrate triaxial tester,the shear tests were carried out on illite clayey silt hydrate-bearing sediments and montmorillonite clayey silt hydrate-bearing sediments with 0%,10%,20%and 30%contents respectively,and the effects of clay mineral types and contents on the strength and deformation characteristics were analyzed.The results show that the existence of clay minerals makes the clayey silt hydrate-bearing sediments exhibit strain hardening behavior with shear shrinkage.The strength value decreases as the clay content rises.The stiffness is less influenced by the clay material.In general,the strength and stiffness of samples containing illite are higher than those of montmorillonite.Because montmorillonite absorbs water to form a thick combined water film,the cohesive force of samples containing montmorillonite is higher than those of illite,and the internal friction angle is opposite.With the increase of clay content,the internal friction angle of clayey silt hydrate-bearing sediments decreases,and the cohesion do not show obvious increasing or decreasing changes.Subsequently,the samples of clayey silt hydrate-bearing sediments containing illite and montmorillonite decomposed to different residual saturation were tested respectively by multi-stage loading shear test.The findings show that as residual hydrate saturation decreases,the strength and stiffness of clayey silt hydrate-bearing sediment decreases,and increases as effective confining pressure increases.However,the stiffness obtained in the multi-stage loading shear test is too large due to the accumulation of stress and strain in the early stage.Therefore,the stiffness value obtained in the multi-stage loading shear test is only used as a qualitative conclusion analysis.The cohesive force decreases as the hydrate decomposes,but the internal friction angle decreases marginally.The main reason is that the clay particles absorb water to form a combined water film to play a lubricating effect.It illustrates that the decrease in strength during hydrate decomposition is largely due to a change in cohesion.Finally,a particle flow numerical model considering capillary water effect was developed based on the characteristics of clayey silt hydrate-bearing sediments in the South China Sea which can capture the main mechanical properties of the sediments.In addition,the mechanical evolution mechanism of clayey silt hydrate-bearing sediments was revealed based on the mesoscopic features.The results show that the contact force chains of the clayey silt hydrate-bearing sediments with strain hardening behavior continuously increase with axial stress,and to bear the axial load,the high contact force chains are vertically distributed.The deformation of samples containing illite is smaller than those of montmorillonite.For the silt hydrate-bearing sediments without clay minerals,it exhibits strain softening behavior and the strong contact force chains reach maximum value at the peak stress.As the strength further decreases,the strong contact force chains gradually weaken and become discontinuous,and finally the sample fails in progressive sliding shear mode.