Study On Mechanical Properties Of Clayey Silty Sand Hydrate-bearing Sediments

The development and utilization of natural gas hydrate resources can effectively alleviate current imbalances in energy supply and demand,as well as address issues related to traditional fossil fuel shortages and environmental pollution.Due to its high sensitivity to temperature and pressure,disturbances during the resource exploitation process may trigger geotechnical disasters such as submarine landslides,excessive subsidence,methane leaks,well instability,and sand boil.Therefore,understanding the mechanical properties of natural gas hydrate reservoirs is crucial as it forms a critical theoretical foundation for the secure and efficient development of hydrate resources.In the study of reservoir mechanical properties,multi-stage triaxial tests serve as an effective approach to address challenges such as time-consuming experiments,randomness of hydrate formation,and inaccurate control of hydrate saturation.This method is applicable for conducting mechanical property tests on in-situ samples that are scarce and costly,especially for hydrate-bearing sediments.Statistical analyses indicate that over 90% of hydrate resources are distributed in marine reservoirs with fine-grained content.Further exploration is required to investigate the applicability,accuracy,and feasibility of multi-stage triaxial tests in determining stress-strain characteristics and mechanical property parameters for reservoirs with fine-grained content.Moreover,the utilization of related data for subsequent establishment of constitutive models warrants additional investigation.In addition,natural reservoirs with high fine-grained content exhibit pronounced heterogeneity.The distribution of hydrates significantly influences the strength and deformation characteristics of hydrate-bearing sediments.Addressing these issues,this study focuses on the reference of the clayey silty sand reservoirs in the Shenhu area of the South China Sea.Combining mechanical experiments,microscopic structural tests,and theoretical analyses,the study investigates the mechanical properties of hydrate-bearing sediments under various loading modes and hydrate distribution conditions and aims to reveal the inherent mechanisms governing their macroscopic mechanical behavior.Additionally,the study works to develop models capable of describing the strength,failure mode,and stress-strain behavior of clayey silty sand sediments.The main work is outlined as follows:(1)Based on drilling data from reservoirs in the Shenhu area of the South China Sea,artificial samples of clayey silty sand were prepared as host sediments.The in-situ formation of hydrate within the specimens was achieved using the gas-saturated method.Simulating the exploitation process by inducing hydrate decomposition to different stages through reducing pore pressure,a series of multi-stage triaxial experiments were conducted to investigate the mechanical response of homogenous hydrate-bearing sediments under the influence of hydrate decomposition and effective confining stress.In addition,the underlying mechanisms were further elucidated based on the microscopic structure of the specimens.Based on experimental data,the MohrCoulomb strength criterion was modified by incorporating changes in saturation induced by hydrate decomposition,aiming to predict the shear strength of reservoirs.By modifying existing formulas and developing empirical formulas to determine model parameters,the Duncan-Chang constitutive model applied to clayey silty sand reservoirs was established.This model takes into account the dynamic variations in reservoir mechanical parameters caused by hydrate decomposition and effective confining stress.The feasibility and accuracy of the constitutive model,established based on multi-stage triaxial experimental data,were validated through the results of single-stage triaxial tests.(2)Single-stage triaxial experiments were conducted on homogeneous methane hydrate-bearing specimens to investigate the strength and deformation behavior of the clayey silty sand specimens under different stages of hydrate decomposition and effective confining stress conditions.Microscopic structural changes were observed through scanning electron microscope(SEM).Strain hardening and softening degrees were defined based on the characteristics of stress-strain curves,and a model predicting the failure mode of hydrate-bearing sediment was established.By comparing stressstrain curves and mechanical property parameters of specimens under single-stage and multi-stage loading conditions,the influence of loading modes on the mechanical behavior of hydrate-bearing sediment was analyzed.The applicable conditions for multi-stage triaxial experiments were clarified.Additionally,by introducing statistical damage theory into the Duncan-Chang constitutive model,a constitutive model reflecting the strain-softening behavior of hydrate-bearing specimens under low effective confining stresses was established.When combined with the constitutive model established earlier in this study,this model can be used to describe the stressstrain characteristics of hydrate-bearing sediments with different failure modes.(3)Clayey silty sand specimens with inhomogeneous hydrate distribution were prepared using both gas-saturated and ice seed methods.With similar saturation degrees within the hydrate-bearing layers,multi-stage triaxial experiments were conducted to examine the influence of thickness proportion and angle of the hydrate-bearing layer on the mechanical properties of the specimens.Potential underlying mechanisms were subsequently analyzed.Based on the strength parameters derived from homogeneous specimens,relationship models were developed for the cohesion and internal friction angle of inhomogeneous specimens with hydrate-bearing layer thickness proportion and angle.Additionally,under conditions of comparable total hydrate saturation,the differences in stress-strain curves and mechanical property parameters between homogeneous and inhomogeneous specimens were analyzed.This underscores the importance and necessity of considering the inhomogeneous characteristics of reservoirs when studying the strength and deformation behavior of hydrate-bearing sediment.(4)Based on the results of multi-stage triaxial experiments on inhomogeneous clayey silty sand specimens,representative specimens with different hydrate distribution characteristics were selected for computerized tomography(CT).Based on CT images,a quantitative analysis of the pore characteristic parameters of the specimens was conducted utilizing Avizo software,examining how changes in hydratebearing layer thickness proportion and angle influence these parameters.The deformation responses of hydrate-bearing and hydrate-free layers under external loads were explored from a pore-scale perspective.This revealed the underlying mechanisms governing the strength and deformation behavior of clayey silty sand reservoirs under the influence of hydrate distribution.