Study Of Microscopic Response Mechanism Of Hydrate-Bearing Sediment During Deformation Process

Natural gas hydrate（NGH）is a clean and efficient new energy source,which is considered as the most promising alternative energy source in the 21^st century due to its huge reserves and wide distribution,and is a strategic high point of energy in the future for all countries in the world.However,the dissociation of NGH during the extraction may lead to the weakening or disappearance of the cementation structure in the reservoir,which in turn causes great changes in the strength and stiffness of the reservoir and may induce engineering geological disasters such as formation subsidence,deformation and wellbore destabilization.To elucidate the microscopic response mechanism of hydrate-bearing sediment deformation is an important prerequisite for the safe exploitation of NGH resources.In this dissertation,a series of visually experimental studies on hydrate-bearing sediment during consolidation,shear and dissociation deformation are carried out based on the self-developed high-pressure low-temperature CT-based hydrate triaxial experiment system,which clarify the evolution of hydrate-bearing sediment microstructure and fabric and reveal the hydrate cementation weakening and failure mechanism,with a view to providing theoretical basis and technical support for the safe exploitation of NGH in China.The main contents are as follows.Firstly,a high-pressure low-temperature CT-based hydrate triaxial experiment system was developed to meet the needs of in-situ mechanical testing and microstructure analysis of hydrate-bearing sediment deformation process,and a series of technical methods were proposed,such as coupling calibration method for image thresholding,quantitative statistical method of micro fabric elements,and shear band development identification method,which provide equipment and technical support for hydrate-bearing sediment consolidation,shear and dissociation deformation visualization experiments and analysis of reservoir cementation weakening failure mechanism.Secondly,CT visualization experiments for the hydrate-bearing sediment during consolidation process were carried out to investigate the effects of factors such as effective stress and local saturation influence on the microstructure evolution and macromechanical behavior of hydrate-bearing sediments.The results show that the consolidation deformation of hydrate sediments mainly occurs in the area with relatively low local hydrate saturation,the count and the average radius of pores decrease rapidly during the deformation process,the area of hydrate-sand contact surface increases rapidly,and the rate of change decreases gradually with the increasing effective stress,and the incremental displacement field inside the hydrate-bearing sediment evolves from stepped cone-shaped distribution to patchy distribution.The above study clarifies the microscopic response mechanism of hydrate-bearing sediment consolidation and deformation,and provides a theoretical and data basis for the analysis of reservoir consolidation and settlement during depressurization exploitation process.Thirdly,CT visualization experiments for the hydrate-bearing sediment during shearing process were carried out to investigate factors such as the spatial distribution of hydrates and effective confining pressure influence on the microstructure evolution and macromechanical behavior of hydrate-bearing sediments.The results show that the hydrate weakening failure is the essential cause of the sediment shear deformation,which is specifically manifested in the processes of“loading-stress concentration-initial crack formation-crack propagation-cementation shedding-cementation crushing”.Under the hydrate saturation,porosity and composition,the sediments with coalescence distribution of hydrates have a thinner and steeper shear band than those sediment with dispersed distribution of hydrate,and are more prone to occur cementation failure.Macroscopically,it presents smaller strength and yield strain,and is prone to occur shear failure.In addition,as the effective confining pressure increases,the occlusion between particles gradually increases,some cracks are closed,and the thickness and inclination of the shear band of hydrate-bearing sediments decrease gradually,and the sediment shows greater the strength and yield strain macroscopically.The above research reveals the cementation failure mechanism of hydrate-bearing sediment during shearing process,and provides a theoretical basis for developing constitutive models of hydrate reservoir.Fourthly,CT visualization experiments for the hydrate-bearing sediment during shearing process were carried out to investigate depressurization and thermal stimulation influence on the microstructure evolution and macromechanical behavior of hydrate-bearing sediments.The results show that the hydrate covered on the sand particle surface dissociates firstly,and the hydrate located at the sand particle contacts dissociates last in both depressurization and thermal stimulation processes.With the depressurization process,the cementation support between sand particles gradually disappears,the sediment is compacted,the distribution of sand particle inclination and azimuth angle gradually homogenizes,and the strength and stiffness of the sediment significantly decreases;meanwhile,the hydrate secondary formation phenomenon occurs during the dissociation process,and the location is mainly concentrated in the hydrate cementation coverage area close to the leading edge of dissociation,which will block the pore and throat,causing local compactness.In addition,due to the effects of particle rearrangement and inhomogeneous distribution of dissociated water during hydrate dissociation process,the strength of dissociated sediments is lower than that of hydrate-free sediments.The above study has explored the hydrate dissociation influence on the microstructure evolution and macroscopic deformation of sediments,which provides theoretical and data support for the safety evaluation of NGH exploitation.Finally,a new hydrate-bearing digital core morphology modeling algorithm was proposed,the cementing and pore-filling hydrate-bearing digital cores were constructed,and the microstructure evolution of the sediment during hydrate formation process was studied.It is found that the pore-throat structure of the sediment is gradually compressed and divided during the growth of hydrate,the tortuosity would increase and the fractal dimension would decrease.At the same time,combined with the discrete element deformation analysis method,a triaxial shear numerical experiment was carried out.It was found that with the progress of the shearing process,the internal force chain of the hydrate gradually changed from pressure to tension,and the fracture surface gradually increased,resulting in the falling off of hydrate from the sand particle surface,causing cementation weakening and failure.Related research has clarified the weakening and failure nature of the hydrate-bearing sediments,and provides an effective method for the next cross-scale analysis of hydrate-bearing sediment deformation.