|Alternative Title||Dynamic and Static Characteristics of Hydrate Sediment and Wellhead Soil Layer Stability Analysis of Horizontal Well Mining|
|Place of Conferral||北京|
Natural gas hydrate is a kind of non-conventional resources with high potential. Study on the exploration and exploitation has been taken as the strategy plan of our country. However, the natural gas hydrate may dissociate and accordingly causes the softening and outburst of the soil layer, which can lead the deformation of the well, and even geological disasters such as marine landslips.
Aiming at the trial production of natural gas hydrate in the South China Sea, a series of experiments are carried out to study the dynamic and static mechanical properties of hydrate-bearing sediments(HBS); Numerical simulation is processed by using the business software FLAC3D to study the stratum deformation and instability around the horizontal well.
Firstly, the triaxle tests were conducted to obtain the dynamic and static mechanical properties of HBS with silt sand and clay as the skeleton by using the apparatus of HBS synthesis and dissociation. The effects of dynamic load amplitude, load frequency and hydrate saturations on HBS were studied. The residual strength after dynamic loading was obtained. The results show that there is a critical relative dynamic loading amplitude (the ratio of loading amplitude to effective confined pressure) for the relative static strength (ratio of residual static strength to initial static strength) of HBS with the skeleton of either silt or clay. That means, once the relative dynamic loading amplitude exceeds the critical value, the curve of residual static strength versus relative dynamic loading amplitude will develop rapidly, and the sediments will change from vibration-induced compaction to vibration-induced failure. The critical values are 0.55 and 0.4 for HBS with the skeleton of silt and clay respectively. Generally, the relative residual strength increases first and then decrease rapidly with the rise of the relative loading amplitude. The relation between the residual static strength and the relative dynamic load amplitude was presented based on the experimental data. It is hoped to provide a reference for the evaluation of safety due to dynamic load, e.g. earthquake during HBS exploitation.
Secondly, by using the integrated experimental platform of triaxial and CT for geotechnique, the macro-mechanical properties of HBS were measured, and the micro characteristics, such as the distribution of hydrates in the pores, the occurrence of the shear band were observed. The effects of saturation of natural gas hydrate and confined pressure were studied. The results show that the stress-strain curve presents the stain softening under high-hydrate saturation. The smaller the effective confining pressure, and the higher the hydrate saturation, the more obvious the strain softening phenomenon and dilation. The distribution of natural gas hydrate in sediments is relative uniform at high saturation while it was in the form of dispersed mass at low saturation. The shear failure initiates from the interface between the gas hydrate and the soils. A single shear band occurs and develops at low saturation, while at high saturation, the cross-type shear bands prevail, the width of shear band becomes small and the angle becomes large.
Finally, the decoupling method is presented to capture the hydrate dissociation, heat conduction and seepage. The initial and boundary conditions are given at different dissociation times. Analysis of the formation deformation caused by natural gas hydrate decomposition by depressurization was carried out. The business software FLAC3D is used to simulate the deformation of the soil layer around a horizontal well, considering the effects of the well length, dissociation length and the slope angle. The results show that with the increase of the dissociation length, the displacement of the soil layer above the well and at the surface of the overlying layer increases linearly. With the increase of the slope angle, the deformation increases in an approximate parabolic type. When the dissociation length is less than 0.1 times the thickness of the natural gas hydrate layer and the slop angle is about 3°, the effects of the well length on the deformation are weak. These results can be taken as the references for the trial exploitation of HBS in the South China Sea.
|孙芳芳. 含水合物沉积物的动静力学特性及水平井开采的井口土层稳定性研究[D]. 北京. 中国科学院大学,2019.|
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