Soil-water characteristic curve and permeability of hydrate-bearing soils
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摘要: 含水合物土的土水特征曲线和渗透系数对分析水合物开采效率和地层稳定性具有重要意义。通过自我改造而成的含水合物土的土水特征曲线的测试装置，测试了含水合物土（黏质粉土和砂土）的土水特征曲线，研究了水合物形成对土体的土水特征曲线的影响规律和机理，并且分析了含水合物土体的非饱和状态下的渗透系数。试验结果分析表明：水合物形成对含水合物土体的土水特征曲线存在明显影响；随着水合物饱和度的增长，可以看到边界效应段明显增大，过渡段土水特征曲线变得逐渐平缓，非饱和残余段对于残余水饱和度更低，但是VG模型仍能有效的描述含水合物土的土水特征曲线；进气值随着水合物饱和度的增加而增大，而残余有效水饱和度则随之减小，这主要是由于水合物形成改变了沉积物内部的孔隙孔径分布特征。在非饱和状态下，由于渗流通道被气体挤占，含水合物土的相对渗透系数随毛细吸力增加而减小，但是在同样的毛细吸力下，越大的水合物饱和度对应的相对渗透系数越小。Abstract: Understanding the soil-water characteristic curve (SWCC) and permeability of hydrate-bearing soils plays a critical role in analyzing the production efficiency and layer stability during hydrate exploitation. Based on the self-improved apparatus, hydrate is formed within clayey silt and sand sediment, and the SWCC of the hydrate-bearing clayey silt and sand is measured. Further, the influence law and mechanism of hydrate formation on the SWCC are investigated, and the permeability of the hydrate-bearing soils at unsaturated state is analyzed. The test results show the hydrate formation has a significant effect on the SWCC of the hydrate-bearing soils. As the hydrate saturation increases, the boundary effect segment remarkably increases, the SWCC changes gently during the transition segment, and the corresponding saturation reduces. However, the VG model is able to address the SWCC of the hydrate-bearing soils. Since the hydrate formation changes the pore-size distribution structure of the hydrate-bearing soils, the gas entry pressure increases but the saturation of the effective residual water decreases with the increasing hydrate saturation. Under the unsaturated state, the relative permeability of the hydrate-bearing soils reduces with the increasing capillary suction as the seepage channel is crowded by gas. At a given capillary suction, the higher hydrate saturation corresponds to the smaller relative permeability.
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图 1 试验装置示意图
Figure 1. Schematic diagram for measuring soil-water characteristic curve of hydrate-bearing soils
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图 7 残余水饱和度和进气值随水合物饱和度的变化关系
Figure 7. Change in residual water saturation and gas entry pressure versus hydrate saturation
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图 9 水合物形成前后含水合物土的内部结构示意图
Figure 9. Schematic diagram for inter-structure of HBS before and after the hydrate formation
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图 10 含水合物土的相对渗透系数（试样A和试样B）
Figure 10. Relative permeability of HBS (Specimens A and B)
表 1 土的基本物性指标
Table 1 Basic physical properties of soils
土样
类型相对质量密度 预制
干密度/
(g·cm-3)液限
wL/%塑限wP/% 塑限指数Ip 不均匀系数Cu 曲率系数Cc 试样A 2.65 1.5 32.8 23.3 9.5 19 1.89 试样B 2.65 1.5 — — — 2.98 1.36 
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表 2 van Genuchten (VG)模型参数(试样A)
Table 2 Model parameters for van Genuchten Model (Soil A)
参数 Srw α/kPa-1 m n R2 Sh=0 0.582 0.110 0.019 46.115 0.9961 Sh=0.35 0.490 0.104 0.019 45.230 0.9936 Sh=0.50 0.358 0.040 0.185 6.034 0.9985 Sh=0.65 0.180 0.035 0.200 5.819 0.9954 注：R2为相关系数。
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表 3 van Genuchten (VG)模型参数(试样B)
Table 3 Model parameters for van Genuchten Model (Soil B)
参数 Srw α/kPa-1 m n R2 Sh=0 0.419 0.218 0.027 34.131 0.9839 Sh=0.35 0.378 0.150 0.014 83.230 0.9882 Sh=0.50 0.311 0.105 0.030 60.051 0.9923
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