南海北部天然气水合物赋存区沉积物渗透性敏感规律试验研究

doi:10.13745/j.esf.sf.2024.4.25

摘要/Abstract

摘要：

天然气水合物主要分布于含有黏土矿物的海洋细颗粒沉积物中。我国南海北部海域沉积物中含有大量的伊利石,揭示其渗透性在降压开采过程中的变化规律对于合理调控水气产出过程非常重要。本文采用取自南海北部水合物试采目标海域的沉积物重塑土样,在不同的孔隙液盐度条件下,进行了界限含水率测定试验、渗流固结试验和稳态渗流试验,获得了土样液限含水率、塑限含水率、塑性指数、渗透系数和渗透率数据,分析了南海沉积物渗透性的应力敏感性和化学敏感性规律。结果表明: 南海沉积物的界限含水率指标均随着孔隙液盐度的增加而降低;南海沉积物的渗透性随着孔隙液盐度的增加而增强,随着应力的增加而减弱,并且应力敏感性程度随着孔隙液盐度的增加而变大,化学敏感性程度随着应力的增加而变小;采用归一化孔隙比获得的渗透系数拟合关系不受孔隙液盐度的影响,便于工程应用。

关键词: 渗透率, 渗透系数, 界限含水率, 敏感性指标, 降压开采

Abstract:

Natural gas hydrates are mainly distributed in fine-grained marine sediments containing clay minerals. For instance, the sediments in the northern region of the South China Sea contain a large amount of illite. Understanding the changes in permeability during the depressurization extraction process is crucial for effectively regulating the production of gas from hydrates. This study utilized remolded soil samples taken from the target hydrate test-production area in the northern South China Sea and conducted limit water content tests, drained consolidation tests, and steady-state seepage tests under different pore fluid concentration conditions. Data on the liquid limit, plastic limit, plasticity index, permeability coefficient, and permeability were obtained. The stress sensitivity and chemical sensitivity of the permeability of South China Sea sediments were analyzed. The results indicated that the limit water content parameters of the South China Sea sediments decreased as the pore fluid concentration increased. The permeability of the sediments increased with the pore fluid concentration, while it decreased with increasing stress. Moreover, the degree of stress sensitivity increased as the pore fluid concentration rose, whereas the degree of chemical sensitivity diminished with increasing stress. The fitting relationship of the permeability coefficient obtained using the normalized void ratio was found to be unaffected by pore fluid concentration, making it convenient for engineering applications.

Key words: hydraulic permeability, hydraulic conductivity, water ratio limit, sensitivity index, depressurization

中图分类号:

宋德坤, 刘乐乐, 王栋. 南海北部天然气水合物赋存区沉积物渗透性敏感规律试验研究[J]. 地学前缘, 2024, 31(6): 405-414.

SONG Dekun, LIU Lele, WANG Dong. Experimental study on the sensitivity of hydraulic permeability of fine-grained sediments sampled from a gas hydrate distribution area in the northern South China Sea[J]. Earth Science Frontiers, 2024, 31(6): 405-414.

图/表 12

图1 南海北部沉积物粒度分布

Fig.1 Grain size distribution of marine sediments sampled from the northern South China Sea

图2 南海北部沉积物矿物组成

Fig.2 Mineral composition of marine sediments sampled from the northern South China Sea

图3 含水合物沉积物三相草图

Fig.3 Three phase diagram of hydrate-bearing sediments

图4 土样渗透性测定试验装置示意图 a—渗流固结试验装置。1—百分表架;2—百分表;3—透水石; b—稳态渗流试验装置。1—溶液箱;2—恒流泵;3—流量计;4—传压板;5—土样;6—盐溶液;7—固结盒;8—底座;9—加压装置。 4—土样;5—围压泵;6—围压腔;7—透水石;8—差压传感器。

Fig.4 The schematic diagram of apparatuses for hydraulic permeability and conductivity measurements

图5 不同溶液盐度条件下土样的界限含水率 a—对塑限含水率影响;b—对液限含水率影响;c—对塑性指数影响。

Fig.5 Water ratio limits of the samples under different salinities of solution

图6 孔隙液盐度对黏土结构影响示意图 a—低盐度条件下蒙脱石颗粒结构示意图;b—高盐度条件下蒙脱石颗粒结构示意图;c—低盐度条件下高岭石颗粒结构示意图;d—高盐度条件下高岭石颗粒结构示意图。

Fig.6 Schematic diagram of the effect of pore fluid salinity on clay fabric

图7 不同孔隙液盐度条件下土样渗透系数随竖向应力变化情况

Fig.7 Vertical stress dependent hydraulic conductivity under different salinities of pore fluid

图8 归一化孔隙比与归一化渗透系数的线性关系

Fig.8 Linear relationship between normalized void ratio and normalized hydraulic conductivity

图9 不同孔隙液盐度条件下渗透率随竖向应力变化情况

Fig.9 Vertical stress dependent hydraulic permeability under different salinities of pore fluid

图10 不同竖向应力条件下渗透率随孔隙液盐度变化情况

Fig.10 Pore fluid salinity dependent hydraulic permeability under different vertical stresses

图11 归一化渗透率敏感系数与归一化应力关系曲线

Fig.11 Relationship between normalized sensitivity coefficient of hydraulic permeability and normalized vertical stress

图12 孔隙比与归一化渗透系数关系曲线

Fig.12 Relationship between void ratio and normalized hydraulic conductivity

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