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EFFECT OF CLAY MINERAL GRAIN CHARACTERISTICS ON MECHANICAL BEHAVIOURS OF HYDRATE-BEARING SEDIMENTS
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摘要: 含水合物的沉积物力学参数是水合物储层稳定性评价的基础数据。我国南海神狐海域含水合物的沉积物中含大量的黏土，深入了解黏土矿物对沉积物力学特性的影响对水合物开采具有十分重要的意义。基于PFC三轴压缩模拟，首先分析了沉积物中不含水合物时，黏土矿物颗粒特征的力学效应，然后分析了水合物对颗粒的胶结作用和围压对沉积物力学特性的影响。结果表明，不含水合物模型的偏应力-应变曲线呈明显的应变硬化特征。黏土矿物的含量、颗粒形状和排列对沉积物三轴压缩特性具有显著影响。黏土矿物含量的增多对沉积物力学强度具有明显的降低作用，黏土矿物形状为条形的沉积物强度和弹性模量要明显高于圆颗粒模型，在细观上受颗粒平均接触数影响，条形黏土颗粒的定向排列使模型的力学参数具有各向异性。水合物对颗粒的胶结作用可显著提高模型的峰值强度和弹性模量，随着颗粒胶结程度的增大和围压的减小，含水合物的沉积物的破坏方式由塑性破坏向脆性破坏转换。Abstract: The mechanical parameters of hydrate-bearing sediments are the key parameters for the stability evaluation of hydrate formation. The hydrate-bearing sediments in Shenhu area of South China Sea contain large amounts of clay mineral. It is of great significance to understand the effects of clay minerals on the mechanical properties of sediments for hydrate mining. Based on triaxial compression simulation in the PFC code,we first analyzed the mechanical effects of clay mineral to the sediment without hydrate. Secondly,we analyzed the cementation effect of hydrate to mineral grains and the influence of confining pressure on the mechanical properties of the sediment. Our results indicated that the deviator stress-strain curve of the model without hydrate shows obvious strain hardening characteristics. The clay mineral content,grain shape and grain arrangement have significant effects on the triaxial compression characteristics of the sediment. The increase in the content of clay mineral has a significant effect on reducing the mechanical strength of sediments. The peak strength and elastic modulus of the sediments with strip-shaped clay grain are significantly higher than those of the sediment with round-shaped clay grain,which are related to the average co-ordination number in the microscopic view. The directional arrangement of the strip-shaped clay grains makes the mechanical parameters of the model to be anisotropic. The cementation effect of hydrate on the grains can significantly increase the peak strength and elastic modulus of the model. With the increase of the interparticle cementation degree and the decrease of confining pressure,the failure mode of the hydrate-bearing sediments changes from plastic failure to brittle failure.
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图 1 我国不同海域的天然气水合物样品(刘昌岭等，2017)
a. 取自南海神狐海域的样品；b. 取自珠江口盆地东部海域的样品
Figure 1. Natural gas hydrate samples from different sea areas in China(Liu et al., 2017)
图 2 颗粒和平行黏结的受力-位移关系(Han et al., 2019)
a. 接触刚度模型；b. 平行黏结模型
Figure 2. Force-displacement behavior of the grain and the parallel bond(Han et al., 2019)
图 3 南海神狐海域沉积物的颗粒组分分布(张辉等，2016)
Figure 3. The grain size distribution of sediments from Shenhu area of South China Sea(Zhang et al., 2016)
图 4 水合物在沉积物孔隙中的3种主要赋存形式
a. 填充模式；b. 持力体模式；c. 胶结模式
Figure 4. Three main distribution forms of hydrate in sediments
图 5 颗粒间胶结程度与半径乘子关系示意图
Figure 5. Relationship between the interparticle cementation degree and radius multiplier
图 6 条形黏土矿物颗粒单元
Figure 6. Striped clay mineral grain unit
a. Se=1.0; b. Se=1.5; c. Se=2.0; d. Se=2.5; e. Se=3.0; f. Se=3.5
图 7 方案1中不同黏土矿物含量模型的偏应力-应变曲线
Figure 7. Deviatoric stress-strain curves of numerical models with different clay mineral content in scenario 1
图 8 方案1中黏土矿物含量对模型宏观力学参数的影响(围压1 MPa)
a. 峰值强度；b. 弹性模量; c. 泊松比
Figure 8. Effects of clay mineral content on model mechanical properties in scenario 1(confining pressure of 1 MPa)
图 9 方案2中黏土矿物形状对模型宏观力学参数的影响(围压1 MPa)
a. 峰值强度; b. 弹性模量; c. 泊松比
Figure 9. Effects of clay mineral grain shape on model mechanical properties in scenario 2(confining pressure of 1 MPa)
图 10 黏土矿物形状与颗粒平均接触数量的关系
Figure 10. Relationship between the clay mineral shape and the average contact number of grains
图 11 方案3中黏土矿物颗粒方向对模型宏观力学参数的影响(围压1 MPa)
a. 峰值强度; b. 弹性模量; c. 泊松比
Figure 11. Effects of clay mineral grain arrangement on model mechanical properties in scenario 3(confining pressure of 1 MPa)
图 12 不同黏土矿物颗粒排列下模型内部接触应力分布(以β=0°和90°模型为例，蓝色：黏土颗粒，黄色：砂颗粒)
Figure 12. Distribution of the contact force under different clay grains arrangement
a. β=0°; b. β=90°
图 13 方案4中不同围压下6种颗粒胶结程度模型的偏应力-应变曲线
Figure 13. Deviatoric stress-strain curves of models with different interparticle cementation degree and confining pressure in scenario 4
图 14 方案4中颗粒胶结程度对模型宏观力学参数的影响(围压2 MPa)
a. 峰值强度; b. 弹性模量; c. 泊松比
Figure 14. Effects of interparticle cementation degree on model mechanical properties in scenario 4 (confining pressure of 2 MPa)
图 15 黏结半径乘子为0.6时不同围压下模型的偏应力-应变曲线
Figure 15. Deviatoric stress-strain curves of model with radius multiplier of 0.6 under different confining pressure
图 16 围压2 MPa时6种颗粒胶结程度不同模型的破裂形态(黑色：张拉裂纹；红色：剪切裂纹)
Figure 16. Failure mode of the models with different interparticle cementation degree(black: tensile cracks, red: shear cracks)
a. λ=0; b. λ=0.2; c. λ=0.4; d. λ=0.6; e. λ=0.8; f. λ=1.0
表 1 PFC三轴压缩试验模型细观参数
Table 1. Micro mechanical parameters of numerical model
参数类型 砂颗粒 黏土颗粒 颗粒 粒径/mm 0.05 0.03 粒径比 1.66 1.66 密度/kg·m-3 2050 1900 模量/MPa 800 400 刚度比 1.5 1.5 摩擦系数 0.7 0.3 平行黏结 法向强度/MPa 5 切向强度/MPa 5 模量/MPa 600 刚度比 1.5 半径乘子λ 1.0 
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表 2 模型细观参数标定结果与试验结果对比
Table 2. Micro mechanical parameters comparison of calibration results and test results
研究对象 水合物含量表征 强度范围/MPa 初始模量范围/MPa 参考文献 试验结果 黏土质粉砂(黏土33%，粉砂67%) 水合物饱和度0~60.0% 5.0~10.0 300~400 张怀文等(2017) 黏土质粉砂(黏土31%，粉砂和砂土69%) 水合物饱和度0~25.7% 1.6~3.2 50~150 Wang et al.(2019) 黏土质粉砂(黏土36%，粉砂和砂土64%) 水合物质量含量4.1%~16.7% 0.5~3.5 100~400 李彦龙等(2020) 标定结果 黏土质粉砂(黏土20%，粉砂和砂土80%) 黏结半径乘子0~1.0 1.8~6.8 325~855
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表 3 4种研究方案设计
Table 3. Four scenarios with different mineral grain characteristics and confining pressure
方案 黏土矿物含量 黏土颗粒长短轴比Se 黏土颗粒倾角β 颗粒黏结半径乘子λ 围压/MPa 方案1 0、10%、20%、30%、40%、50% 1 随机 0 1 方案2 20% 1.0、1.5、2.0、2.5、3.0、3.5 随机 0 1 方案3 20% 3.0 0°、30°、60°、90° 0 1 方案4 20% 1 随机 0、0.2、0.4、0.6、0.8、1.0 1、2、5
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