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摘要:
为探究细观尺度下岩石破坏机理，结合数字图像处理（DIP）技术和颗粒流代码（PFC）方法提出了一种带有表征矿物属性新的建模方法，利用常规三轴压缩实验条件下获取的宏观力学参数和破坏形态对建立的离散元数值模型进行了校正，分析了地应力条件下岩石矿物属性对岩石破坏演化的影响，探究了非均质性结构对微裂行为的影响。结果表明：常规三轴压缩实验加载过程主要分为裂纹闭合期、弹性变形期、裂纹生长期、裂纹爆发期；由选取的3块泥页岩样品计算机断层扫描（CT）切片模拟结果表明，岩石的非均质性对岩石微裂纹的产生和岩石物理力学参数均有一定的影响，非均质性越弱，峰值强度（
σ max）和弹性模量（E ）越高；从岩石裂纹扩展分布来看，非均质性的增强使得微裂纹分布更加复杂；从岩石微裂纹空间分布来看，微裂纹更加倾向于产生在白云石类矿物和石英类矿物的边界之间。研究成果对油气地下深部开采、防灾工程具有重要的参考意义。Abstract:Objective To investigate the failure mechanism of rocks at the mesoscopic scale,
Methods this study proposes a novel modeling method that combines digital image processing (DIP) technology and particle flow code (PFC) to characterize mineral properties. The discrete element numerical model was calibrated using macroscopic mechanical parameters and failure modes obtained from conventional triaxial compression tests. Under geostress conditions, the influence of rock mineral properties on rock failure evolution and the effect of heterogeneous structures on microcracking behavior were analyzed.
Results The results show that the loading process of conventional triaxial compression tests can be divided into four stages: Crack closing stage, elastic deformation stage, crack growth stage, and crack explosion stage. Simulations based on computed tomography (CT) slices of three shale samples indicate that rock heterogeneity has a certain impact on the generation of rock microcracks and rock physical-mechanical parameters: Weaker heterogeneity corresponds to higher peak strength (
σ max) and elastic modulus (E ). In terms of crack propagation distribution, increased heterogeneity leads to a more complex microcrack distribution. From the perspective of the spatial distribution of rock microcracks, microcracks tend to occur preferentially at the interfaces between dolomite and quartz minerals.Conclusion The research findings provide important reference significance for deep underground oil and gas exploitation and disaster prevention engineering.
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Key words:
- mud shale /
- image processing /
- rock mechanics /
- heterogeneity /
- microcracking behavior
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图 3 模型组成(a)、平行黏结模型本构关系(b)、平行黏结模型的力−位移规律和破坏包络线(c)、岩石的破坏包络线[32](d)、PFC颗粒破坏模式[33](e)
gs. 接触面积相关因子；$\bar{k}_{\mathrm{n}} $. 法向刚度；$\bar{k}_{\mathrm{s}} $. 切向刚度；$ \overline{\sigma}_{\mathrm{c}} $. 黏结强度；$\overline{c} $. 黏聚力；$ \overline{\varphi} $. 内摩擦角；μ. 摩擦系数；$ \overline{F}/\overline{A} $. 单位面积上受到的力（抗拉强度）；$ \overline{\tau} $. 剪切强度；$\overline{\tau}_{\mathrm{max}} $为破坏时受到的剪切应力；$\overline{\tau}_{\mathrm{c}} $为岩石脆-塑到破坏变化过程中剪切强度的指向大小；σ为抗拉强度；$\overline{F}、{\overline{F}}_{\rm{n}}、{\overline{F}}_{\rm{s}}、{\overline{M}}_{\rm{a}} $分别为平行键力、平行键法向分力、平行键剪切分力、平行键合力矩的赋值；$\hat{n} $. 法向力方向；$ \hat{t}_{\mathrm{c}} $. 弯矩法向方向；$ \hat{t}_{\mathrm{s}} $. 剪切分量方向；1. 单轴拉伸；2. 单轴压缩；3. 脆塑转变；4.临界状态
Figure 3. Model composition (a), constitutive relation of parallel bond model (b), force-displacement law and failure envelope of the parallel bond model (c), rock failure envelope (d) and PFC particle failure pattern (e)
图 4 基于CT图像的离散元模型二维重建
Figure 4. Two-dimensional reconstruction of the discrete element model based on CT images
图 5 参数校准结果
蓝紫色到红色表示越靠近红色区域破坏状态越剧烈；界面表示模拟结果的断裂面
Figure 5. Parameter calibration results
图 6 破坏过程中不同阶段划分及应力指标确定
Figure 6. Division of different stages and determination of stress indexes in the failure process
图 8 常规三轴压缩实验和数值模拟应力应变曲线
Figure 8. Stress-strain curves of the conventional triaxial compression test and numerical simulation
图 9 泥页岩样品V1（a1～f1）、V2（a2～f2）和V3（a3～f3）数值模拟裂隙演化过程
Figure 9. Numerical simulation fracture evolution process of shale sample V1(a1-f1), V2(a2-f2) and V3(a3-f3)
图 10 泥页岩样品V1(a1～h1)、V2(a2～h2)和V3(a3～h3)数值模拟的微裂纹分布及其与CT 结果比较图
a1～a3. 数值模拟图；b1～b3. CT图；c1～e1，c2～e2，c3～e3. 数值模拟局部放大图；f1～h1，f2～h2，f3～h3. CT局部放大图
Figure 10. Micro fracture distributions of numerical simulation of shale sample V1(a1-h1)、V2(a2-h2) and V3(a3-h3) and their comparison with CT results
表 1 离散元细观参数标定值
Table 1. Calibration values of the discrete element mesoscopic parameters
元素 参数 石英类 白云石类 金属矿物 颗粒尺寸 最小粒径Rmin/mm 0.15 0.15 0.15 粒径比Rmax/Rmin 1.66 1.66 1.66 黏结模型参数 颗粒接触模量E*/GPa 62 58 80 颗粒刚度比k* 1.5 1.5 2.8 摩擦系数μ 0.5 0.5 0.5 平行黏结模量Ec/GPa 62 58 80 平行黏结刚度比kc 1.5 1.5 2.8 抗拉强度pb-ten/MPa 120 115 165 黏聚力pb-coh/MPa 135 130 182 光滑节理模型参数 法向刚度sj_kn/GPa 250000 切向刚度sj_ks/GPa 78000 摩擦系数sj_fric 0.45 抗拉强度sj-ten/MPa 50 黏聚力sj-coh/MPa 65 
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