Preliminary numerical simulation of rock perforation cracking
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摘要: 岩石射孔作业后孔眼周围裂缝分布规律对后续压裂有不可忽视的影响。选取射孔围岩的横切面为研究对象，将三维射孔侵彻过程简化为二维扩孔过程。考虑岩石细观非均匀性，设细观强度参数服从韦布尔分布。应用拉伸破坏准则和Mohr-Coulomb压剪破坏准则，并用模量折减法处理单元开裂，从而用FEPG软件实现了有限元数值模拟。模拟结果表明：射孔后的岩石可根据裂缝产生原因及分布由内而外划分为四个区域：压剪破坏区、拉伸破坏集中区、拉伸破坏扩展区和未破坏区。分析了不同射孔弹规格及围压条件下裂纹分布变化规律。与室内模拟实验结果进行对比分析，初步验证了模型的有效性。Abstract: The distribution of cracks around the rock hole after perforation has a great influence on the subsequent fracturing. A cross-section of the target is selected as the researching object. The process of three-dimensional penetration is simplified into a two-dimensional reaming process. In terms of the mesoscopic heterogeneity of the rock, the strength parameters of meso-units are set to obey the Weibull probability distribution. The tensile failure criteria and the Mohr-Coulomb compression shear failure criteria are applied, and modulus reduction method is applied to deal with cracking. Then FEPG software is used to achieve a finite element method (FEM) simulation. The simulation results show that according to the causes and distribution of cracks, the perforated rock can be divided into four regions from the inside to the outside: compression shear damage zone, tensile damage concentration zone, tensile damage propagation zone and undamaged zone. The variations of crack distribution under different loading and confining pressure conditions are analyzed. Compared with the results of laboratory simulation experiments, the validity of the model is preliminarily verified. The research results lay the foundation for subsequent research work.
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图 6 侵彻孔直径7 mm围压2 MPa时岩石切片破坏形态
Figure 6. The rock damage form with 7 mm perforating charge under 2 MPa confining pressure
图 7 不同侵彻孔直径下岩石切片裂纹分布图
Figure 7. Distribution of rock crack number with perforating charge of different diameters
图 8 侵彻孔径9 mm围压为0时岩石切片裂缝形态
Figure 8. The rock damage form with 9 mm charge under no confining pressure
图 9 不同围压下岩石切片裂纹分布图
Figure 9. The distribution of rock crack number under different confining pressures
图 14 X1-B面不同缝宽裂缝数目分布图
Figure 14. The distribution of crack with different width on X1-B surface
图 15 数值模拟与物理实验靶材破坏形态对比图
Figure 15. Comparison of damage forms between physical experiment and numerical simulation
图 16 数值模拟与物理实验裂缝数目分布图
Figure 16. Crack number distribution by physical experiment and numerical simulation
表 1 模拟计算参数
Table 1. Parameters of simulation calculation
参数
名称弹性模
量/GPa抗拉强
度/MPa内摩擦
角/(°)内聚力/
MPa泊松比 侵彻孔
直径/mm参数大小 40 10 17 20 0.3 11 
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表 2 天然页岩与水泥试样数据对比表
Table 2. Comparison of performance parameters between natural shale and cement sample
试样种类 渗透率/(10−6 μm2) 孔隙度/% 天然页岩 4.0 4.1 水泥试样 6.7 3.3
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表 3 水泥试样尺寸及编号表
Table 3. Size and number of cement samples
围压/
MPa试样直径/
mm试样长度/
mm试样
编号上端面
编号下端面
编号30 200 200 X1 X1-A X1-B 300 X2 X2-A X2-B 500 X3 X3-A X3-B 20 200 200 Y1 Y1-A Y1-B 300 Y2 Y2-A Y2-B 500 Y3 Y3-A Y3-B
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