Micromechanical characteristics and controlling mechanism of deep shale: a case study of well JYA in Pingqiao block, Fuling area
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摘要: 四川盆地涪陵地区深层页岩气具有构造复杂、地应力高、地应力差大、地层温度高、致密化程度高、低孔低渗、孔渗变化规律复杂等地质特征，不同井之间产量差异大，原因之一是对于深层页岩气储层的地质力学特征和控制机理认识不足，适合压裂的甜点区间识别不准确。因此，对涪陵地区五峰组—龙马溪组海相页岩进行研究，围绕深层页岩气储层的微观地质力学特征与控制机理这一关键科学问题，通过微观岩石力学实验、数字光斑实验、X射线衍射、总有机碳含量、SEM扫描电镜等5个系列实验设计，结合数字图像处理技术，精细刻画了龙马溪组页岩在加载条件下的应力场和位移场变化及微观裂纹扩展过程，分析了龙马溪组页岩的变形与破裂特征。实验测得深层页岩总有机碳含量约为4.2%，石英含量为55.4%，黏土矿物含量为26.9%，明确了深层页岩的微观损伤变化的5个过程，即压密、弹性、裂纹均匀扩展、裂纹扩展破坏及脆性破坏。在石英等脆性矿物及有机质等软组分的控制作用下，深层页岩微观破裂具有多种裂缝扩展模式。同时，计算了深层页岩样品的断裂韧性指数，其中Ⅰ型断裂韧性指数为18.279 $\mathrm{MPa} \cdot \sqrt{\mathrm{m}}$, Ⅱ型断裂韧性指数为1.243 $\mathrm{MPa} \cdot \sqrt{\mathrm{m}}$。实验得到的断裂韧性指数可应用于评价深层页岩的脆性，也可为深层页岩的压裂改造提供指导。Abstract: The deep shale gas in the Fuling area is characterized by complex structures, high crustal stress, significant stress differences, high formation temperatures, high compaction levels, low porosity, low permeability, and complex porosity-permeability variation patterns. One reason for the significant production differences between wells is the insufficient understanding of the geomechanical features and controlling mechanisms of deep shale gas reservoirs, and the inaccurate identification of sweet spots for hydraulic fracturing. This study focuses on the marine shale of the Wufeng-Longmaxi formations in the Fuling area, investigating the micromechanical characteristics and controlling mechanisms of deep shale gas reservoirs through five series of experiments: micro rock mechanics experiments, digital speckle experiments, X-ray diffraction, total organic carbon content measurement, and scanning electron microscopy (SEM). Coupled with digital image processing technology, the changes in stress field and displacement field, and microcrack propagation processes in the Longmaxi shale under loading conditions were meticulously depicted. The deformation and fracture characteristics of the Longmaxi shale were analyzed. Experimental results indicated that the total organic carbon content in the deep shale is approximately 4.2%, with quartz content at 55.4% and clay mineral at 26.9%. The study identified five stages of microdamage evolution in deep shale: compaction, elasticity, uniform crack propagation, crack propagation failure, and brittle failure. Under the influence of brittle minerals such as quartz, and soft components such as organic matter, the microcracks in deep shale exhibit various propagation modes, including transgranular, intergranular, and laminated layer cracks. Additionally, the fracture toughness indices of the deep shale samples were calculated, with Mode Ⅰ index being 8.279 $\mathrm{MPa} \cdot \sqrt{\mathrm{m}}$ and Mode Ⅱ index being 1.243 $\mathrm{MPa} \cdot \sqrt{\mathrm{m}}$. These experimentally obtained fracture toughness indices can be applied to evaluate the brittleness of deep shale, providing guidance for deep shale fracturing modification.
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Key words:
- micromechanical characteristics /
- brittleness characteristics /
- fracturing /
- deep shale /
- Fuling area
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图 2 四川盆地涪陵地区平桥区块构造及井位
Figure 2. Structure and well location of Pingqiao block in Fuling area, Sichuan Basin
图 3 四川盆地涪陵地区JYA井综合柱状图
Figure 3. Comprehensive column diagram of well JYA in Fuling area, Sichuan Basin
图 4 四川盆地涪陵地区深层页岩微观力学实验样品及仪器照片
Figure 4. Micromechanical samples of deep shale in Fuling area and experimental instruments, Sichuan Basin
图 5 四川盆地涪陵地区深层页岩微观损伤实验流程
Figure 5. Flow chart of microscopic damage experiment of deep shale in Fuling area, Sichuan Basin
图 6 四川盆地涪陵地区深层页岩微裂纹数字化处理流程
Figure 6. Digital processing flow for micro-fractures in deep shale from Fuling area, Sichuan Basin
图 7 四川盆地涪陵地区深层页岩位移—载荷曲线
Figure 7. Displacement and load curves of deep shale in Fuling area Sichuan Basin
图 8 四川盆地涪陵地区深层页岩微裂纹SEM图像
Figure 8. SEM images of microcracks in deep shale in Fuling area, Sichuan Basin
图 9 四川盆地涪陵地区深层页岩(样品3)低加载力前后SEM图像
a.纹层(加载前)；b.纹层(加载后)；c.黏土矿物(加载前)；d.黏土矿物(加载后)；e.有机质(加载前)；f.有机质(加载后)
Figure 9. SEM images of deep shale before and after loading in Fuling area, Sichuan Basin
图 10 四川盆地涪陵地区深层页岩微观裂纹数字图像
Figure 10. Digital image of microcracks in deep shale in Fuling area, Sichuan Basin
图 11 四川盆地涪陵地区深层页岩微观裂纹扩展SEM图像
Figure 11. SEM images of microcrack growth in deep shale in Fuling area, Sichuan Basin
图 12 四川盆地涪陵地区深层页岩微裂纹扩展模式
Figure 12. Microcrack propagation mode of deep shale in Fuling area, Sichuan Basin
图 13 四川盆地涪陵地区深层页岩微观结构影响裂纹扩展模式
Figure 13. Microcrack propagation mode influenced by micro structure in deep shale in Fuling area, Sichuan Basin
图 14 四川盆地涪陵地区深层页岩损伤过程位移场特征
Figure 14. Displacement field characteristics during damage process of deep shale in Fuling area, Sichuan Basin
图 15 四川盆地涪陵地区深层页岩损伤过程应变场特征
Figure 15. Strain field characteristics during the damage process of deep shale in Fuling area, Sichuan Basin
图 16 四川盆地涪陵地区深层页岩测井参数及脆性评价结果
Figure 16. Logging parameters and brittleness evaluation of deep shale in Fuling area, Sichuan Basin
表 1 四川盆地涪陵地区深层页岩微观力学参数
Table 1. Micromechanical parameters of deep shale in Fuling area, Sichuan Basin
样品编号 长/mm 宽/mm 高/mm 受力面积/mm2 峰值载荷/N 单轴抗压强度/MPa 绝对位移/mm 1 12.37 3.20 3.63 11.62 1 340 115.32 0.15 2 12.19 3.63 3.52 12.78 1 850 144.78 0.20 3 12.25 3.59 3.53 12.67 500 0.09 4 12.18 3.65 3.39 12.37 1 000 0.07 
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表 2 四川盆地涪陵地区深层页岩微观裂纹参数
Table 2. Microcrack parameters of deep shale in Fuling area, Sichuan Basin
裂纹序号 裂纹面积/μm2 裂纹长度/μm 裂纹宽度/μm 裂纹周长/μm 方位角/(°) 1 27 555.00 1 763.31 56.82 3 672.25 6.74 2 8 379.00 732.75 38.72 1 757.58 1.81 3 11 982.00 980.00 11.12 2 285.61 2.75 4 7 888.00 694.66 10.78 1 676.18 63.18 5 3 536.00 400.87 7.44 1 078.27 7.02 6 1 003.00 129.45 7.52 366.75 7.85
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表 3 四川盆地涪陵地区深层页岩断裂韧性参数
Table 3. Fracture toughness parameters of deep shale in Fuling area, Sichuan Basin
序号 峰值载荷/N 抗拉强度/MPa 断裂韧性/($\mathrm{MPa} \cdot \sqrt{\mathrm{m}}$) KIC0 KⅡC0 KC0 1 1 220 11.65 27.52 1.53 14.53 2 1 200 11.45 26.57 1.50 14.04 3 1 100 10.50 22.10 1.37 11.74
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