Research on Interlayer Interference and the Fracture Propagation Law of Shale Oil Reservoirs in the Dongying Sag
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摘要: 东营凹陷页岩油储量丰富，但储层物性差，纵向含油层系多而薄，多为灰泥岩互层。为了准确描述东营凹陷页岩油储层层间应力干扰机理及水力压裂裂缝的扩展规律，利用非线性有限元法建立了基于渗流–应力–损伤耦合的多薄互层分层压裂模型，模拟分析了不同排量、压裂液黏度及不同上、下隔层厚度下的裂缝扩展形态、规律和诱导应力场，研究了裂缝扩展形态与诱导应力场的关系，并对压裂施工参数进行了优化。模拟结果表明：随着水力裂缝扩展，应力干扰区域越来越大；当排量为9～12 m3/min、黏度为20 mPa∙s时，裂缝尖端诱导应力大，易连通天然裂缝，压裂改造效果明显；上部隔层厚度大于2.50 m、下部隔层厚度大于4.50 m时，极少出现穿层现象。研究结果可为东营凹陷页岩油储层后续的水力压裂施工提供理论支撑。Abstract: The Dongying Sag is rich in shale oil reserves, but poor in reservoir physical properties. The sag has many thin oil-bearing layer sequences in the vertical direction, which are mostly interlayered with limestone and mudstone. To precisely describe the law for the interlayer interference and the fracture propagation of the shale oil reservoirs in the Dongying Sag, a separate-layer fracturing model based on seepage-stress-damage coupling was built with the nonlinear finite element method. The morphology and law for fracture propagation, and induced stress field were analyzed considering different flow rates and viscosities of fracturing fluid, and different thicknesses of the upper and lower isolation layers. On this basis, the fracturing parameters were optimized. Simulation results show that the stress interference area grows along with the propagation of hydraulic fractures. When the flow rate is 9–12 mm3/min and the viscosity is 20 mPa∙s, the induced stress at the tips of fractures is high. In this case, natural fractures are prone to be connected and good stimulation results can be achieved. In addition, layer crossing is rare when the thickness of the upper isolation layer is greater than 2.5 m and that of the lower one is greater than 4.5 m. The results can provide theoretical support for the subsequent hydraulic fracturing of shale oil reservoirs in the Dongying Sag.
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Keywords:
- shale oil /
- interlayer interference /
- hydraulic fracturing /
- fracture propagation /
- induced stress /
- Dongying Sag
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图 2 Cohesive黏弹性孔压单元的损伤判断依据
Figure 2. Failure criterion for the Cohesive viscoelastic pore-pressure elements
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图 3 灰泥互层渗流–应力–损伤耦合分层压裂几何模型和离散模型
Figure 3. Geometric model and discrete model of separate-layer fracturing based on seepage-stress-damage coupling for interlayers of limestone and mudstone
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图 4 数值模拟与现场实测的破裂压力
Figure 4. Simulated fracturing pressure and field measured fracturing pressure
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图 5 不同压裂液排量下的裂缝形态
Figure 5. Fracture morphology under different flow rates of fracturing fluid
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图 6 不同压裂液排量下裂缝宽度和裂缝高度的变化
Figure 6. Variation of fracture width and height under different flow rates of fracturing fluid
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图 7 不同压裂液排量下的最小水平主应力
Figure 7. Minimum horizontal principal stress under different flow rates of fracturing fluid
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图 9 不同压裂液排量下储/隔层中不同路径最小诱导应力沿x轴的变化情况
Figure 9. Variations in minimum induced stress along the x axis in different paths of reserve layers/isolation layers under different flow rates of fracturing fluid
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图 10 不同压裂液黏度下的裂缝形态
Figure 10. Fracture morphology under different viscosities of fracturing fluid
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图 11 不同压裂液黏度下的最小水平主应力
Figure 11. Minimum horizontal principal stress under different viscosities of fracturing fluid
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图 12 不同压裂液黏度下储层、隔层中不同路径最小诱导应力沿x轴的变化情况
Figure 12. Variations in minimum induced stress along the x axis in different paths of reserve layers/isolation layers under different viscosities of fracturing fluid
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图 13 不同上隔层厚度对应的最小水平主应力
Figure 13. Minimum horizontal principal stress with different thicknesses of the upper isolation layer
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图 14 不同下隔层厚度对应的最小水平主应力
Figure 14. Minimum horizontal principal stress with different thicknesses of the lower isolation layer
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图 15 隔层2和隔层3中不同厚度下x轴方向最小水平诱导应力的变化情况
Figure 15. Variations of minimum horizontal induced stress along the x axis with different thicknesses in the 2nd and 3rd isolation layers
表 1 各层位的地层物性力学参数
Table 1 Physical and mechanical parameters of formations in different layers
层位 岩性 弹性模量/GPa 泊松比 渗透率/mD 孔隙度，% 孔隙压力/MPa 垂向应力/MPa 最大水平主
应力/MPa最小水平主
应力/MPa隔层4 泥岩 24 0.28 0.02 3.2 38.21 80.53 78.52 73.06 储层3 灰岩 25 0.27 2.67 6.5 38.21 80.73 71.61 67.25 隔层3 泥岩 24 0.28 0.08 1.8 38.21 80.93 77.33 72.98 储层2 灰岩 25 0.27 1.53 6.8 38.21 81.43 70.41 66.39 隔层2 泥岩 24 0.28 0.01 3.1 38.21 82.16 76.74 72.38 储层1 灰岩 25 0.27 1.21 8.1 38.21 84.83 71.61 67.75 隔层1 泥岩 24 0.28 0.15 1.7 38.21 84.92 78.74 73.48 
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表 2 水力裂缝Cohesive单元的基本参数
Table 2 Basic parameters of Cohesive elements of hydraulic fractures
层位 t0n/MPa t0s/MPa Gcs/（Pa·m） 储层 1 1 26.7 隔层 2 2 54.0
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