Numerical simulation of mesoscopic deformation and failure for glutenite in Triassic Baikouquan Formation, Junggar Basin
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摘要: 我国西部准噶尔盆地三叠系百口泉组砂砾岩致密油资源丰富。因砂砾岩储层中含有众多成分及强度大小不一的砾石，导致砂砾岩力学特征受砾石的形状、尺寸及物性影响较大，进而影响砂砾岩油藏的压裂裂缝复杂程度和压裂改造效果。鉴于此，针对准噶尔盆地百口泉组砂砾岩储层特征，建立了随机不规则多边形砾石的生成方法，并基于颗粒离散元法建立了砂砾岩力学数值模型，研究了典型砾石含量和分布对砂砾岩细观力学特征的影响机理。砂砾岩变形破坏数值模拟结果表明：低强度砾石对裂缝延伸的屏蔽作用较弱，裂缝大多呈“穿砾”模式，而高强度砾石对裂缝的屏蔽作用较强，裂缝更多呈“绕砾”模式；围压增加，岩石抗压强度明显增加，峰值应变能及滑移能也呈线性增加，且应变能增长显著；含不同砾石组合的砂砾岩储层中，剪切微裂缝数量随围压的增加而增加，且具有线性特征关系，高围压下含高强度砾石的砂砾岩具有更显著的塑性及延性特征，且存在明显的二次破坏现象；随低强度砾石的减少或高强度砾石的增加，砂砾岩的弹性模量增大，且抵抗变形能力增强，而不同砾石组合情况下，围压对砂砾岩弹性模量的影响较小；砂砾岩宏观破坏带的形成发育过程很大程度受控于内部细观结构，且受围压、砾石类型(力学强度)等的影响较大。Abstract: The Triassic Baikouquan Formation glutenite in the Junggar Basin of western China is rich in tight oil resources. Due to the presence of gravels with varying composition and strength in the glutenite, the mechanical characteristics of the glutenite are significantly affected by the shape, size, and physical properties of the gravels, which in turn affects the fracture complexity and fracturing reconstruction effectiveness of the glutenite reservoirs. In view of this, a method for generating random irregular polygonal gravels was established based on the reservoir characteristics of Baikouquan Formation, Junggar Basin. A mechanical numerical model of the glutenite was created based on particle discrete element method to study the influence mechanism of typical gravel content and distribu-tion on the meso-mechanical characteristics of glutenite. The results show that low-strength gravels have a weak shielding effect on fracture propagation, with most cracks exhibiting a "penetrating gravel" pattern. In contrast, high-strength gravels exhibit a stronger shielding effect, leading to more "bypass gravel" fracture patterns. With increased confining pressure, the compressive strength of the rock significantly increases, along with peak strain energy and slip energy, both showing a linear increase with strain energy being particularly pronounced. In glutenite reservoirs with different gravel combinations, there is a strong linear relationship between the number of shear microcracks and the confining pressure. The glutenite reservoirs with high confining pressure and high-strength gravels exhibit more obvious plastic and ductile characteristics, along with evident secondary failure phenomena. With the decrease of low-strength gravels or the increase of high-strength gravels, the elastic modulus of the glutenite increases, enhancing its resistance to deformation. However, the influence of confining pressure on the elastic modulus of glutenite is minimal across different gravel combinations. The formation and development of macroscopic failure zones in glutenite are largely controlled by the internal meso-structure and are greatly affected by confining pressure and gravel type (mechanical strength).
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图 2 准噶尔盆地三叠系百口泉组砂砾岩照片
Figure 2. Photograph of glutenite from Triassic Baikouquan Formation, Junggar Basin
图 3 准噶尔盆地三叠系百口泉组砂砾岩三轴(围压40 MPa)压缩模拟结果
Figure 3. Simulation results of glutenite triaxial compression (confining pressure 40 MPa) in Triassic Baikouquan Formation, Junggar Basin
图 4 不同围压下砂砾岩三轴压缩破坏形态
Figure 4. Triaxial compression failure patterns of glutenite under different confining pressures
图 5 不同围压微裂缝数量及能量曲线的演化过程
Figure 5. Evolution of number and energy curves of microcracks under different confining pressures
图 6 不同围压砂砾岩轴向压缩模拟结果对比
Figure 6. Comparison of axial compression simulation results of glutenite under different confining pressures
图 7 不同围压下不同砾石比例砂砾岩三轴压缩破坏形态
Figure 7. Failure patterns of glutenite with different gravel ratios under different confining pressures in triaxial compression
图 8 不同围压下不同砾石比例砂砾岩三轴压缩微裂缝数量及能量曲线
Figure 8. Number and energy curves of glutenite with different gravel ratios under different confining pressures in triaxial compression
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