Micro scopic heterogeneity and mechanical anisotropy of the laminated shale in Longmaxi Formation
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摘要: 层状页岩的微观非均质性及力学各向异性对研究井壁稳定以及水力裂缝扩展形态具有重要意义。为了向页岩优化钻井、压裂工艺参数提供一定的理论和试验依据，对沿不同角度取心的页岩试样开展单轴压缩实验，配合场发射扫描电镜、原子力显微镜观测实验和波速测试等，研究龙马溪组层状页岩微观非均质性及力学各向异性特征，并讨论这些物理力学特征对水力裂缝形态的影响规律。结果表明：受层理面的影响，龙马溪组页岩呈现出较强的微观非均质性和宏观力学各向异性特征。具体的，微观孔隙结构特征方面，随着观测方向与层理方向之间夹角β的增大，微观孔隙结构的发育程度逐渐增加，说明气体的储集和空间呈增加趋势；宏观力学特征方面，单轴压缩条件下，随着加载方向与层理方向间夹角θ的增加，页岩试样的破坏模式从贯穿层理面的张拉破坏，先转变为剪切破坏，再变为劈裂-剪切混合破坏；龙马溪组层状页岩的单轴抗压强度、泊松比随着θ的增加呈现出先减小后增大的"U"形各向异性模式，弹性模量、横纵波速则逐渐减小，胶结程度较弱的页岩层理面会先于基质体发生破坏，进而显著影响岩石整体的力学性质；页岩微观非均质性及力学各向异性特征在一定程度上影响压裂过程中水力裂缝的扩展行为，以及停泵后压裂液的渗流路径。研究结果可为页岩压裂工艺参数优选提供一定依据。Abstract: As a clean energy, the commercial development and utilization of shale gas affect the global energy landscape. The microscopic heterogeneity and mechanical anisotropy of laminated shale have crucial significance for studying wellbore stability and the hydraulic fracture (HF) geometry. In order to provide experimental and theoretical bases for the optimization of drilling and fracturing parameters in the field, the microscopic heterogeneity and mechanical anisotropy of Longmaxi laminated shale were studied. The uniaxial compressive experiments, field-emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM) observations and wave velocity tests were conducted on shale samples cored at different angles. Moreover, the effect of microscopic heterogeneity and mechanical anisotropy on the geometry of HFs was discussed. The results suggest that the bedding planes influence microscopic heterogeneity and mechanical anisotropy in the Longmaxi laminated shales. For the microscopic heterogeneity, with increasing angle between observation direction and bedding planes, the development degree of micropores increased. As observed from FE-SEM and AFM images, the distribution of mineralogical components and organic matter-hosted pores shows strong heterogeneity in microscopic scale, indicative of increasing gas storage capacity (
Fig. 1 ,2 ).FE-SEM images of shale samples with different coring angles (β is the angle between the observation direction and bedding direction in the FE-SEM images. The figures show that in the direction parallel to the beddings, the shale matrix is cemented well. With increasing bedding angle, the development degree of micro pore structure increases gradually) AFM images of shale samples with different coring angles (γ is the angle between the observation direction and the bedding direction in the AFM tests. The results are similar to the FE-SEM tests) As for the mechanical anisotropy, under the uniaxial compression, the failure mode and mechanical parameters were different due to the different bedding angles. With increasing angle (θ) between the loading direction and bedding direction, the failure mode gradually changed from tensile failure perpendicular to the bedding planes, to shear failure, and then to the "splitting-shearing" mixed failure (Fig. 3 ). With increasing θ, the uniaxial compressive strength and Poisson's ratio of Longmaxi laminated shales display a "U-shaped" anisotropic model that is characterized by a first decrease and a subsequent increase. While the elastic modulus and S-P wave velocity shows a decreasing trend, the bedding planes of shale with weak cementation will be damaged before the rock matrix, which will significantly affect mechanical properties of the whole rock. The microscopic heterogeneity of shales influences the anisotropy of mechanical properties to a certain extent. The varying development degree of micropore structure in different bedding directions will indirectly affect the mechanical properties by affecting the strain and cementation degree of matrix during the uniaxial compression experiments.Typical failure patterns and fracture geometry of shale samples with different coring angles Effect of shale anisotropy on the initiation of hydraulic fracture (a.The HF propagates along the bedding plane near the well after initiation; b.The HF propagates directly to the sample boundary after initiation) The microscopic heterogeneity and mechanical anisotropy of shales can affect the HF behavior during hydraulic fracturing, and the fluid seepage flow paths under the shutoff of pumps. The development of natural fracture planes near the wellbore will induce HF propagation. It is suggested that micro-fractures are relatively developed in the direction perpendicular to beddings. The developed micro-fractures not only create conditions for the initiation of HFs, but also provide channels for the seepage of fracturing fluid after the pump is stopped. The results provide theoretical basis for the parameter optimization of laminated shale hydraulic fracturing. -
图 2 取样示意图及成品
a.取心示意图；b.不同取心角度的试样示意图；c.部分加工完成的样品
Figure 2. Diagram of the specimen preparation process and the experimental specimens
图 3 不同取心角度页岩试样的FE-SEM测试结果
Figure 3. FE-SEM images of the shale specimens at different coring angles
图 5 不同取心角度页岩试样AFM测试结果
Figure 5. AFM images of the shale specimens at the different coring angles
图 6 不同取心角度页岩试样破坏模式及裂缝形态(θ为加载方向和层理方向之间的夹角)
Figure 6. Typical failure patterns and fracture geometry of the shale specimens at different coring angles
图 7 抗压强度、弹性模量、泊松比随θ的变化规律
Figure 7. Variation of uniaxial compressive strength, elastic modulus and Poisson′s ratio with the θ
图 9 页岩各向异性对于裂缝起裂的影响
a.裂缝起裂后沿近井层理面扩展；b.水力裂缝起裂后直接延伸至试样边界
Figure 9. Effect of the shale anisotropy on the initiation of hydraulic fracture
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