Abstract Shale formations demonstrate distinct characteristics, such as a wide spectrum of pore size from micro-scale to nano-scale, limited pore connectivity and ultra-low permeability. Despite extensive research work over recent years to characterize flow properties of shale samples, the interplay between pore connectivity and permeability still remains to be understood. In this study, numerical methods were used in tandem with experimental data to characterize and evaluate pore connectivity of shale samples. To evaluate permeability and pore connectivity in the shale matrix, three-dimensional (3D) pore structure constructed by stacking scanning electron microscopy (SEM) images of an Eagle Ford sample is used. First, static petrophysical properties of the shale sample such as total and the connected porosity, pore size distribution and geometric tortuosity are calculated and evaluated. Next, pore connectivity is assessed using Euler-Poincare Characteristics (EPC) method. Finally, fluid flow through the sample is simulated using Lattice Boltzmann Method (LBM) to predict single-phase permeability of the whole system. Results indicate that total porosity of the studied Eagle Ford shale sample is around 12.5% and slightly decreases as the thickness of the digital sample increases. On the other hand, the connected porosity of the sample decreases resulting in a 50% reduction when the sample thickness increases from 1 μm to 6 μm. Moreover, the calculated permeability is 17.6 md and 1.17 μd for the sample thickness of 1 μm and 6 μm, respectively; i.e. 15,000 times reduction is observed when the digital sample thickness increases. Consistent with the permeability results, the pore connectivity determined through EPC method is strongly (and inversely) correlated with the sample thickness and 80% reduction of the pore connectivity is realized when the sample thickness is increased from 1 μm to 6 μm. The results from this study will provide new insights into how pore structure characteristics scale up and help improve the better estimation of permeability in shale formations. In addition, the outcome of this work has some important implications to fluid flow research studies in shale formations.

中文翻译：

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页岩样品孔隙连通性与渗透率的相互作用

摘要 页岩地层表现出明显的特征，例如从微米级到纳米级的广泛孔径、有限的孔隙连通性和超低渗透率。尽管近年来为表征页岩样品的流动特性进行了大量研究工作，但孔隙连通性和渗透率之间的相互作用仍有待了解。在这项研究中，数值方法与实验数据结合使用，以表征和评估页岩样品的孔隙连通性。为了评估页岩基质中的渗透率和孔隙连通性，使用了通过叠加 Eagle Ford 样品的扫描电子显微镜 (SEM) 图像构建的三维 (3D) 孔隙结构。首先，页岩样品的静态岩石物理特性，例如总孔隙度和连通孔隙度，计算和评估孔径分布和几何弯曲度。接下来，使用欧拉-庞加莱特征 (EPC) 方法评估孔隙连通性。最后，使用格子玻尔兹曼方法 (LBM) 模拟通过样品的流体流动，以预测整个系统的单相渗透率。结果表明，所研究的 Eagle Ford 页岩样品的总孔隙度约为 12.5%，并且随着数字样品厚度的增加而略有下降。另一方面，当样品厚度从 1 μm 增加到 6 μm 时，样品的连通孔隙率降低，导致减少 50%。此外，对于 1 μm 和 6 μm 的样品厚度，计算的渗透率分别为 17.6 md 和 1.17 μd；即当数字样品厚度增加时观察到 15,000 倍的减少。与渗透率结果一致，通过 EPC 方法确定的孔隙连通性与样品厚度呈强（反）相关，当样品厚度从 1 μm 增加到 6 μm 时，实现了 80% 的孔隙连通性降低。这项研究的结果将为孔隙结构特征如何扩大提供新的见解，并有助于更好地估计页岩地层的渗透率。此外，这项工作的成果对页岩地层中的流体流动研究具有重要意义。这项研究的结果将为孔隙结构特征如何扩大提供新的见解，并有助于更好地估计页岩地层的渗透率。此外，这项工作的成果对页岩地层中的流体流动研究具有重要意义。这项研究的结果将为孔隙结构特征如何扩大提供新的见解，并有助于更好地估计页岩地层的渗透率。此外，这项工作的成果对页岩地层中的流体流动研究具有重要意义。