Interplay between pore connectivity and permeability in shale sample

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. •Permeability and petrophysical properties of Eagle Ford shale sample are analyzed as a function of sample thickness.•Permeability is estimated from 3D SEM images using LBM simulations.•The impact of pore connectivity and geometric tortuosity on permeability reduction was compared.•