High-Resolution Three-Dimensional Characterization of Pore Networks in Shale Reservoir Rocks

The ever-growing demand for energy, relatively high price of hydrocarbons, and recent advances in production technologies have brought tight hydrocarbon-bearing reservoirs into attention as a potential source of energy. However, the displacement physics at nano and micro scales and their impact on fluid flow in these rocks is poorly understood. The unconventional rocks, such as shale rocks, are highly heterogeneous, fine-grained, and their representative elementary volume is uncertain. In order to identify flow pathways in the pore network of these rocks, it is essential to characterize nanopores and their connectivity. This can be achieved using high-resolution 3D imaging technique provided by Focused Ion Beam milling and Scanning Electron Microscopy (FIB-SEM). In this technique, a sequence of 2D cross sectional images, spaced evenly through a region of bulk specimen, is acquired. The stack of 2D images is then re-constructed into a 3D digital gray-scale representation of the sample volume. In this study, a reservoir rock sample from a major shale oil reservoir is selected for high-resolution imaging and statistical analysis. Rock specimens, 1 to 2 cm in dimensions, are cut from different locations of the reservoir core from which a high-resolution 2D map and multiple 3D FIB-SEM images are obtained. The digital images are then visualized, segmented, and analyzed to obtain porosity, pore size distribution, pore aspect ratios, spatial distribution of organic/total porosity, and total organic content. We find that the majority of the pores are below 100 nm in radius for this rock. In addition, the total visible porosity and total organic content are in the range of 1 to 2% and 8 to 14 vol.%, respectively. Chemical composition and mineralogy of the samples are also evaluated by Energy Dispersive X-Ray Spectroscopy (EDS) analysis. Furthermore, 3D pore networks are extracted from the FIB-SEM images; pore connectivities are examined; and permeabilies are calculated by solving the Stokes equation numerically using the finite volume method. It is observed that the pore connectivity for these rocks is poor, resulting in low permeabilities ranging from 1 to 6 μD. Finally, the impact of calculated parameters on fluid flow in unconventional rocks is discussed.