Assessing the utility of FIB SEM images for shale digital rock physics
Abstract Shales and other unconventional or low permeability (tight) reservoirs house vast quantities of hydrocarbons, often demonstrate considerable water uptake, and are potential repositories for fluid sequestration. The pore scale topology and fluid transport mechanisms within these nanoporous sedimentary rocks remain to be fully understood. Image informed pore scale models are useful tools for studying porous media: a debated question in shale pore scale petrophysics is whether there is a representative elementary volume (REV) for shale models? Furthermore, if an REV exists, how does it differ among petrophysical properties? We obtain three dimensional (3D) models of the topology of microscale shale volumes from image analysis of focused ion beam scanning electron microscope (FIB SEM) image stacks and investigate the utility of these models as a potential REV for shale. The scope of data used in this work includes multiple local groups of neighboring FIB SEM images of different microscale sizes, corresponding core scale (milli and centimeters) laboratory data, and, for comparison, series of two dimensional (2D) cross sections from broad ion beam SEM images (BIB SEM) which capture a larger microscale field of view than the FIB SEM images; this array of data is larger than the majority of investigations with FIB SEM derived microscale models of shale. Properties such as porosity, organic matter content, and pore connectivity are extracted from each model. Assessments of permeability with single phase, pressure driven flow simulations are performed in the connected pore space of the models using the lattice Boltzmann method. Calculated petrophysical properties are compared to those of neighboring FIB SEM images and to core scale measurements of the sample associated with the FIB SEM sites. Results indicate that FIB SEM images below ~5000 um3 volume (the largest volume analyzed) are not a suitable REV for shale permeability and pore scale networks; i.e. field of view is compromised at the expense of detailed, but often unconnected, nanopore morphology. Further, we find that it is necessary to acquire several local FIB SEM or BIB SEM images and correlate their extracted geometric properties to improve the likelihood of achieving representative values of porosity and organic matter volume. Our work indicates that FIB SEM images of microscale volumes of shale are a qualitative tool for petrophysical and transport analysis. Finally, we offer alternatives for quantitative pore scale assessments of shale.