Abstract
Modeling of fluid migration in shale nanoscale systems is hampered by knowledge gaps in rock-fluid affinity, storage and transport processes under confinement, phase behavior, organic/inorganic interplay, and scale translation complexities. Experimental insight is often lacking at the relevant scale. Our objective is to combine multiple imaging techniques with nanoscale resolution to better understand how nanopore networks contribute to the transport and storage properties of shale samples.
With exceptional nanometer resolution, electron microscopy techniques alone can resolve nanostructures. We integrate datasets collected from a Barnett shale on different platforms such as Scanning Electron Microscopy (SEM), Focused Ion Beam - Scanning Electron microscopy (FIB-SEM) and Scanning Transmission Electron Microscopy (STEM) to provide high-resolution insight into nanopore networks.
FIB-SEM microscopy is used to reconstruct a 3D volume where organic matter, pores and minerals can be identified and segmented. Thin sections are then micro-machined by FIB-SEM, mounted on a TEM grid and thinned down to electron transparency (about 100 nm). High-magnification micrographs highlighting composition are then acquired by Scanning Transmission Electron Microscopy (STEM) in High Angle Annular Dark Field mode (HAADF). We observe a variability in pore sizes, with some large (i.e. on the order of 100-200nm) pores that go all the way through TEM lamellas and a multitude of well-resolved nanopores down to 1nm. Mineralogy and pore size distributions are generally consistent in both FIB-SEM and 2D STEM observations. In order to understand whether the smallest pores form a connected network, we acquire tilt series by STEM tomography. Processed images are segmented using machine learning pixel classification and tomograms are reconstructed and analyzed. On both FIB-SEM and STEM datasets, we measure fundamental storage and transport properties (porosity, permeability, PSD, connectivity, mineralogy) and compare results across scales and techniques.
This study represents one of the first few utilizations of integrated FIB-SEM and STEM for high-resolution imaging of the shale organic matter and the structurally associated mineral fabric. Visualization of connected pore networks and interfaces will help to better understand the contribution of the nanoscale to the overall transport and storage properties of shales.
