CCUS 2022

Summary

Laura Froute, Anthony R. Kovscek, Stanford University

CO₂ sequestration is crucial to curbing emissions from the energy sector. Currently, depleted shale formations and depleted conventional reservoirs with an overlying shale caprock constitute prime targets for geological sequestration due to their wide availability, seal efficiency, and high storage potential. The shale caprock properties controlling containment include its petrophysical, geometric, and geochemical attributes.

Characterization of the microscopic details of the fabric of shales underpins all understanding of subsurface containment processes. With exceptional nanometer resolution, electron microscopy techniques alone offer the magnification and contrast needed to resolve shale fabric at dimensions < 500 nm. The objective of this study is to apply high-resolution electron tomography with nanoscale resolution to shed light upon the texture, morphology, and composition of shale nanopore structures and their interplay with sc-CO₂.

We use Scanning Transmission Electron Microscopy (STEM) to provide high-resolution insight into the nanoporosity of a shale sample. Tilt series are acquired by STEM tomography in High Angle Annular Dark Field mode (HAADF). Processed images are segmented using machine learning pixel classification and 3D volumes are reconstructed and analyzed. Visualization of connected pore networks and mineral interfaces provide physical observations and quantitative measurements of shale caprock properties controlling confinement. In particular, we examine fundamental storage and transport properties such as porosity, permeability, mineralogy, and characterize pore shapes, types, and connectivity.