Multiscale Imaging in Understanding Pore Structure and Alignment of Seal Rock for Long Term CO2 Containment

Abstract

Abstract Carbon capture and storage (CCS) provides a viable option to reduce CO2 emissions into the atmosphere. In CCS, CO2 is planned to be safely stored in suitable subsurface formations, providing sufficient storage capacity and sealing integrity. Analysing caprock properties like porosity and permeability is important in ensuring long-term CO2 containment. Caprocks are mainly composed of mudrocks that are characterised by nanometre to micron scale structures (pores and particles). A combination of medical and micro-CT scanning, Scanning Electron Microscopy (SEM), Focused-Ion-Beam (FIB), and Broad-Ion-Beam (BIB) are available to produce high-resolution 3D and 2D images. These can help to determine porosity down to a nanometre resolution. They can however also be used in pore network modelling which allows the computation of flow properties. In this study, core samples were CT scanned to determine the presence of fractures. This was followed by micro-CT and SEM scanning for higher-resolution images of the larger pores. To improve understanding of the porosity and permeability of the mudrocks analysed, samples were milled by BIB and imaged by SEM producing high-resolution 2D images (up to 5 nm) and allowing for detailed microstructural analysis at a scale of 2 mm2. We then identified different areas representing sample heterogeneity in terms of pore structure and permeability. These areas were then analysed using FIB-SEM with resolution down to 10 nm to obtain 3D models. Image analysis was done to identify the pore structure and alignment with the principal stress directions. The total porosity (within the resolution of the measurements) has been integrated with other experimental measurements and will become important parameters in the correlation of data determined on drill cuttings with core data and well logs. We hypothesize that upscaling from cuttings and core samples to well scale is feasible if an improved understanding of sample heterogeneity, method limitation, and the integration of experimental data with pore scale and pore network modelling is achieved. This will support de-risking of CO2 leakage across low permeability sealing formations.