Multiscale Characterisation of Complex Carbonate Rocks for Geological CO2 Storage

Abstract Existing porosity-permeability models predominantly rely on bulk and large-scale properties, often neglecting multiple length scales, particularly the nano-to-micron scale. Such multi-scale pore types are inherent to carbonate rocks. Nano-scale imaging is crucial for interpreting the structure of microporosity, which consists of micropores that contribute up to 96% of the total pore size distribution in carbonates. This study aims to characterise the pore space of complex carbonate rocks across multiple scales using multiscale imaging techniques and establish a more accurate porosity-permeability relationship for CO2 storage sites. Multiscale X-ray and SEM tomography were performed on different sample sizes. 3D images were acquired using X-ray micro-CT at low resolution, followed by high-resolution 3D imaging of a sub-plug region of interest and mini-plug samples. Qualitative image processing was conducted to segment into distinct phases. Subsequent quantitative image processing involved pore space analyses, subvolume extraction and numerical pore-scale simulations. The porosity-permeability relationships were upscaled from the nano-scale to the meso-scale using the Darcy-Brinkman-Stokes equation. The image-based upscaling models were then applied and compared with the petrophysical-derived models. This study reveals the presence of interconnected micropores within the rock matrix during carbonate rock formation, highlighting the importance of micropores in enhancing the porosity-permeability relationship. By employing an image-based protocol that enables multi-resolution imaging of the same sample, the complex pore structures and fluid dynamics within carbonate rocks can be revealed, thereby enhancing the accuracy of CO2 injection models and improving storage efficiency. Consequently, the study can be utilised in upscaling routines to provide critical insights into the feasibility and optimisation of geological CO2 storage.