3D characterisation of potential CO₂ reservoir and seal rocks

Digital core analysis at multiple scales incorporating X-ray micro-computed tomography (μCT) imaging in different states in 3D, and registration of 2D SEM and SEM-energy-dispersive X-ray spectra (EDS) images into the 3D tomograms, offers an extensive and unique toolbox for characterising potential CO₂ reservoir and seal candidates. μCT imaging allows the calculation of connected porosity, and subsequently properties such as permeability, formation factor, Archie's cementation component, drainage capillary pressure, and Swi can be determined digitally and pore-throat network models can be generated. Sub-micron scale features in the 3D image can be directly correlated with high-resolution scanning electron microscope (SEM) images using 2D-to-3D image registration. Additionally, the in situ mineralogy can be quantified by using an automated mineral and petrological analysis (SEM-EDS) system. The mineralogy determined in 2D by SEM-EDS can then be interpolated into the 3D image block for the direct identification of minerals with contrasting X-ray attenuation. The 3D data can be readily displayed using 3D visualisations that show the pore connectivity, 3D mineralogy, geological structures, and incorporating the pore-throat network model, SEM, and 2D in situ mineral map. Additionally the porosity and flow pathways of a potential seal rock can be characterised at the nanoscale (pores 10-30 nm) using focussed ion beam SEM (FIBSEM) imaging. The behaviour of the potential reservoir and seal rocks during interaction with supercritical CO₂ and water can be directly investigated by coupling digital core analysis with a high-pressure cell. Multiple images can be collected of the same plug before, during and after interaction with CO₂ and water to directly characterise in 3D the CO₂ trapping, and changes to the pore/throat geometries and mineralogy owing to interactions with the CO₂.