Caprock and overburden characterisation: avoiding leakage of CO2 from CCS sites

Fine-grained lithologies directly above carbon capture and storage (CCS) reservoirs, commonly act as a sealing caprock for the injected CO2 and are of great importance in restricting upwards fluid escape. Despite their importance, they are usually poorly characterised in terms of sedimentology, pore structure, or diagenetic processes, and how all these processes relate to each other for assessing caprock quality and integrity. Caprocks should be a concern at all CCS sites as they may be texturally and mineralogically unstable in the presence of high CO2 pressure. For that reason, this thesis aims to characterise two caprocks to help understand long term top seal leakage potential of the injected CO2 by presenting a full chain of caprock characterisation and geochemical modelling. The principal aim of the characterisation is to compare and contrast two CCS top seals, the Lower Cretaceous Rodby Shale that is calcite rich, caprock to the Captain Sandstone, and the Palaeocene Lista Shale that is calcite free, caprock to the East Mey Sandstone. Caprock characterisation was undertaken to investigate the mineralogy, pore systems, and surface area of the Lower Cretaceous Rodby Shale, caprock to the Captain Sandstone, and the Palaeocene Lista Shale, caprock to the East Mey Sandstone, at the UK’s planned Acorn and East Mey CCS sites. Cores from both shales were logged and analysed by X-ray diffraction, light microscopy, and scanning electron microscopy to explore the origin of pore types. Grain size was measured using laser particle size analysis. Nitrogen adsorption and mercury intrusion porosimetry were used to characterise the surface area and pore network in terms of pore body and throat sizes. The key findings of this thesis show that the Rodby is smectite-rich with abundant amount of calcite, quartz silt, and small amounts of albite, kaolinite, and chlorite. The calcite in the Rodby Shale Formation was sourced from benthic microfossils (foraminifera), that have locally recrystalised to pore filling cement. The Lista is smectite-rich but calcite free, with quartz silt, and minor amounts of albite, kaolinite, and chlorite. The mean pore body and throat for the Rodby Shale Formation is 17 nm, and for the Lista Shale Formation it is between 17 and 18 nm, putting both the Rodby and Lista Shale Formations in the mesopore range and indicating a dominance of slit-like pores. Leakage risk assessment was undertaken on the Rodby and Lista Shale Formations for caprock integrity after CO2 injection. Results show that both caprocks have good potential to act as an effective barrier for CO2 escape based on the calculation of diffusion and post-breakthrough advection rates, along with sealing and stability assessments. Diffusion of CO2 into the caprock was studied further by modelling the effect of diffusion on the Rodby and Lista Shale Formation using geochemical modelling software called PHREEQC. Depending on the caprocks’ mineral composition, geochemical reaction could occur and affect their physical properties, such as porosity and permeability. A one-dimensional CO2 reactive transport (diffusion) model controlled by kinetics was developed to investigate the mutual influences of diffusion and reaction on caprocks. The model targeted the base of the caprock and simulated CO2 over 3000 years at a present-day burial temperature of 65 ˚ C. Two primary chemical reactions were identified within the Rodby and Lista Shale models: the local dissolution and precipitation of calcite, as well as the replacement of albite by kaolinite. Variations in porosity and pH were also observed. In the Rodby Shale, porosity increased by 3% in the cell closest to the CO₂-rich reservoir, while in the Lista Shale, it rose by 1%. However, these increases in porosity do not extend significantly into the caprock, as materials released through dissolution processes in the cell adjacent to the reservoir diffuse into the caprock, resulting in a zone of porosity reduction. Significantly, the slight decrease in porosity in certain regions of the Rodby and Lista Shale Formations could improve their sealing capacity. To conclude, from laboratory analysis and geochemical modelling, the Rodby and Lista Shale Formations are likely to represent good seals for the CO2 injected into the underlying reservoirs.