Influence of Rock Heterogeneity in CO2 Geo-Sequestration
Creators: name: Daraei Ghadikolaei, Farshad
Publisher: name: The Australian National University
Year: 2023

description: Among the viable strategies outlined by the Intergovernmental Panel on Climate Change (IPCC) for atmospheric emission reduction strategies and technologies, geological storage of CO2 (CO2 geo-sequestration) holds an enormous promise with the potential to have significant impacts on emissions and atmospheric CO2 reduction. Of the various geological storage options, sandstone aquifers are potential geological formations for storing captured CO2 because they possess characteristics that are conducive to safe and effective storage. However, predicting the behaviour of scCO2 in rocks is challenging largely due to the complex interactions between various fluids and minerals, in particular in the presence of structural heterogeneities presented by real rocks. The focus of this research is directed towards understanding the role of rock heterogeneity on the spatial distribution of scCO2 in sandstone rocks. I present results from in-situ experiments, where sub-surface conditions are created in the laboratory to simulate CO2 geo-sequestration process. High resolution X-ray micro-computed tomography scans were acquired throughout the experiments to resolve pore scale features and fluid distribution in the system. The results combine experiments, 2D-3D-4D imaging and simulations. The results show that trapped CO2 in the rock is influenced by a range of structural and morphological features at the pore scale. I measure that rock heterogeneity has a significant impact on the CO2 phase connectivity, which consequently affects the storage capacity of the rock. I propose that the more structural heterogeneity and stratification at the core scale reflects the stronger capillary heterogeneity in the system along with a relative variability in pressure profile. I find that a sample with narrow distribution of the Wetting Index, which implies a homogeneous wetting state, can have a higher amount of trapped scCO2 after imbibition and vice versa. The findings indicate that the wetting state of pores varies with the pore size in my rock samples. I quantify non-wetting phase connectivity via an invariant topological measure, Euler Characteristic or Euler number (calculated in 3D). The results indicate that increasing mobilisation of the non-wetting fluid during imbibition is related with high connectedness of this phase and vice versa. The results indicate that, the ratio between buoyancy (gravity force) and capillary force which also called the Bond numbers influence residual saturation. Accordingly, residual scCO2 saturation controlled by porous structure (more specifically pore sizes), larger pore sizes result in dramatically lower residual saturation. I lastly conduct two sets of cyclic brine-CO2 injection experiment. The findings illustrate that trapped scCO2 increases after each imbibition for the first set of cyclic injections experiment (injection rate of 0.1 ml/min) but decreases for the second set (injection rate of 1 ml/min). The pore-by-pore wettability analysis of the first series of cyclic injection experiment (0.1 ml/min rate) show that the sample's wettability after each cycle alter to more hydrophobic conditions as the average value of wetting index increases. In contrast, pore-by-pore wettability study for cyclic injections for the second series (injection rate of 1 ml/min) exhibit that pore wettability changes towards a more hydrophilic state after each cycle. Overall, this thesis enhances our understanding of CO2 geo-sequestration by elucidating the intricate relationships between geological heterogeneity and scCO2 behaviour in sandstone rocks. The insights gained contribute valuable knowledge for optimizing CO2 storage strategies and advancing efforts to combat atmospheric CO2 levels effectively.