Viscous Fingering Dynamics in Carbon Capture: Impact of CO2 Solubility and Relative Permeability Selection in CCS

Summary Carbon capture and storage (CCS) has emerged as a critical strategy in the global effort to mitigate greenhouse gas emissions and combat climate change. In the context of CCS, the interaction between injected CO2 and resident fluids in geological formations is of paramount importance. Given the significant viscosity ratio between water and supercritical CO2, the flow dynamics in these systems are expected to be affected by viscous fingering (VF). Viscous fingering is an instability that occurs when a lower viscosity fluid displaces a higher viscosity fluid in a porous medium. If VF does occur, then it has important implications for the efficiency and long-term behaviour of CO2 storage operations. The dissolution of carbon dioxide in water also plays a vital role in enhancing the integrity of the carbon storage process. However, questions remain regarding the influence of CO2 solubility on viscous fingering dynamics. This study employs numerical simulations to demonstrate the presence of viscous fingering during carbon dioxide injection in storage formations. We investigate the impact of CO2 solubility on the development of fingering patterns and breakthrough times using a two-dimensional heterogeneous permeability model. In our simulation model, we maintained a constant bottomhole gas injection rate across various pressure conditions while investigating the effects of CO2 solubility. We observed a consistent delay in breakthrough time when CO2 solubility was activated, compared to simulations without solubility. However, the magnitude of this delay diminished at higher pressures. At higher pressures, although the absolute quantity of dissolved CO2 increases, the proportion of CO2 in the supercritical state also rises significantly. The presence of a larger fraction of supercritical CO2 at elevated pressures maintains a higher-pressure gradient within the system. Consequently, this mitigates the reduction in pressure drop that was more pronounced in lower pressure scenarios where dissolution effects were relatively more dominant. The inclusion of CO2 solubility in our numerical simulation model does not prevent the formation of viscous fingers. However, it significantly alters the morphology of the fingering patterns, with these changes being pressure-dependent. Additionally, we examined the impact of viscous fingering on CO2 dissolution in water by comparing conventional relative permeability with relative permeability that accounts for viscous fingering effects. Indeed, we will present results showing that none of the published CO2/water relative permeabilities lead to the correct viscous fingering behaviour in CO2 - water displacement at the large scale.