Predicting CO₂ migration in the shallow subsurface: the role of heterogeneity and mass transfer

Abstract

Carbon capture and storage (CCS) is an important technology for the net-zero transition. The success of CCS relies on the security of storage, as unintended release of CO₂ from a storage site can impact water resources, release stored CO₂ to atmosphere and reduce the value of carbon credits associated with storage. Given the potential impacts, robust monitoring techniques are required, supported by a deep understanding of the complexities of CO₂ migration in the shallow subsurface. The objective of this research is to better understand the impacts of subsurface heterogeneity and multicomponent mass transfer on the fate and migration of CO₂ using numerical models. Simulations of a bench-scale CO₂ injection in saturated homogeneous sand were conducted using a coupled continuum-discrete approach using the Electrothermal Macroscopic Invasion Percolation (ET-MIP) model. ET-MIP was shown to accurately describe the experimental gas velocity, aqueous plume height and gas fingering behaviour, and demonstrated that multicomponent mass transfer impacts the persistence, distribution and development of the gas phase. A sensitivity study was conducted, and demonstrated that gas velocity and distribution were sensitive to the critical gas saturation and grid size. A binary tree algorithm was implemented for MIP including gas channel fragmentation and mobilization (MIP-FM) to improve performance of ET-MIP. The new algorithm showed a 76% decrease in overall run time when implemented in ET-MIP, and enables the use of ET-MIP in larger domains. Gas migration in realistic 3D sedimentary structures was simulated to understand the impact of cm-scale heterogeneities. Realistic sedimentary structures with cm-scale bedding and lamina were generated stochastically with varied entry pressure characteristics, and gas migration was simulated using MIP. The ensemble of gas migration results was analyzed and it was established that increased grain size contrast between the bedding and lamina (a function of both grain size and grain sorting of both materials) causes a transition from ganglia-dominated flow to pool-dominated flow and enhanced lateral migration from the source. These results highlight the challenges in gas migration monitoring, as small variations in heterogeneity can significantly alter gas migration pathways. Lastly, the effects of subscale heterogeneity, groundwater flow and gas composition on gas migration were investigated by simulating CO₂ and noble gas mixtures in heterogeneous domains with varied aqueous flow rates. Results showed that both heterogeneity and aqueous flow rate can influence the vertical migration of CO₂, as a significant portion of the CO₂ will dissolve. Additionally, dissolved noble gas ratios were shown to be sensitive to multicomponent mass transfer during injection and post-injection, as less soluble noble gases such as He will preferentially partition to the gaseous phase. The results of this thesis highlight the complexity of CO₂ migration in the shallow subsurface. Subscale heterogeneity can impact gas distribution, dissolution, and cause extensive lateral migration. Multicomponent mass transfer can impact the evolution of dissolved gas concentrations including noble gas ratios, and the persistence of gas in the subsurface. Ultimately, this research will help to improve monitoring and verification of CCS, through advancements in numerical modelling techniques and data interpretation. These findings extend beyond CCS, and can be applied to other fields of research including methane or hydrogen leakage and groundwater remediation techniques such as air-sparging or in-situ thermal remediation