Microscopic study of salt precipitation dynamics during the injection process of CO2 storage in saline aquifers
Global climate change, driven by CO₂ emissions, has caused a 1.1 °C rise in temperatures and a 30% increase in ocean acidity. The Paris Agreement targets net-zero emissions by 2050. Carbon Capture, Utilization, and Storage (CCUS), especially geological CO₂ storage in saline aquifers, can store billions of tons of CO₂. However, salt precipitation during injection can clog pores and reduce permeability. Understanding the dynamics of this process, influenced by injection flow rate, pore structure, and brine properties, is crucial for optimizing CCUS strategies. This thesis examines the impact of salt precipitation on CO₂ geological storage in saline aquifers using microfluidic experiments and molecular dynamics simulations. It analyses the dynamics and mechanisms of salt precipitation, examines key influencing factors such as injection flow rate, pore structure, and solution type, and proposes multiple feasible strategies to mitigate its adverse effects on storage efficiency and stability. The key research conclusions are as follows: (1) Salt Precipitation Dynamics Mechanism and Quantitative Analysis. Salt precipitation dynamics involve four main processes: nucleation, growth, migration, and blockage. Nucleation leads to two crystal types: in high-saturation regions (e.g., brine pool or bridge), large bulk crystals form and clog pores quickly, while in low-saturation regions (e.g., brine film), porous aggregated crystals with larger voids reduce blockage risk. Brine migration, driven by capillary backflow, intensifies localized precipitation. Ex-situ precipitation, caused by mobile bulk crystals moving away through residual brine, worsens pore blockage. Salt precipitation dynamics are quantified in three stages: 1) the nucleation stage, where small crystal clusters form; 2) the constant growth stage, where bulk crystals increase rapidly, heightening blockage risk; and 3) the decline stage, where complex, intertwined structures develop, exacerbating pore blockage. (2) Factors Influencing the Salt Precipitation. Salt precipitation dynamics are influenced by the CO₂ injection flow rate, pore structure, and brine composition. The injection flow rate has a critical threshold: low rates limit crystallization due to insufficient gas solubility, while high rates disrupt nucleation, reducing precipitation. The optimal rate balances brine capillary backflow and evaporation, enhancing salt precipitation. Smaller pores trap more brine, increasing in-situ precipitation and blockage, while pore heterogeneity creates preferential flow paths, intensifying precipitation. Brine composition also significantly affects the process. Monovalent salts, like NaCl, form porous, mobile crystals that migrate with brine, leading to ex-situ precipitation and pore blockage. In contrast, divalent salts, such as CaCl₂, form stable, bulk crystals that restrict evaporation, resulting in in-situ precipitation and localized blockage. Together, these factors determine the extent and impact of salt precipitation on CO₂ geological storage efficiency. (3) Mitigation Strategies for the Salt Precipitation. Key strategies to mitigate salt precipitation in CO₂ geological storage include optimizing the injection plan, employing multilayer injection, pre-flushing with low-salinity solutions, wellbore modification, and using slowrelease inhibitors. These strategies help control the salt precipitation process, reduce pore blockage, and maintain reservoir stability. This study provides a comprehensive analysis of salt precipitation dynamics in CO₂ geological storage and proposes effective mitigation strategies, offering significant theoretical insights and practical solutions to optimize injection processes, enhance storage efficiency, and ensure long-term reservoir stability for climate change mitigation. Key words: CO₂ Geo-Sequestration; Saline aquifers; Salt Precipitation; Microfluidic Experiments; Molecular Dynamics Simulations; Dynamics Mechanisms
Networked Digital Library of Theses and Dissertations
Faculty of Graduate Studies and Research, University of Regina