[H-SC06] CCUS (Carbon Dioxide Capture, Utilization, and Storage) for Climate Mitigation

[HSC06-14] Coalescence Behavior of CO₂ Nanobubbles in Geological Storage Systems

*YUE FU¹, YUNFENG LIANG¹, WUQUAN LI¹, TAKESHI TSUJI¹ (1.The University of Tokyo)

Keywords:CO2 nanobubbles, Geological carbon storage, Coalescence mechanisms, Molecular dynamics simulation

The stability and interaction of CO₂ nanobubbles (NBs) are crucial for geological carbon storage. However, the fundamental mechanisms governing NB coalescence and dissolution under subsurface conditions remain poorly understood, particularly regarding the influence of ionic composition, bubble size, and number density (i.e., bubble-bubble separation distance). A recent study has indicated that, at high pressure of about 20 MPa, NB can reach a concentration on the order of 1.0~5.0×10¹⁷ bubbles/mL with a bubble size of around 6 nm, indicating bubble-bubble separation distance is around 12.5 ~ 21.5 nm [1]. However, direct observation of NB behavior for bubbles with a small size of around 6 nm under high-pressure conditions is not trivial. This study explores coalescence mechanisms governing their behavior under geological conditions based on molecular dynamics simulations. We prepared CO₂ bubble systems with two identical equilibrated NBs (~6 nm) in CO₂ saturated water and 1 mol/kg brines (NaCl, KCl, CaCl_2, and MgCl₂) with separation distances of 10.5, 11.5, and 12.5 nm, all simulations were conducted under 313.15 K at 20 MPa.
The simulation shows that NB evolution encompasses diffusive mass transfer-driven size redistribution and bubble-bubble interaction that led to two types of coalescence behavior. Ostwald ripening [2] (Type A, shown in Fig. 1 a-c) contributes significantly to nanobubble size redistribution. Laplace pressure differences lead to gradual transfer of CO₂ molecules from smaller bubbles to larger ones. Due to their higher internal pressure, smaller bubbles dissolve faster, with CO₂ diffusing through surrounding liquid and redepositing on larger bubbles, promoting their growth. Brownian motion-induced collisions (Type B, shown in Fig. 1 d-f) play a dominant role in bubble interactions, where coalescence occurs when van der Waals forces surpass electrostatic repulsion at distances of 10-20 nm. The initial bubble-bubble separation distance, ionic environment, and coalescence types significantly impact coalescence lifetimes. For Brownian motion-induced collisions, in CO₂ saturated water, an 11.5 nm separation results in a 73.5 ns coalescence lifetime, decreasing to 25 ns in NaCl solutions, while divalent ion solutions (Ca²⁺, Mg²⁺) maintain extended coalescence lifetimes of 195.5 ns and 265.5 ns, respectively. These results indicated that divalent ions significantly influence nanobubble stability by reducing CO₂ transfer rates, effectively suppressing coalescence over longer lifetimes (with Mg²⁺ the longest coalescence lifetime). Surprisingly, we observed Ostwald ripening type of coalescence in KCl solution. Two NBs were initially identical, they came out with different sizes after 10s of ns. It exhibited a longer lifetime of CO₂ NBs at 345 ns, which indicated that Ostwald ripening is a slower process, primarily governed by dissolution-redeposition dynamic equilibrium initially with two identical equilibrated NBs. Further study should be performed for Ostwald ripening in different solutions with distinct sizes of NBs.
In general, a higher separation distance, and a longer coalescence lifetime can be anticipated. The ongoing simulation with a separation distance of 12.5 nm supports a significantly elongated coalescence lifetime. Further analysis of molecular mobility confirms that diffusion coefficients correlate with ionic composition. CO₂ molecules in CO₂ saturated water exhibit a diffusion coefficient of 2.49 × 10^-9 m²/s, decreasing to 2.20 × 10^-9 m²/s in the MgCl₂ solution, which is in line with the extended coalescence lifetime in the MgCl₂ solution. Radial distribution function (RDF) analysis showed that Mg²⁺ forms the most structured hydration layer, with a peak intensity of 8.328 at 0.208 nm, and Ca²⁺ with a peak intensity of 5.176 at 0.24 nm, while Na⁺ 3.516 at 0.234 nm and was K⁺ 2.087 at 0.276 nm, which illustrated that Mg²⁺ reinforced nanobubble stability and prevented rapid coalescence the most.
These findings provide essential insights for optimizing CO₂ sequestration strategies. Carefully controlling ionic composition, nanobubble sizes, and a high number density of nanobubbles to ensure optimal bubble-bubble separation distances can enhance storage efficiency. The study bridges molecular-scale mechanisms with practical applications in carbon capture and long-term storage.
[1]. H. Wang, T. Lawal, S. H. Achour, K. Sheng, and R. Okuno, Aqueous nanobubble dispersion of CO₂ at pressures up to 208 bara. Energy Fuels 2023, 37, 19726–19737.
[2]. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Ostwald ripening”.