Migration behavior and lifetime of CO2 micro-nano bubbles in shallow aquifer

Abstract

In light of the application of CO₂ micro-nanobubbles (MNBs) in distributed carbon dioxide capture and storage (CCS), a series of experiments were conducted to investigate the stability of CO₂ MNBs. Prior to the in-situ assessments, foundational laboratory experiments were performed to evaluate the stability of the MNBs. Subsequently, a small-scale in-situ CO₂ MNB injection test was conducted to measure the CO₂ MNB density in the extraction well.

The bubble density was measured using a resonance mass spectrometer, which effectively discerns bubbles from solid particles. Furthermore, the behavior of the injected CO₂ MNB water was monitored through electric resistivity surveys. The findings revealed that CO₂ MNBs and O₂ MNBs exhibit low ζ-potentials at low pH values. Regarding bubble density, the CO₂ MNB remained relatively stable at a pH of 4, proximate to the point of supersaturation. As time elapsed following injection, the bubble density in the extraction wells of the in-situ CO₂ MNB water injection experiments steadily increased, implying the replacement of groundwater in the aquifer by injected CO₂ MNB. The resistivity survey effectively delineated the migration area of the CO₂ MNB water, indicating that CO₂ MNBs could persist in the aquifer even up to one day post-injection. Laboratory measurements of ζ-potential and bubble density further corroborate the complete displacement of water in the aquifer by CO₂ MNB water, leading to a reduction in porewater pH and ultimately facilitating the stable retention of CO₂ MNBs.