The carbon dioxide can rise toward the surface, turn into gas bubbles and vent to the atmosphere, defeating the purpose of the whole grand scheme. Even worse, if the liquid-to-gas conversion happens suddenly, the gas can bubble up in a plume and erupt---a potential hazard.
Small-scale ocean experiments have been done to investigate how the carbon dioxide (CO2) actually would behave, but such experiments are too costly and time consuming to carry out under a wide range of ocean conditions. However, a new theoretical model developed by University of Michigan researcher Youxue Zhang can be used to explore the fate of CO2 injected into oceans under various temperature and pressure conditions. Zhang's model shows that liquid CO2 would have to be injected to a depth of at least 800 meters (about a half mile) and possibly as much as 3,000 meters (nearly two miles) to keep it from escaping.
Eruptions from injected CO2 are a serious concern, Zhang said, "because carbon dioxide is known to have driven deadly water eruptions." In 1986, a CO2-driven eruption in Cameroon's Lake Nyos killed some 1,700 people, as well as animals in the area; two years earlier, a smaller release of CO2 from Lake Monoun in the same country resulted in 37 human deaths. The deaths were not directly caused by the explosions, but resulted from carbon dioxide asphyxiation. "Carbon dioxide is denser than air, so it settled down and flowed along the river valley, choking people and animals to death."
The challenge in designing CO2 injection strategies is figuring out how to keep droplets of the liquid from rising to 300 meters---the approximate depth at which, depending upon temperature and pressure, liquid CO2 become
Source:University of Michigan