Utilizing environmentally friendly nanobubble technology for universally applicable offshore CO ₂ geological storage in the Asia–Pacific region: a perspective from a preliminary laboratory experiment

Despite a growing demand for advancing carbon capture and storage (CCS), various concerns persist that hinder the practical application of offshore CCS in the Asia–Pacific region, including site selection, safety, environmental conditions and cost. In this paper, we examine the possible application of low-cost and environmentally friendly nanobubbles (NBs) for offshore CCS by investigating the gas solubility and stability of CO 2 NBs. We studied the effectiveness of CO 2 NBs in CCS through laboratory experiments by injecting water containing CO 2 NBs into the tested porous sand media and measuring the temporal change of dissolved CO 2 (DCO 2 ) concentration within the pore water. We prepared several different porous media with varying porosities ( ϕ = 0.48, 0.50, 0.55 and 0.60) and grain sizes (median diameters of 180 and 350 µm) of sand particles. The DCO 2 concentration measured immediately after generating CO 2 NBs in pure water for 20 min under open-air conditions was 1051 ± 202 ppm. The concentration agreed well with that theoretically estimated from the bubble diameter and number density of generated CO 2 NBs. We also monitored DCO 2 within the pore water for six weeks after adding the CO 2 NB water into the given sand media. Although the DCO 2 concentration in the pore water of sand media decreased with time, our results revealed that the reduction rates of DCO 2 concentration were lower when the porosity was higher. Notably, the DCO 2 concentration at ϕ = 0.60 was as high as 160 ± 10 ppm even after six weeks, highlighting remarkable results given that our experiment was carried out in open-air conditions. We suggest that bulk CO 2 NBs contribute to increasing DCO 2 in the early stage, and that surface NBs, favoured for being pinned on sand particle surfaces, play a role in sustaining a high DCO 2 in the later phase, likely later than about two weeks in the studied environment. These findings may underscore the effectiveness of NBs for universally applicable offshore CCS, even in shallow subsurface.