© 2019 Elsevier B.V. Carbon dioxide capture and storage is an efficient technology to reduce CO2. A candidate CO2 reservoir is a sub-seabed aquifer under the cap rock. However, there is a risk of CO2 leakage even though such probability is low. Once CO2 seeps at the seafloor, it forms bubbles/droplets and dissolves during their rise. The rising speed and the dissolution rate significantly depend on the bubble/droplet size. Interestingly, past observations of natural seepage of CO2 indicated that the bubble/droplet sizes were not very different, regardless of the seepage depths. In this study, a numerical method to simulate 3D solid-liquid-gas three-phase flow in the unconsolidated particle layers was developed and applied to elucidate the influences of gas flux, porosity, and particle size on the formations of gas channels and subsequent bubbles. The results suggested that the bubble size did not depend on the flux, but on the gas channel size at the level of gas flux expected in possible seepage events.