Numerical study on CO2 chimney formation in subsea sedimentary layers

In Carbon dioxide Capture and Storage (CCS), gas-hydrate trap mechanism is attracting attention as an alternative and backup storage method. This method applies CO₂ hydrate formed under low-temperature and high-pressure conditions to contain CO₂ in a geological formation. However, understanding gas chimney structures, which can be a leakage pass through a hydrate stability zone (HSZ), is essential for evaluating hydrate trap potential. Conventional research on gas chimneys has mostly focused on methane gas. The objective of this study is to numerically investigate the mechanism of CO₂ chimney formation using a two-phase flow simulator that considers gas hydrate formation within subsea sediments: how CO₂ chimneys are formed and how the differences in permeability, CO₂ chamber thickness, and salinity affect the formation of chimney structures. The results showed that the structure and formation mechanism of CO₂ chimney differ significantly from that for methane. Dissolution of CO₂ increased the density of water within the assumed cylindrical high-permeability region (HPR), generating a downward current. CO₂ that rose outside the HPR was blocked by hydrate and moved upwards along the side of the HPR towards the seabed. As a result, the CO₂ chimney had a hollow, cylindrical structure.