CO₂ Hydrates Could Provide Secondary Safety Factor in Subsurface Sequestration of CO₂

Subsurface storage of carbon dioxide (CO₂) is regarded as a short to medium term solution for reducing greenhouse gas emissions. However, there are concerns with respect to the integrity of seals in subsurface storage of CO₂ and the risks associated with leakage to ocean and atmosphere. In this paper, we report the results of experimental laboratory simulation of CO₂ leakage from subsurface storage sites and the self-sealing mechanism of CO₂ hydrates in subsea sediments, using an experimental setup specifically constructed for this work. The results demonstrate that the sequestrated CO₂ migrated upward and formed hydrates with the pore water in the sediment when the pressure and temperature conditions in the sediments were inside the hydrate stability zone. The CO₂ hydrate formation slowed down the CO₂ diffusion rate by several times to 3 orders of magnitude. The upward migrating CO₂ tended to form hydrate at the base of the hydrate stability zone. On the geological time scale the CO₂ hydrate formation could create a low-permeability secondary cap layer which greatly restricts further upward CO₂ flow, should a leakage occurs. This potential “self-sealing” and “self-healing” process could be an important criterion in the selection of suitable sites for geological storage of CO₂.