Can CO2 hydrate formation act as a safety factor for subsurface storage of CO2?

CO2 is now recognised by the majority of scientists as the main greenhouse gas responsible for global warming. Many countries are now working towards reducing emissions of CO2 to the atmosphere. This is being achieved by three principal means: (1) the use of renewable energy sources, (2) increasing the efficiency of industrial plants using fossil fuels, and (3) carbon capture and storage (CCS).It is believed that in the short to medium term, fossil fuels will remain the principal source of the World’s energy. Furthermore, the initial routes to a hydrogen economy will also require fossil fuels. Therefore, in addition to increasing the efficiency of existing fossil fuel plants, carbon capture and storage (CCS) is likely to be essential for reducing the risk of potentially catastrophic global climate changes in the future.There are several options in storing CO2 in geological structures. These include CO2 injection into depleted oil/gas reservoirs, CO2 use in EOR processes, subsea sequestration in the form of gas hydrates, coal-bed disposal and aquifer disposal. However, there are concerns with respect to the integrity of the seal in subsurface storage and potential leakage of CO2 to ocean (reducing the pH) and atmosphere.Gas hydrates are crystalline compounds formed as a result of combination of water and gas molecules at suitable temperature and pressure conditions. Many gases, including methane and CO2 can form gas hydrates.We propose that gas hydrate formation could play an important role in CO2 storage in offshore environments. Depending on the system temperature, CO2 can form gas hydrates at water depths greater than 200 m. Therefore, any CO2 released from geological structures could be converted into solid hydrates in subsea sediments, providing an additional seal and safety factor against any CO2 leakage to ocean/atmosphere. If proved, this could provide a further criterion for choosing suitable disposal sites and help improve public acceptability.Our aim is to examine whether: (1) CO2 hydrate formation in sediments could prevent/reduce the release of CO2 to the ocean/atmosphere from geological structures, and (2) whether a two-tracer system (one hydrate forming and one non-hydrate forming) could be used to monitor/detect CO2 leakage and efficiency of a CO2 hydrate seal.As a secondary objective, we want to investigate the stability of CO2 hydrates in marine sediments, for evaluating the possibility of CO2 being stored as CO2 hydrates in marine sediments (and possibly permafrost). It is argued that methane hydrates have been stable for thousands of years, therefore, it should be possible to store CO2 in the form of hydrates.As a further secondary objective, it is proposed to conduct a limited number of tests on the possibility of replacing methane with CO2 in the hydrate structure, i.e., storing CO2 and producing methane from methane gas hydrate reservoirs. Worldwide occurrences of methane gas hydrates (comparable to total fossil fuels) have inspired the scientific community to investigate the ways to recover this resource of low-carbon energy. The main recovery methods proposed include depressurization, thermal stimulation, inhibitor injection, or various combinations of these. However, such methods may induce instability in hydrate-rich sediments in addition to requiring a significant heat input to dissociate hydrates. As yet, no economical production technique has been developed.