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

二氧化碳现在被大多数科学家认为是导致全球变暖的主要温室气体。许多国家正在努力减少向大气中排放二氧化碳。实现这一目标的主要途径有三个：（1）使用可再生能源;（2）提高使用化石燃料的工厂的效率;（3）碳捕获和储存（CCS）。据信，在中短期内，化石燃料仍将是世界能源的主要来源。此外，氢经济的最初路线也需要化石燃料。因此，除了提高现有化石燃料发电厂的效率外，碳捕获和储存（CCS）可能对降低未来潜在灾难性全球气候变化的风险至关重要。这些措施包括将二氧化碳注入枯竭的石油/天然气储层、在EOR过程中使用二氧化碳、以气体水合物形式进行海底封存、煤层处置和含水层处置。然而，地下储存的密封完整性和CO2泄漏到海洋（降低pH值）和大气中的可能性令人担忧。天然气水合物是水和气体分子在合适的温度和压力条件下结合形成的结晶化合物。包括甲烷和CO2在内的许多气体都可以形成天然气水合物，我们认为天然气水合物的形成在海洋环境中CO2的储存中起着重要的作用。取决于系统温度，CO2可以在大于200 m的水深处形成气体水合物。因此，从地质结构释放的任何CO2都可以转化为海底沉积物中的固体水合物，从而提供额外的密封和安全因素，防止CO2泄漏到海洋/大气中。如能证实，这可作为选择合适弃置地点的进一步准则，并有助提高公众的接受程度。我们的目的是研究：（1）沉积物中CO2水合物的形成可以防止/减少CO2从地质构造释放到海洋/大气中，（2）双示踪剂体系（一种水合物形成和一种非水合物形成）可用于监测/检测CO2泄漏和CO2水合物密封的效率。作为次要目标，我们希望研究海洋沉积物中CO2水合物的稳定性，以评估CO2作为CO2水合物储存在海洋沉积物（可能还有永久冻土）中的可能性。有人认为，甲烷水合物已经稳定了数千年，因此，应该可以以水合物的形式储存CO2。作为另一个次要目标，建议对水合物结构中用CO2取代甲烷的可能性进行有限数量的测试，即，储存CO2和从甲烷气体水合物储层生产甲烷。甲烷水合物（相当于化石燃料的总量）在世界范围内的出现激发了科学界研究如何回收这种低碳能源资源。提出的主要开采方法包括降压、热刺激、抑制剂注入或这些方法的各种组合。然而，这种方法除了需要大量的热输入来解离水合物之外，还可能引起富含水合物的沉积物的不稳定性。到目前为止，还没有开发出经济的生产技术。

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.