Unusual CO₂ hydrate formation in porous media: Implications on geo-CO₂ storage laboratory testing methods

CO₂ sequestration in geological sediments as hydrates is a promising technique currently being explored by research to permanently store CO₂. To achieve this, studies on site selection and best experimental practices are useful to provide recommendations for understanding and implementing the process. In this study, the kinetics of CO₂ hydrate formation was evaluated in quartz sand at 4 MPa and 274.15 K to gain insight on selecting a suitable location that provides high CO₂ hydrate storage capacity. The study was conducted using a stainless-steel autoclave cell and a constant cooling method in pure water and brine system (3.3 wt% NaCl). The study revealed that CO₂ hydrate forms faster in quartz sand achieving about 87% CO₂ to hydrate conversion in pure water as opposed to 55% CO₂ to hydrate conversion in the brine system. The high hydrate conversion achieved in pure water is promising to provide a potential storage capacity of CO₂ in sediments. However, visual confirmation revealed that a high percentage of the hydrates in the pure water system formed at the top of the sand bed within the reactor which is unusual and undesired for practical CO₂ storage field applications. Interestingly, the hydrate formed in the brine system was within the porous media pore spaces. The results thus suggest that hydrate storage techniques experimental testing should be conducted in reactors with visual capabilities to gain a proper understanding of the location of the hydrates within the porous media.