Characteristics of formation and dissociation of CO2 hydrates at different CO2-Water ratios in a bulk condition

Abstract

Hydrate formation during an offshore oil/gas drilling operation can yield serious problems, such as plugging in the pipeline and damages at the wellhead. On the other hand, the idea of using CO2 hydrate as a self-trapping mechanism has been emerging for sequestering carbon dioxide in the offshore sediments. In this regard, we conducted a fundamental experimental study to investigate the characteristics of CO2 hydrate's formation-and-dissociation under the different CO2-water ratios, stimulation method, and dissociation temperature, as well as the density and size of CO2 bubbles after the dissociation of CO2 hydrate. The overall results show that the amount of CO2 in the bulk condition exerted a significant impact on the characteristics of the formation and dissociation of CO2 hydrate. In the CO2-limited condition, CO2 was insufficient to saturate water, and thus, nucleate and form hydrate crystals. On the other hand, in the water-limited condition, the amount of CO2 was able to fully saturate the aqueous phase and lead to the nucleation and formation of hydrate crystals. As the volumetric ratio of CO2 increased, longer melting and equilibrium time was monitored, which suggests that the formed CO2 hydrate could be denser and more stable. Besides, a denser population of smaller CO2 bubbles resulted after the hydrate dissociation, which implies more intense shearing of CO2 molecules in the water phase. Higher dissociation temperature from 8 to 22 °C led to smaller and more bubble formation, while differences in the mixing speed (60 or 120 rpm) and style (full- or half-tumbling) resulted in a negligible effect on CO2 hydrate formation and dissociation. Further study on the CO2-water-solid system will help to understand the formation and dissociation characteristics of CO2 hydrate under the different bulk and porous medium condition.