Hydrate as a by-product in CO leakage during the long-term sub-seabed sequestration and its role in preventing further leakage.

Subsurface sequestration of CO produced by industrial production is an effective way to control the excessive emission of greenhouse gases and alleviate the potential global warming. However, CO leakage is likely to occur in its long-term sub-seabed sequestration, posing a great threat to the marine environment and the marine ecology. Previous investigations mainly focused on the implementation of CO sequestration project, ignoring the subsequent potential environmental hazards such as CO leakage, let alone the post-treatment of these accidents. In the present work, secondary sequestration mechanism of hydrate-bearing sediment for leaked CO was explored, and the effect of two important factors (hydrate saturation and sediment thickness) on it was then analyzed. It is expected to provide reference for exploring engineering measures to secondary sequestrate the leaked CO and avoid the catastrophic environmental accidents. The experimental results demonstrate that the role of hydrate in secondary sequestration for leaked CO is mainly due to its filling and occupation of the migration channels in sediment. In addition, due to the secondary sequestration of CO hydrate, change in seawater pH value caused by dissolution of leaked CO in water can be significantly weakened. Besides, with the increasing hydrate saturation and sediment thickness, CO hydrate plays a progressively obvious role in secondary sequestration of CO and avoiding great change in the marine environment. In this way, the leaked CO can be secondary sequestrated by designing/optimizing the characteristics of hydrate-bearing sediment. The investigation demonstrates that most of the leaked CO can be secondary sequestrated only when the hydrate saturation exceeds 0.30 and/or thickness of hydrate-bearing sediment exceeds 3.0 cm. The experimental investigation herein can provide technical support for avoiding environmental disasters caused by the leakage of long-term sequestrated CO.