Abstract

Increasing anthropogenic CO₂ in the atmosphere causes global warming and subsequent environmental changes, which may lead to an increase in natural disasters jeopardizing human society. Prompt technological development for CO₂ capture and sequestration is required in the international community. In this study, we performed CO₂ emission and shallow-type methane hydrate decomposition experiments at the Joetsu Knoll, offshore Joetsu, Niigata, Japan, as pilot studies to test feasibility of CO₂ sequestration and methane recovery using methane-CO₂ replacement in shallow-type methane hydrates. An isobaric cylinder pump and probe with a built-in heater (“Heat sonde”) were developed to inject CO₂ in deep-sea, high-pressure conditions. Before injecting CO₂ into a methane hydrate located in deep-sea sediments, we attempted CO₂ emission directly into deep-seafloor. In the experiment, liquid CO₂ was emitted at the head of Heat sonde, however, the isobaric cylinder pump became clogged during operation. The result reveals that precipitates of CO₂ hydrate, which are generated during mixing of inflow seawater and outflow liquid CO₂, blocked flow lines of the isobaric cylinder pump and Heat sonde. This suggests that our developed instruments must be improved for future work. We also observed the collapse of an exposed methane hydrate layer at the seafloor upon contact with the Heat sonde in our experiment.