ABSTRACT

The morphological features of gas hydrate formation in gas-seawater systems were observed using a prove-type microscope video camera. The hydrate was synthesized in a pressure chamber with pressure-tight glass windows for observations. Video observations showed the details of hydrate formation processes; after the hydrate formation started, hydrate films grew on the gas-seawater interfaces such as the surfaces of water droplets and gas bubbles; finally, massive hydrates were formed in bulk water. The implementation of imaging techniques is expected to have an advantage for investigation of the hydrate formation process because the hydrate formation would be initiated under non-equilibrium conditions.

INTRODUCTION

Gas hydrates are ice-like crystalline solids made from water and guest gas such as methane and carbon dioxide (CO2), and, in general, thermodynamically stable under high-pressure and low-temperature conditions. Throughout 20th century, methane hydrate was the target which should be removed from natural gas pipeline for flow assurance; in recent decades, however, it has been expected as an unconventional natural gas resource (Buffett, 2000; Sloan, 2003; Koh et al., 2011). Moreover, CO2 hydrate would play a key role in CO2 capture and storage (CCS), which is one of greenhouse gas control technologies.

Methane hydrate occurs naturally in permafrost and under deep seafloor in the world (Kvenvolden, 1995; Buffett, 2000; Hester and Brewer, 2009; Chong et al., 2016). Marine methane hydrate is buried in two types of deposits; one is called stratigraphic-type hydrate, which is contained in turbidite layers beneath the seafloor, and another is called shallow accumulation, which is buried as massive chunks in muddy sediments (Matsumoto et al., 2011). To extract methane gas from the stratigraphic-type hydrate, depressurization method, where the hydrates are dissociated to water and methane gas due to depressurization by upwelling water in the well drilled into the turbidite layers, was employed at offshore gas production tests performed in the eastern Nankai Trough, off Honshu Island of Japan (Yamamoto, 2015; Konno et al., 2016). In contrast, a different exploitation method for the shallow accumulation, where the hydrates are excavated using submersible mining machinery on the seafloor and conveyed as slurry, a mixture of water and hydrate particles, through a riser pipe to a support vessel, was proposed (Nakajima et al., 2020). During the conveying, the hydrates can be partially dissociated because of changes in pressure and temperature in the pipe, which provides a solid-gas-liquid multiphase flow. Moreover, methane hydrates can be formed again according to changes in pressure and temperature, which can cause blockage in the pipe (Aman, 2021; Zhang et al., 2022).

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