Methane Concentration Above Methane Seep and Methane Hydrate Coated Methane Gas Bubble

Abstract:

In recent studies, it has been reported that natural gas (mainly methane gas) is seeping from seafloor. This gas is called “methane seep” and has been developing as an unconventional natural gas resource. Gas bubbles rise to sea surface in high temperature in shallow areas. Because methane is water-soluble gas, part of methane dissolve into sea water, and methane concentration of the area rise. In deep sea, the temperature is low and the pressure is high. Therefore, seeped methane bubble is coated by methane hydrate membrane. Methane hydrate is solid clathrate hydrate that is stable under low temperature and high pressure. Coated bubble causes different behavior from general gas bubble on the rising because of its solid surface. In order to collect gas as natural gas resource, it is necessary to design facilities and installation based on the behavior of the bubbles coated with solid membrane. In this study, sea water above methane seep was collected, and methane concentration was measured. Then, morphology change of methane hydrate coated bubble was observed in the laboratory experiment to understand the behavior of seeped methane gas. Furthermore, the purpose of this study is to use the result to contribute to methane seep development.

In the survey voyage in 2019, sea water above methane seep was collected in the Sea of Japan. Water sample was collected at intervals of about 25 m using Niskin water sampler in the area at a water depth of 900 m. Methane concentrations of the samples were measured using gas chromatography. As a result, sample collected at the depth of 850 m, 675 m and under 250 m showed higher concentration. At 850 m, methane may have dissolved before transition to methane hydrate. The temperature is lower under 250m and higher than the equilibrium condition of methane hydrate which allows the surface to dissociate and dissolve into sea water. Methane concentration may have increased due to the dissolution.