Mechanism of methane migration in oceanic hydrate system: Insights from microfluidic investigations

Methane vents and gas hydrate systems coexist extensively in many regions of the world's oceans, posing challenges to the predictions of global carbon budgets and climate. However, the understanding of how methane gas migrates through deep-sea hydrate-bearing sediments and ultimately enters the overlying seawater is still incomplete. To address this issue, we used microfluidic technology to study the pore-scale mechanisms of free gas methane migration under hydrate-forming conditions. The microfluidic device is used to simulate the porous medium in submarine reservoir. The nucleation and growth of hydrates in saturated solution under the condition of methane and water flow were observed. Results indicate that during seepage, methane bubble surfaces continuously undergo the formation and detachment of tinny hydrate fragments, acting like an upward-flowing hydrate seeder. These flowing hydrate-bearing bubbles and hydrate fragments are capable of penetrating the entire oceanic hydrate system. Once captured elsewhere, these hydrate seeds, like other kinetic promoters, can significantly accelerate hydrate evolution at certain locations. Meanwhile, flowing methane repeatedly collides with the trapped hydrate fragments, either leading to the continuous flow of free methane or adsorption to the surface of the trapped hydrate fragments and eventually nucleation. Furthermore, when hydrates occupy the majority of pore channels, free methane gas can migrate through the water film between hydrates and the matrix. These three migration mechanisms may all contribute to the sustained emission of methane gas in natural seafloor hydrate systems.