Kinetic Characteristics Of Hydrate Formation During Methane Seepage In The "Haima" Cold Seeps

Methane seepage is prevalent in deep-sea environments and is an important link that affects global methane and carbon balance.Studying the ultimate fate of seafloor methane seepage processes is an important basis for understanding the ocean and the global carbon cycle.The formation of natural gas hydrates is a special process that blocks the free emission of methane from the seafloor,and the kinetic characteristics of hydrate generation during methane bubble seepage have rarely been studied.Based on the in-situ marine environment of "Haima" cold seeps in the South China Sea,this thesis mainly studied hydrate formation behavior under different systems and different methane fluxes,to accurately depict the formation kinetics of methane bubbles released from the deep seafloor,which can provide fundamental support for assessing the potential of oceanic methane sequestration and coping with climate change.The main contents of this study are as follows:(1)The characteristics of gas hydrate formation from methane bubbles in pure water systems were investigated in both continuous and intermittent seeping modes,laying the groundwork for further experimental investigation of hydrate formation kinetics.Bubble flow could be divided into steady flow and unsteady flow according to the methane flux magnitude.There might be a critical methane flux between steady flow and unsteady flow(0.002-0.014 mol/min),which made the hydrate nucleation time shortest,mainly affected by the total surface area and residence time of seepage bubbles in the same time.Even under high pressure driving force of 8.1 MPa,not all methane bubbles could form hydrate.Due to the formation and consumption of methane gas in the bubble,the pressure difference between the inside and outside of the hydrate film caused the hydrated bubsble to collapse.Raman spectra showed that the distribution of dissolved methane was not uniform in the reactor,which provided evidence for the random nucleation process of hydrate.Under the seeping flux of 0.010?0.036 mol/min and the gas injection time of 20 min,the methane fixation rate increased from 41.1% to 65.5%.At 0.019 mol/min seeping flux,the methane fixation rate increased by more than 2 times when the injection time increased by 6 times.The results showed that the methane fixation rate increases with the increase of injection time and seeping flux.(2)The hydrate formation characteristics of methane bubbles under different continuous seepage fluxes in simulated seawater system were further studied.The results showed that in the methane bubble leakage-gas dissolution phase before hydrate formation,the methane dissolution rate accelerateed with increasing methane flux,and the difference mainly originated from the enhanced mass transfer perturbation.In the range of flow rates investigated in the experiment,hydrate nucleation time did not show a significant correlation with the methane flux and dissolved methane concentration,but the hydrate nucleation sites tended to appear at the gas-liquid contact surface or reactor surface,showing obvious interfacial effects.After hydrate formation,a large number of flocculent hydrates often appeared in the system.The amount of hydrate transformation was closely related to the concentration of dissolved gas in the system at the initial formation of the hydrate.The transformed hydrated bubbles would not only have the phenomenon of hydrate membrane collapse,but also more hydrate bubbles rupture and coalesce to form larger hydrate bubbles with the extension of seepage time.Raman spectroscopy was used to further analyze the transformation process of the methane state in the water phase during hydrate formation.The results showed that the dissolved methane content decreased rapidly from saturated or near-saturated concentration to 0.089?0.116 mol/L after hydrate formation,and its rate was related to methane flux.However,the signal of hydrate could not be detected during the initial formation of hydrate until the fine hydrate grains gather to a certain extent.The final methane fixation rate was not correlated with the change in methane flux.When the seeping flux was 0.029 mol/L and 0.008 mol/L,the methane fixation rate was as high as 55%,while for other fluxes,the methane fixation rate is 23-38%,indicating that there might be an optimal leakage rate for the highest methane fixation rate.At the same time,the carbon sequestration capacity in the form of hydrate under in-situ methane leakage could be evaluated according to the gas absorption value and methane fixation rate.