Microfluidic insights: Methane hydrate dynamics in distinct wettable confined space

Natural gas (main methane CH₄) hydrates are prospective energy resource that occurs in micro-channels of hydrate-bearing sediments. CH₄ gas is produced by hydrate dissociation into gas/water via depressurization. However, pore-scale characteristics of gas/water/hydrate in these micro-channels during CH₄ hydrate dynamics are lacking to understand hydrate transitions. This work investigated morphological CH₄ hydrate formation/dissociation with gas/water movement in microfluidic chips with gas-rich, water-rich and moderate micro-pores. Results showed hydrate nucleated at gas/water interfaces homogenously in hydrophilic pores, while heterogeneously in hydrophobic pores under quiescent conditions of 82.4–83.3 bar and 0.9–1.4 ℃. In hydrophilic, hydrate nuclei preferably grew from interfaces towards gas phase by consuming available gas. The hydrate patterns varied from coarse films to smooth crystals totally in gas-rich pores while partially in water-rich/moderate pores. More favorable water diffusion dominated in continuous gas flows of gas-rich hydrophilic system, causing the highest hydrate saturation of 86.3% after formation. In hydrophobic, hydrate growth developed sufficiently into hydrate crystals in water-rich pores owing to enough gas/water contacts with separated minor gas phases, while insufficiently into hydrate films in moderate pores because localized pressure variations triggered hydrate dissociation. During depressurization, higher initial dissociation pressures of 30.5–44.9 bar with shorter dissociation time of 12.5 h in hydrophilic pores were advantaged than those of 27.5–32.9 bar with 18.9 h in hydrophobic pores. These slower dissociation rate with lower dissociation pressures suggested enhanced hydrate stability in hydrophobic system and this was determined by disadvantaged high gas density at interfaces. Additionally, hydrate reformation in hydrophobic system was due to insufficient gas/water diffusions and localized pressure variations. These findings of kinetics and micromorphology during CH₄ hydrate dynamics are beneficial to understanding mechanisms of CH₄ hydrate transitions in distinct wettability.