Formation of methane nano-bubbles during hydrate decomposition and their effect on hydrate growth

Abstract

Molecular dynamic simulations are performed to study the conditions for methane nano-bubble formation during methane hydrate dissociation in the presence of water and a methane gas reservoir. Hydrate dissociation leads to the quick release of methane into the liquid phase which can cause methane supersaturation. If the diffusion of methane molecules out of the liquid phase is not fast enough, the methane molecules agglomerate and form bubbles. Under the conditions of our simulations, the methane-rich quasi-spherical bubbles grow to become cylindrical with a radius of ∼11 Å. The nano-bubbles remain stable for about 35 ns until they are gradually and homogeneously dispersed in the liquid phase and finally enter the gas phase reservoirs initially set up in the simulation box. We determined that the minimum mole fraction for the dissolved methane in water to form nano-bubbles is 0.044, corresponding to about 30% of hydrate phase composition (0.148). The importance of nano-bubble formation to the mechanism of methane hydrate formation, growth, and dissociation is discussed.

Decomposition dynamics of dodecahedron and tetrakaidecahedron structures in methane hydrate by molecular simulations

“…An ice layer will be formed at the interface of methane hydrate thus to prevent the diffusion of methane and water molecules and slow down the decomposition. When methane molecules have diffused into liquid water region, methane bubbles may be formed because of the low methane solubility in water . The presence of methane bubbles may reduce the concentration of methane in the water and thus to increase the decomposition rate of solid hydrate.…”

Generation of micro- and nano-bubbles in water by dissociation of gas hydrates

“…As the released CH 4 molecules are dissolved into water prior to the bubble formation when CH 4 hydrate is dissociating in water, as shown experimentally [29] and theoretically [20][21][22][23], it is reasonable that P g in CH 4 MNB should be less than P d .…”

“…Molecular dynamic simulations recently predicted the formation of NBs in solution after gas hydrate dissociation [20][21][22][23]. However, it is still difficult to directly confirm the existence of slightly lower than that expected theoretically.…”

“…3). Thus, the MNB formation can be interpreted not only by the aggregation of CH 4 molecules dissolved in water [20][21][22][23], but also by the shrinkage of larger bubbles formed during the hydrate dissociation.…”

Some current challenges in clathrate hydrate science: Nucleation, decomposition and the memory effect

“…It is known that gas diffusion in a quiescent decomposing hydrate system is slow and that this can lead to significant concentration gradients. Recent computer simulations of methane hydrate decomposition [23,57,58] show that when methane is injected from the melting solid hydrate into the aqueous phase, the solution does indeed become supersaturated in methane. Does supersaturation imply the presence of only a high concentration of dissolved molecules or is there more to it?…”