Dissociation of Methane Hydrate in Aqueous NaCl Solutions

Abstract

Molecular dynamics simulations of the dissociation of methane hydrate in aqueous NaCl solutions are performed. It is shown that the dissociation of the hydrate is accelerated by the formation of methane bubbles both in NaCl solutions and in pure water. We find two significant effects on the kinetics of the hydrate dissociation by NaCl. One is slowing down in an early stage before bubble formation, and another is swift bubble formation that enhances the dissociation. These effects arise from the low solubility of methane in NaCl solution, which gives rise to a nonuniform spatial distribution of solvated methane in the aqueous phase. We also demonstrate that bubbles form near the hydrate interface in dense NaCl solutions and that the hydrate dissociation proceeds inhomogeneously due to the bubbles.

Clathrate hydrates: recent advances on CH4 and CO2 hydrates, and possible new frontiers

“…Not much effect was observed on the dissociation. Yagasaki et al 111 conducted molecular dynamics (MD) simulations for CH4 hydrate dissociation in the presence of NaCl. They found two competing phenomena: NaCl slightly slows the early stage of hydrate dissociation, before the formation of a methane bubble, but it decreases CH4 solubility in water, which causes the formation of methane bubbles.…”

“…These simulations were consistent with those of Baez and Clancy, 119 who showed that even hydrate dissociation, not only hydrate nucleation, is stochastic, and those of others confirming the importance of the transport of guest gases away from the hydrate. 111,138 The occupancy of the hydrate affects the rate of dissociation, 137,139 as well as the size of the guest molecules and their interactions with the water molecules in the cage. 140 Adiabatic MD simulations showed that, because the hydrate dissociation is endothermic, it is possible that the local temperature decreases upon dissociation, regenerating hydrate cages from the melt.…”

The dynamic behavior of gas hydrate dissociation by heating in tight sandy reservoirs: A molecular dynamics simulation study

“…Previous studies focused on hydrate phase behavior [9][10][11], nucleation and growth [12][13][14][15][16][17][18][19][20][21][22][23][24], transport properties [25,26], and chemical inhibition [27][28][29][30][31][32][33][34][35][36][37]. In addition, the kinetics of methane hydrates dissociation within the water phase have been widely studied using MD simulation and discussed in detail in previous studies [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53]. The dissociation processes of NGHs is described as a two-step process: first, the enhanced diffusive behaviors of the host water molecules in the hydrate crystals lead to unit cell size increases and distortions that ultimately break down the lattices, and second, methane molecules escape from these incomplete cages and aggr...…”

Methane hydrate: shifting the coexistence temperature to higher temperatures with an external electric field

“…Several studies have focused on the capabilities of various techniques and molecular models to describe the experimental conditions required for the coexistence of liquid water-methane hydrate, [6][7][8][9][10][11][12][13] whereas the current ability to perform simulations within microseconds [14,15] has allowed for the direct study of the nucleation and crystallisation processes, [16][17][18][19][20][21][22][23][24][25] the dissociation processes, [26][27][28][29] and even how these processes are affected by various factors, such as bubble formation [30] or the presence of NaCl in an aqueous solution. [31] It is expected that this knowledge will contribute to the development of better criteria for choosing among the various chemical inhibitors of hydrate formation that are available [32] for specific situations or even to modification of these inhibitors and the design of new ones. Physical methods can also be used to prevent hydrate formation or to destroy an already existing clog; a more practical alternative than heating or depressurising is to apply an external electric field.…”