Nonequilibrium adiabatic molecular dynamics simulations of methane clathrate hydrate decomposition

Abstract

Nonequilibrium, constant energy, constant volume (NVE) molecular dynamics simulations are used to study the decomposition of methane clathrate hydrate in contact with water. Under adiabatic conditions, the rate of methane clathrate decomposition is affected by heat and mass transfer arising from the breakup of the clathrate hydrate framework and release of the methane gas at the solid-liquid interface and diffusion of methane through water. We observe that temperature gradients are established between the clathrate and solution phases as a result of the endothermic clathrate decomposition process and this factor must be considered when modeling the decomposition process. Additionally we observe that clathrate decomposition does not occur gradually with breakup of individual cages, but rather in a concerted fashion with rows of structure I cages parallel to the interface decomposing simultaneously. Due to the concerted breakup of layers of the hydrate, large amounts of methane gas are released near the surface which can form bubbles that will greatly affect the rate of mass transfer near the surface of the clathrate phase. The effects of these phenomena on the rate of methane hydrate decomposition are determined and implications on hydrate dissociation in natural methane hydrate reservoirs are discussed.

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

“…Figure shows the time dependence of pressure in the simulation system; it can be seen that the variation of pressure might not be negligible. However, the tendency is extremely similar to the pressure curves simulated by Alavi and Ripmeester at the initial temperature of 310 K under the initial pressure of 10 MPa . In Figure , the pressure of the system increases gradually with the escape of methane gas.…”

“…The hydrate absorbs heat supplied by water. Due to the nonequilibrium characteristic of decomposition, there appears a temperature drop in the whole system . And decomposition no longer continues when the temperature reaches the equilibrium temperature of the system .…”

“…Due to the nonequilibrium characteristic of decomposition, there appears a temperature drop in the whole system. 13,14 And decomposition no longer continues when the temperature reaches the equilibrium temperature of the system. 15 When the temperature falls below the freezing point of liquid water, the decomposition process will present the self-protective effect.…”

Prediction of the phase equilibria of methane hydrates using the direct phase coexistence methodology

“…An accurate knowledge of the thermodynamic and kinetic properties of methane hydrate is a primary goal for a number of studies. MD studies considering methane hydrates have been reported as early as 1983 with the pioneering work of Tse et al 44 Subsequently, a significant number of MD studies examining methane hydrates have been reported focusing on topics such as hydrate nucleation, [45][46][47][48][49][50] formation and growth, 51,52 dissociation, [53][54][55][56][57][58] inhibition, 59-61 heat transfer effects, 62-64 the effect of hydrate contact with solid surfaces, 65-68 methane replacement by carbon dioxide, [69][70][71][72] and the calculation of transport properties such as thermal conductivity, [73][74][75] or thermodynamic properties such as three-phase equilibrium conditions. [76][77][78][79] In the area of calculation of the three-phase equilibrium conditions for pure methane hydrate which is the main focus of the current study, there is a number of molecular simulation studies that have provided important insights of the mechanisms involved.…”

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

“…Dissociation of hydrates is usually simulated with the hydrate phase either in contact with liquid water along two parallel planes or in some cases with these phases surrounded by a silica phase to mimic the hydrate decomposition in natural deposits. [19][20][21][22][23][24][25][26][27][28] It has been observed that during methane hydrate decomposition, the guest methane molecules are released into the liquid water phase and form methane-rich regions. [29][30][31][32][33][34] It was also recently reported that formation of methane bubbles reduces the dissolved methane concentration in water thus increasing the dissociation rate of the solid hydrate.…”