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

Abstract

The direct phase coexistence method is used for the determination of the three-phase coexistence line of sI methane hydrates. Molecular dynamics (MD) simulations are carried out in the isothermal-isobaric ensemble in order to determine the coexistence temperature (T3) at four different pressures, namely, 40, 100, 400, and 600 bar. Methane bubble formation that results in supersaturation of water with methane is generally avoided. The observed stochasticity of the hydrate growth and dissociation processes, which can be misleading in the determination of T3, is treated with long simulations in the range of 1000-4000 ns and a relatively large number of independent runs. Statistical averaging of 25 runs per pressure results in T3 predictions that are found to deviate systematically by approximately 3.5 K from the experimental values. This is in good agreement with the deviation of 3.15 K between the prediction of TIP4P/Ice water force field used and the experimental melting temperature of ice Ih. The current results offer the most consistent and accurate predictions from MD simulation for the determination of T3 of methane hydrates. Methane solubility values are also calculated at the predicted equilibrium conditions and are found in good agreement with continuum-scale models.

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

“…For this reason, we decided to use the system with the TIP4P/Ice water model and a single LJ centre for methane to study hydrate formation under an external electric field. The direct coexistence method possesses an inherent degree of stochasticity, [59,69] which is why our result at 400 bar is slightly different from the results of Conde and Vega (−5K) and Michalis et. al.…”

“…Finally, for electric field magnitudes above 0.9Vnm−1 the system suffered a rapid melting that lasted for only the first 2 ns of the simulation. It could be hypothesized that the observation of this effect might be a result of using the direct coexistence method, which possesses an inherent degree of stochasticity, [59,69] However, this stochasticity applies only in a small region near the coexistence temperature; however, when we continued increasing the temperature at the same pressure in the presence of an electric field, the same effect was observed at 305K even though the range of magnitudes at which the system crystallised was reduced to 0.3-0.8Vnm−1; see Figure 6(b). Even at 310 K; see Figure 6(c).…”

“…In addition, Tung et al [58] used the same methodology but with a different initial configuration, and using the TIP4P-Ew [39] water potential and the OPLS-AA force field for methane, they found that factors such as the solubility of methane in water, the mass transport of methane via diffusion and the affinity of methane for incomplete water cages at the interface are three key factors that drive the growth rate of methane hydrate. More recently, Michalis et al [59] used the direct phase coexistence method in a system that consisted of a solid hydrate slab, two liquid water slabs surrounding the hydrate slab and one gaseous methane slab. Using the TIP4P/Ice water model in combination with the OPLS-UA force field for methane, they found that the statistical analysis of multiple independent runs per pressure and long-term simulations were necessary to compensate for the inherent stochasticity of the method.…”

“…The same combination of models and the same direct coexistence method, used in Refs. [9,59], were employed in this work to extend the studies on their ability to reproduce the experimental hydrate liquid water gas coexistence temperatures under various different pressures, and to explore the shift produced by an external constant electric field, using intensities in the range from 0.1 to 0.9Vnm−1.…”

Multiphase non equilibrium pipe flow behaviors in the solid fluidization exploitation of marine natural gas hydrate reservoir

“…[23][24][25] There is no study on the flow behavior of wellbore multiphase flow with coupling hydrate decomposition. Moreover, the research on hydrate decomposition mainly focuses on decomposition temperature and pressure condition, [26][27][28][29][30] decomposition rate model, and prediction. [31][32][33][34] The dynamic decomposition behavior of hydrate particles in the condition of increasing temperature and decreasing pressure is not studied.…”