[Show abstract][Hide abstract] ABSTRACT: We simulated decomposition of structure I methane hydrate (H) with all cages filled in contact with two reservoirs (pools) of liquid water (W) which in turn are in contact with either two methane gas reservoirs (G), or with vacuum (V), under constant volume–constant energy conditions. By adding gas or empty spaces to the simulation box we allow the released methane to diffuse out of the liquid phase and into the gas phase similar to what happens during methane hydrate dissociation. The evolution of the released methane molecules during the hydrate dissociation process was carefully monitored. We found that some of the released methane gas reaches the gas phase and contributes to the increase of gas pressure on the hydrate phase. As the hydrate dissociates, liquid water phase becomes supersaturated with methane, methane molecules aggregate, and spherical regions of high concentration of methane form which we identify as “nano-bubbles”. These nano-bubbles grew to a specific size range which depends on simulation conditions and remained stable in the liquid phase for the duration of the simulations (5 ns).
[Show abstract][Hide abstract] ABSTRACT: We study the hydrated silica–water interface in the presence of methane or carbon dioxide gas with molecular dynamics simulations. The simulations are performed with a limited amount of water, which forms a meniscus between two hydroxylated silica surfaces separated by 40 to 60 Å. Simulations were performed with the remaining space of the simulation cell left empty or filled with different numbers of methane or carbon dioxide gas molecules. The meniscus is used to determine the contact angle between water and silica in the absence and presence of the gases. The distribution profiles of the water and gas phases are determined over the duration of the simulation. The water number density in the layers adjacent to the silica is higher, and these layers are more structured and less mobile compared with water layers far from the surface. Additionally, the concentrations of the gases are significantly higher at the liquid and silica interfaces than in other locations in the gas phase. We speculate that the enhanced concentration of gases at the interface along with the extended contact area (curved meniscus compared with flat interface in the absence of silica surfaces) between water and guest molecules at the meniscus as well as lesser mobility of water molecules near the silica surface may provide a mechanism for the heterogeneous nucleation of the clathrate hydrate in water-wetting porous medium.
The Journal of Physical Chemistry C. 11/2012; 116(47):24907–24915.
[Show abstract][Hide abstract] ABSTRACT: We use constant energy, constant volume (NVE) molecular dynamics simulations to study the dissociation of the fully occupied structure I methane hydrate in a confined geometry between two hydroxylated silica surfaces between 36 and 41 Å apart, at initial temperatures of 283, 293, and 303 K. Simulations of the two-phase hydrate/water system are performed in the presence of silica, with and without a 3 Å thick buffering water layer between the hydrate phase and silica surfaces. Faster decomposition is observed in the presence of silica, where the hydrate phase is prone to decomposition from four surfaces, as compared to only two sides in the case of the hydrate/water simulations. The existence of the water layer between the hydrate phase and the silica surface stabilizes the hydrate phase relative to the case where the hydrate is in direct contact with silica. Hydrates bound between the silica surfaces dissociate layer-by-layer in a shrinking core manner with a curved decomposition front which extends over a 5-8 Å thickness. Labeling water molecules shows that there is exchange of water molecules between the surrounding liquid and intact cages in the methane hydrate phase. In all cases, decomposition of the methane hydrate phase led to the formation of methane nanobubbles in the liquid water phase.
The Journal of Physical Chemistry B 03/2012; 116(10):3188-97. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We recently performed constant energy molecular dynamics simulations of the endothermic decomposition of methane hydrate in contact with water to study phenomenologically the role of mass and heat transfer in the decomposition rate [S. Alavi, J.A. Ripmeester, J. Chem. Phys. 132 (2010) 144703]. We observed that with the progress of the decomposition front temperature gradients are established between the remaining solid hydrate and the solution phases. In this work, we provide further quantitative macroscopic and molecular level analysis of the methane hydrate decomposition process with an emphasis on elucidating microscopic details and how they affect the predicted rate of methane hydrate decomposition in natural methane hydrate reservoirs. A quantitative criterion is used to characterize the decomposition of the hydrate phase at different times. Hydrate dissociation occurs in a stepwise fashion with rows of sI cages parallel to the interface decomposing simultaneously. The correlations between decomposition times of subsequent layers of the hydrate phase are discussed.
The Journal of Chemical Thermodynamics 01/2012; 44(1):13–19. · 2.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The formation of methane hydrate in an unconsolidated bed of silica sand was investigated and spatially resolved by employing the magnetic resonance imaging technique. Different sand particle size ranges (210–297, 125–210, 88–177, and <75 μm) and different initial water saturations (100, 75, 50, and 25%) were used. It was observed that hydrate formation in such porous media is not uniform, and nucleation of hydrate crystals occurs at different times and different positions inside the bed. Also, hydrate formation was found to be faster in a bed with lower water content and smaller particle size. Decomposition of hydrate by thermal stimulation at constant volume showed that the dissociation front moves radially inward starting from the external surface of the hydrate formation vessel.
Energy & Fuels 06/2011; 25(7). · 2.85 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Constant energy molecular dynamics simulations are used to study the role of mass and heat transfer in the decomposition of methane hydrate. The rate of methane hydrate decomposition in a constant energy simulation is affected by heat and mass transfer arising from the breakup of the hydrate framework and release of the methane gas into the liquid phase. As the hydrate undergoes endothermic dissociation, temperature gradients are established between the remaining solid hydrate and the solution phases. Hydrate dissociation occurs in a concerted fashion with rows of sI cages parallel to the interface decomposing simultaneously. This leads to the release of large amounts of methane gas near the solid-liquid interface which can form bubbles that affect the rate of mass transfer between the phases. These phenomena can affect the rate of methane hydrate decomposition in natural methane hydrate reservoirs.
Canadian Unconventional Resources and International Petroleum Conference, Calgary, Alberta, Canada; 10/2010