Methane Hydrate Nucleation:
Choreography and Cadence from Molecular Simulations
Matthew R. Walsha, Carolyn A. Koha, E. Dendy Sloana, Amadeu K. Suma, and David T. Wua,b*
a
Center for Hydrate Research, Department of Chemical Engineering, b Department of Chemistry
Colorado School of Mines, Golden, CO 80401, USA
Abstract
Methane hydrates are ice-like solids that form at low temperatures and high pressures, with methane
molecules encaged in a hydrogen-bonded water lattice. They have been of long-standing interest to the
oil and gas industry because of their tendency to plug pipelines, as a resource in the vast seafloor hydrate
sediment deposits, and recently in the containment of the Gulf oil spill. The control of hydrate formation
in these and other situations can be guided by moleular level knowledge of the hydrate nucleation process.
The nucleation of hydrates is a rare event that occurs in nanoseconds at a random location on a nanometer
length scale, and thus a molecular understanding of the process has been difficult to obtain experimentally
or by simulation.
By employing large-scale microsecond long molecular dynamics simulations, we have captured a
mechanism for spontaneous hydrate nucleation, revealing an intricate cooperative dance between methane
and water molecules [1], and analyze the development of cage ordering during the process of hydrate
nucleation. We discuss our findings with respect to different simulation system conditions (temperature,
pressure) and system geometry (methane nanobubble and bulk methane). The system geometry is found
to be important in affecting the concentration of methane in water and consequently inducing the
probability of hydrate nucleation – a methane nanobubble causes hydrates to nucleate more readily,
whereas nucleation in a bulk methane system requires longer times. By comparing different system
geometries we also discuss possible roles for the interface in hydrate nucleation. Knowledge of the phase
boundary, necessary for determining the subcooling driving force, was calculated by thermodynamic
integration for the molecular potentials used [2]. Analysis of trajectories under different driving forces
and geometries allowed an estimate of the nucleation rate as a function of methane concentration.
Acknowledgement: The authors acknowledge support from the US NSF under Grants CBET-0933856, NSFMRSEC DMR0820518, from US DOE-BES award DE-FG02-05ER46242, and from the Center for Hydrate
Research Consortium at the Colorado School of Mines (BP, Champion, Chevron, ConocoPhilips, ExxonMobil,
Haliburton, Multichem, Nalco, Petrobras, Schlumberger, Shell, SPT Group, StatoilHydro, and Total). The
simulations were carried out on facilities at the Golden Energy Computing Organization at the Colorado School of
Mines, using resources acquired with financial assistance from NSF and the National Renewable Energy
Laboratory.
References
1.
Walsh M. R., Koh C. A., Sloan E. D., Sum A. K. and Wu D. T. Microsecond Simulations of Spontaneous
Methane Hydrate Nucleation and Growth, Science, 326, 1095-1098, 2009.
2. Jensen L., Thomsen K., von Solms N., Wierzchowski S., Walsh M. R., Koh C. A., Sloan E. D., Wu D. T. and
Sum A. K. Calculation of Liquid Water-Hydrate-Methane Vapor Phase Equilibria from Molecular Simulations,
J. Phys. Chem. B, 114, 5775-5782, 2010.
3. Walsh M. R., Rainey J.D., Lafond P.G., Park D.-H., Beckham G.T., Jones, M.D., Lee K.H., Koh C. A., Sloan E.
D., Wu D. T. and Sum A. K. The cages, dynamics, and structuring of incipient methane clathrate hydrates,
PhysChemChemPhys, to appear.
4. Walsh M. R., Beckham, G.T., Koh C. A., Sloan E. D., Sum A. K. and Wu D. T. Methane Hydrate Nucleation
Rates from Molecular Dynamics Simulations: Effects of Aqueous Methane Concentration, Interfacial
Curvature, and System Size, J Phys Chem C, to appear.