(661a) The Role of Molecular Level Modeling in Gas Hydrate Studies

For decades, researchers have been
able to model and simulate systems of different scales independent of each
other; however, the ability to bridge multiple time and length scales between
such models has only recently begun to be intensively investigated. Natural gas
hydrates (GH) provides an ideal model system on which to develop multiscale
modeling techniques. The formation properties of hydrates are governed by
molecular-level interactions between the gas and water molecules as well as the
host media, yet hydrate reservoirs in nature can span tens of meters vertically
and kilometers horizontally. Over the past few decades, many studies using
Molecular-Level Modeling (MLM) have been conducted on gas hydrates and
molecular dynamics (MD) simulations have been shown to correlate with
experimental data in many areas such as phase equilibria, inhibition, cage
occupancy, diffusion, and vibrational spectra to name a few examples.

Many successful research programs
have been based on the constant integration of accurate theoretical modeling
and laboratory studies. Concepts developed through simulations can enable a
fundamental understanding of the processes in the laboratory while the lab can
expose gaps in the fundamental knowledge eager for computational enlightenment.
A few examples of how researchers at NETL, The University of Pittsburgh, and
West Virginia University are using MLM for hydrate systems and their connection
to hydrates, and their recoverability, in nature are presented. This work was
performed as part of NETL's Institute for Advanced Energy Solutions
(NETL-IAES).

Models of gas hydrate responses
to production stimulation do not include expansion due to either thermal effects
or guest effects for gas hydrates.  Currently, the compressibility and thermal
expansivity of ice are used in the absence of any data on the subject.  To
address this problem, NETL, Pitt, and WVU has examined the temperature,
pressure, and compositional dependence of the crystal lattice distortion of gas
hydrates using molecular dynamics¹. MD simulations were used to study the lattice dimensions (or lattice constant) of various sI and sII hydrates at different temperatures and pressures.  Temperature and guest size have a large influence on the lattice dimensions while pressure and guest-guest interaction energy have a relatively smaller but significant effect. The lattice constant, l, has been shown to be a function of hydrate guest composition, the fractional occupancy of the cavities and the repulsive nature of the guest molecule. This work highlights the importance of precise lattice dimensions in calculating hydrate equilibria and demonstrates the role that molecular dynamic simulations can play in determining the effect of temperature, pressure and guest size on l. The primary function of such calculations is to create better thermodynamic models for hydrate equilibria, but the calculations can also provide a way to determine the
lattice expansion or contraction that occurs during production methods that
involve, for example, temperature change or gas exchange (CO₂ for CH₄).

The thermal behavior of hydrates has
attracted considerable attention since the discovery that the thermal conductivities
of hydrates are very different from that of ice.  In addition, gas hydrate
thermal conductivity is anomalously low, and general thermal properties are
relatively insensitive to temperature, in contrast to the behavior expected for
crystalline materials.²  The Jordan Group at NETL and the University of Pittsburgh has recently carried out non-equilibrium MD (NEMD) simulations of the thermal conductivity of methane hydrate and for the empty hydrate cage. They have found that, except at low (T ≤ 50 K) temperatures, the calculated thermal conductivities of methane hydrate and the empty cage system are nearly the same, ruling out host-guest coupling as a dominant factor for the anomalous thermal conductivity of methane hydrate.²  This finding is remarkable in that although the thermal conductivity of methane hydrate can differ from ice Ih by as much as 15 times, the difference in thermal conductivity between an empty and a methane-filled hydrate is not significant. 

Recent work at NETL and WVU on
the equilibrium of mixed CO₂-CH₄ hydrates has illustrated
that molecular-level processes result in macroscopic properties. The phenomenon
of mixed hydrate lattice strain is a perfect example of a phenomenon in which
macroscopic properties are governed by molecular-level events, and can be
studied carefully using molecular-level modeling. In the case of CO₂-CH₄
hydrate equilibrium, these equilibrium pressure errors could lead to large
differences in predictions of CO₂-CH₄ exchange processes
and should be incorporated into reservoir simulation algorithms. As recently
reported by Anderson and Velaga⁶, most thermodynamic models for hydrate dissociation pressure drastically overpredict the occupancy of CO₂ in the small cage of the hydrate lattice compared to NMR data obtained near the hydrate equilibrium pressure⁷. Through the use of ab initio calculations we have been able to develop a CO₂-H₂O potential that results in improved accuracy of CO₂ and CO₂-CH₄ thermodynamic predictions⁶.

Currently, MD simulations are
being performed at NETL and WVU to determine the dissolution rate and mechanism
for CH₄ and CO₂ hydrates. In 2006, Nihous and Masutani⁸ suggest that the concentration of the hydrate guest species at the interface between the desorption film and diffusive boundary layer may be much lower than ambient solubility. Preliminary MD simulations support this conclusion. Additionally, in the area near the hydrate-water interface, an increase in the solubility of CO₂ may aid in increasing the rate of CO₂ dissolution compared to CH₄. An understanding of the dissolution of gas hydrates in contact with undersaturated water is important to determining the long-term stability of gas hydrate reservoirs.

At the nano-, or molecular-,
scale, molecular modeling can be utilized to estimate key parameters such as
thermal expansion, isothermal compressibility, thermal conductivity, and
dissolution rates necessary for larger-scale models. These models can be
extended to include relevant real-world conditions such as sediment composition
and porosity and integrated into the reservoir simulation models at various
degrees of detail, allowing the reservoir models to compute methane hydrate
production scenarios over decade time scales.

References

(1)        Zhang,
M.; Anderson, B. J.; Warzinski, R. P.; Holder, G. D. ?Molecular dynamics
simulation of hydrate lattice distortion?; Prepr. Pap.-Am. Chem. Soc., Div.
Fuel Chem., 2009, Salt Lake City, UT.

(2)        Jiang,
H.; Myshakin, E. M.; Jordan, K. D.; Warzinski, R. P. The Journal of Physical
Chemistry B 2008, 112, 10207.

(3)        Klauda,
J. B.; Sandler, S. I. Chemical Engineering Science 2003, 58,
27.

(4)        Sloan,
E. D.; Koh, C. A. Clathrate hydrates of natural gases, 3rd ed.; CRC
Press: Boca Raton, FL, 2008.

(5)        Anderson,
B. J. Fluid Phase Equilibria 2007, 254, 144.

(6)        Velaga,
S.; Anderson, B. J. ?Phase equilibrium and cage occupancy calculations of CO₂
hydrates using an ab initio intermolecular potential?; Prepr. Pap.-Am. Chem.
Soc., Div. Fuel Chem., 2009, Salt Lake City, UT.

(7)        Ripmeester,
J. A.; Ratcliffe, C. I. Energy & Fuels 1998, 12, 197.

(8)        Nihous,
G. C.; Masutani, S. M. Chemical Engineering Science 2006, 61,
7827.