(197bu) A Molecular Dynamics Study of Growth and Dissociation of Mixed Methane + Carbon Dioxide Gas Hydrates

Gas hydrates are crystalline structures that are made of water (host) cages encapsulating the gas (guest) molecules. Methane hydrates are mostly found in the deep sea and permafrost regions; and are a source of fossil fuel where the estimated amount of carbon content trapped in hydrate reservoirs globally is twice that present in conventional fossil fuels[1]. Methane recovery from hydrate reservoirs using CO₂ injection has gained attention in the past few decades owing to its potential to suffice energy requirements and sequester carbon dioxide[1]. However, due to differences in hydrate forming conditions between methane and carbon dioxide hydrates full recovery of methane from the reservoirs cannot be achieved, as result residual quantities of mixed (CO₂ + CH₄) hydrates, CH₄ hydrates, CO₂ hydrates, CH₄ and CO₂ gases may remain in the hydrate reservoir. Although methane hydrates and carbon dioxide hydrates have been extensively studied in the literature, not many studies are available demonstrating the behavior of mixed gas hydrates[2], [3]. Therefore, in our work, we are studying the effect of various parameters, such as salinity, CO₂ concentration in the aqueous liquid phase, and temperature which shall affect the stability of mixed gas hydrates in the reservoir. To achieve this, we have performed molecular dynamics simulations (by employing GROMACS open source software package) of the mixed hydrate in contact with liquid water (H-L_w phase) in the NpT ensemble at a fixed pressure of 30.5 bar and temperatures of 250 and 273 K. We have observed the effect of addition of salts in the aqueous phase, supersaturation of CO₂ in the aqueous phase, and combined effect of both, on mixed hydrate behavior at both 250 and 273 K. Simulations have been performed using all-atom forcefields for water (TIP4P/2005), carbon dioxide (TraPPE Flexible), methane (TraPPE-EH Flexible) and NaCl to capture a reliable and effective description of the interactions present in the system. At low temperatures (250 K) in the presence of supersaturated CO₂ in the aqueous phase, growth of CO₂ hydrate has been observed. Order parameter analysis has been used to estimate the variation in the thickness of the hydrate with time and eventually the velocity of the hydrate front during growth has been calculated. The interface width at both interfaces has been observed to be invariant with time. However, at the same low-temperature condition, when salt was present along with supersaturated CO₂ in the aqueous phase, the growth of the hydrate is limited up to a few nanoseconds, and eventually, gas nanobubbles were formed. This confirms the role of salts in inhibiting the growth of the mixed hydrate. Hydrogen bonding and local order parameter analysis suggest that salts disrupt the hydrogen bonding between water molecules required to form the hydrate cages, thereby inhibiting the growth of the hydrate. Two-dimensional mass density profiles are used to understand the mechanism of nanobubble growth by measuring its change in radius with time. At low-temperature conditions, when neither CO₂ nor salts were present in the aqueous phase, the formation of ice was detected. However, when only salts and no CO₂ were added to the aqueous phase at the same temperature condition, no significant drop in the fraction of solid water was observed. This result is interesting by itself as it suggests a possible difference in the effect of salts in the presence and absence of an aqueous solution supersaturated with CO₂ at low temperatures. At the high-temperature condition (273 K), dissociation of the mixed hydrate was observed. However, when the liquid water phase was supersaturated with CO₂ at the same condition, a cylindrical nanobubble was observed. This is due to decreased solubility of CO₂ in the aqueous phase at high temperatures. Here, we attempt to explain the role of a mixture of guest molecules in the hydrates using interaction energies, the possible difference in the mechanism of growth and dissociation by studying interfaces, and the role of salt concentration in growth and dissociation by looking at hydration shells around ions and orientation of water molecules in the hydration shells. The work presented here will help to gain molecular insights for efficient methane recovery from and carbon dioxide sequestration in the hydrate reservoir by carbon dioxide injection.

References

[1] W. N. Wei, B. Li, Q. Gan, and Y. Le Li, “Research progress of natural gas hydrate exploitation with CO2 replacement: A review,” Fuel, vol. 312, p. 122873, Mar. 2022, doi: 10.1016/J.FUEL.2021.122873.

[2] S. Sarupria and P. G. Debenedetti, “Homogeneous Nucleation of Methane Hydrate in Microsecond Molecular Dynamics Simulations,” J Phys Chem Lett, vol. 3, no. 20, pp. 2942–2947, Oct. 2012, doi: 10.1021/jz3012113.

[3] S. Sarupria and P. G. Debenedetti, “Molecular dynamics study of carbon dioxide hydrate dissociation,” Journal of Physical Chemistry A, vol. 115, no. 23, pp. 6102–6111, Jun. 2011, doi: 10.1021/jp110868t.