Accounting for twice the carbon content of conventional fossil fuels, natural gas hydrate reservoirs are found in the deep sea and permafrost regions. [1] The extraction of CH
4 from hydrate reservoirs by CO
2 injection has received considerable attention in the past few decades as this method has the potential to sequestrate CO
2 and produce CH
4. [1] Nonetheless, complete extraction of CH
4 is not possible because of differences in hydrate-forming conditions between CH
4 and CO
2. Thus, residual quantities of mixed (CO
2 + CH
4) hydrates, CH
4 hydrates, CO
2 hydrates, CH
4 and CO
2 gases may remain inside the hydrate reservoir. Both CH
4 and CO
2 hydrates have been studied extensively, however, literature related to the stability of mixed gas hydrates is limited. [2,3] Hence, in this work, we have performed molecular dynamics (MD) simulation of mixed (CO
2 + CH
4) hydrates to understand the effect of various parameters, such as salinity, CO
2 supersaturation in the aqueous phase, and temperature on the stability of mixed (CO
2 + CH
4) hydrates. Open-source package GROMACS has been employed to perform the MD simulations of mixed (CO
2 + CH
4) hydrates in contact with liquid water (Lw-H phases) in isobaric isothermal (NPT) ensemble. To understand the effect of temperature, salinity, and carbon dioxide supersaturation on mixed gas hydrate behavior, we have considered four cases which are: Case-1: Lw-H phases, Case-2: Lw-H phases with supersaturated CO
2 in the aqueous phase, Case-3: Lw-H phase with NaCl present in the aqueous phase, and Case-4: Lw-H phases with both CO
2 and NaCl present in the aqueous phase. All four cases were run at 30.5 bar and temperatures of 250 K and 273 K. All-atom forcefields models for water (TIP4P/2005), carbon dioxide (TraPPE-EH), methane (TraPPE-EH), and NaCl have been used to capture a better description the system. Various analyses such as order parameter analysis, interface analysis, hydrogen bonding, clustering analysis, and density profiles have been used to capture the growth/dissociation of hydrates and the effects of added NaCl and CO
2. We observe the growth of CO
2 hydrate in Case-2 (supersaturated CO
2) at a low-temperature condition. However, at the same condition when NaCl is present along with CO
2 (Case-4), the growth of CO
2 hydrate becomes limited, and a nanobubble of CO
2 is formed. This showcases the effect of salinity i.e., inhibiting the growth of the mixed hydrate. At low-temperature, regardless of the presence of salt in the Lw phase for Case-1 and Case-2, ice formation in the Lw phase is 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 CO
2 at low temperatures. At the condition of high temperature, mixed hydrate dissociation is noticed. 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.
