(679h) Molecular Insights into Methane Hydrate Dissociation: Role of Methane Nanobubble Formation

For efficient CH₄ extraction from gas hydrate reservoirs (through the chemical injection method and/or combined extraction method) it is important to understand the mechanism of CH₄ gas hydrate dissociation. Due to their high surface area and good thermal conductivity nanoparticles are considered a potential green additive in chemical injection methods. However, depending upon the type of nanoparticles, its effect will vary in promoting/inhibiting the formation or dissociation of the hydrates. [1] Thermodynamically, the phase equilibrium of gas hydrate is slightly affected by nanoparticles [1] though nanoparticles can play a role similar to kinetic hydrate inhibitors and anti-agglomerates. [2] Jiao et al. observed that the presence of the hydrophilic silica nanoparticles promotes the CO₂ hydrate dissociation and with the increase in mass fraction of nanoparticles gas production also increases. [3] Thus, there is a possibility to use nanoparticles as an additive in the combined extraction method to improve the efficiency. Therefore, in this work, we have performed molecular dynamics (MD) simulations to study the hydrophilic silica nanoparticle effect on methane hydrate dissociation and identify the underlying physics of CH₄ dissociation. Silica being ‘sand’ is already present inside the reservoir in some form, making it an environmentally benign additive that will not have any adverse effect on the reservoir environment. MD studies related to nanoparticles in gas hydrate are very limited. [2,4,5] None of these publications investigated the effect of nanoparticles on the dissociation mechanism of methane hydrate. Hence, we have carried out MD simulations in the constant pressure and temperature (NPT) ensemble to investigate the dissociation of CH₄ hydrate in the presence and absence of ~1, ~2, and ~3 nm diameter hydrophilic silica nanoparticles at 100 bar and 310 K. Local order parameters for the host molecules, hydrogen bonding, interface analysis, cluster size distribution, and density plots are used to quantify the dissociation process. Interface analysis is crucial for understanding growth or dissociation phenomena in a biphasic system. We have computed the fraction of solid (Fs) along the Z coordinate of the simulation box. The interface location, the width of the interface, and hydrate thickness at each time frame are determined through curve fitting of the Fs vs. Z data; these data help to understand the dissociation trend of hydrate structure. We find that the formation of CH₄ nanobubble has a strong influence on the dissociation rate; after the initial hydrate dissociation, the rate of dissociation slows down till the formation of CH₄ nanobubble. The CH₄ hydrate dissociation process can be divided into three regimes i.e., regime-A: the initial stage of dissociation, regime-B: where the dissociation rate slows down, and regime-C: the final stage of dissociation corresponding to which a stable CH₄ nanobubble appears that affects the further dissociation process by increasing the dissociation rate. We find the critical concentration and size limit to form CH₄ nanobubble to be ~0.04 mole fraction of CH₄ and approximately 40 to 50 CH₄ molecules respectively. Additionally, we have also performed constant pressure and temperature Gibbs ensemble Monte Carlo simulations (GEMC-NPT) to determine the solubility of CH₄ and the chemical potential of H₂O and CH₄. The equilibrium state in our MD simulation is an aqueous phase in contact with the vapor phase, properties of this equilibrium state (i.e., density of both phases and solubility) are consistent with the results obtained from GEMC simulation. The liquid phase chemical potential of both H₂O and CH₄ in the presence and absence of the nanoparticle is nearly the same indicating that the effect of this additive will not be significant. Interactions between the water molecules and the nanoparticle in our MD simulations are indicated by the formation of the hydration shell around the nanoparticle due to the formation of hydrogen bonds between water molecules and the hydroxyl group of the silica nanoparticle. However, the contact between the hydrophilic silica nanoparticle and the interface is infrequent due to which no significant effect of the nanoparticle on the dynamics of methane hydrate dissociation is noted.

References:

[1] Zhang, W., Li, H.Y., Xu, C.G., Huang, Z.Y. and Li, X.S., 2022. Research progress on the effects of nanoparticles on gas hydrate formation. RSC advances, 12(31), pp.20227-20238.

[2] Min, J., Kang, D.W., Lee, W. and Lee, J.W., 2020. Molecular dynamics simulations of hydrophobic nanoparticle effects on gas hydrate formation. The Journal of Physical Chemistry C, 124(7), pp.4162-4171.

[3] Jiao, L.J., Wan, R.C. and Wang, Z.L., 2021. Experimental Investigation of CO₂ Hydrate Dissociation in Silica Nanoparticle System with Different Thermal Conductivity. International Journal of Thermophysics, 42, pp.1-18.

[4] Zhang, Z., Kusalik, P.G., Wu, N., Liu, C. and Ning, F., 2022. Molecular Insights into the Impacts of Calcite Nanoparticles on Methane Hydrate Formation. ACS Sustainable Chemistry & Engineering, 10(35), pp.11597-11605.

[5] Cao, P., 2023. Structural stability evolutions of CH₄ and CO₂ hydrate-sand nanoparticle systems. Journal of Molecular Liquids, 370, p.121041.