Abstract
Molecular dynamic simulations were performed to determine the elastic constants of carbon dioxide (CO2) and methane (CH4) hydrates at one hundred pressure–temperature data points, respectively. The conditions represent marine sediments and permafrost zones where gas hydrates occur. The shear modulus and Young’s modulus of the CO2 hydrate increase anomalously with increasing temperature, whereas those of the CH4 hydrate decrease regularly with increase in temperature. We ascribe this anomaly to the kinetic behavior of the linear CO2 molecule, especially those in the small cages. The cavity space of the cage limits free rotational motion of the CO2 molecule at low temperature. With increase in temperature, the CO2 molecule can rotate easily, and enhance the stability and rigidity of the CO2 hydrate. Our work provides a key database for the elastic properties of gas hydrates, and molecular insights into stability changes of CO2 hydrate from high temperature of ~5 °C to low decomposition temperature of ~−150 °C.
Highlights
The replacement of CH4 by CO2 in gas hydrate-bearing sediments has received great attention[14,15,16,17,18,19,20,21,22] since it may enable long-term storage of CO2, which could mitigate the influence of global warming and ocean acidification, and facilitate CH4 recovery as a potential future energy resource
For the hydrate synthesized at the proximity of phase equilibrium pressure, the refined model show that CO2 occupied almost all (>99%) of the large cages and roughly 2/3 of small cages[33, 35,36,37]
Molecular dynamics (MD) simulations can provide insights on hydrates at the molecular level with linkages to macroscopic phenomena[17,18,19, 22, 40,41,42,43,44,45,46,47,48,49,50], where interactions between guest and host are of particular importance
Summary
The replacement of CH4 by CO2 in gas hydrate-bearing sediments has received great attention[14,15,16,17,18,19,20,21,22] since it may enable long-term storage of CO2, which could mitigate the influence of global warming and ocean acidification, and facilitate CH4 recovery as a potential future energy resource. Molecular dynamics (MD) simulations can provide insights on hydrates at the molecular level with linkages to macroscopic phenomena[17,18,19, 22, 40,41,42,43,44,45,46,47,48,49,50], where interactions between guest and host are of particular importance Those simulation studies provided valuable information on free energy changes for CO2 replacing CH4 from gas hydrate[17,18,19], structural changes induced by various guest molecules[41], dissociation[42], nucleation[43, 44], thermal conductivity[41, 45], mechanical properties[46,47,48,49], and NMR spectra[50]. The aim of the work is to obtain a priori knowledge on the elastic properties of two different gas hydrates for the purpose of monitoring their distributions in the field during CO2 replacement of CH4 from gas hydrate deposits
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