Stability of carbon dioxide and methane hydrates in water in presence of small driving forces using MD simulations

Gas hydrates are a potential source of energy as a large fraction of natural gas worldwide is stored in the form of gas hydrates. Methane hydrates are widely distributed in sediments along the continental margins and contain more energy than all other fossil fuel reserves in the world. However, methane is also a potential greenhouse gas which could play a major role in global climate change. Accurately characterizing the stability of methane and CO 2 hydrates in water can help us understand their effects on earth\u27s environment and also the feasibility of long term CO2 sequestration in the sediments under the ocean floor. Hydrate stability can be better predicted by understanding the phenomena related to the hydrate dissolution in water. Under the hydrate stability conditions, the concentration difference of hydrate forming gases between the hydrate and water phases should be an important factor affecting the hydrate stability as oceanic hydrates are exposed to undersaturated seawater.;In this work, the dissolution of methane and CO2 hydrates have been studied and compared to one another in the presence of water using molecular dynamic simulations. The average lattice constant for structure I and II methane hydrate was calculated for two different potentials of methane OPLS and GROMACS and compared to the experimental value to validate these potentials. Methane hydrate dissociation in presence of water was also studied at these 2 potentials of methane and also at the Anderson et al. model. The effect of temperature and pressure on hydrate dissociation was studied. The TIP4P model for water and Harris and Yung model for CO2 were used in all of the simulations of CO2 hydrate in this study. Molecular dynamic simulations were done on methane and CO2 hydrates at 275 K and 50 bar to study the effect of changing the small cage occupancy in the hydrate and the percentage level of gas saturation in the water phase on hydrate dissolution in water. A higher amount of dissolution occurred in the case of CO2 hydrates compared to methane hydrates in all the simulations. Methane hydrate was found to be more stable at cage occupancies close to 100% while CO2 hydrate was found to be stable at small cage occupancies close to 0% with large cage completely occupied. The amount of dissolution was found to increase with a decrease in the level of gas saturation in the water phase and the dissolution seemed to be driven by a difference between gas concentration and its solubility in water phase for both methane and CO 2 hydrates. In all the simulations on methane hydrate, water molecules in the liquid phase close to hydrate-water interface were found to be regrouping to become tetrahedrally bonded which is predicted to be indication towards a hydrate growth. An increase in CO2 solubility in water was observed in all the simulations on CO2 hydrate the reason for which is not yet understood. A structural change of water molecules in the liquid phase in the presence of hydrate is proposed to be a reason