Carbon dioxide induced bubble formation in a CH4–CO2–H2O ternary system: a molecular dynamics simulation study

Abstract

The extraction of methane from its hydrates using carbon dioxide involves the decomposition of the hydrate resulting in a CH₄–CO₂–H₂O ternary solution. Using classical molecular dynamics simulations, we investigate the evolution of dissolved gas molecules in the ternary system at different concentrations of CO₂. Various compositions considered in the present study resemble the solution formed during the decomposition of methane hydrates at the initial stages of the extraction process. We find that the presence of CO₂ aids the formation of CH₄ bubbles by causing its early nucleation. Elucidation of the composition of the bubble revealed that in ternary solutions with high concentration of CO₂, mixed gas bubbles composed of CO₂ and CH₄ are formed. To understand the role of CO₂ in the nucleation of CH₄ bubbles, the structure of the bubble formed was analyzed, which revealed that there is an accumulation of CO₂ at the interface of the bubble and the surrounding water. The aggregation of CO₂ at the bubble–water interface occurs predominantly when the concentration of CO₂ is high. Radial distribution function for the CH₄–CO₂ pair indicates that there is an increasingly favorable direct contact between dissolved CH₄ and CO₂ molecules in the bubble–water interface. It is also observed that the presence of CO₂ at the interface results in the decrease in surface tension. Thus, CO₂ leads to greater stability of the bubble–water interface thereby bringing down the critical size of the bubble nuclei. The results suggest that a rise in concentration of CO₂ helps in the removal of dissolved CH₄ thereby preventing the accumulation of methane in the liquid phase. Thus, the presence of CO₂ is predicted to assist the decomposition of methane hydrates in the initial stages of the replacement process.