(251b) Understanding the Stability of Mixed Hydrates Containing Propane, Ethane, and Methane Under Deep Water Conditions

Methane hydrates are globally distributed in sediments along the continental margins and potentially contain more energy than all fossil fuel reserves. However, methane is also a potential greenhouse gas which could play a major role in global climate change. Understanding the stability of gas hydrates can help us to understand their role in the climate change. Three main factors affect the stability of hydrates: Temperature (T), Pressure (P) and composition. Hydrates become unstable when they are exposed to pressures and temperatures outside the hydrate stability zone (HSZ) in a process commonly called dissociation. However, hydrates can also become unstable even when the pressure and temperature are within the HSZ but the concentrations of the hydrate forming gases are below their fully-saturated levels in the surrounding water phase. This process can be described as dissolution. In situ observations of marine outcrops of gas hydrates indicate that these hydrates exposed to surrounding seawater are more stable than predicted using diffusion-controlled models based on the surrounding methane saturations. Naturally-occurring hydrates may not be simple structure I methane hydrates and may contain higher hydrocarbons like ethane, propane etc., which forms structure II (sII) hydrates. Therefore, these mixed hydrates may act to stabilize the hydrates. In this work, the dissolution of sII hydrates in the presence of water has been studied using molecular dynamics (MD) simulations to understand if and how the presence of ethane and propane may stabilize the hydrate. Lattice constants for sII hydrates were calculated and compared to experimental values to validate the OPLS potentials used for the hydrocarbon guest molecules. The effect of higher hydrocarbons, such as ethane and propane, on the stability of gas hydrate was studied by changing the composition in the hydrate phase keeping the methane composition constant in the large cage and small cages. Also, the effect of methane composition was also studied by changing the methane composition in large and small cages.