Model Comparison of the CH4/CO2/Water System in Predicting Dynamic and Interfacial Properties

The CO2 storage in deep saline aquifers and enhanced oil/gas recovery techniques have an increasing demand of the knowledge about dynamic properties of natural gas components such as methane and carbon dioxide, as well as the interfacial properties of multiphase fluids such as CH4/water and CO2/water systems. Molecular dynamics simulation is a very important and efficient method to predict dynamic and interfacial properties. The force-field parameters are most important for the accuracy of molecular dynamics simulation. Although there are many potential energy models for CO2/CH4/water, the single model often has high accuracy only for a specific characteristic. It is necessary to find or develop a combined potential energy model that can describe the set of properties with high accuracy simultaneously. In this study, a comprehensive comparison of the quality of different CO2/CH4/water force fields in predicting dynamic and interfacial properties was conducted. The models we selected here included seven different CO2 models (three rigid models and four flexible models), two CH4 models (a united atom (UA) model and an all-atom (AA) model), and three flexible water models. It was found that the rigid CO2 model performs better than its flexible version in density but has little difference in self-diffusion. The UA model of CH4 slightly outperformed the AA model both in density and self-diffusion, and the UA model can save computing resources. The accuracy of interfacial properties depends more on which water model was adopted, and it shows that the F3C water model is the best choice within the water models selected. These results might be helpful to select suitable force fields to further investigate the dynamic and interfacial properties of CO2/CH4/water systems.