Bulk and Interfacial Properties of Brine or Alkane in the Presence of Carbon Dioxide, Methane, and Their Mixture

We present an overview of the molecular simulations performed to understand the two-phase behavior of brine or alkane in the presence of CH4, CO2, and their mixture at reservoir conditions. The simulation results of bulk and interfacial properties of these systems compared well with experimental data and theoretical estimates obtained using, for example, density gradient theory based on CPA (with Debye–Hückel electrostatic term) and PC-SAFT EoSs. Here, CO2 is preferentially dissolved in the water-rich or alkane-rich phase and enriched at the interface from the CH4/CO2 equimolar mixture. The fact that the interfacial enrichment in CO2 was much higher than that of CH4 explained the relatively steep decrease in the interfacial tension (IFT) with pressure in brine+CO2 and alkane+CO2 systems. IFTs of brine+CH4+CO2 and alkane+CH4+CO2 systems decreased with increasing mole fraction of CO2 in the CH4/CO2-rich phase. Solubilities of CH4 and CO2 in the water-rich phase decreased with the addition of salt (salting-out effect). This effect followed the order NaCl < CaCl2. IFTs of brine+CH4/CO2 systems linearly increased as the salt concentration increased. Here, the larger slopes in the presence of CaCl2 are due to stronger hydration and repulsion from the interface of Ca2+ ions. Solubilities of CH4 and CO2 in the alkane-rich phase generally decreased with increasing alkane size n c. These solubilities were relatively lower in the cycloalkane-rich phase, while linear and branched alkanes gave similar results. IFTs of alkane+CH4/CO2 systems increased as n c increased and a relatively high IFT was obtained in cycloalkane+CH4/CO2 systems.