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, CO₂is preferentially dissolved in the water-rich or alkane-rich phase and enriched at the interface from the CH₄/CO₂equimolar mixture. The fact that the interfacial enrichment in CO₂was much higher than that of CH₄explained the relatively steep decrease in the interfacial tension (IFT) with pressure in brine+CO₂and alkane+CO₂systems. IFTs of brine+CH₄+CO₂and alkane+CH₄+CO₂systems decreased with increasing mole fraction of CO₂in the CH₄/CO₂-rich phase. Solubilities of CH₄and CO₂in the water-rich phase decreased with the addition of salt (salting-out effect). This effect followed the order NaCl < CaCl₂. IFTs of brine+CH₄/CO₂systems linearly increased as the salt concentration increased. Here, the larger slopes in the presence of CaCl₂are due to stronger hydration and repulsion from the interface of Ca²⁺ions. Solubilities of CH₄and CO₂in 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/CO₂systems increased as n_cincreased and a relatively high IFT was obtained in cycloalkane+CH₄/CO₂systems.