Characterization of phase structures of a supercritical water/supercritical carbon dioxide/heavy oil system with molecular dynamics simulations

Abstract

In this work, phase structures of a supercritical water (SCW)/supercritical carbon dioxide (scCO2)/heavy oil system are evaluated and characterized by performing molecular dynamics (MD) simulations. The MD simulation processes are initialized with structures of an oil droplet-in-SCW + scCO2 mixture, and its resultant structure, radial distribution function (RDF), solvation free energy, and cohesive energy density (CED) were then integrated to analyze the simulation results. The SCW and scCO2 are found to disperse into the oil drop and induce the oil swelling through stretching the oil molecules. Both saturates and aromatics can be easily dissolved in an SCW + scCO2 mixture due to the attractive interaction between them, whereas resins and asphaltenes are intended to conduct self-assembly due to their repulsion to the SCW + scCO2 mixture and high CED, resulting in a pseudo-one phase structure of an SCW/scCO2/heavy oil system. The presence of scCO2 can enhance the miscibility of heavy oil with an SCW + scCO2 mixture through intensifying the affinity between heavy oil and solvent(s) as well as destroying the hydrogen bonds by diluting the SCW. Elevating temperature is adverse to the miscible process between heavy oil with an SCW + scCO2 mixture on account of the reduced density of such a mixture. A moderate increase in pressure facilitated dissolution of heavy oil in an SCW + scCO2 mixture; however, an excess increase in pressure can lead to a suppressed miscibility between heavy oil and an SCW + scCO2 mixture since the hydrogen bonding is significantly recovered.