Molecular dynamics simulation of heavy oil dissolution in supercritical water and multi-component thermal fluid

Abstract

Supercritical water (SCW) and multi-component thermal fluid (MCTF) are both effective injection agents for heavy crude oil recovery, in which the dissolution and miscibility of heavy oil is supposed to be the key mechanism for enhanced recovery and in situ upgrading. In this paper, molecular dynamics simulations were carried out to investigate the dissolution and miscibility characteristics of heavy oil droplets in SCW and MCTF. The results indicated that SCW exhibited a higher dissolution rate for heavy oil at low solvent density (0.1–0.4 g cm⁻³), while MCTF dominated at high solvent density (0.5 g cm⁻³). The dissolution characteristics of heavy oil were mainly determined by density, temperature, and the structure of the oil molecule. The increase in solvent density inhibited the dissolution of heavy oil, while the effect of temperature was determined using the competitive relationship between the decrease in solvent solubility and the increase in solute dissociation. The increase in carbon atoms was detrimental to the solubility of the oil, while the isomerization of chain alkanes and the change in the heteroatom in asphaltenes did not significantly affect the solubility. The addition of lighter components could improve the dissociation of heavier components. Electrostatic attraction interaction was the main cause of poor dissociation of polar oil molecules. At 623–723 K, the optimal density of spontaneous miscibility of supercritical water and heavy oil was 0.3 g cm⁻³, while that of the multi-component thermal fluid was 0.5 g cm⁻³.