Molecular Insight into the Extraction Behaviors of Confined Heavy Oil in the Nanopore by CO2/C3H8in Huff-n-Puff Process

The residual oil present in localized low-permeability pores in heavy oil reservoirs is a limiting factor in oil recovery. In this study, molecular simulations were used to comprehensively describe the flow mechanisms and potential performance when using CO2 and C3H8 as solvents to assist the extraction of oil in quartz nanopores. We showed that CO2 and C3H8 effectively reduced residual oil in pores during the soaking stage. The adsorption of CO2 on the SiO2 surface broke the heavy oil away from it, significantly reducing interaction between the surface and the oil and allowing it to flow out. C3H8 broke heavy oil down into individual molecules or small clusters, aiding the oil to flow out of the pore. With insufficient soaking, the reflux phenomenon of oil during the depletion stage caused a high residual oil amount for both the CO2 and C3H8 simulations. With sufficient soaking within the CO2 simulation, the CO2 desorbed from the surface and escaped from the oil, generating a gas phase within the pore that provided a driving force to push the oil out. Sufficient soaking within the C3H8 simulation did not significantly change the amount of residual oil during the depletion stage. The adhesion of residual oil on the pore surface prevented the oil from flowing out of the pore with C3H8. The synergistic behavior of the CO2/C3H8 mixture was explored to better understand the advantages of combining different solvent extraction mechanisms. A combination of 70% CO2 + 30% C3H8 effectively extracted heavy oil during the soaking stage and had the best recovery in the depletion stage. This study revealed the mechanism of flow interaction between solvents and heavy oil under confined conditions at the molecular level. Furthermore, this study provides helpful information to guide the enhancement of heavy oil recovery.