Hybrid CO₂ thermal system for post-steam heavy oil recovery: Insights from microscopic visualization experiments and molecular dynamics simulations

The hybrid CO₂ thermal technique has achieved considerable success globally in extracting residual heavy oil from reserves following a long-term steam stimulation process. Using microscopic visualization experiments and molecular dynamics (MD) simulations, this study investigates the microscopic enhanced oil recovery (EOR) mechanisms underlying residual oil removal using hybrid CO₂ thermal systems. Based on the experimental models for the occurrence of heavy oil, this study evaluates the performance of hybrid CO₂ thermal systems under various conditions using MD simulations. The results demonstrate that introducing CO₂ molecules into heavy oil can effectively penetrate and decompose dense aggregates that are originally formed on hydrophobic surfaces. A stable miscible hybrid CO₂ thermal system, with a high effective distribution ratio of CO₂, proficiently reduces the interaction energies between heavy oil and rock surfaces, as well as within heavy oil. A visualization analysis of the interactions reveals that strong van der Waals (vdW) attractions occur between CO₂ and heavy oil molecules, effectively promoting the decomposition and swelling of heavy oil. This unlocks the residual oil on the hydrophobic surfaces. Considering the impacts of temperature and CO₂ concentration, an optimal gas-to-steam injection ratio (here, the CO₂: steam ratio) ranging between 1:6 and 1:9 is recommended. This study examines the microscopic mechanisms underlying the hybrid CO₂ thermal technique at a molecular scale, providing a significant theoretical guide for its expanded application in EOR.