A CO ₂ -Viscosity Reducer Synergistic Strategy for Enhancing Thin-Layer Heavy Oil Recovery: 2D Visualization and CO ₂ Storage Study

The vertical–horizontal well steam-assisted gravity drainage (VH-SAGD) technology demonstrates great potential in heavy oil production but faces high emissions and low development efficiency. Therefore, this study first introduced CO2 and a viscosity reducer into VH-SAGD to enhance oil recovery and reduce carbon emissions. A high-temperature and high-pressure two-dimensional visualization apparatus was employed to conduct a series of CO2- and viscosity reducer-assisted VH-SAGD experiments. The enhancement mechanisms of CO2 and the viscosity reducer in VH-SAGD were investigated from multiple aspects. More importantly, this study innovatively revealed the advantages of the composite system coupled with VH-SAGD technology for CO2 storage. The results indicated that CO2 and the viscosity reducer significantly enhanced the performance of VH-SAGD. However, VH-SAGD assisted solely by either CO2 or the viscosity reducer could not fully realize the well-pattern advantages of VH-SAGD. The recovery factors were 64.05% and 59.00%, respectively, both lower than the 71.45% achieved by the combined assistance of CO2 and the viscosity reducer. The synergistic effect of CO2 and the viscosity reducer resulted in optimal heat sweep and viscosity reduction ranges. This is because the injection strategy enabled effective viscosity reduction across the high-temperature zone, the mobile oil region, and the cold oil zone simultaneously. Furthermore, in the CO2 and viscosity reducer coassisted VH-SAGD experiments, the CO2 sequestration efficiency reached 16.4%, which is 1.18 times that achieved by CO2-assisted VH-SAGD. Consequently, the composite system with VH-SAGD improved oil recovery and increased CO2 storage. The results have important implications for economically and environmentally sustainable heavy oil development.