Experimental investigation on enhanced oil recovery of heavy oil via cyclic supercritical water stimulation

With the gradual depletion of conventional oil resources, the exploitation of heavy oil is essential to alleviate the energy crisis, as fossil energy remains predominant in global energy supply. The feasibility of cyclic supercritical water (SCW) stimulation is confirmed by oil field tests, while the enhanced oil recovery (EOR) mechanisms and the dynamic characteristics of displacement are still unclear owing to the scarceness of relevant studies. In this study, an experimental system with a design temperature up to 450 °C and pressure up to 30 MPa was developed to simulate the cyclic SCW stimulation process. A customized two-dimensional sand pack core with equitable temperature measuring arrangement can monitor the dynamic evolution of temperature field. The results of the core experiments indicated that the injection of SCW obviously enhances oil production, with the ultimate oil recovery factor up to 75.91%, which was 18.85% higher than that of steam. The formation and expansion of the SCW gravity override was found to be one of the mechanisms for boosting oil production. At the end, the swept volume coefficient of SCW was enlarged from 5.39% to 10.53%, which was 2.99 times that of steam. The large expansion of the gravity override zone of SCW in the vertical and radial directions changes the heat and mass transfer law, raises the core temperature, and expands the swept volume. Further, the condensation of supercritical water in the soak stage caused pressure difference (up to 11.12 MPa) near the well driving distant heavy oil to flow towards. These findings have the potential to deepen the understanding of the EOR mechanisms of SCW stimulation and provide theoretical guidance for improving oil production in oilfields.