Sub- and supercritical water upgrading of heavy oil: A review of laboratory-scale research on upgrading performance and physicochemical mechanism

In order to alleviate the pressure on the international energy supply, people began to turn their attention to the abundant reserves of heavy oil resources. However, the high density and viscosity of heavy oil, along with their rich content of heteroatoms pose significant challenges for extraction and subsequent refining. Therefore, upgrading heavy oil has become a necessity. Sub- and supercritical water, proposed as both a thermal carrier and an organic solvent for heavy oil upgrading, can significantly increase the yield of light components while reducing coke formation. This method effectively overcomes the high coke yield and cost drawbacks of traditional heavy oil upgrading technologies, emerging as a promising, environmentally friendly and efficient alternative. Herein, we systematically overview the state-of-the-art progress in sub- and supercritical water upgrading technology for heavy oil from multi-scale perspectives, mainly including upgrading performance, reactions, and mechanisms. We also discussed the current challenges and future research directions for this technology. This review provides a comprehensive and accessible reference for the mechanistic analysis of sub- and supercritical water upgrading technology for heavy oil, guiding its industrial promotion and application.