Abstract: In-situ heavy oil upgrading is one of the efficient development techniques for reducing heavy oil viscosity and improving oil quality.In this framework,a key research direction is to develop industrial hydrogen donors with strong hydrogen-supplying capability,abundant availability,reasonable prices and environmental friendliness, in addition to developing efficient catalysts.Investigation was conducted on the mechanisms and application progress of novel hydrogen donors-including inorganic, organic, biomass-based sources-used both at home and abroad,and assessment was made on the applicable conditions of each hydrogen donor in in-situ heavy oil upgrading.The results indicated that in the subcritical water conditions of steam-injected heavy oil recovery,heavy oil molecules containing aliphatic hydrogen or active C-H bonds could release hydrogen through water-gas shift reactions with water;however,due to limited operating temperatures,the hydrogen-supplying capacity remained constrained.Alkanes,alkyl aromatics,and distillates,although effective as organic hydrogen donors, were costly and thus impractical for heavy oil recovery.In contrast,biomass-derived hydrogen donors(e.g.from plants and algae)had more obvious applicable advantages due to their abundant raw materials and carbon-reduction potential.Using the intrinsic hydrogen in renewable biomass as a donor is expected to be an important direction for enhancing future heavy oil upgrading technologies.This study has important implications for promoting green,low-carbon heavy oil development,reducing carbon emissions,and improving oilfield development efficiency.

Key words: hydrogen donor, heavy oil, in-situ upgrading, biomass, catalyst