Hot Compressed Water for Conversion and Upgrading of Unconventional Oil Resources: State of the Art, Perspectives, and Future Directions

Unconventional oil resources (oil shale, oil sands, extra heavy oil, and natural bitumen) and residues (atmospheric and vacuum residua) can exploit hot compressed water (HCW: T > 250 °C and P > 4 MPa) in the reservoir (in situ) or reactor on the ground (ex situ), where all mass transfer limitations are relaxed by adaptation in thermophysical properties of HCW as a fluid. In the case of in situ conversion and upgrading, injecting HCW may not only reduce the oil viscosity by heating the reservoir but also make dissolved heavy distillates, avoiding coking and more reduction in carbon loss with ultimate enhanced heavy oil recovery (EOR). Compared to the widely utilized steam flooding approach, HCW injection as a solvent and heat carrier is projected to have better recovery efficiency in EOR, especially in deep reservoirs, resulting in significant energy savings, heavy oil recovery, and in situ upgrading. The role of HCW, as an active hydrogen donor, becomes more prominent by introducing suitable homogeneous (oil-soluble) and heterogeneous catalysts in an upgraded system that led to improved catalytic conversion and inhibition of coke formation on the catalyst surface. The activity and stability of the catalyst in HCW along with the synergy for the creation of active hydrogen from HCW during in situ/ex situ heavy oil upgrading is the core of the current study. Generally, the reviewed literature works confirm the high potential of the application of HCW for in situ upgrading and conversion of unconventional oil resources. Furthermore, additional research is crucial for the development of more efficient catalysts in this context.