Liquid Catalysts (LCs) have been used in surface upgrading of heavy oil for a long time, but their in-situ application in heavy oil reservoirs has been considered only recently. There are four main types of LCs for in-situ applications: water-soluble, oil-soluble, mineral, and dispersed. This paper reviews various types of water-soluble liquid catalysts applied for in-situ upgrading of heavy oil. These LCs have shown great potential in terms of reducing viscosity and in-situ upgradation of heavy oil, thus facilitating recovery and transport of heavy oil in experimental studies and in the field pilot implementation. These studies show that the water-soluble LCs have great potential in becoming an economically and environmentally competitive alternative enhanced oil recovery technique to the established conventional heavy oil recovery EOR techniques, such as thermal and solvent injection. However, further research is needed in studying and developing suitable LCs in terms of their behaviour under the heavy oil reservoir pressure and temperature conditions, and to develop efficient water-soluble LCs with desirable properties.
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