Increasing demand and dwindling supply of crude oil have spurred efforts towards enhancing heavy oil recovery. Recently, applications of ionic liquids (ILs) for heavy oil recovery and catalytic upgrading have been reported. However, ILs are generally considered too expensive for industrial applications. Moreover, certain types of ILs are non-biodegradable and toxic. An alternative class of ILs called Deep Eutectic Solvents (DESs) have recently been discovered and investigated upon. A DES generally composed of two or more components that are capable of self-association, to form eutectic mixture with a melting point lower than that of each individual component. DESs are non-toxic, biodegradable, recyclable, non-flammable, environmentally friendly and cheap. Therefore, DESs have found application in fields like extraction/separation, solvent development/reaction medium, hydrometallurgy, etc. In this study, and for the first time to our knowledge, the effectiveness of two different types of DESs, Choline Chloride:Glycerol (1:2) and Choline Chloride:Urea (1:2), in enhancing heavy oil recovery was thoroughly investigated. Effect of the two DESs diluted with formation brine on emulsification, altering wettability, spontaneous imbibitions, surface and interfacial tensions and tertiary residual heavy oil saturation reduction are studied experimentally at different temperatures. Heavy oil with 16 °API and formation brine from one of the Omani heavy oil field were utilized. The core flood experiments were conducted at reservoir condition and using Berea sandstone core plugs.The results showed that the two DESs did not make emulsion with the heavy oil. They increased the IFT of oil-brine system. Moreover, the two DESs altered the wettability of the sandstone rock surfaces from liquid-wetting towards intermediately air-wetting conditions at oil-air-rock system. The core flooding runs at different temperatures showed high potential of using these DESs for enhancing heavy oil recovery; where 14-30% of water flooded residual heavy oil was recovered after injecting the DES solvents as tertiary recovery stage. Wettability alteration phenomenon and viscose forces were known as the main mechanisms of enhancing the heavy oil recovery by DESs injection.
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