Formulation and evaluation of a new multi-functional fracturing fluid system with oil viscosity reduction, rock wettability alteration and interfacial modification

•A multi-functional fracturing fluid system was formed, the field proved it can increase the primary production by 20–25%.•The contribution of improving liquidity and reducing viscosity to heavy oil recovery was clarified and compared.•The contribution of reducing interfacial tension in different degrees to enhancing heavy oil recovery is explored.•The influence of wettability inversion on interfacial tension was elucidated.