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. Low permeability heavy oil reservoir has the characteristics of poor physical properties and poor oil mobility, and its primary production is very limited.Based on the synergistic mechanism, the multi-functional composite fracturing fluid system HC-5 was constructed in this study, which combines fracturing, viscosity reduction, wettability change, and interface modification. Taking a low permeability heavy oil block in Changqing Oilfield as an example, the oil recovery of the new multifunctional composite fracturing fluid system was evaluated. The results show that the viscosity reduction should not be used as a single evaluation index for enhanced oil recovery in heavy oil reservoirs, and the improvement degree of the water–oil mobility ratio is more significant.Wettability reversal can change capillary resistance into displacing force and affect the influence of interfacial tension.The interfacial tension is not the smaller the better, the contribution of reducing interfacial tension to enhancing oil recovery has decreased after reaching a low value (10−2 mN/m), even lower than the contribution of mobility ratio, so it is not recommended to blindly pursue extremely low interfacial tension and the synergistic effects of various reasons should be played to obtain the best result.In addition, by adding polymer, the viscoelasticity can be used to increase the microscopic pull force and expand the macroscopic sweep volume. The field test of 10 volumetric fracturing wells provides that the fracturing fluid system can significantly increase the primary production of the well by 20–25%.