Abstract: Multi-stage volume fracturing of horizontal wells is common in the development of tight oil reservoirs,where the matrix near wellbore is fully cut by and in large-scale contact with the fracture network,resulting in reverse imbibition of fracturing fluid or water into the matrix through the fracture network.Therefore,the reverse imbibition effect shouldn't be ignored in both fracturing and water-flooding.In addition,due to the difference of flow pattern between tight and ordinary reservoirs,the factors contributing to the reverse imbibition effects are accordingly different in the two distinct reservoirs.To accurately simulate the imbibition in fractured tight oil reservoirs,we apply the Dissipative Particle Dynamic(DPD)method to verify the existence of boundary layers during the imbibition in nano-scale pore throats in tight reservoirs,and discover the variation pattern of the boundary layer thickness;then calculate the critical seepage parameters with boundary layers considered via the pore network model,mainly including permeability,capillary pressure curve and relative permeability curve;and finally incorporate these parameters into the numerical simulation model to characterize complex fracture networks in tight reservoirs.The simulation results show that the boundary layers serve to greatly reduce imbibitions-the imbibition recovery in tight reservoirs with boundary layers is only 3/8 of that without considering imbibition.In conclusion,the boundary layer in tight reservoirs deserves great attention;otherwise,the flow potential of these reservoirs may be overestimated.

Key words: boundary layer, dual media, reverse imbibition, numerical simulation, volume fracturing, tight oil, reservoir development