Effect of Retained Fracturing Fluid on the Imbibition Oil Displacement Effciency of Tight Oil Reservoir

The flowback of fracturing fluid directly affects the production rate of a reservoir in the later stage of production. In laboratory studies, the basic physical properties of reservoir core and fracturing fluid were measured. Based on the measurement, factors affecting automatic imbibition and key factors affecting oil/water replacement by fracturing fluid were studied through experiment. The optimum imbibition recovery rate and the maximum imbibition rate for feld use were determined in laboratory studies. Differences between imbibition and displacement are explained by nuclear magnetic experiment. The experimental results have shown that influencing factors affected imbibition rate in an order of:interfacial tension > permeability > viscosity of crude oil > salinity. The imbibition recovery rate increased with increase in permeability, while the amplitude of increase was gradually reducing.When the viscosity of the simulated oil was in a range of 1.204-4.864 mPa·s, the imbibition recovery rate increased with decrease in the viscosity of the simulated oil. When the salinity of the imbibition fluid was in a range of 1 000-80 000 mg/L, the imbibition recovery rate increased with increase in salinity, When the concentration of cleanup additive in imbibition fluid was in a range of 0.005%-5%, i.e., the interfacial tension was in a range of 0.316-10.815 mN/m, the use of the imbibition fluid containing 0.5% cleanup additive (interfacial tension 0.869 mN/m) got the maximum imbibition recovery rate. Static imbibition experimental resultshave shown that higher interfacial tension did not necessarily result in higher recovery rate; instead, there was an optimum interfacial tension at which the amount of bypassed oil was minimized, and the imbibition recovery rate was maximized. This fnding provides a guide to enhance recovery rate from tight reservoirs.