Expanding Sweep Volume Effect and Mechanism of Pulse Injection Improving Viscous Fingering

Abstract

Summary Viscous fingering of the injected fluid will significantly reduce the swept volume. Increasing the injected fluid viscosity can impress viscous fingering, but the reservoir and economy limit its application. Exploring a simple, effective, and universal viscous fingering impression method can effectively improve secondary oil recovery efficiency. This paper focuses on the effect and mechanism of pulse injection to expand the swept volume and provides experimental guidance for related pilot applications. Microfluidic and core fluidity experiments were conducted using large-scale glass chip and artificial cores, respectively. Including four constant-velocity and nine pulse microfluidic experiments under four injection rates (40, 1000, 300, and 5000 nL/min), three pulse frequencies (0.1, 1, and 10 Hz), and three pulse amplitudes (0.25, 0.5, and 0.75). Image recognition can quantitatively characterize the expanding swept volume efficiency of pulse injection. The macroscopic effect of pulse injection was verified by core fluidity experiments, and the mechanism of pulse injection expanding the swept volume was given from two aspects of force analysis and invasion front backflow. The results of constant injection experiments show that increasing the injection rate will cause the invasion front to transition from capillary fingering to viscous fingering, but the final swept volume first increases and then decreases. Pulse injection can transform the invasion front morphology from a multi-branched structure to a thicker one- or two-channel structure. Moderate pulse frequency and high pulse amplitude are beneficial to pulse injection to expand the swept volume. When the frequency is 1 Hz (amplitude is 0.5), and the amplitude is 0.75 (frequency is 1Hz), the swept volume can be expanded by 9.38% and 12.51%, respectively. The results of core flooding experiments show that pulse injection can increase water permeability. Pulse injection can generate fluctuation of injection pressure by the periodic change of injection rate, equivalent to inputting additional energy into the injection system. The additional pressure gradient is the fundamental reason for promoting the expanding throat flow and swept volume. Besides, the pulse injection leads to the inhomogeneous backflow of the invasion front, shortening the distance of multiple flow channels and delaying the formation of dominant channels. This work innovatively use low-frequency pulse injection as a universal method to improve viscous fingering and propose the mechanisms of pulse injection improving pore throat flow, which can provide an experimental basis for research in related fields.