Investigation of plugging performance and enhanced oil recovery of multi-scale polymer microspheres in low-permeability reservoirs

Advanced water injection has become an effective method for developing low-permeability reservoirs. However, after a period of high yield from fracturing, the injected water flows rapidly along high-capacity channels, which greatly reduces the sweep efficiency of reservoirs. In this work, the plugging process by microspheres and nanospheres from the fracture to the matrix was simulated through core experiments. The plugging performance of multi-scale microspheres (microspheres and nanospheres) on the core matrix was evaluated by measuring the plugging rate and other parameters. Enhanced oil recovery after synergistic plugging with multi-scale microspheres was used as the evaluation index, a sensitivity analysis of injection parameters was performed, and the best combination of injection parameters was optimized by orthogonal experiments. The experimental results showed that the plugging performance of nanospheres is mainly reflected in the filling of the core matrix, while the plugging performance of microspheres is mainly reflected by retention at the injection end of the core (as opposed to the fractured end face). The plugging rate of nanospheres reached 78.83%, which is much higher than the 30.28% rate achieved with microspheres. The maximum oil recovery after plugging was 19.12%, which corresponds to the following combination of injection parameters: total injection of 0.6 pore volume (PV), injection ratio between nanospheres and microspheres of 2:1, injection rate of 1.5 mL·min⁻¹, and aging time of 6 days. Among the injection parameters, the injection ratio between nanospheres and microspheres had the greatest influence on the recovery of waterflooding after plugging, followed by the total injection of multi-scale microspheres. The injection rate and aging time had little effect.