Experimental Investigation of CO 2 Huff-and-Puff Enhanced Oil Recovery in Fractured Low-Permeability Reservoirs: Core-Scale to Pore-Scale

CO 2 huff-n-puff is regarded as an effective method to improve the recovery of low permeability and tight oil reservoirs. To understand the impact of CO 2 huff-n-puff on crude oil mobilization in tight reservoirs with different fracture scales, this study conducted CO 2 huff-n-puff nuclear magnetic resonance (NMR) and microscopic visualization experiments, focusing on how varying fracture apertures and densities affect the efficiency of the CO 2 huff-n-puff. The results show that in scenarios with a single fracture, larger fracture apertures significantly boost oil mobilization within the fracture and the surrounding matrix. For instance, increasing the aperture from 20 μm to 70 μm improved the recovery factor by 9.20%. In environments with multiple fractures, greater fracture density enhances reservoir connectivity, and increases the CO 2 sweep area, and the complex fracture model shows a 4.26% increase in matrix utilization compared to the simple fracture model. Notably, the improvement in recovery due to multi-scale fractures is most significant during the first two huff-and-puff cycles, with diminishing returns in subsequent cycles. Overall, increasing both fracture size and density effectively enhances crude oil mobilization in tight reservoirs. These findings provide valuable insights into improving the recovery efficiency of CO 2 huff-and-puff techniques in tight oil reservoirs.