Influence of fracture on CO₂ huff-n-puff efficiency and pore-scale shale oil mobilization in shale reservoirs

CO 2 huff and puff (HnP) is widely recognized as an effective strategy for enhancing shale oil recovery. However, a comprehensive understanding of its multi-scale performance in fractured shale reservoirs remains limited. To bridge this gap, this study investigates the enhanced oil recovery (EOR) potential of CO 2 HnP, with a particular focus on the influence of fractures. A series of large-diameter core HnP experiments, combined with online nuclear magnetic resonance (NMR) analysis, were conducted to evaluate both the macroscopic production characteristics and microscopic pore-scale mobilization characteristics under with and without fractures conditions. The results demonstrated that the presence of fractures significantly enhances shale oil recovery by increasing the CO 2 injection capacity, facilitating expansion, and interconnection of isolated fractures, and promoting deeper CO 2 diffusion into shale matrix, thereby improving seepage capacity. Additionally, fractures increase the total recovery rates in micropores and mesopores by 8.59% and 10.26%, respectively, while their effect on macropores is negligible (only a 0.14% increase). These findings suggest that fractures play a crucial role in mobilizing oil within smaller pores. This study provides valuable insights into the mechanisms of CO 2 HnP in a matrix-fracture system and highlights the importance of fracture networks in optimizing shale oil recovery.