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

Abstract CO2 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 CO2 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 CO2 injection capacity, facilitating expansion, and interconnection of isolated fractures, and promoting deeper CO2 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 CO2 HnP in a matrix-fracture system and highlights the importance of fracture networks in optimizing shale oil recovery.