Microscopic Flow Characteristics of Immiscible CO2 Flooding and CO2 Foam Flooding After Water Flooding in Fractured Porous Media: A Visual Investigation

Quantitative pore-scale multiphase flow characteristics through CO2 flooding and CO2 foam flooding after water flooding in oil-wet fractured porous media are hardly investigated and unclear. Therefore, micro-scale visualization displacement experiments of CO2 flooding and CO2 foam flooding after water flooding were conducted in this study. Firstly, four micro-scale visualization models are established based on practical sandstone cores. Secondly, micro-scale visual displacement experiments were performed in the conducted porous media. Thirdly, the qualitative analyses of multiphase flow characteristics in different micromodels were carried out. Finally, the volumetric sweep efficiency (VSE), oil recovery factor (ORF) and residual oil distribution were quantitatively analyzed. Results indicate that both the parameters of fractures and displacement methods significantly affect the multiphase flow and residual oil distribution. The residual oil after displacement can be classified into four types: cluster-shaped oil, dead corner oil, oil film and columnar oil. The residual oil after water flooding was mainly cluster-shaped oil and dead corner oil, indicating that sweep efficiency is the main factor restricting ORF of water flooding. Both CO2 flooding and CO2 foam flooding displaced most of the cluster-shaped residual oil and dead corner residual oil after water flooding, while CO2 foam flooding yielded better performance due to the blocking capability of foam system on high permeability areas. The fracture improved the connectivity of micromodels, leading to higher VSE and ORF, but also to earlier fluids breakthrough. Different from wide fracture, narrow fracture significantly improved VSE and ORF. In conclusion, reasonable fracturing and CO2 foam flooding are advantageous to further enhance oil recovery after water flooding in oil reservoirs.