Effect of Fracture-Matrix Interaction on CO₂-Oil Displacement via Dual-Permeability Microfluidics

Fractured shale reservoirs pose a major challenge for CO2-enhanced oil recovery (CO2-EOR) due to the complex interaction between high-permeability fractures and the low-permeability matrix. To better understand the mechanisms of the CO2-oil displacement in fractured shale, we conducted microfluidic experiments in a dual-permeability chip. We examined how the pressure difference and phase conditions affect the CO2-oil displacement efficiency and residual oil. Under immiscible conditions, a critical differential pressure was required for the CO2 to penetrate the matrix. Below this threshold, only oil in the fracture can be recovered. Increasing differential pressure enhanced both the sweep depth of CO2 and the ultimate recovery of oil. Conversely, under miscible conditions, efficient molecular diffusion allows CO2 to readily penetrate the matrix even at low pressure differentials, achieving a 27.88% higher ultimate recovery compared with immiscible displacement at a ΔP of 0.10 MPa. Three typical types of residual oil were discovered in the matrix, including clustered, columnar, and filmy. Clustered and columnar oils account for the majority of residual oil in the matrix, while filmy oil increases with pressure differences. This work directly visualizes and quantifies how the interplay of pressure, miscibility, and capillary forces at the fracture-matrix interface governs displacement efficiency, providing a fundamental framework for optimizing the CO2-EOR strategies in fractured reservoirs.