Influences of diffusion and advection on dynamic oil-CO2 mixing during CO2 EOR and storage process: Experimental study and numerical modeling at pore-scales

The dynamic oil-CO2 mixing and miscible flow in porous media are complex and important phenomena that occurs during CO2 injection for enhanced oil recovery (EOR) and geological storage. In this study, microfluidic experiments at pore-scale were conducted to simulate and investigate the mixing and flow behavior of oil and CO2 in porous media with dead-end pores. The non-uniform oil-CO2 mixing and bi-directional diffusion (CO2 into oil and oil components into CO2) in dead-end pores were observed. Based on the experimental observation, a novel oil-CO2 miscible flow model was established. The model can describe the effects of diffusion, flow velocity distribution, fluid properties change, and pore structures on oil-CO2 mixing and oil displacement. The diffusion coefficient between CO2 and oil in porous media was figured out by matching the experimental results using the new model. The modeling results indicate that diffusion plays an important role in oil-CO2 mixing, especially in deep dead-end pores. Without diffusion, over 70% of oil components would remain in their original place during CO2 flooding. In complex porous media, advection induced by CO2 flow dominates oil displacement in the early stage of CO2 flooding, which can reduce the average oil mole fraction by 24%. Then diffusion increasingly influences the oil-CO2 mixing and oil displacement, reducing the average oil mole fraction by over 35%. Without diffusion, much of the dead-end, column, and corner oil would remain in place in the oil reservoir. During CO2 flow in complex porous media, reducing oil viscosity due to mixing with CO2 can decrease the flow resistance in the main flow channels, which in turn can decrease the fluid flowing through the surrounding pores and is not conducive to oil displacement. In practice, even though a high CO2 injection rate can produce oil quickly, it might affect the overall oil recovery factor and lower the CO2 utilization efficiency.