In the challenge of decarbonization, an economical option for carbon capture, utilization, and storage is carbon dioxide enhanced oil recovery (CO2-EOR) and sequestration in depleted oil reservoirs. During the CO2-EOR processes, pore structures have significant effects on miscible flow performance. The permeability and heterogeneity are investigated by magnetic resonance imaging for seepage characteristics in this study. Furthermore, the dispersion coefficient and Peclet number are calculated by the error function for dispersion characteristics. The whole displacements are relatively stable with piston-like fronts in homogeneous cores while quite unstable with fingering fronts in heterogeneous cores. The results exhibited that the mixing zone length, mixing zone velocity, recovery factor, dispersion coefficient, and Peclet number are significantly affected by heterogeneity but less affected by permeability in the miscible displacement process. This indicates that the seepage and dispersion are more affected by the heterogeneity rather than the permeability. Heterogeneity is a more important parameter than permeability. To further investigate the micromechanism of supercritical CO2 miscible flows, the Lattice-Boltzmann method (LBM) is also used for local pore-space simulation, and the results showed that the fronts in local space are stable. The oil saturation results of the LBM simulation and BZ-04 experiment are closed at the A–B stage before the breakthrough time, and LBM is a good method for dealing with the microflow in miscible displacement. Therefore, more insight should be focused on heterogeneity rather than permeability. It is important to determine an effective parameter to represent heterogeneity in the future. Our study could support the application of oil recovery engineering.

Gravity plays an important role in enhanced oil recovery and groundwater hydrology. A two-dimensional visual homogeneous micromodel was used in this study to describe the role of gravity in displacement processes. A theoretical analysis is proposed for three flow modes, i.e., vertical-upward, vertical-downward, and horizontal displacements, in which water and decane are used for the displacing and the displaced phases, respectively. A relatively compact displacement front was obtained at high flow rates in the three displacement modes, and the front gradually became unstable with a decrease in the flow rate. Compared with horizontal displacement, in vertical-upward displacements, gravity can hinder the evenness of the flow and aggravate the front finger formations at the inlet. This process forces the heavier displacing phase to expand horizontally at the midpoint and weakens the front's fingers. In the vertical-downward displacement process, two states occurred at the same low flow rate: stable flow and unstable flow. Unstable flows occurred more frequently with a decrease in the flow rate. To better understand the role of gravity in displacement, we proposed a theoretical prediction model for the flow state transition of the three displacement modes by combining the capillary force, viscous force, and gravity based on pore-filling events. Finally, to predict the final recovery factor for various displacement modes, four dimensionless formulations were produced using the capillary number, the gravity number, the bond number, and the viscosity ratio.