The Impact of the Double Cross-Phase Diffusion Mechanism on Oil Recovery during CO Injection into Fractured Rocks: A Simulation Study.

In this research, a new diffusion mechanism called "double cross-phase diffusion" is introduced and applied to simulate the non-equilibrium gas injection process into fractured rocks. This new mechanism represents additional multicomponent gas diffusion into the crude oil through the water phase, existing in porous media as initial water saturation. Therefore, a lab-scale simulator, by implementing the generalized Fick's law of multicomponent diffusion, is developed and used for predicting the experimental data of oil recovery during CO injection in chalk fractured rocks in the presence of initial water saturation. The results revealed a significant difference in the oil recovery predicted by the model when the double cross-phase diffusion mechanism is considered. The transient behavior of produced oil composition, predicted by the simulation model, is matched well with the experimental data. The portion of active oil recovery mechanisms in the system has been evaluated for the first time and it was observed that the molecular diffusion mechanism induced 75.4% of the total oil transfer rate in the initial time oil recovery, in which 23.1% of this value was supplied by the double cross-phase diffusion mechanism, which is an interesting finding. Results of sensitivity analysis showed that by increasing the initial water saturation, the impact of the double cross-phase diffusion mechanism on oil recovery increases. In contrast, the transferred rate by the diffusion mechanism decreases from 85.4% to 60.8% when matrix permeability increases from 0.1 to 10 mD. The results of this work illustrate that the double cross-phase diffusion mechanism introduced in this study plays a significant role in the simulation results since the water is responsible for accelerating the diffusivity of CO into the crude oil and, in consequence, increasing the oil recovery.