Molecular Insights into the EOR Mechanism of Water, CO₂, and CO₂–WAG Flooding in Heterogeneous Nanochannels: A Molecular Dynamic Simulation

In tight oil exploitation, the water flooding, CO₂ flooding, and CO₂ water gas alternate (CO₂-WAG) flooding are three commonly adopted methods to enhance the oil recovery (EOR), and the heterogeneous of reservoir has crucial influence on the oil sweep volume and oil displacement efficiency. In this work, molecular dynamic simulation was employed to investigate the displacement behavior in these three flooding methods in the heterogeneous tight reservoir. First, a single nanochannel was used to investigate the different displacement performances in these three flooding methods. Then a double nanochannel model were constructed to mimic the heterogeneous tight reservoir. The threshold injection pressure of three flooding modes was calculated. The number of displaced oil molecules was used to evaluate the oil displacement efficiency. Simulation results showed that the threshold injection pressure gave the following order: CO₂ flooding < CO₂-WAG flooding < water flooding. In double nanochannel systems, the injecting water passed through the large-sized nanochannel, and the oil inside the small-sized nanochannel could not be displaced in water flooding and CO₂-WAG flooding. In CO₂ flooding, some oil molecules inside two nanochannels were displaced, and the oil displacement efficiency in the large-size nanochannel was higher than that in the small-size nanochannel. The comparison of these three flooding methods showed that the CO₂-WAG flooding has priority over the other two flooding methods, exhibiting both low threshold injection pressure of gas flooding and high oil displacement efficiency of water flooding; consequently, high EOR could be achieved. Our work was helpful to deeply understand the microscopic oil displacement processes of different flooding methods, and it has reference value for the development of tight oil reservoirs.