Molecular insight into the oil displacement mechanism of CO₂ flooding in the nanopores of shale oil reservoir

With the increasing demand for petroleum, shale oil with considerable reserves has become an important part of global oil resources. The shale oil reservoir has a large number of nanopores and a complicated mineral composition, and the effect of nanopore confinement and pore type usually makes the effective development of shale oil challenging. For a shale oil reservoir, CO₂ flooding can effectively reduce the oil viscosity and improve the reservoir properties, which can thus improve the recovery performance. In this study, the method of non-equilibrium molecular dynamics (NEMD) simulation is used to simulate the CO₂ flooding process in the nanoscale pores of shale oil reservoir. The performance difference between the organic kerogen slit nanopore and four types of inorganic nanopores is discussed. Thus, the effects of nanopore type and displacement velocity on the nanoscale displacement behavior of CO₂ are analyzed. Results indicate that the CO₂ flooding process of different inorganic pores is different. In comparison, the displacement efficiency of light oil components is higher, and the transport distance is longer. The intermolecular interaction can significantly affect the CO₂ displacement behavior in nanopores. The CO₂ displacement efficiency is shown as montmorillonite, feldspar > quartz > calcite > kerogen. On the other hand, it is found that a lower displacement velocity can benefit the miscibility process between alkane and CO₂, which is conducive to the overall displacement process of CO₂. The displacement efficiency can significantly decrease with the increase in displacement velocity. But once the displacement velocity is very high, the strong driving force can promote the alkane to move forward, and the displacement efficiency will recover slightly. This study further reveals the microscopic oil displacement mechanism of CO₂ in shale nanopores, which is of great significance for the effective development of shale oil reservoirs by using the method of CO₂ injection.