Molecular simulation of the dynamic distribution of complex oil components in shale nanopores during CO2-EOR

Due to the high clay content of shale reservoirs, developing shale oil by water flooding is difficult. CO2 is easier to inject into formations than water, significantly increasing shale oil production by reducing the viscosity of shale oil and increasing the gas-oil ratio. Moreover, CO2 flooding can also effectively reduce emissions. However, CO2 flooding changes the formation fluid's phase behavior and composition. This causes the fluid to redistribute in the nanopore. Based on the field data, it can be seen that the changes of formation fluid components are obvious in the three stages of CO2 injection, the initial state of production and depressurization production. Therefore, this paper uses the molecular dynamics method to study the changes in adsorption layer thickness, adsorption ratio of components, interaction energy, and self-diffusion coefficient of fluids in organic and inorganic pores under these three stages. The results show that the adsorption layer thickness gradually increases during CO2 flooding. After CO2 injection, the fluid components in kerogen are significantly desorbed. The desorption of the components in the small pore size of illite, feldspar, and calcite is insignificant; the desorption of the components in the large pore size is significant, and the desorption of the components in quartz is insignificant. The interaction energy between fluid and pores decreases first and then increases during CO2 flooding, and the types of interaction energy between pores and fluids differ. The results show that CO2 injection can increase shale oil production. New ways to enhance oil recovery are needed to increase shale oil production in the later stage of depressurization production.