Mechanism of CO2 enhanced oil recovery in kerogen pores and CO2 sequestration in shale: A molecular dynamics simulation study

The strong interactions between kerogen and CO2 give an opportunity to enhance shale oil recovery (EOR) by CO2 injection, and also reduce greenhouse gas emissions through CO2 capture and geological storage. Understanding the mechanism of CO2 enhanced shale oil recovery is important significance for achieving optimum shale oil exploration and development. In this work, the oil storage behavior and mechanism of CO2 enhanced shale oil recovery in kerogen pores are studied by using molecular dynamics (MD) simulations. For oil storage, the density curves are calculated and the results show that it can be found that there are two adsorption layers near the wall, and the slight fluctuations density near the two opposite wall presents different trends due to the roughness of the walls surface. For flooding behavior, CO2 molecules are easily dissolved into the oil phase and drive out most of the oil within the kerogen slit pores after 3 ns with differential pressure of 10 MPa for our model. The higher differential pressure corresponds to earlier CO2 breakthrough and smaller value of oil recovery. Oil molecules located in smaller pores require a longer flooding time to be displaced. The displacement is the main mechanism of oil recovery in nanoscale kerogen pores for CO2 flooding. The diffusion coefficients of CO2/oil and interaction energy are calculated and analyzed. Further, the CO2 storage capacity in shale formations are computed and its value is 466 kg/m3. This work reveals oil storage behavior and the mechanism of CO2 flooding in shale reservoirs, and the results are significant for the CO2 enhancement of oil recovery, and for CO2 capture and storage in kerogen pores.