Microscopic Mechanism of CO2 Huff-N-puff Promoting Shale Oil Mobilization in Nanopores

CO2 huff-n-puff is a promising approach for enhanced recovery and sequestration of CO2 in shale reservoirs. Here, we investigate three stages of CO2 huff-n-puff promoting shale oil mobilization from organic-inorganic nanopores by molecular dynamics simulation. We first defined the captured state of shale oil based on interaction energy and density. The results show that the capture range of kerogen for shale oil was 3.5 times that of kaolinite, but due to the presence of active components, the adsorption density and interaction energy of shale oil on the surface of kerogen were lower than those of kaolinite. In the CO2 soaking stage, although CO2 has a desorption effect on shale oil near two kinds of walls, stripping shale oil near the inorganic surface was more effective than the kerogen surface. In addition, when the CO2 adsorbed on the surface of kaolinite slightly deviates from the equilibrium position, the attraction effect of kaolinite on CO2 will be transformed into the repulsion effect, making CO2 adsorption unstable and easy desorption. In the CO2 puff stage, an interesting result in the 10 MPa CO2 puff pressure, as opposed to the ideal model, demonstrates a significant capacity of CO2 to efficiently dissociate the "bullet head" structure of the medium component blocking the pore exits through a synergistic effect of miscible phase action, viscosity reduction, and swelling, resulting in a notable enhancement of overall shale oil recovery by more than 37%. This work first investigates the CO2 huff-n-puff mobilization of shale oil from multiple stages and effectively reveals the promoting effects of CO2 on different components of shale oil in each stage of huff-n-puff.