CO2 Extraction of Shale Oil from Nanoscale Variable-Diameter Blind Pores from a Molecular Dynamic Perspective

Shale oil, despite its substantial reserves and significant development potential, presents a great extraction challenge. The existence of micro- and nanopores within shale formations renders conventional recovery methods largely ineffective, resulting in a limited recovery factor. With stronger adsorption, CO2 enables it to enter the micro–nanopores, enhancing oil recovery. Current research has primarily focused on studying the flow mechanisms of CO2 and oil within the pores and the mechanisms of enhanced oil recovery. Research on the mechanism of enhanced oil recovery in pore tips and blind pores is limited. In our study, in order to investigate the mechanism of enhanced oil recovery, variable-diameter blind pores at the nanoscale are constructed. The influences of displacement times, system temperature, injection pressure, and the pore size combination on CO2-enhanced oil recovery are systematically investigated using molecular dynamics simulations. The simulation results show that the mechanism of CO2-enhanced oil recovery in variable-diameter pores is different from that in the traditional single-pore-size structure. In the variable-diameter pores, CO2 molecules preferentially enter the narrow region in the model. The oil in the wide region (the larger-diameter sections of the blind pores, as opposed to the narrower throats) is mobilized and recovered after the CO2 blockage has been generated. In addition, elevated system temperature improves intermolecular thermal motion and interactions. With the system temperature increasing, the CO2 molecules can enter the nanopore more readily, ultimately enhancing oil recovery. Higher injection pressure serves as a powerful driving force for CO2, allowing more CO2 molecules to enter the nanopore. However, the ultimate recovery will decrease if the injection pressure continues to increase after 20 MPa. This study provides support and recommendations for the investigation of the mechanisms of EOR using CO2 in nanoscale variable-diameter blind pores.