Molecular Dynamics Simulation of Water Film Effect on CO2 Flooding in High-Water-Cut Reservoirs

Abstract Residual oil in reservoirs is trapped by surrounding water, forming a dead-end residual oil sealed by a water film. Enhancing the recovery rate of this residual oil is an urgent issue that requires resolution. To overcome the shielding effect of the water film in dead-end pores, CO2 injection is strategically employed to destabilize the aqueous structure, thereby facilitating the oil mobilization. Molecular dynamics simulations are employed to investigate the dynamic behavior of the CO2–water film interface within a CO2–water film–oil three-phase system under bulk conditions. We systematically investigated the effects of pressure, water film thickness, salt type, and salt concentration on the CO2–water film interface dynamics. Results indicate that an increased water film thickness extends breakthrough time across all pressure conditions. There exists a thickness threshold beyond which the breakthrough time undergoes an exponential rise, effectively preventing CO2 penetration. Moreover, it is demonstrated that the promotion or inhibition effect of different salts on this process is primarily determined by their electrostatic interactions with water molecules, and there exists an optimal concentration for all salts. These findings indicate that the CO2–water film interface dynamics can be modulated by altering the salinity. This work offers valuable guidance for enhancing the recovery from dead-end residual oil reservoirs.