Study on the Migration Characteristics of Oil Under CO2 Flooding in Tight Reservoir Based on Molecular Dynamics

Summary The increasing challenges of high water content and declining productivity in water-injected tight reservoirs underscore the need for a better understanding of residual oil recovery mechanisms and the enhancement of oil mobility to improve recovery rates. Carbon dioxide injection has demonstrated significant potential in mobilizing residual oil; however, the precise mechanisms driving this process remain insufficiently understood. This study systematically examines the migration dynamics of residual oil displaced by CO2 under unsaturated oil conditions within nanopores of tight reservoirs. Moreover, the research emphasizes the pivotal role of pore structure roughness in optimizing oil recovery efficiency. The findings reveal the following: (1) Phase mixing between CO2 and decane increases molecular spacing, resulting in volumetric expansion. Hydrogen bonding is the dominant mechanism driving CO2 adsorption on hydroxylated quartz surfaces, which directly leads to the desorption of membrane-like residual oil, exhibiting a distinct layer-by-layer peeling behavior. (2) The displacement of droplet-like residual oil in pore throats requires overcoming high-energy barriers, with CO2 influencing the process in three key aspects: (i) enhancing droplet deformability through phase mixing; (ii) shielding liquid-solid interface interactions via a molecular shell, thereby significantly reducing fluid migration resistance through liquid-liquid slip; and (iii) improving oil-water interfacial properties and reducing interfacial tension (IFT). (3) The effect of pore roughness r on droplet migration primarily manifests as changes in inertial forces. As r increases, alterations in flow field characteristics occur, leading to vortex formation and backflow. These findings provide new insights into the microscopic mechanisms of residual oil mobilization, offering valuable implications for enhancing recovery in tight reservoirs.