Impact of interlayer mass transfer on CO2 flooding performance in heterogeneous tight reservoirs

CO2 flooding can effectively enhance oil recovery in tight oil reservoirs. However, the mobilization characteristics and interlayer mass transfer mechanism of CO2 and crude oil in heterogeneous tight reservoirs with permeability ratio (PR) remain unclear. We simulated heterogeneous tight reservoirs by fabricating artificial PR cores and conducted CO2 flooding experiments under different miscibility states. Moreover, nuclear magnetic resonance was employed to distinguish and quantify the mobilization characteristics of high-permeability layers (HPLs) and low-permeability layers (LPLs), revealing interlayer mass transfer mechanisms. The results indicated that, compared with those of immiscible and partially miscible flooding, miscible flooding yielded a higher overall recovery, with the recovery continuing to increase after CO2 breakthrough. This is primarily due to the significant increase in the recovery efficiency of the LPL. At this stage, displacement becomes less effective, and interlayer mass transfer mechanisms, such as extraction and diffusion between the HPL and LPL, become the dominant drivers of oil recovery. In the miscible state, oil–gas interfacial tension vanishes, thus enhancing diffusion and extraction effects. This significantly promotes interlayer mass transfer, effectively mobilizing oil in the LPL. The contributions of interlayer mass transfer and displacement to oil mobilization in the LPL during miscible flooding were quantified, revealing that the final contribution of the former is 1.69 times greater than that of the latter. This verifies the interlayer mass transfer as the primary mechanism for mobilizing oil in the LPL after CO2 breakthrough, emphasizing its critical role in recovery processes.