Abstract

CO2 flooding plays a crucial role in enhancing oil recovery (EOR) in heterogeneous reservoirs; however, the reservoir’s vertical rhythmic characteristics and permeability gradients significantly influence the flooding efficiency. Unlike homogeneous reservoirs, CO2 migration in heterogeneous reservoirs is jointly controlled by gravitational overburden and differences in flow conditions, which can easily lead to gas short-circuiting and the formation of dominant flow paths, thereby reducing the degree of crude oil mobilization. However, systematic research on the mechanisms of CO2 flooding under different rhythm types and permeability difference remains relatively scarce. In this study, two-dimensional large-scale physical model experiments were conducted using stratified core plates with a planar size of 30 × 30 cm2 and a single-layer thickness of 1 cm. The experiments were performed at 70 °C and 18 MPa, corresponding to the target reservoir conditions, with CO2 injected from the inlet side and outlet pressure controlled by a backpressure valve. Under these conditions, CO2 remained in the supercritical state during displacement. These experiments were designed to comparatively investigate the effects of reservoir rhythm and permeability contrast on pressure distribution, CO2 migration patterns, and crude oil mobilization. The study elucidated the mechanisms by which reservoir heterogeneity influences the effectiveness of CO2 flooding. The results show that the positive rhythmic unit delays upward CO2 migration and gas breakthrough because of the low-permeability top layer, resulting in the highest ultimate oil recovery of 73.35%. In contrast, the reverse rhythmic unit promotes rapid CO2 breakthrough through the high-permeability top layer and forms dominant flow paths, causing insufficient mobilization of the middle and bottom layers and yielding the lowest oil recovery of 51.03%. In the sandwich-type rhythmic unit (low–high–low permeability configuration), the interaction between the high-permeability middle layer and gravity override enhances mobilization in the top and middle layers, whereas oil mobilization in the bottom layer remains limited. Under interlayer conditions, increasing the permeability contrast from three-fold to five-fold strengthens preferential flow in the high-permeability layer and reduces oil recovery from 65.58% to 60.99%.

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